AC DRIVE BASICS

MOTOR OUTPUT

LINE INPUT

All AC Drives convert “fixed” voltage and frequency into “variable” voltage and frequency, to run 3-phase induction motors.

Types of AC Drives In today’s marketplace, there are 3 basic AC Drive categories: • Open loop “Volts / Hz” Drives

• Open loop “Sensorless Vector” Drives

• Closed loop “Flux Vector” Drives All are Pulse-Width-Modulated (PWM) Some manufacturers offer 2-in-1 & 3-in-1 Drives, combining these attributes.

V/Hz

SENSORLESS

VECTOR

FLUX VECTOR

Open loop “Volts / Hz” Drives

V o l 230 t s

Motor Nameplate V/Hz

460

0

st oo B e r qu o T

30 900

60 1800 (Base)

• Motor voltage is varied linearly with frequency • No compensation for motor & load dynamics • Poor shock load response characteristics

Hz RPM* *( 4-pole motor)

Sensorless & Flux Vector Drives

V o l 230 t s

Motor Nameplate V/Hz

460

0

30 900

60 1800 (Base)

Hz RPM* *( 4-pole motor)

• Motor voltage is varied linearly with frequency, with dynamic self-adjustments • V/Hz compensation for motor & load dynamics • Excellent shock load response characteristics & high starting torque

AC Motor Torque & HP vs. Speed

%

100

Torque

T & HP

HP

50

0

30 900

• Motor Torque is constant to base speed • HP varies proportionally to speed

60 1800

Hz RPM

Pulse-Width-Modulated Inverter Basic Power Circuit AC to DC Rectifier

AC Input

DC to AC Inverter

DC Filter

DC Bus Caps

AC Output IGBTs

All PWM inverters (V/Hz, Vector & Sensorless Vector) share similar power circuit topologies. AC is converted to DC, filtered, and inverted to variable frequency, variable voltage AC.

M

PWM Power Circuit: AC to DC Converter Section AC to DC Rectifier

AC Input

DC Filter

DC Bus Caps

+ -

Input Reactor (option) DC Reactor

The AC input is rectified and filtered into fixed-voltage DC • Certain manufacturer’s units contain an integral DC reactor (choke) as part of the DC filter. • Adding an external AC input reactor will yield similar benefits. •

Both reduce harmonics, smooth and lower peak current.

Power Switches The IGBT: (Insulated Gate Bipolar Transistor) An IGBT is a hybrid between a MOSFET and a Bi-polar Darlington Transistor. COLLECTOR

=

GATE

EMITTER

• An IGBT can switch from “OFF” to “ON” in less than a microsecond. • Amplified logic signals drive the high-impedance GATE. Application Issues: • A 1 microsecond state-change will generate a 1 MHz RF pulse. • Dv/dt (rapid voltage changes) can stress motor insulation systems.

SWITCH

PWM Power Circuit: DC to AC Inverter Section Vu-v

DC to AC Inverter

DC Filter

AC Output + -

IGBTs

U

M

V W

Imotor IGBT Firing Signals An IGBT (Insulated Gate Bipolar Transistor) is a high-speed power semiconductor switch. IGBTs are pulse-width modulated with a specific firing pattern, chopping the DC voltage into 3phase AC voltage of the proper frequency and voltage. The resulting motor current is near-sinusoidal, due to motor inductance.

IGBT Switching Issues CONDITION Controller-to-motor lead length > 125’

RESULT

Nuisance trips from Output reactor installed capacitive coupling to near controller ground

Reflected (standing) wave phenomena

Nuisance trips; Motor insulation damage from voltage doubling

Carrier frequency in 2 to10Khz range

Motor acoustic noise

High dV/dT from fast Motor insulation damage from voltage doubling switching R.F. & Electromagnetic interference

SOLUTION

Interference with other equipment; telecommunications

Output reactor; Improved motor insulation Higher carrier or “quiet” algorithm Improved motor insulation RFI/EMI input filter; shielded motor cable; separate ground conductor

Basic V/HZ Control Circuit: Input, and Control Signals

V DC Bus current & voltage

IGBT Firing Signals

Operator Interface AC MOTOR DRIVE 0.75 KW HEALTH

Speed reference

200 V S E EQ

v 1.3 LO CA REF L PROG L

M

R JOG RUN

F W RE D V STOP RESET RESET

PWM microprocessor controller

Motor current & voltage

f

Flux Vector Control Elements Input, and Control Signals

Encoder

DC Bus voltage

Manmachine Interface AC MOTOR DRIVE

Speed and / or Torque reference

0.75 KW HEALTH

200 V S E EQ

v 1.3 LO CA REF L PROG L

M

R JOG RUN

F W RE D V STOP RESET RESET

IGBT Gating Signals

PWM microprocessor controller with Vector algorithm

Motor current & voltage

AC VECTOR CONTROL LOOPS AC Vector Drive Speed Loop Speed Error Speed Reference

Encoder

Torque Loop

Torque Ref.

