Elec Power Electronics Indiabix 4j6g4x

Excel Review Center - Power Electronics 1.

A cycloconverter can be A. B. C. D.

step down step up step down or step up - Ans none of the above

Explanation: The output frequency can be more or less than input frequency. 2.

In the below figure the average load current is 15 A. The rms value of transformer secondary current is

A. B. C. D.

15 A 10.61 A - Ans 7.5 A 14.14 A

Explanation: RMS secondary current = 3.

= 15/1.414 = 10.608 A.

In a 3 phase fully controlled converter the firing frequency is A. B. C. D.

3 times the line frequency 6 times the line frequency - Ans 9 times the line frequency 12 times the line frequency

Explanation: It is a 6 pulse converter. 4.

Two thyristor of same rating and same specifications A. B. C. D.

will have equal turn on and turn off periods will have equal turn on but unequal turn off periods may have equal or unequal turn on and turn off periods - Ans will have unequal turn on and turn off periods

Explanation: Turn on and turn off times of similar SCRs may not be same.

1


The amount of dielectric heating is inversely proportional to frequency. A. B.

True False - Ans

Explanation: It is proportional to frequency. 6.

A semiconverter feeding an RLE load operates in A. B. C. D.

first quadrant only - Ans first and second quadrants first and fourth quadrants first and third quadrants

Explanation: Semiconvertor operates only in first quadrant. 7.

The phenomenon of overlap in converter operation due to A. B. C. D.

source resistance source inductance - Ans both source resistance and source inductance high value of firing angle

Explanation: Source inductance delays turn off and hence causes overlap. 8.

A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is A. B. C. D.

20 W 50 W - Ans 92 W 128 W

Explanation: . 9.

For a 12 pulse operation, the two 6 pulse units are fed by A. B. C. D.

Y-Y Δ-Δ Y-Y Y-Δ

and Y - Y transformers and Δ - Δ transformers and Y - Δ transformers - Ans and Y - Δ transformers

Explanation: 30° phase difference is necessary.

2


In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is A. B. C. D.

p-a P - Ans p+a p - 2a

Explanation: Since conduction is from a to p + a, period of conduction is p in each half cycle. 11.

An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be A. B. C. D.

50 A 25 A less than 50 A but more than 25 A - Ans less than 25 A

Explanation:

. 12.

The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, vbecomes A. B. C. D.

about 100 times about 10 times about 3 times - Ans about twice

Explanation: v ∝ (i)1/4 ∝ (100)1/4 ≈ 3. 13.

In a step down chopper using pulse width modulation, T on = 3 x 10-3 and Toff = 1 x 10-3s. The chopping frequency is A. B. C. D.

333.33 250 - Ans 500 1000

Explanation: .

3


Thyristors are suitable for dc circuit breakers but not for ac circuit breakers. A. B.

True False - Ans

Explanation: They are suitable for both dc and ac circuit breakers. 15.

Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time. A. B. C. D.

Both A and R are correct and R is correct explanation of A - Ans Both A and R correct but R is not correct explanation of A A is correct but R is wrong A is wrong but R is correct

Explanation: Low turn off time enables fast switching. 16.

The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by A. B. C. D.

forward characteristics of thyristors reverse recovery characteristics of thyristors - Ans both forward and reverse recovery characteristics of thyristors none of the above

Explanation: Value of C is proportional to maximum difference between reverse charge of SCRs in series. 17.

A chopper A. B. C. D.

converts constant voltage dc into ac and then into variable voltage dc converts constant voltage dc into variable voltage dc directly - Ans converts ac of one frequency into ac of another frequency converts ac to dc

Explanation: A chopper is a dc-ac converter. It converts a fixed dc voltage into a variable dc voltage. 18.

Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous. A. B. C. D.

Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A A is correct but R is wrong - Ans A is wrong but R is correct

4

Excel Review Center - Power Electronics Explanation: If duty cycle is low, current may become discontinuous. 19.

In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally A. B. C. D.

Equal - Ans different equal or different different but sometimes equal

Explanation: Firing angles are kept equal to get symmetrical output voltage. 20.

An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is A. B. C. D.

16% maximum 40% of maximum - Ans 60% of maximum 84% of maximum

Explanation: Vl = V a. Therefore V l2 - V2 (a). Heating is proportional to a. 21.

A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is Is and rms value of load current in Irms. Then A. B. C. D.

Is = Is = Is = Is =

Irms irrespective of the value of firing angle - Ans 0.5 Irms irrespective of the value of firing angle Irms if firing angle is less than 30° Irms if firing angle is more than 30°

Explanation: . 22.

The terminals of a power BJT are called A. B. C. D.

emitter, base, collector - Ans emitter, base, drain source, base, drain source, base, collector

Explanation: In a BJT terminals are emitter, base and collector irrespective of rating. 23.

In single phase half wave regulator, the average current over one full cycle

5

Excel Review Center - Power Electronics A. B. C. D.

is always positive may be positive or negative may be negative is always negative - Ans

Explanation: The input current during positive half cycle is less than that during negative half cycle. 24.

The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance R C (along with parallel connected diode) A. B. C. D.

limits charging current limits discharging current - Ans limits both charging and discharging currents none of the above

Explanation: Discharge current flows through Re . 25.

A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current A. B. C. D.

are similar are not similar - Ans may be similar or dissimilar are similar only if firing angle is zero

Explanation: When freewheeling diode conducts output current exists but output voltage is zero. 26.

Current derating may be necessary when thyristors are operating in A. B. C. D.

series Parallel - Ans series or parallel none of the above

Explanation: Since current does not divide equally, current derating is necessary. 27.

If duty cycle is a and the load is a resistance R, the rms value of output voltage is A. B. C.

aV a V - Ans a2V

6

Excel Review Center - Power Electronics

D. Explanation: RMS output voltage VL = a V. 28.

For a BJT a = 0.98, then β = A. B. C. D.

0.02 0.5 4.9 48 - Ans

Explanation:

29.

Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR A. B. C. D.

Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A A is correct but R is wrong A is wrong but R is correct - Ans

Explanation: Power transistor does not have high 30.

rating.

A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is A. B. C. D.

4 5 - Ans 6 8

Explanation: . 31.

In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be A. B. C. D.

400 V 800 V 200 V - Ans 2002 V

7

Excel Review Center - Power Electronics Explanation: In bridge connection PIV is half of that in M-2 connection. 32.

A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be A. B. C. D.

about 350 rpm about 250 rpm about 175 rpm about 125 rpm - Ans

Explanation: Average output voltage ∝ cos a and therefore reduced to 0.707 of the value for a = 0. Speed is nearly proportional to voltage. 33.

In a multiphase chopper, all choppers operate together. A. B.

True False - Ans

Explanation: They operate in a sequence. 34.

The terminals of a power MOSFET are called A. B. C. D.

emitter, base, collector source, gate, drain - Ans source, base, drain emitter, gate, drain

Explanation: In a MOSFET terminals are source, gate, drain irrespective of rating. 35.

Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current. Reason (R): The duration of surge current is very small. A. B. C. D.


Explanation: Since duration of surge current is small, the associated energy is small. 36.

A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is a. The period of conduction of SCR and free wheeling diode respectively are A.

a < ωt < p and p < ωt < (2p + a) - Ans

8

Excel Review Center - Power Electronics B. C. D.

a < ωt < p and p < ωt < 2p a < ωt < (p + a) and (p + a) < ωt < 2p a < ωt < p and p < ωt < (2p - a)

Explanation: SCR conducts from a to p and free wheeling diode from p to (2p + a). 37.

Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse. Reason (R): Transistor find widespread application in power electronic circuits. A. B. C. D.


Explanation: Transistors are used but not very widely. 38.

In a thyristor the gate current is increased, then A. B. C. D.

anode current will increase anode current will decrease anode current will remain constant - Ans anode current may increase or decrease

Explanation: Gate current does not affect the magnitude of anode current. 39.

Thyristors are not suitable for logic circuits. A. B.

True False - Ans

Explanation: Thyristors can be used for logic circuits. 40.

A single phase half wave rectifier is feeding a resistive load. Input voltage v = Vm sin ωt. The output dc voltage is Vdc and output rms voltage is Vrms . If firing angle is 180°, Vdc and Vrms respectively are A.

0 and 0 – Ans

B. C.

0 and – Vm

D. Explanation:

9


Put a = 180° in Eqs Otherwise also a = 180° means no conduction. Hence zero output voltage. 41.

The number of leads in an SCR are : A. B. C. D.

2 3 - Ans 4 5

Explanation: Anode, cathode and gate. 42.

In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = Vm sin ωt. The expression for dc output voltage is

A.

B.

C.

- Ans

D. Explanation: Due to inductive load the period of conduction is a to p + a in each half cycle. 43.

A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be A. B. C. D.

6 μs less than 6μs more than 6 μs - Ans either 6 μs or less

Explanation: Inductive load increases the turn on time. 44.

The total number of leads in reverse conducting thyristor is

10


2 3 - Ans 1 4

Explanation:

45.

Assertion (A): A TRIAC is a bidirectional SCR. Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions. A. B. C. D.


Explanation: Since a TRIAC can conduct in both directions it is bi directional SCR. 46.

If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then A. B. C. D.

Vdc1 = 0.5 Vdc2 for all values of firing angle - Ans Vdc1 = Vdc2 for all values of firing angle Vdc1 ≤ 0.5 Vdc2 for all values of firing angle Vdc1 ≥ 0.5 Vdc2 for all values of firing angle

Explanation: and Vdc = 47.