PWM

Torque Regulator

Speed Regulator

Torque Reference Actual Torque

Torque Calculator

Freq. & Voltage Reference

Firing

Frequency

Speed

Typical AC Induction Motor Speed / Torque Curve “Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor Breakdown point: Maximum torque motor can produce before locking rotor

225 Starting Torque

%T

175

Pull-Up Torque

Full load operating point (100% current & torque)

LO AD

150

1750 RPM (nameplate)

100 Synchronous “no-load” speed 1800 RPM

Speed

(50 rpm)

SLIP

Typical AC Induction Motor Current & Torque Curves “Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor Starting (inrush) current

650

Breakdown current:

400

maximum level when motor locks rotor (stalls)

225

%T %I

175 150

Linear range: 40-150% load (operating range in which current is

100

proportional to torque)

Speed

AC Motor Speed / Torque Curve family on Inverter Power 225

%T

Motor base speed: 1750 RPM Peak Inverter Torque (150 -200%)

175 150 100

100% load torque operating line

Slip (50 rpm)

Speed

Slip (50 rpm)

At any applied Frequency, an induction motor will slip a fixed RPM at rated load.

AC MOTOR FORMULA SYNCHRONOUS SPEED SYNC RPM

=

120 x Frequency # of Poles

VOLTS / HERTZ V/Hz =

Example: 4-pole motor SYNC RPM = 120 x 60 / 4poles = 1800 RPM

Motor Line Volts Motor Frequency

Example: 460 V, 60 Hz motor V/Hz = 460/60 = 7.66 V/Hz

MOTOR SLIP %SLIP =

SYNC RPM - FULL LOAD RPM X 100 SYNC RPM

VOLTS FREQUENCY V/Hz 460

60

7.66

Example: 1750 RPM motor

345

45

7.66

% Slip = (1800 - 1750) / 1800 x 100 = 3% Slip

230

30

7.66

115

15

7.66

7.66

1

7.66

AC MOTOR SIZE Frame size is directly related to base RPM, for a given Horsepower Example: 15 HP motors of different base speeds

Base RPM

3600 (2-pole)

1800 (4-pole)

1200 (6-pole)

Frame Size

215

254

284

Torque

22.5 lb-ft

45 lb-ft

67.5 lb-ft

Amps

18.5

18.7

19.3

How Slip Compensation improves speed regulation Example: Motor under load at 30 Hz BEFORE 175

30 Hz curve Full load 30 Hz operating point (100% current & torque)

% 150 T

850 RPM

100

AFTER 175

% 150 T

New 31.7 Hz curve 900 RPM

100

Sync. or “no-load” 30 Hz speed

950 RPM

900 RPM

Speed

Slip (50 rpm)

A motor will lose 50 rpm under full load with 30 Hz applied frequency, slipping from 900 to 850 RPM.

Speed

Slip (50 rpm)

By sensing current and other variables, SLIP COMP will apply 31.7 Hz to the motor, restoring the speed to 900 RPM.

Induction Motor Advantages • Low cost (compared with DC) • Wide availability • Low maintenance - no brushes or commutator • Rugged design - can be used in harsh environments • Low inertia rotor designs • High electrical efficiency • Wide speed ranges • No separately-powered field windings • Good open-loop performance

Elements of an Induction Motor: The Rotor No direct electrical connections are made to the rotor. All forces are magnetically induced by the stator, via the air gap.

Rotor Bar Current Cast aluminum rotor bars Carry induced current (skewed bars shown)

Cast aluminum end rings Electrically s rotor bars at both motor ends

Laminations of high-silicon content steel Low-eddy current loss magnetic medium

Elements of an Induction Motor: The Stator Stator Core Lamination stack of notched steel plates

Elements of an Induction Motor: Stator Windings (4-pole) Steel Laminations

wye or delta connection types

Slots

Stator Windings

Elements of an Induction Motor: The Stator (4-pole) t Rotating magnetic field

The stator induces magnetic lines of flux across the air gap, into the rotor

Induction Motor Slip SLIP = (ωs - ωr ) / ωs

ω ω

rotor

stator

• Motor slip is proportional to load torque. • Stator speed is known by frequency • Rotor speed is measured with an encoder (Vector). • Rotor speed can be approximated, knowing motor and bus current (Sensorless Vector algorithm)

Rotor Magnetic Field Dynamics: SLIP creates TORQUE When rotor speed is near stator speed (light load), few stator flux lines are cut . Rotor bar current and slip frequency are low. Magnetic Flux Lines

Magnetic Flux Lines

R

ot or B

ar C

ur r

en t

Magnetic Flux Lines

Light Load

Heavy Load

As the rotor slips, rotor bar current slip frequency increases, resulting in greater rotor field strength (more torque).

Induction Motor Equivalent Circuit Air Gap

Rotor

Stator

V

Stator Resistance

Leakage Reactance

R1

XLR

Rotor Reactance

XR XM

Magnetizing Reactance

RLOAD = R / Slip* 2 *(R2 is rotor bar resistance)

Although there is no physical connection between rotor and stator, the induced field causes the motor model to behave as if there is.

Motor Current Vectors

Magnetizing Current

lC Tota

Total Current is the Vector sum of Magnetizing and Torque-producing current, which are at a right angle to each other.

nt e r r u

Torque-Producing Current

Stator Stator Resistance

R1 Total Current

Leakage Reactance

XLR Magnetizing Current

Air Gap

Rotor Rotor Reactance

Torque Current

XR XM

RLOAD

Motor Current Vectors Magnetizing Current

al t t To ren r Cu

LIGHT LOAD

• Magnetizing current is reactive (low p.f.) • Measured (total) motor current is not a good indicator of load level.

TorqueProducing Current

Magnetizing Current

• High % of total current is “magnetizing” current

lC Tota

nt e r r u

&

Torque-Producing Current

Magnetizing Current

T

MEDIUM LOAD

ent r r u C otal

Torque-Producing Current

HEAVY LOAD

• Most of total current is torque-producing • Motors run at high power factor • Total motor current is proportional to load level.

Autotuning on Sensorless Vector Drives FACT: Most motor electrical parameters are difficult to obtain from the manufacturer. ROTOR RESISTANCE ROTOR REACTANCE MAGNETIZING CURRENT STATOR RESISTANCE LEAKAGE REACTANCE

?????

Not typically found on motor nameplate

A Sensorless Vector AUTOTUNE function makes the job easy: 1. Enter nameplate motor parameters (base speed, full load amps, voltage, frequency, power factor). 2. Run the ‘AUTOTUNE ‘ function. The controller will pulse the motor & determine approximate motor electrical characteristics for SENSORLESS VECTOR Operation. 3. The S-V algorithm can now compute torque- and magnetizing current vectors for more precise motor control.

Facts about Induction Motors Most AC motors are designed to be used in fixed speed (across-the-line) operation. • Rotor bar design, cooling impellers, insulation systems have been designed for 60 Hz sine-wave power. • When operated on an inverter, performance and reliability may be compromised: » Insulation systems may break down from stresses of IGBT PWM power. » Cooling efficiency from shaft-driven fan will limit low speed range » Motor harmonics will reduce Service Factor rating. » Peak running torque is less than optimum.

Inverter-Duty Induction Motors Many motor manufacturers have introduced lines of motors they call “Inverter Duty” or “Vector Duty”. Features and characteristics vary between manufacturers.

Typical features found on Inverter-Duty Motors • High Dielectric strength wire insulation - Thermal-ezeTM (one brand) resists pin-hole punctures caused by IGBT dV/dT switching stresses. • Better Cooling - Efficient shaft-fan designs, constant-speed fans, and overframing. • Optimized rotor design - Bar profile designs suited for inverter, not linestart duty. • Tach-mounting provisions - Easy, non-drive end mounting of encoders for Vector Duty operation. • Wider speed ranges - Designs for above-base speed operation and custom V / Hz ratios

AC Induction Motors Common Rotor Bar Shapes & Effects All have nearly the same performance at full load At locked rotor... Low resistance Low reactance High amps Average torque

• • • •

High resistance Average reactance Average amps Average torque

• • • •

High resistance High reactance Low amps Low torque

ACROSS-THE-LINE OPERATION Best for Inverter

TORQUE & AMPS

• • • •

SPEED

AC Induction Motors Effecting Base Speed through Volts / Hz Design Motors on inverters don’t have to be wound for “60 Hz” • Optimal power delivery occurs if voltage peaks at base speed • Lowest amps occur at peak voltage . • Drive price / component cost is related to amps. Example of a 4-pole “550 RPM” base speed motor: Stator is wound for 460V @ 20 Hz V/Hz = 460/20 = 23 3:1 CONSTANT HP 460 NAMEPLATE BASE SPEED

VOLTS

0

20 600

40 1200

60 1800

Hz RPM (sync.)