(1 + cos a).

A semi converter operates in two quadrants. A. B.

True False - Ans

Explanation: It operates only in quadrant.

11


Figure shows a chopper feeding RLE load, The free wheeling diode conducts when

A. B. C. D.

thyristor is on thyristor is off - Ans both when thyristor is on and thyristor is off partly when thyristor is off and partly when thyristor is on

Explanation: FD is forward biased only when thyristor is off. 49.

Assertion (A): An ideal rectifier should have unity harmonic factor Reason (R): An ideal rectifier should have unity displacement factor. A. B. C. D.


Explanation: Ideal rectifier should have zero harmonic factor and unity displacement factor. 50.

A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about A. B. C. D.

127 120 110 100

V V - Ans V V

Explanation: Using Eq

= 0.933. Hence Vdc for 30° firing angle = 0.933 x 127 = ≈ 120 V.

12


A dc circuit breaker must use A. B. C. D.

natural commutation forced commutation - Ans both natural and forced commutation either natural or forced commutation

Explanation: Natural commutation is possible only in ac circuits. 2.

In a single phase full wave regulator feeding R-L load, the minimum firing angle can be zero. A. B.

True - Ans False

Explanation: Range of control of a is 0 ≤ a ≤ p. 3.

An RC snubber circuit is used to protect a thyristor against : A. B. C. D.

false triggering failure to turn on switching transients - Ans failure to commutate

Explanation: Especially high 4.

during switching.

A DIAC has two terminals, anode and cathode. A. B.

True False - Ans

Explanation: They are designated as T1 and T2 because diac can conduct in both directions. 5.

A single phase semiconverter has a freewheeling diode. If the firing angle is a and the load is purely resistive, the periods of conduction and freewheeling respectively are A. B. C. D.

p - a and 0 - Ans p - a and a p + a and 0 p + a and a

Explanation: Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation

13


. A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When a = 0, speed is 600 rpm. If a = 90°, the speed is likely to be A. B. C. D.

about 300 rpm - Ans about 150 rpm about 75 rpm about zero

Explanation: Average output voltage is reduced to 50% if a = 90°. Hence speed is also reduced to 50%. 7.

A static induction thyristor A. B. C. D.

is a majority carrier device is a minority carrier device - Ans either a majority carrier or minority carrier device has a total number of 2 leads

Explanation: It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low. 8.

A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then one half wave of input will give rise to A. B. C. D.

one half wave of output one full wave of output - Ans two full waves of output either (b) or (c)

Explanation: Since output frequency is 2f, one half wave of input must equal one full wave of output. 9.

Which of these triggering methods for series connected SCRs use LASCR in the gate circuit? A. B. C. D.

Resistance triggering Simultaneous triggering Sequential triggering Optical triggering - Ans

Explanation: Optical triggering uses LASCR.

14


A 3 phase ac regulator uses A. B. C. D.

3 thyristors 6 thyristors - Ans 9 thyristors 12 thyristors

Explanation: Two thyristors for each phase. 11.

The average load current supplied by a thyristor depends on A. B. C. D.

firing angle - Ans firing frequency magnitude of gate current all of the above

Explanation: The firing angle controls the period of conduction and hence average load current. 12.

A dc chopper is feeding an RLE load. The maximum steady state ripple is

A.

- Ans

B. C. D. Explanation: (Imax - Imin) max = 13.

.

In a 3 phase full wave regulator feeding a star connected resistance load the input voltage is 400 V line to line. The firing angle is 160°. The line to line output voltage would be A. B. C. D.

400 V about 100 V about 20 V Zero - Ans

Explanation: In a three-phase full wave regulator, output voltage is zero if a ≥ 150°.

15


A thyristor is reverse biased. A positive gate pulse is applied. The thyristor A. B. C. D.

will be turned on may or may not turn on will not turn on - Ans will turn on after sometime

Explanation: Thyristor can turn on only when it is forward biased. 15.

A single phase series converter is used for A. B. C. D.

high voltage output - Ans high current output high voltage and high current output improving the circuit efficiency

Explanation: Series connection increases the voltage. 16.

A single phase half wave controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR A. B. C. D.

is forward biased is reverse biased - Ans may be forward biased or reverse biased forward biased initially but reverse biased afterwards

Explanation: Free wheeling diode is forward biased only when SCR is reverse biased. 17.

An SBS can conduct is one direction only. A. B.

True False - Ans

Explanation: It is a bilateral switch. 18.

A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is A. B. C. D.

3 4 6 - Ans 10

Explanation: .

16


Assertion (A): An inverter does not require forced commutation Reason (R): A series inverter is a forced commutation inverter. A. B. C. D.


Explanation: Some inverters require forced commutation. 20.

In a single phase full wave converter (M - 2 connection), the rms transformer voltage is 300 V across each half of secondary. The PIV for each thyristor is A. 300 V B. C. 6002 V - Ans D. Explanation: PIV is twice the peak voltage across each half of secondary.

21.

The number of doped regions in a SCR is A. B. C. D.

2 3 4 - Ans 5

Explanation: p = n = p = n. 22.

All inverters use forced commutation. A. B.

True False - Ans

Explanation: Line commutated inverters are also used especially in HVDC. 23.

In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time. A. B.

True False - Ans

Explanation: In 180° mode three thyristors conduct simultaneously.

17


In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then A. B. C. D.

average thyristor current will be less than 20 A but rms thyristor current will be more than 20 A both the average and rms thyristor currents will be less than 20 A Ans both the average and rms thyristor currents will be more than 20 A rms thyristor current will be 20 A and average thyristor current will be 10 A

Explanation: Each thyristor conducts for 50% of time. 25.

A reactor is used in a dual converter operating in non-circulating current mode. A. B.

True False - Ans

Explanation: Reactor is used only for circulating current mode. 26.

Which of these commutation methods uses an auxiliary SCR? A. B. C. D.

Class A Class B Class C - Ans Class D

Explanation: Class C commutation uses an auxiliary SCR and it is also called auxiliary commutation. 27.

A thyristor needs protection against

A. high B. high C. both high

and high

D. either high

or high

- Ans

Explanation: Both high

and high

can destroy SCR.

18


Two semiconverters are connected in series to form a single phase series converter. The input v = Vm sin ωt and a1 = a2 are the firing angles of the two converters. If a1 = a2= 0, the dc output voltage is

A. B.

C.

- Ans

D. Explanation: Output voltage is 29.

for each.

In integral cycle control the period of on-off control should be more than mechanical time constant of load. A. B.

True False - Ans

Explanation: It should be less than mechanical time constant. 30.

A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is A. B. C. D.

zero about 1 V same as input voltage - Ans twice the input voltage

Explanation: Before triggering the whole of applied voltage is across SCR and load current is zero. 31.

In a 3 phase full wave regulator feeding resistance load connected in star, the possible range of firing angle a is A. B. C. D.

0≤ 0≤ 0≤ 0≤

a ≤ 180° a ≤ 150° - Ans a ≤ 120° a ≤ 90°

19

Excel Review Center - Power Electronics Explanation: Since output voltage is zero if a ≥ 150°, the possible range of firing angles is 0 ≤ a ≤ 150°. 32.

Assertion (A): A dual converter may be circulating current type or non circulating current type. Reason (R): In circulating current dual converter only one converter can be operated at one time. A. B. C. D.


Explanation: In circulating current dual converter both converters operate together. 33.

Current chopping leads to A. B. C. D.

over voltage - Ans over current both over voltage and over current either over voltage or over current

Explanation: e=L 34.

. In current chopping

is high. Hence e is high.

A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of load current is A. B. C. D.

40 A 20 A - Ans 10 A 5A

Explanation: Two resistances are in series at any time. 35.

In an SCR, the anode current is controlled by A. B. C. D.

gate current only external circuit only - Ans both gate current and external circuit none of the above

Explanation: Once SCR has been triggered the gate has no control. Only external resistance controls the anode current. 36.

The rated current of a thyristor is 40 A. The holding current will be about :

20


20 A 1A 100 mA 10 mA - Ans

Explanation: The holding current is much less then 1% of rated current. 37.

A cycloconverter is A. B. C. D.

ac-dc converter dc-ac converter dc-dc converter ac-ac converter - Ans

Explanation: It changes ac at frequency f into ac at a frequency different from f. 38.

The number of leads in a power semiconductor diode are A. B. C. D.

2 - Ans 3 4 2 or 3

Explanation: A diode has 2 leads irrespective of its rating. 39.

If ton and toff are the turn on and turn off times of a thyristor, then A. B. C. D.

ton and toff are almost equal ton >> toff toff >> ton - Ans ton is about twice toff

Explanation: Turn on time is less than 3 μs while turn off time is between 10-300 μs. 40.

Assertion (A): The gate current of an SCR is always in the form of pulses. Reason (R): The gate losses, in pulse triggering, are low. A. B. C. D.


Explanation: Gate current in the form of pulses is commonly used because gate losses are low.

21


UJT can be used for A. B. C. D.

firing an SCR - Ans turning off an SCR both firing and turning off an SCR none of the above

Explanation: UJT relaxation oscillator is used to provide gate pulses to SCR for triggering. 42.

In a series inverter supplying a load resistance R, the commutating elements L and C should be such that

A. B.

- Ans

C. D. Explanation: Load should be under damped. 43.

In a full wave converter (M - 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be A. B. C.

30 A 152 A 15 A – Ans

D. Explanation: In both the connections the output voltage and peak thyristor current are the same. Both are full wave rectifiers. 44.

Induction heating systems require low frequency inverters. A. B.

True False - Ans

Explanation: It uses high frequency inverter.

22


A single phase half wave controlled rectifier circuit is feeding on RL load. The circuit has a freewheeling diode. The input voltage is v = Vm sin ωt. If I0 is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is

A.

B.

- Ans

C. D. Explanation: Load is short-circuited by free wheeling diode. 46.

HVDC transmission systems generally use A. B. C. D.

12 pulse converters - Ans 6 pulse converters 3 pulse converters either 12 or 6 pulse converters

Explanation: To reduce ripple, 12 pulse converters are used in HVDC. 47.

A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is a. During one cycle of input voltage, the total duration of conduction of freewheeling diode is A. B. C. D.

a 2 a - Ans 4a 0.5 a

Explanation: Freewheeling diode conducts for a° in each half cycle. 48.

During forward blocking state of SCR, the voltage and current respectively are A. B. C. D.

high and high low and low high and low - Ans low and high

Explanation: Voltage is high but current is nearly zero.

23


In a 3 phase full converter the average load current is 150 A. The average thyristor current is A. B. C. D.

150 A 75A 50 A - Ans 25 A

Explanation: Each thyristor conducts for one-third of one cycle. 50.

The two modes of operation of three phase bridge inverters are known as A. B. C. D.

60° mode and 120° mode 90° mode and 120° mode 120° mode and 180° mode - Ans 120° mode and 240° mode

Explanation: In 180° mode each thyristor conducts for 180° and in 120° mode each thyristor conducts for 120°. 1.

In a dual converter both converters work as rectifiers. A. B.

True False - Ans

Explanation: One operators as rectifier and the other as inverter. 2.

A single phase full wave regulator feeds R-L load. The best gating signal is A. B. C. D.

short duration pulses long duration pulses pulse train - Ans either (a) or (b)

Explanation: To ensure proper triggering pulse train in necessary because load is R - L. 3.

A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if A. B. C. D.

load torque is constant load torque increases with increase in speed - Ans load torque decreases with increase in speed either (a) or (b)

Explanation: If load torque is constant, the change in speed is very small.

24


A cycloconverter cannot give a frequency higher than input frequency. A. B.

True False - Ans

Explanation: Step up cycloconverter can give a frequency higher than input frequency. 5.

A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is A. B. C. D.

114°C 164°C 94° C 84° C - Ans

Explanation: . 6.

In a single phase dual converter, a1, and a2 are firing angles of the two converters. Then

A. a1 - a2 = B. a1 + a2 = p C. a1 - a2 = p D. a1 + a2 = Explanation: Since one converter is rectifying and the other is inverting Vdc1 = -Vdc2 or a1 + a2 = p. 7.

The number of gates in an SCR is A. B. C. D.

8.

1 - Ans 3 4 6

The waveshape of output voltage of half bridge inverter is A. B. C. D.

sinusoidal Square - Ans triangular either (a) or (b)

25


A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be A. B. C. D.

about 1350 rpm about 720 rpm - Ans about 360 rpm either (b) or (c)

Explanation: In this case synchronous speed at 50 Hz is 1500 rpm. Therefore synchronous speed at 25 Hz is 750 rpm. Since slip is about 3 - 5%, speed is about 720 rpm. 10.

The gate current required to turn on an SCR is A. B. C. D.

few amperes a few mA - Ans almost equal to anode current about 50% of anode current

Explanation: Only a very small gate current is required to trigger SCR into conduction.

11.

A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct A. B. C. D.

from 0 to 180° when v > 50 V - Ans from 0 to 90° When v < 50 V

Explanation: Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V. 12.

The symbol in figure is for

A. B. C. D. 13.

DIAC P-N junction diode Schottky diode - Ans None of the above

In a 3 phase semiconverter, the firing frequency is A.

equal to line frequency

26


3 times the line frequency - Ans 6 times the line frequency 12 times the line frequency

Explanation: Since the circuit has 3 thyristors, firing frequency is three times the line frequency. 14.

Present day HVDC converters are all A. B. C. D.

3 pulse converters 6 pulse converters 12 pulse converters - Ans either 6 pulse or 12 converters

Explanation: To reduce ripple 12 pulse converters are used. 15.

A single phase full converter gives maximum and minimum output voltage at firing angles of A. B. C. D.

0 and 180° respectively - Ans 180° and 0° respectively 0 and 90° respectively 90° and 0° respectively

Explanation: Output voltage is maximum if a = 0 and is minimum if a = 180°. 16.

A thyristor has its gate current zero. To start conduction the forward voltage must exceed A. B. C. D.

about 0.5 V forward breakover voltage - Ans on state voltage peak working off state forward voltage

Explanation: With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage. 17.

Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR Reason (R): Voltage across a capacitor can not change instantaneously. A. B. C. D.


Explanation: Since voltage across capacitor cannot change instantaneously it protects SCR

27


against high 18.

.

In dielectric heating, the thyristor circuits consist of A. B. C. D.

rectifier-chopper combination controlled rectifier ac regulator rectifier-inverter combination - Ans

Explanation: Input ac is rectified and then converted to high frequency ac. 19.

On load tap changer using thyristors can have a maximum of two taps. A. B.

True False - Ans

Explanation: Any number of taps are possible. 20.

Assertion (A): A cycloconverter may be line commutated or forced commutated Reason (R): A step up cycloconverter requires forced commutation. A. B. C. D.

21.


In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is

A. B. C. D.

200 V 2(200) V 2(400) V - Ans 800 V

Explanation: PIV = 2(2 x 200).

28


Assertion (A): A GTO circuit is more compact than SCR circuit Reason (R): A GTO can be turned off by negative gate pulse. A. B. C. D.


Explanation: Since commutation circuit is not required, GTO circuit is compact. 23.

In a single phase full wave ac regulator, the firing angles in the two half cycles A. B. C. D.

24.

are always equal - Ans are sometimes equal are never equal may be equal or unequal

In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ωt. The output dc voltage for resistive load

A.

B.

C.

- Ans

D. Explanation: SCRs conduct from a to p in each half cycle. 25.

As regards single phase dual converter A. B. C.

the change in operation from one quadrant to the other is faster in non-circulating current mode the change in operation from one quadrant to the other is faster in circulating current mode - Ans the time response to change in operation from one quadrant to other is the same in both the modes

29

Excel Review Center - Power Electronics D.

26.

the time response to change in operation from one quadrant to the other may be faster in any of the two modes

To protect an SCR against high di/dt we connect A. B. C. D.

a capacitance in series with SCR a capacitance in parallel with SCR an inductance in parallel with SCR an inductance in series with SCR - Ans

Explanation: current through inductance cannot change instantaneously. 27.

In a parallel inverter A. B. C. D.

28.

the commutating capacitor C is across the primary of transformer and the load is across the secondary - Ans the commutating capacitor and load are connected across the secondary of transformer the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer none of the above

A silicon controlled switch has : A. B. C. D.

2 leads 3 leads 4 leads - Ans 5 leads

Explanation: Anode, cathode, anode gate and cathode gate. 29.

A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is A. B. C. D.

20 mH about 31 mH - Ans about 51 mH about 92 mH

Explanation: .

30.

A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is A.

Vm

30


0.5 Vm Zero - Ans 0.5 Vm - voltage drop across inductance

Explanation: During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero. 31.

In a semi converter feeding a resistive load, the firing angle is a. Each thyristor conducts for A. B. C. D.

p radians (p - a) radians - Ans (p + a) radians (p - 0.5 a) radians

Explanation: The conduction is from a to p. 32.

Between which points does UJT exhibit negative resistance characteristics A. B. C. D.

33.

between peak and valley points - Ans before peak point after peak point after valley point

A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is A. B. C. D.

2° C/W 1.4° C/W - Ans 0.7°C/W 2.2° C/W

Explanation: . 34.

A single phase electric heater uses phase controlled thyristors. The firing angle is 90°. The degree of heating is A. B. C. D.

25% of maximum 50% of maximum - Ans 75% of maximum none of the above

Explanation: Conduction occurs only during 50% of wave. 35.

Which of these commutation methods is used in AC-DC converters?

31


Class A Class C Class D Class F – Ans

Explanation: Class F is natural commutation AC = DC converter use natural (or line) commutation. 36.

When thyristors are connected in series and parallel, it may be necessary to have : A. B. C. D.

current derating voltage derating both current and voltage derating - Ans none of the above

Explanation: Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating. 37.

A single phase half wave regulator uses A. B. C. D.

2 diodes and 2 thyristors 2 thyristors 1 diode and 1 thyristor - Ans 2 diodes

Explanation: Regulation is possible only in positive half cycle. 38.

A Shockley diode can be used A. B. C. D.

in place of SCR as a trigger switch for SCR - Ans in place of UJT both in place of SCR and as a trigger switch for SCR

Explanation: It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor. 39.

An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about A. B. C. D.

2 μs 5 μs - Ans 20 μs 50 μs

32

Excel Review Center - Power Electronics Explanation: Duration of gate pulse should be slightly more than turn time of SCR. 40.

In a multiphase chopper, the different choppers operate in A. B. C. D.

41.

series and all simultaneously parallel and one at a time - Ans parallel and simultaneously either (b) or (c)

Explanation: Only one chopper operates at one time. In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is A. B. C. D.

30° 60° 90° - Ans 120°

Explanation: Vdc ∝ (1 + cos a). Maximum possible output voltage occurs when a = 0. 42.