Motor Operation above Base Speed Motor base speed: 1750 RPM (4-pole) 60 Hz curve

225

% 175 T 150 Base

100

120 Hz curve Peak Inverter Torque (150 -200% current) 100% current operating line

50 1800

Speed

Slip (50 rpm)

3600 Slip (50 rpm)

• Above base speed, continuous torque declines to 50% at 2 x base. • Peak Inverter (overload) torque declines even more rapidly. • Motor slip increases, for a given torque level.

Motor Operation above Base Speed Constant Voltage 460 “Field Weakened Range”

Torque

α

V/Hz

Frequency increases above base speed, but voltage levels off.

V

60

120

Hz 100

%T & HP 50

Constant Horsepower

Constant Torque

W PO E RS HO

ER

Reduced Torque

Hz

60

120

The result is increased speed with weakened torque, or constant HP operation. Above 2:1 , motor torque drops sharply & operation is not recommended.

AC V/Hz Drives Pro’s & Con’s Advantages

Limitations

• Simple, “look-up table” control of voltage and frequency

• Low dynamic performance on sudden load changes

• Good speed regulation (1-3%)

• Limited starting torque

• No motor speed needed

• Lacks torque reference capability

• Multi-motor capability

• Overload limited to 150%

Best for General Purpose & Variable Torque Applications: • Centrifugal Pumps & Fans • Conveyors • Mixers & Agitators • Other light-duty non-dynamic loads

AC Sensorless Vector Drives Pro’s & Con’s Advantages

Limitations

• High starting torque capability (150% @ 1 Hz)

• Speed regulation may fall short in certain high performance applications

• Improved speed regulation (< 1%)

• Lacks zero-speed holding capability

• No motor speed needed

• Multi-motor usage defaults to V/Hz operation

• Self-tuning to motor • Separate speed and torque reference inputs

• Torque control in excess of 2 X base speed may be difficult

Suitable for all General Purpose, Variable Torque and moderate to high performance applications • Extruders • Winders and unwind stands • Process lines

AC Closed-Loop Vector Pro’s & Con’s Advantages

Limitations

• Ultra-high torque and speed loop performance & response

• Requires encoder

• Excellent speed regulation to .01%

• May require vector motor for full performance benefits

• Full torque to zero speed • Extra-wide speed range control

• Single motor operation only

• 4-quadrant (regenerative) operation requires additional hardware

Best for High Performance Applications: • Converting applications • Spindles & Lathes • Extruders • Other historically DC-applications

Variable Torque Applications: Centrifugal Pumps & Fans Flow, Torque & Horsepower

100%

T = K x (RPM)2 HP = K x (RPM)3

80%

50%

ow Fl

l Vo r o

•

Load varies with the square of the speed

e um

• HP varies with the cube of the speed • Ideally suited for AC Drives • Energy savings benefits: only 50% power required at 80% flow

e qu r To

rs Ho

Speed

e

er w po

• AC Drives replace inefficient dampers, guide vanes and valves

80%

100%

Variable Torque Applications: Centrifugal Fan Energy Savings

Power Consumption

100%

Damper Control Power

Throttling air volume mechanically with dampers or inlet guide vanes is an inefficient control method.

50%

AC Drive Power

Flow

100%

Variable Torque Applications: Centrifugal Pumps & Fans 100%

Since load torque diminishes rapidly below base speed, the Drive always appears lightly loaded.

Load Torque

Base

RPM 100%

Volts

CO

N TA S N

T

T

Most drive controllers have a special “variable torque” V/Hz profile selection that further cuts down on magnetizing current at light loads. Since magnetizing current is purely reactive, motor losses are reduced .

UE Q OR

VA

Hz

E BL A RI

T

QU R O

E

60

Regenerative Operation of AC Motors Example: 1750 RPM motor on 60 Hz power

LOAD TORQUE & CURRENT

Current

+100%

Synchronous Speed 1800 RPM

Motoring 1750

1850

Regenerating

-100%

Regen Breakdown

SPEED

4-Quadrant Operation of AC Motors on Inverter Power Clockwise TORQUE

REVERSE REGENERATING

FORWARD MOTORING

- RPM

+ RPM

REVERSE MOTORING

FORWARD REGENERATING

CounterClockwise TORQUE

Conditions for Regenerating on an AC Motor AC Motors regenerate when pulled faster than their sync speed at the applied frequency. At 60 Hz, if a motor is pulled faster than 1800 RPM*, the motor will behave as an induction generator.

Regeneration conditions: • Overhauling loads • Fast deceleration of high inertial loads • Stopping on a timed-ramp • Cyclic loads or eccentric shaft loading PULL

ROTATION

WEIGHT

* 1750 RPM base speed at 60 Hz

AC Drive Regeneration Energy Flow:

AC Input

TWO - WAY

ONE - WAY

DC Bus Caps

+ _

IGBTs

M

• Current flows back into the DC bus, via the IGBT switching & back diodes. • AC Drive front-end rectifier is unidirectional; energy cannot flow back into the AC line. • Some returned energy is dissipated in losses in the capacitors, switches, and motor windings (10-15%). • Excessive regeneration can cause problems, such as DC Bus Overvoltage.

DB is ACTIVE when: • Motor has an overhauling load • Fast decel of high-inertial load • Stopping in ramp-to-rest mode

+ _

DBR

DC Bus Caps

SIGNAL

AC Input

V DC

Dynamic Braking on AC Drives

DYNAMIC BRAKING CONTROL

M

DB is NOT ACTIVE when: • Decelerating a frictional load • Stopping in coast-to-rest mode • Drive is disabled or if power is removed

DYNAMIC BRAKING is typically an option for AC Drives A seventh IGBT, integrally mounted, is modulated when DC Bus voltage is excessive. Resistor Grids (external on ratings 5 HP & above) dissipate the excess energy. DB is duty-cycle limited to a set number of stopping operations

Dynamic Braking on AC Drives: Application Considerations DB is not failsafe: if the drive faults or power is removed, DB will not function. DB only operates when the drive is running: in coast-rest or stand-by, DB is inactive. DB should not be used in EMERGENCY STOPPING: the drive will continue on a timed ramp, producing torque the entire time. DB is suitable for intermittent operation only: other regenerative solutions exist for long-term overhauling loads

Application of AC Drives on a Common DC Bus + M

M

M

AC Drives on a Common DC Bus: Theory of Operation +

AC DRIVE

REGEN

NET POWER Net power usage is minimal, due to the efficient use of returned energy.

AC DRIVE MOTORING

AC DRIVE REGEN

AC DRIVE MOTORING

As individual drives regenerate, the returned energy is redistributed to motoring drives via the common DC bus.

AC Drives on a Common DC Bus: Typical Connection Diagram THERMAL- MAG BREAKER

INPUT LINE REACTOR

AC DRIVE

AC DRIVE

AC DRIVE

SEMICONDUCTOR FUSES INTERLOCKED DC OR

Line Regenerative AC Drives V DC

BI-DIRECTIONAL POWER FLOW

LINE

M

IGBT Firing Signals

IGBT Firing Signals

CONVERTER

INVERTER

LOAD

PWM microprocessor controller • • • • •

Two sets of 6 - IGBT bridges Gating control for both sets Converter IGBTs modulate on when bus voltage is excessive. More complex regulator design More conducted noise to power line

Cost of drive is 1.8 times standard non-regen AC Drive

Multi-motor Applications Motor amps must total less than controller amp capability • Each motor must have its own overload • Drive must be in the “V/Hz” control mode

AC DRIVE (V/Hz mode)

• Motor speeds will be within slip-speed range, with respect to each other.

30 HP 38 Amps

• Interlock output ors to drive run logic, when used.

2 hp 2.8 amps

3 hp 3.9 amps

OVERLOAD S

10 hp 12 amps

2 hp 2.8 amps

Total HP = 25 Total Amps = 32.6

3 hp 3.9 amps

5 hp 7.2 amps

Application of or By on AC Drives

MAIN CB

Provides back-up, across-the-line operation of motor • Single-speed operation on line only (must have mechanical control in place) • Motor overloads are mandatory.