A high frequency series inverter consists of A. B. C. D.

a number a number a number a number

of series inverters in series of series inverters in parallel - Ans of parallel inverters in parallel of parallel inverters in series

Explanation: A high frequency series inverter has many series inverters in parallel. Only one inverter operates at one time and they operate in fixed sequence. 43.

In an SCR, the anode current consists of A. B. C. D.

only holes only electrons both electrons and holes - Ans either electrons or holes

Explanation: Both electrons and holes contribute to current. 44.

In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is

33


A. B.

15 A - Ans 152 A

C. D.

30 A

Explanation: Because of highly inductive load, output current is nearly constant. 45.

A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load A. B. C. D.

46.

rectification 0.35 - Ans rectification 0.35 rectification 0.35 rectification 0.35

efficiency will be more than 0.6 and TUF will be more than efficiency will be more than 0.6 but TUF will be less than efficiency will be less than 0.6 and TUF will be less than efficiency will be less than 0.6 but TUF will be more than

Assertion (A): A high dv/dt may cause triggering of a thyristor. Reason (R): A high dv/dt results in charging current in the thyristor. A. B. C. D.


Explanation: The charging current may cause triggering. 47.

Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V. Reason (R): When an SCR is conducting, the junction temperature is about 200°C.

34



Explanation: Junction temperature is about 120°C. Hence R is wrong. 48.

When the core of a saturable reactor is saturated, the inductance is high. A. B.

True False - Ans

Explanation: Since core is saturated, change of flux is small and inductance is low. 49.

In controlled heating circuits, thyristors act as A. B. C. D.

rectifier inverter ac regulator - Ans chopper

Explanation: The output voltage is controlled ac. 50.

In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current A. B. C. D.

are always similar - Ans can never be similar may be similar or dissimilar are similar only if a = 0

Explanation: For resistive load output current is always proportional to output voltage. 1.

A chopper cannot be line commutated. A. B.

2.

Natural commutation can be used in : A. B. C. D.

3.

True - Ans False

DC circuits only AC circuits only - Ans both AC and DC circuits none of the above

In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected.

35

Excel Review Center - Power Electronics A. B. 4.

True False - Ans

A single phase half wave rectifier circuit has a free wheeling diode. The free wheeling diode will conduct only if A. B. C. D.

load load load load

is purely resistive is purely inductive is combination of R and L is purely inductive or combination of R and L - Ans

5.

McMurray Bedford full bridge inverter uses

6.

A. natural commutation B. auxiliary commutation C. complementary commutation - Ans D. any of the above Which converter is used in HVDC transmission? A. B. C. D.

7.

Assertion (A): The turn on time of SCR is about 1 to 4 ms. Reason (R): The turn off time of SCR is about 10 to 300 ms. A. B. C. D.

8.

3 pulse 6 pulse 12 pulse - Ans 24 pulse

Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A - Ans A is correct but R is wrong A is wrong but R is correct

An n pulse rectifier is fed by a source having an inductance L. If load current is I0, the voltage drop due to overlap is A.

- Ans

B. C. D.

9.

Which chopper circuit operates in all the 4 quadrants? A. B. C. D.

A C D E - Ans

36


A single phase cycloconverter feeds a resistance heating load. The heating power is provided by A. B. C. D.

11.

only the fundamental component of output wave only higher harmonics in the output wave both fundamental and higher harmonics in the output wave - Ans either (a) or (b)

A 3 phase fully controlled bridge converter is fed by a 3 phase system having phase voltage v = Vm sin ωt. The firing angle is a. The dc output voltage is

A.

- Ans

B.

C. D 12.

Which of these commutation methods uses an external pulse for commutation? A. B. C. D.

13.

In a single phase full wave regulator feeding an R-L load, the power factor angle of load is θ. The range of control of firing angle a is A. B. C. D.

14.

0≤ θ≤ 0≤ θ≤

a≤p a ≤ p - Ans a ≤ (p - θ) a ≤ (p + θ)

A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be A. B. C. D.

15.

Class A Class C Class E - Ans Class F

0.9 0.8 0.75 - Ans 0.6

A relaxation oscillator uses A. B.

tunnel diode UJT - Ans

37

Excel Review Center - Power Electronics C. D. 16.

In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are A. B. C. D.

17.

is zero is 60° is more than 60° - Ans is more than 90°

In the circuit of figure, the load current when thyristor is off

A. B. C. D. 20.

4 quadrants 3 quadrants 2 quadrants - Ans 1 quadrant

In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle A. B. C. D.

19.

1 2 3 - Ans 4

A single phase full converter can operate in A. B. C. D.

18.

both tunnel diode and UJT PIN diode

is constant increases exponentially decreases exponentially - Ans zero

A full bridge inverter is fed by a battery of 30 V. The rms value of output voltage is A. B. C. D.

15 V 152 V 30 V - Ans 302 V

38


In brushless excitation system of modern alternators, the main exciter is A. B. C. D.

22.

The two-transistor model of a thyristor consists of two transistors : A. B. C. D.

23.

will have positive part only - Ans will have both positive and negative parts may have both positive and negative parts will have negative part if load is inductive

In a stud mounted SCR, the stud is A. B. C. D.

27.

p layer to which anode is connected n layer nearest to the anode terminal - Ans p layer nearest to the cathode terminal outside n layer

In a 3 phase full converter, the firing angle is less than 60°. The instantaneous output voltage A. B. C. D.

26.

is constant may be constant or may depend on gate current decreases as gate current is increased - Ans increases as gate current is increased

The gate lead, in a thyristor is welded to : A. B. C. D.

25.

one n-p-n and other p-n-p - Ans both p-n-p both n-p-n one n-p-n and other UJT

In a thyristor, the forward breakover voltage A. B. C. D.

24.

dc generator ac generator - Ans either dc or ac generator induction generator

in thermal with heat sink insulated from heat sink in electrical and thermal with heat sink - Ans in either electrical or thermal with heat sink

In pulse width modulation of chopper A. B. C. D.

T is kept constant and Ton is varied - Ans Ton is kept constant T is varied both T and Ton are varied either T or Ton is varied

39


In a full bridge inverter, the waveshape of load current depends on A. B. C. D.

29.

The characteristics of which device has peak point and valley point A. B. C. D.

30.

relative values of load parameters R, L and C - Ans relative values of load parameters R and L relative values of load parameters L and C gating frequency

SCR PUT - Ans SCS Shockley diode

In a step down chopper using time ratio control, the input voltage is V and duty cycle is a. The load is purely resistive having resistance R. The average and rms load currents are A.

B.

- Ans (B)

C.

D.

31.

In a thyristors are i = kva. The value of a is A. B. C. D.

32.

In an ordinary thyristor A. B. C. D.

33.

about 0.5 about 2 about 5 - Ans about 20

turn turn turn turn

on and turn off times are equal on time is more than turn off time off time is more than turn on time - Ans on time may be equal, less than or more than turn off time

Thyristors connected in series need : A. B. C. D.

static equalizing circuit dynamic equalizing circuit both static and dynamic equalizing circuits - Ans none of the above

40



A. B. C. D. 35.

If 4 quadrant operation is required we need A. B. C. D.

36.

= Pac for all values of firing angles = Pac if firing angle is less than 180° < Pac for all values of firing angles < Pac if firing angle is less than 180° - Ans

UJT PUT Both UJT and PUT - Ans UJT, PUT and SBS

In integral cycle control of ac regulators, the period of integral cycle control, i.e., (m + n) T should be A. B. C. D.

39.

Pdc Pdc Pdc Pdc

Which of the following devices exhibit negative resistance characteristics? A. B. C. D.

38.

dual converter - Ans full converter semi converter none of the above

A single phase half wave converter is feeding a resistive load. Output dc power is Pdc and total power delivered to load is Pac then A. B. C. D.

37.

SUS SBS SCS - Ans None of the above

less than mechanical or thermal time constant of the system - Ans more than the mechanical or thermal time constant of the system more or less than the mechanical or thermal time constant very high

All other parameters remaining the same, a 3 phase bridge inverter with 180° mode of operation A. B. C.

has the same peak thyristor current as in 120° mode of operation has higher peak thyristor current than in 120° mode of operation - Ans has lower peak thyristor current than in 120° mode of operation

41

Excel Review Center - Power Electronics D. 40.

As compared to BJT, MOSFET has A. B. C. D.

41.

True - Ans False

In the circuit of figure, the waveshape of load current when thyristor conducting is

A. B. C. D. 45.

hVBB (h + 1) VBB hVBB + VD - Ans VD

A single phase bridge type cycloconverter uses eight thyristors. A. B.

44.

average output voltage - Ans rms output voltage average or rms output voltage depending on load neither of the above

A voltage VBB is applied across the terminals of a UJT. The emitter voltage at peak point is A. B. C. D.

43.

lower switching losses and higher conduction losses - Ans higher switching losses and higher conduction losses higher switching losses and lower conduction losses lower switching losses and lower conduction losses

If a step down chopper is fed by a dc voltage V with T on and Toff are on and off periods then is

A. B. C. D. 42.

any of the above

constant increasing exponentially - Ans decreasing exponentially either (b) or (c)

In the basic series inverter circuit, the source supplies load

42

Excel Review Center - Power Electronics A. B. C. D. 46.

A current source inverter has simple commutation circuit. A. B.

47.


Assertion (A): Typical value of thermal resistance from source to sink of an SCR is about 0.3° C/W Reason (R): Heat sinks of thyristors are generally made of aluminium. A. B. C. D.

1.

2f F - Ans 0.5 f 0.25 f - Ans

Assertion (A): A transistor has lower voltage drop during conduction as compared to thyristor Reason (R): Transistors are manufactured in very high voltage and current ratings. A. B. C. D.

50.

is capacitor - Ans is capacitor and inductor is capacitor, inductor and auxiliary thyristor any of the above depending on load

In a half bridge inverter each thyristor is gated at a frequency f. The output frequency is A. B. C. D.

49.

True - Ans False

In load commutated chopper, the commutating element A. B. C. D.

48.

only in positive half cycle - Ans both in positive and negative half cycles only in part of positive half cycle in positive half cycle and part of negative half cycle


Multiphase choppers are used in A. B. C. D.

low power circuits high power circuit - Ans both low and high power circuits medium power circuits

43


In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is a. Each thyristor conducts for a period A. B. C.

p - Ans a p–a

D. 3.

In a parallel inverter A. B. C. D.

4.

A step up chopper can give an output voltage A. B. C. D.

5.

3 full waves of input 3 half waves of input - Ans 6 full waves of input 6 half waves of input

A half bridge inverter A. B. C. D.

8.

50°C 80°C 120°C - Ans 200°C

A bridge type single phase cycloconverter changes the frequency f to f/3. Then one half wave of output contains A. B. C. D.

7.

higher than input voltage - Ans lower than input voltage both higher and lower input voltage all of the above

The specified maximum junction temperature of thyristors is about A. B. C. D.

6.

each thyristor is turned on twice during each cycle - Ans each thyristor is turned on once during each cycle each thyristor is turned on either once or twice in each load cycle each thyristor may be turned on upto four times in each load cycle

needs a 3 wire dc supply - Ans may use a two wire or 3 wire dc supply has higher output voltage if fed from 3 wire dc supply has higher efficiency if fed from 3 wire dc supply

A 3 phase series inverter has A. B. C. D.

12 thyristors 6 thyristors - Ans 3 thyristors 3 or 6 thyristors

44


In figure, thyristor Th is in off state. When thyristor is turned on, the peak thyristor current can be

A. B. C. D. 10.

HVDC systems are preferable for only long distances. A. B.

11.

C. D.

the maximum repetitive surge current in which the SCR can withstand the maximum non-repetitive surge current in which the SCR can withstand - Ans the maximum repetitive or non repetitive surge current in which the SCR can withstand the maximum surge current in which the SCR can withstand if both surge current and steady current exist together

Which of the following devices has metal-silicon junction? A. B. C. D.

13.

True - Ans False

Surge current rating of an SCR is A. B.

12.

5A 10 A 15 A - Ans 20 A

General purpose power diode Schottky diode - Ans SCR MOSFET

In a single phase full wave converter (M-2 connection) feeding on R-L load the input voltage is v = Vm sin ωt. The expression for rms output voltage is

A.

45


B.

- Ans (B)

C.

D. 14.

A dc chopper is feeding an RLE load. The condition for maximum ripple is A. B. C. D.

15.

V and I respectively of ohm's law I and V respectively of ohm's law - Ans R and V respectively of ohm's law I and R respectively of ohm's law

A string of series connected thyristors needs equalising circuit to A. B. C. D.

19.

I V - Ans 5V 20 V 50 V

In thermal ohm's law, temperature and power loss are analogous to A. B. C. D.

18.

the commutating elements L and C are in parallel the commutating elements L and C are in series with load - Ans the commutating L and C may be in series or parallel with the load none of the above

When a power diode is conducting, the voltage drop across it is about A. B. C. D.

17.

1 - Ans 0.8 0.5 0.5

In a series inverter A. B. C. D.

16.

a= a= a= a<

ensure proper firing ensure proper current division ensure proper voltage division - Ans none of the above

A surge voltage A. B.

has very high magnitude and very long duration has very high magnitude and very small duration

46


The minimum gate current which can turn on SCR is called : A. B. C. D.

21.

two 3 phase semiconverters connected in parallel two 3 phase semiconverters connected in antiparallel two 3 phase full converters connected in antiparallel- Ans two 3 phase full converters connected in parallel

Which inverter circuit uses coupled inductors? A. B. C. D.

26.


A 3 phase dual converter consists of A. B. C. D.

25.

SCR SUS SBS - Ans UJT

Assertion (A): A half wave controlled rectifier has poorer efficiency and higher ripple factor than a full wave controlled rectifier. Reason (R): The use of a freewheeling diode in half wave controlled rectifier circuit improves the waveform of load current and circuit power factor. A. B. C. D.

24.

3, 3, 4, - Ans 3, 3, 3 2, 3, 3 3, 3, 5

Which of these devices can conduct in both directions? A. B. C. D.

23.

trigger current - Ans holding current junction current breakover current

The total number of leads in SUS, SBS and SCS respectively are A. B. C. D.

22.

has very high magnitude and very small duration and can be positive or negative - Ans can be only positive

Modified McMurray full bridge inverter McMurray Bedford half bridge inverter Modified McMurray Bedford half bridge inverter Both (b) and (c) – Ans

When an SCR is in forward blocking mode, the applied voltage A.

appears across only one junction - Ans

47

Excel Review Center - Power Electronics B. C. D. 27.

In sinusoidal PWM for inverters the width of different pulses are not the same. A. B.

28.

in first quadrant only in second quadrant only in either first or fourth quadrant - Ans in all the quadrants

UJT PUT - Ans SCS SBS

80% or more - Ans around 50% around 20% around 5%

If the frequency is less than the damped frequency in a series- inverter A. B. C. D.

32.

operate operate operate operate

The efficiency of a chopper circuit is about A. B. C. D.

31.

can can can can


A. B. C. D. 30.

True - Ans False

A class D chopper A. B. C. D.

29.

is equally shared by two junctions is equally shared by three junctions may appear across one or more junctions

the distortion of output voltage waveform is low the distortion of output voltage waveform is high - Ans the distortion of output voltage may be high or low depending on the load impedance none of the above

The output frequency of a cycloconverter can be changed by changing the firing angle. A. B.

True False - Ans

48


Mc Murray full bridge inverter uses A. B. C. D.

34.

firing firing firing firing

angle is less than 60° angle is less than 90° angle is more than 60° - Ans angle is more than 90°

must be isolated - Ans may or may not be isolated should be preferably, isolated should not be isolated

The circuit in figure is for

A. B. C. D. 38.


In a single phase full wave ac regulator using two thyristors only, the gate circuits of the two thyristors A. B. C. D.

37.

2 diodes 4 diodes 4 diodes - Ans 8 diodes

In a 3 phase full converter, the instantaneous output voltage can have negative part if A. B. C. D.

36.

and and and and

Assertion (A): A DIAC has four layers but only two terminals. Reason (R): A DIAC can conduct in both directions. A. B. C. D.

35.

4 thyristors 4 thyristors 8 thyristors 8 thyristors

resonant commutation line commutation auxiliary commutation complementary commutation - Ans

A fully controlled bridge converter is feeding an RLE load. For some circuit conditions firing angle is 47°. If connections of battery are reversed, the firing angle is likely to be

49

Excel Review Center - Power Electronics A. B. C. D. 39.

In a single phase half wave converter feeding resistive load and full wave converter (M - 2 connection) feeding a resistive load the circuit conditions are similar. The firing angle is a. Then the waveshapes of voltages across thyristors A. B. C. D.

40.

are similar - Ans are not similar may be similar or dissimilar are similar only for some values of a

In a 3 phase full converter feeding a highly inductive load, the average load current is 150 A. The peak current through thyristor is A. B. C. D.

41.

47° less than 47° more than 47° - Ans either more or less than 47°

150 A - Ans 75 A 50 A 300 A

A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of thyristor current is A.

B.

- Ans (B)

C.

D.

42.

The main utility of SCR is due to A. B. C. D.

43.

gate triggering - Ans high temperature triggering triggering high breakdown voltage

In a single phase full wave ac regulator the firing angles during both half cycles are equal. A. B.

True - Ans False

50


In a 3 phase full wave regulator feeding a star connected resistance load, firing angle is 75°. Then the number of thyristors conducting at one time is A. B. C. D.

45.


A. B. C. D. 46.

1 2 - Ans 3 4

SUS SBS Shockley diode - Ans UJT

The turn on characteristics of a power transistor can be assumed to be linear, as shown in the below figure . During turn on time T, voltage reduces linearly from V to 0 and current rises linearly from 0 to I. The energy loss during time T is

A. VIT B. C. D.

- Ans

51


47.

The circuit in figure is for

A. B. C. D. 48.

A single phase half bridge inverter required to feed R-L loads needs A. B. C. D.

49.

for an SCR are in the range

1 to 10 25 to 500 – Ans 0.1 to 1 1000 to 2000

When a dc chopper feeds an RLE load, the load current, during steady state operation A. B. C. D.

1.

2 thyristors 4 thyristors 2 thyristors and two diodes - Ans 4 thyristors and 4 diodes

Typical values of of A. B. C. D.

50.

resonant commutation self commutation - Ans load commutation auxiliary commutation

remains constant varies between maximum and minimum values - Ans may remain constant or vary is constant if R is constant

In a single phase full wave ac regulator feeding R-L load, the gate pulses A. B. C. D.

are always of short duration may be of short duration should be rectangular with width (p - a) - Ans cannot be rectangular with width (p - a)

52


In a semi converter feeding a resistive load, A. B. C. D.

3.

B. C. D.

Class A Class B Class C Class D – Ans

dc output voltage proportion dc output voltage dc output voltage dc output voltage proportion

and rms output voltage decrease in the same and rms output voltage decrease - Ans and rms output voltage increase and rms output voltage increase in the same

A cycloconverter for single phase operation has A. B. C. D.

8.

is high a is low E is high none of the above - Ans

A single phase half wave rectifier is feeding a resistive load. As firing angle is increased A.

7.

16 8 4 2 - Ans

Which of these commutation methods uses two leads? A. B. C. D.

6.

and input current are similar only if

A dc chopper is feeding an RLE load. The current can become discontinuous if

A. B. C. D. 5.

and input current are always similar and input current can never be similar and input current are similar only if

In a single phase semi-converter the number of thyristors is A. B. C. D.

4.

the waveshapes of input voltage the waveshapes of input voltage the waveshapes of input voltage firing angle is zero - Ans the waveshapes of input voltage firing angle is high

one positive group and one negative group converters - Ans two positive group and two negative group converters three positive group and three negative group converters any of the above

A chopper is a high efficiency circuit.

53

Excel Review Center - Power Electronics A. B.

9.

The difference between resonant commutation and self commutation is A. B.

C. D. 10.

True - Ans False

both these methods are based on entirely different principles - Ans in resonant commutation, the commutating elements carry load current but in self commutation, the commutating elements do not carry load current resonant commutation uses L and C but self commutation uses only capacitor none of the above

If 0.5 T is the time period of oscillations and T off is the time between turn off of one thyristor and turn on of second thyristor, the frequency of output f of a series inverter is

A.

- Ans (A)

B. C.

D. 11.

In a series inverter A. B. C. D.

12.

A NAND gate has 3 inputs and one output. The number of thyristors required for this gate are A. B. C. D.

13.

the commutating elements carry full load current - Ans the commutating elements carry upto 50% of load current the commutating elements do not carry load current any of the above

1 2 3 - Ans 3 or 4

Which of these devices is used in relaxation oscillator? A. B. C. D.

UJT – Ans SCR DIAC TRIAC

54


In auxiliary commutated chopper fed by voltage V and having L and C commutating elements, the peak value of capacitor current is

A. B.

C.

- Ans

D. 15.

In a single phase semiconverter feeding a highly inductive load, the load current is A. B. C. D.

16.

Two thyristors of the same rating and specifications A. B. C. D.

17.

will have equal on state voltage drops will have equal forward blocking voltage for the same current will have different on state voltage drops may have equal or different on state voltage drops and equal or different forward blocking voltage's for the same current - Ans

A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the starting torque is 100 N-m. At 25 Hz, the starting torque A. B. C. D.

18.

sinusoidal nearly constant - Ans absolutely constant rectangular

will be less than 100 N-m will be more than 100 N-m - Ans may be more or less than 100 N-m depending on motor parameters very small

In a three phase to single phase full wave bridge cycloconverter, the number of thyristors is A. B. C. D.

36 18 12 - Ans 8

55


A single phase half wave converter is feeding-a resistor load. The input voltage is v = Vmsin ωt. If firing angle is zero, the output dc voltage V dc and output rms voltage Vrmsrespectively

A.

B.

- Ans

C.

D. 20.

In a cycloconverter it is possible to vary A. B. C. D.

21.

A single phase full bridge inverter for R-L loads needs A. B. C. D.

22.

low harmonic content high harmonic content - Ans low or high harmonic content depending on firing angle zero harmonic content

Thyristors connected in series need static and dynamic stabilizing circuits. A. B.

24.

4 thyristors 4 thyristors and 4 diodes - Ans 4 thyristors and 2 diodes 8 thyristors

In the basic series inverter circuit, the source current has A. B. C. D.

23.

only output frequency only output voltage both output voltage and frequency but not simultaneously both output voltage and output frequency simultaneously – Ans

True - Ans False

In a controlled rectifier a freewheeling diode is necessary if the load is A. B. C. D.

Inductive - Ans resistive capacitive any of the above

56


For an illumination control circuit it is advisable to use a matched TRIAC-DIAC pair. A. B.

26.

True - Ans False

A single phase converter is fed by a source having an inductance L. If load current is I0, the decrease in output voltage due to overlap is

A. B.

C. - Ans D.

27.

Assertion (A): An ac regulator uses natural commutation Reason (R): An ac regulator can be used for static on load tap changing of transformer. A. B. C. D.

28.

When a UJT is used as a relaxation oscillator the current should be A. B. C. D.

29.

less than Ip more than Iv more than Ip but less than Iv - Ans negative

In auxiliary commutated chopper, the charging and discharging time of capacitor A. B. C. D.

30.


are constant depend on load current - Ans may be constant or may depend on load current none of the above

Which chopper circuit uses saturable reactor? A. B. C. D.

Auxiliary commutated Jones chopper Morgan chopper - Ans Load commutated

57


A thyristor circuit is feeding an R-L load. The turn on time can be reduced by A. B. C. D.

32.

A single phase semiconverter is supplying RLE load and has a freewheeling diode across the load. The firing angle is a. The freewheeling diode conducts for A. B. C. D.

33.

decreasing R decreasing L - Ans increasing L decreasing R and L together

a° in each half cycle - Ans (p - a)° in each cycle a° in one full cycle (p + a)° in each half cycle

In a full bridge inverter fed by a battery of voltage V, the rms value of fundamental component of output voltage is A.

B. - Ans C.

D.

34.

In a single phase full wave regulator feeding a resistive load, the average current over one full cycle A. B. C. D.

35.

Two single phase semi-converters are connected in series to form a series converter. The input is v = Vm sin ωt and a1, a2 are the firing angles. If a1 = 0, the dc output voltage is A. B. C. D.

36.

is always zero - Ans cannot be zero may be sometimes zero is always positive

Vm/pi (1 + cos a2) Vm/pi (2 + cos a2) Vm/pi(3 + cos a2) - Ans Vm/pi(4 + cos a2)

The total number of leads in MOS controlled thyristor is A. B. C.

2 3 - Ans 4

58


Assertion (A): Chopper always requires forced commutation Reason (R): Morgan chopper uses a saturable reactor. A. B. C. D.

38.

50% of peak reverse current - Ans 25% of peak reverse current 10% of peak reverse current zero value

Assertion (A): McMurray Bredford half bridge inverter uses complementary commutation Reason (R): Three phase inverter can have two modes of operation. A. B. C. D.

40.


In a power diode the reverse recovery time is the time from the instant the forward current is zero to the instant when reverse recovery current has decayed to A. B. C. D.

39.

2 or 3


A capacitor C farads is charged to a voltage V. It discharges through a thyristor, a diode and an inductance L. The peak current in the circuit is A. VCL B.

- Ans

C. D.

41.

Which of these devices have four layers? A. B. C. D.

42.

UJT SCR - Ans TRIAC SCR and TRIAC

When UJT is used for thyristor gate signal, the output of UJT is A. B.

sine wave square wave

59


The stud in a stud mounted SCR acts as A. B. C. D.

44.

Vrms 0.5 Vrms (Vrms - Vdc)0.5 (Vrms2 - Vdc2)0.5 - Ans

A UJT exhibits negative resistance characteristics when A. B. C. D.

48.

True False – Ans

In rectifier circuits, the rms value of ac component of output voltage is equal to A. B. C. D.

47.

True - Ans False

Half bridge and full bridge inverters of one type use different commutation methods. A. B.

46.

anode - Ans cathode gate anode or cathode

In a single phase full wave regulator feeding R-L load, short gate pulse are not suitable. A. B.

45.

saw tooth wave - Ans either (a) or (c)

Current is less than valley current Current is less than peak current Current is more than peak current but less than valley current - Ans None of the above

A dc chopper feeds an RLE load and has a duty cycle of 1. The load current will be such that A. B. C. D.

49.

- Ans Imax > Imin Imin > 0 Imin < 0

In rectifier circuits, ripple factor RF and form factor FF are related as A. B. C. D.

RF RF RF RF

= (FF2 - 1)0.5 - Ans = FF = FF - 1 = (FF - 1)0.5

60


If we connect two silicon unilateral switches in antiparallel we get A. B. C. D.

1.

Assertion (A): SUS is a PUT and avalanche diode connected between gate and cathode Reason (R): SBS has two SUS inbuilt together and connected in anti-parallel A. B. C. D.

2.

and MOSFET are current controlled devices is voltage controlled and MOSFET is current controlled device and MOSFET are voltage controlled devices is current controlled and MOSFET is voltage controlled device -

three times output frequency six times output frequency - Ans twice the output frequency equal to output frequency

A 3 phase fully controlled converter is a A. B. C. D.

6.

BJT BJT BJT BJT Ans

In a three phase series inverter, the firing frequency is A. B. C. D.

5.

2 Vm - Ans Vm 0.5 Vm 0.25 Vm

Which statement is correct? A. B. C. D.

4.


In a single phase full wave controlled rectifier using centre tap transformer, the voltage across each half of secondary is V m sin ωt. The peak inverse voltage is A. B. C. D.

3.

SCR SBS - Ans SCS UJT

3 pulse converter 6 pulse converter - Ans 2 pulse converter 12 pulse converter

In a half bridge inverter the gate pulses of the two thyristors have a phase displacement of A. B. C.

60° 90° 120°

61


A fully controlled bridge converter can operate in quadrants A. B. C. D.

8.

natural commutation forced commutation either natural or forced commutation - Ans both natural and forced commutation together


line commutation forced commutation either (a) or (b) - Ans none of the above

Inverters find applications in A. B. C. D.

13.

in second quadrant only in first or third quadrant in all the four quadrants - Ans in either second or third quadrant

The commutation method in an inverter, is A. B. C. D.

12.

operate operate operate operate

Assertion (A): A fully controlled bridge converter can operate in first and fourth quadrant. Reason (R): A semi converter is cheaper than a full converter. A. B. C. D.

11.

can can can can

A cycloconverter uses A. B. C. D.

10.

1 1 and 2 1 and 3 1 and 4 - Ans

A class E chopper A. B. C. D.

9.

180° - Ans

HVDC transmission UPS variable speed ac drives all of the above - Ans

When a thyristor is conducting, the voltage drop across it is about A. B. C. D.

1 V - Ans 10 V 100 V 0.1 V

62


The use of free wheeling diode in controlled rectifier improves the waveshape of load current. A. B.

15.

As the firing of a thyristor is delayed, the output voltage of a controlled rectifier decreases. A. B.

16.

True - Ans False

Assertion (A): A GTO can be turned on and turned off by gate pulses Reason (R): A GTO can be turned off by negative gate pulse. A. B. C. D.

17.

True - Ans False


In a half bridge inverter circuit fed by a battery of voltage V, the rms value of fundamental component of output voltage is A. 2 V B.

- Ans

C. D. 18.

In a full bridge single phase inverter feeding RLC load A. B. C. D.

19.

one thyristor and one diode conduct simultaneously two thyristors conduct simultaneously two thyristors or two diodes conduct simultaneously - Ans four thyristors conduct simultaneously

It is possible to use a freewheeling diode in A. B. C. D.

single phase half wave controlled rectifier as well as in single phase full wave controlled rectifier (M - 2 connection) single phase half wave controlled rectifier but not in single phase full wave controlled rectifier (M - 2 connection) single phase full wave controlled rectifier (M - 2 connection) but not in single phase half wave controlled rectifier all controlled rectifier circuits - Ans

63


Assertion (A): When the anode current of an SCR starts increasing during turn on process, the anode cathode voltage drop starts decreasing. Reason (R): During turn on the reverse biased junction in SCR breaksdown. A. B. C. D.

21.

In a certain inverter circuit each thyristor carries 26.67 A for 60°, 13.33 A for 120° and zero A for 180° of the cycle. The rms current of thyristor is A. B. C. D.

22.

UJT - Ans PUT Shockley diode TRIAC

In a single phase full converter fed by a source having inductance, the number of thyristors conducting during overlap is A. B. C. D.

25.

0.1 0.2 0.6 - Ans 0.9


A. B. C. D. 24.

20 A 13.33 A - Ans 6.67 A 3.33 A

The factor η for a UJT is about A. B. C. D.

23.


1 2 3 4 - Ans

The cut in voltages for germanium and silicon diodes respectively as A. B. C.

0.3 V and 0.7 V - Ans 0.7 V and 0.3 V 0.6 V and 0.9 V

64


An SCR is triggered at 80° in the positive half cycle, only. The rms anode current is 25 A. If the firing angle is changed to 160°, the rms current is likely to be A. B. C. D.

27.

True - Ans False

A class A chopper has an input voltage of 200 V. T on = 1.5 ms and Toff = 4.5 ms. The duty cycle, average output voltage and ripple factor respectively are A. B. C. D.

29.

25 A 12.5 A less than 25 A less than 12.5 A - Ans

The output wave of a cycloconverter consists of a number of segments derived from the input wave. A. B.

28.

0.5 V and 0.7 V

0.5, 100 V 0.25, 50 V 0.25, 50 V 0.5, 100 V

and 1 and 3 - Ans and 1 and 3

In a parallel inverter the turn off time tq of thyristor and values of commutating elements L and C are related as A. tq = pLC

30.

B. tq =

LC

C. tq =

LC - Ans

D. tq =

LC

A TRIAC can be turned on : A. B. C. D.

31.

by applying a positive signal to gate by applying a negative signal to gate by applying either positive or negative signal to gate - Ans none of the above

Which of the following devices has two gates? A. B. C. D.

SCR SCS - Ans SUS SBS

65


32.

In auxiliary commutated chopper, the peak value of thyristor current is A. B. C. D.

33.


A. B. C. D. 34.

equal to peak value of load current equal to peak value of capacitor current sum of peak value of load current and capacitor current - Ans none of the above

SUS SBS - Ans SCS Shockley diode

If peak value of input voltage is Vm and firing angle is a, the average output voltage for single phase half wave and single phase full wave regulators are A.

- Ans

B.

C.

D.

35.

The static equalisation circuit for thyristors connected in series is A. B. C. D.

36.

one resistor in series with each thyristor one resistor in parallel with each thyristor - Ans one resistor and one capacitor in parallel with each thyristor none of the above

A cycloconverter is a group of controlled rectifiers.

66

Excel Review Center - Power Electronics A. B. 37.

A single phase full wave regulator feeds a resistive load. The firing angle of thyristors is 90°. The input power factor is A. B. C. D.

38.

anode cathode, gate T1 T2, gate - Ans T1, T2, anode T1, T2, T3

A step down cycloconverter has an input frequency f. If it uses natural commutation, the output frequency A. B. C. D.

40.

1 0.707 lagging - Ans 0.5 lagging 0.2 lagging

The leads in a TRIAC are denoted by A. B. C. D.

39.

True - Ans False

must be a sub-multiple of input frequency - Ans can be any frequency must be an even sub-multiple of input frequency must be an odd sub-multiple of input frequency

In a single phase full wave regulator, the rms current in each thyristors is I r The run load current is equal to A. 2/Ir B. 2 Ir - Ans C. Ir D.

41.

Lead mounting for SCR is used for A. B. C. D.

42.

rating less than mA rating about 1 A or less - Ans rating of 20 A or less than 20 A rating 100 A or less

A load commutated chopper circuit has A. B. C. D.

one thyristor two thyristors four thyristors - Ans six thyristors

67


Assertion (A): The output voltage of an inverter can be controlled by PWM technique Reason (R): In a current source inverter the input current is constant. A. B. C. D.

44.

A TRIAC is equivalent to A. B. C. D.

45.

is absolutely constant decreases with increase in load current increases slightly with increase in load current - Ans none of the above

The advantage of modified series inverter with two inductors and two capacitors is A. B. C. D.

49.

even when load is resistive only when load is inductive only when load is inductive or capacitive - Ans for all types of loads

When a thyristor is conducting, the voltage drop across it A. B. C. D.

48.

Class A Class B Class C Both class A and Class B – Ans

In a half bridge inverter, the freewheeling diodes are needed A. B. C. D.

47.

two thyristors in series two thyristors in parallel - Ans one thyristor and one diode one thyristor and one transistor

Which of the following commutation methods is the overall circuit under damped? A. B. C. D.

46.


the intermittent operation of battery is avoided - Ans efficiency of circuit is more the output frequency is low all of the above

An SUS is A. B. C.

exactly similar to SCR but with higher rating similar to UJT PUT and avalanche diode connected in anti parallel - Ans

68


Which chopper circuit operates in quadrants 1 and 4? A. B. C. D.

1.

A B D - Ans E

In a 3 phase bridge inverter, the gating signals for the three phases have a phase difference of A. B. C. D.

2.

SCR and avalanche diode connected in parallel

120° - Ans 60° 240° any of the above

Assertion (A): The circuit of figure thyristor will conduct for 180° during positive half cycle if it is continuously fired.

Reason (R): A thyristor can conduct only when it is forward biased. A. B. C. D. 3.

The total number of leads in SCR, DIAC and TRIAC respectively are A. B. C. D.

4.


3, 2, 3 - Ans 2, 3, 3 3, 3, 2 3, 2, 4

A single phase semiconverter is feeding a highly inductive load and has a freewheeling diode across the load. The waveshape of input current is A. B. C. D.

sinusoidal rectangular - Ans sinusoidal or rectangular triangular

69


In the below figure, the load is highly inductive. The firing angle is a for each of the thyristors. The periods of conduction of Th 1 and Th2 respectively are

A. B. C. D. 6.

A crowbar circuit is generally needed for protection under fault conditions. A. B.

7.

Forward A. B. C. D.

8.

True - Ans False rating of an SCR is the maximum rate of rise of anode voltage which will trigger the SCR the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is applied the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is not applied - Ans either (b) or (c)

The number of leads in a DIAC are A. B. C. D.

9.

0 to p and p to 2p a to p and p + a to 2p a to p + a and p + p to 2p + a - Ans a to p - a and p + a to 2p – a

2 - Ans 3 4 6

The condition for underdamped oscillations in a series RLC circuit is

A. B.

- Ans

C.

70


D. 10.

The method of triggering SCS is A. B. C. D.

11.

The number of thyristors in a single phase fully controlled bridge rectifier is A. B. C. D.

12.

B. C. D.

output transformer is essential output transformer is not essential - Ans output transformer cannot be connected output transformer, if used, can improve the efficiency of the circuit

100% efficiency, zero ripple factor, zero harmonic factor and unity displacement factor - Ans 100% efficiency, zero tipple factor, zero harmonic factor and zero displacement factor 100% efficiency, zero ripple factor, unity harmonic factor and unity displacement factor 100% efficiency, unity ripple factor, zero harmonic factor and unity displacement factor

When thyristors are connected in parallel, the current distribution may become non-uniform due to A. B. C. D.

15.

an an an an

An ideal rectifier should have A.

14.

16 8 4 - Ans 2

In a bridge inverter A. B. C. D.

13.

application of positive pulse at cathode gate or negative pulse at anode gate - Ans application of pulse at anode gate application of pulse at cathode gate application of low voltage at anode

inductive effect of current carrying conductors - Ans capacitive effect of current carrying conductors both inductive and capacitive effects none of the above

A single phase fully controlled bridge converter is fed by a voltage V m sin ωt. The average and rms values of output voltage are A.

- Ans

B.

71


16.

C.

cos a and Vm

D.

cos a and 2 Vm

In a single phase half wave ac regulator feeding a resistive load, the rms load current A. B. C. D.

17.

is always positive - Ans may be positive or negative is always negative can be even zero

Which of the following is correct for a BJT? A. - Ans (A) B.

C. D.

18.

In a bridge inverter with single pulse width modulation A. B. C. D.

19.

In a modified series inverter with two inductors and two capacitors, the load current A. B. C. D.

20.

the output frequency is equal to frequency of reference signal - Ans the output frequency is equal to frequency of control signal either (a) or (b) neither (a) nor (b)

has 2 parallel paths - Ans has only one parallel path can have three parallel paths has four parallel paths

The number of layers in a Shockley diode are A. B. C. D.

2 3 4 - Ans 2 or 3

72


In a modified series inverter A. B. C. D.

22.

Assertion (A): Light triggering is very suitable for HVDC transmission. Reason (R): Light triggering has the advantage of complete electrical isolation of gate circuit. A. B. C. D.

23.

D.

trigger current holding current - Ans breakover current none of the above

When a single phase dual converter is operating in circulating current mode A. B. C. D.

26.

operate in first quadrant operate in second quadrant class A chopper operates in first quadrant while class B operates in second quadrant - Ans may operate in either first or second quadrant

To turn off an SCR, it is necessary to reduce its current to less than : A. B. C. D.

25.


Class A and class B chopper A. B. C.

24.

the maximum output frequency is less than frequency of damped oscillations the output frequency can be even higher than the frequency of damped oscillations - Ans the output frequency cannot exceed the frequency of damped oscillations any of the above

both converters are working as rectifiers simultaneously one converter is always working as rectifier and the other as inverter Ans only one converter is working at one time both converters are either working as rectifiers or as inverters simultaneously

The load circuit of a single phase full bridge inverter A. B. C. D.

must be underdamped must be overdamped may be underdamped or overdamped may be any type of load - Ans

73


When an SCR is reverse biased A. B. C. D.

28.

A dc series motor is coupled to a load whose torque is proportional to speed. It is fed by a single phase ac supply through a full converter. When speed is 500 rpm, firing angle is 45°. It is desired to raise the speed. It will be necessary to A. B. C. D.

29.

4 semiconductor layers and 3 junctions - Ans 3 semiconductor layers and 2 junctions 2 semiconductor layers and 2 junctions any of these depending on the rating

A fully controlled bridge converter is feeding an RLE load. The load current is I 0. The input voltage is Vm sin ωt. The KVL equation to find firing angle a is

A. B. C. D. 32.

is a capacitor is a capacitor and an inductor is a capacitor and an auto transformer - Ans none of the above

A LASCR has A. B. C. D.

31.

decrease the firing angle - Ans increase the firing angle either decrease or increase the firing angle depending on the required speed decrease the load torque

In Jone chopper, the commutating element A. B. C. D.

30.

two junctions are reverse biased and one junction is forward biased. Ans all the three junctions are reverse biased. one junction is reverse biased and two junctions are forward biased. any of the above depending on the magnitude of reverse bias.

cos a = E + I0R - Ans cos a = E + I0R cos a = E - I0R cos a = E - I0R

A single phase cycloconverter with centre tapped input transformer requires A. B.

2 thyristors 4 thyristors - Ans

74


8 thyristors either 2 or 4 thyristors

The duty cycle of a step down chopper is A. - Ans

B.

C.

D.

34.

In a 3 phase bridge inverter with 120° mode of operation the. number of thyristors conducting at one time are A. B. C. D.

35.

1 2 - Ans 3 4

A rectifier is fed by a transformer. The rms voltage and current at secondary of transformer are Vs and Is. Output dc power is Vdc Idc and output ac power is (Vrms ) (Irms), Transformer utilisation factor TUF is

A.

- Ans (A)

B.

C.

D. 36.

Assertion (A): A class C chopper can operate in first and second quadrants Reason (R): A class B chopper operates only in second quadrant. A. B.

Both A and R are correct and R is correct explanation of A Both A and R correct but R is not correct explanation of A - Ans

75


A single phase dual converter consists of A. B. C. D.

38.

B. C. D.

a schottky diode has lower cut in voltage and lower reverse saturation current a schottky diode has higher cut in voltage and higher reverse saturation current a Schottky diode has lower cut in voltage and higher reverse saturation current - Ans a schottky diode has higher cut in voltage and lower reverse saturation current

From which instant is the turn off time of a thyristor defined A. B. C. D.

43.

SCR is clamped on one side of disc shaped heat sink SCR is clamped between two disc shaped heat sinks - Ans The SCR has low current rating None of the above

As compared to a p-n junction diode A.

42.

decrease the output voltage - Ans increase the output voltage decrease the output voltage and increase the output current increase the output voltage and decrease the output current

In disc mounting of SCR A. B. C. D.

41.

a capacitor connected in series with thyristor a capacitor connected in parallel with thyristor a resistor and capacitor in series connected across thyristor - Ans none of the above

The effect of source inductance in a 3 phase fully controlled bridge converter is to A. B. C. D.

40.

two fully controlled converters connected in parallel two fully controlled converters connected in antiparallel - Ans two fully controlled semiconverters connected in antiparallel two fully controlled semiconverters connected in parallel

The snubber circuit consists of A. B. C. D.

39.

A is correct but R is wrong A is wrong but R is correct

when anode voltage becomes zero when anode current becomes zero - Ans when both anode voltage and current become zero none of the above

In a single phase cycloconverter with centre tapped transformer

76


44.

all thyristors carry currents for equal duration the durations of currents in all thyristors may or may not be equal Ans the durations of currents in all thyristors cannot be equal the duration of currents in all thyristors must be equal for satisfactory operation

If Ar and Ac are the amplitudes of reference and control signals, the modulation index Mis A.

- Ans

B.

C.

D.

45.

Thyristors can be used for control of A. B. C. D.

46.

Assertion (A): Thyristors can be used in controlled heating, excitation systems of alternators, speed control of motors and HVDC Reason (R): A static var system using thyristors is very commonly used in high voltage ac systems. A. B. C. D.

47.


In a dual converter operated in circulating current mode, the circulating current flows A. B. C. D.

48.

dc separately excited motor dc shunt motor dc series motor all of the above – Ans

when load current is zero when load current is non-zero even when load current is zero - Ans none of the above

For a single phase half wave converter feeding a resistive load, the firing angle a. The waveshape in below figure is for

77


A. B. C. D. 49.

A class B chopper operates in third quadrant. A. B.

50.

D.

the waveshapes of output voltage and load current are square the waveshapes of output voltage and load current are sinusoidal the waveshapes of output voltage is square but the waveshape of load current is nearly sinusoidal - Ans none of the above

In a semi converter feeding a resistive load A. B. C. D.

2.

True False - Ans

In a full bridge circuit supplying an RLC under damped circuit A. B. C.

1.

output voltage input voltage voltage drop across thyristor none of the above - Ans

the waveshapes of output voltage and similar - Ans the waveshapes of output voltage and similar the waveshapes of output voltage and for low values of firing angle the waveshapes of output voltage and dissimilar

output current are exactly output current can not be output current are similar only output current may be similar or

In an n stage high frequency series inverter having a time period T, each stage is active for a time interval of A. T B.

- Ans

C. D

78


In a 3 phase semiconverter, the output voltage becomes discontinuous for A. B. C. D.

4.

5.

A.

cos a = RI0 + E – Ans

B.

cos a = RE0 + E

C.

cos a = E0 - RI0

D.

cos a = E - RI0

In integral cycle control of ac regulator, the input power factor is equal to

auxiliary commutation - Ans complementary commutation natural commutation any of the above

A fully controlled bridge converter feeds a highly inductive load such that the load current can be assumed to be constant. The wave shape of input current is A. B. C. D.

8.

duty cycle a a2 a - Ans (a)1/3

McMurrary full bridge inverter uses A. B. C. D.

7.

angle > 30° angle > 60° - Ans angle > 90° angle > 120°

A single phase fully controlled bridge converter is feeding an RLE load. Input voltage is Vmsin ωt and firing angle is a. If average current is I0, then

A. B. C. D. 6.

firing firing firing firing

dc sinusoidal rectangular - Ans none of the above

A static excitation system uses A. B. C. D.

transformer and thyristors - Ans transformer and ac exciter dc exciter and ac exciter transformer and dc exciter

79


The application of gate signal decreases the break over voltage of on SCR. A. B.

10.

The power factor at the input terminals of a cycloconverter is generally A. B. C. D.

11.

RF RF RF RF

= FF2 - 1 = (FF2 - 1)0.5 - Ans = FF - 1 = (FF - 1)0.5

Modified Mc Murray full bridge inverter uses A. B. C. D.

13.

low and leading low and lagging - Ans high and leading high and lagging

Ripple factor RF and form factor FF are related as A. B. C. D.

12.

True - Ans False

4 thyristors 4 thyristors 8 thyristors 8 thyristors

and and and and

2 diodes 4 diodes 4 diodes 8 diodes - Ans

Assertion (A): Gate triggering is the most commonly used method for triggering of an SCR. Reason (R): Even a small gate current is sufficient to turn on an SCR. A. B. C. D.


.

80

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