INVERTER DISCONNECT

• ors are interlocked to prevent inverter back-feed.

AC DRIVE

• Popular in HVAC / VT applications. • Not recommended on “inverter duty only” motors (high inrush current). OFF INVERTER

INVERTER OR

BY OR

BY MOTOR OVERLOAD

TYPICAL 3-POSITION SELECTOR SWITCH

AC Drives and Power Factor Motor P.F. = .70 (Light Load)

AC INPUT P.F. = .96

REACTIVE FLOW

AC Input

M

AC Drives inherently correct motor Power Factor • Reactive current bi-directionally flows between the inductive motor and bus capacitors. • Input PF has no relationship to motor PF. • Since input current is in-phase with voltage, input displacement PF is always near unity.

Never use power factor correction capacitors with AC Drives!!!

DC DRIVE BASICS

A1

Armature

A2 F1 F2

LINE INPUT

Field

MOTOR OUTPUT

DC Drives convert AC line voltage into variable DC voltage with an SCR phase-controlled bridge rectifier, to power the DC motor ARMATURE. A separate field supply provides the motor with DC FIELD excitation.

Inside the DC Motor (Shunt Field Design)

F1

N F2

A1

A2

S

The commutator & brushes keep armature flux in a fixed position relative to the field, which guarantees the torque force is always perpendicular to field magnetization.

Typical DC Motor Armature Current & Torque Curves

100

%T % IDC

0

-100

-200

NO LOAD

200

MOTORING RPM

REGENERATING

Armature current is directly proportional to torque throughout the loading range.

DC Motor Torque & HP vs. Speed

% 100

FIELD WEAKENED RANGE 4:1

FULL FIELD CONSTANT TORQUE

CONSTANT HORSEPOWER

75

50

HO RS EP OW ER

TORQUE & HORSEPOWER

Motor nameplate: 250 / 1000 RPM

TORQUE @ 100% ARMATURE AMPS 2 : 1 FIELD WEAKENING 3 : 1 FIELD WEAKENING

25

4 : 1 FIELD WEAKENING

250 Base Speed

500

SPEED (RPM)

750

1000 Max.Speed

Power Switches The SCR: (Silicon Controlled Rectifier) a.k.a. - “Thyristor” ANODE

CATHODE

GATE • Extremely robust solid-state switch / 40+ year proven track record • Key element in DC Drive power circuit • Simple pulse gating turns on current flow • Device has self-turn-off when reverse biased • Stud-mount, hockey-puck and encapsulated 2-, 4- and 6-pack types available in certain sizes and ratings.

TRIGGER

+

Application Issues: AC Line Notching on DC Drives AC Input

Commutation notches are caused by the transfer of current from one SCR to another.

V ph-ph

The notches can cause misfiring on drives common to the same power line.

Solution: Installation of a small (25-50 uH range), 3-phase reactor on each DC controller will prevent cross-talk and other related problems.

Elements of a DC Drive: Non-regenerative type

A1

F1

AC Input

F2

SCR Firing Signals

Field Control Signals

Line current

AC MOTOR DRIVE

Speed or Torque Reference

0.75 KW HEALTH

200 V S E EQ

v 1.3 LO CA REF L PROG L

M

R JOG RUN

F W RE D V STOP RESET RESET

Operator Interface

Microprocessor controller

A2 Motor voltage

Tachometer (closed-loop)

Elements of a DC Drive: Regenerative type AC Input

F

F

R

F

R

F

R

R

F

F

SCR Firing Signals FWD/MOT Line current AC MOTOR DRIVE

Speed or Torque Reference

0.75 KW HEALTH

200 V S E EQ

v 1.3 LO CA REF L PROG L

M

R JOG RUN

F W RE D V STOP RESET RESET

Operator Interface

A1

R F1

R

F2 Field Control Signals

REGEN/REV

Microprocessor controller

A2 Motor voltage

Tachometer (closed-loop)

Dynamic Braking on DC Drives A1

M

DBR

F1

F2 A2 M

Braking Power

M

time

• Dynamic Braking Resistors are shunted across the motor armature in a STOP or ESTOP mode. • Motor counter-EMF (back voltage from motor, acting as generator) appears across resistor grids. • Voltage diminishes as resistors dissipate energy. • Braking Power diminishes exponentially with motor slowdown: P = V2/R

Not failsafe: DB will not function if field supply is absent (i.e. - if power is lost)

DC Regenerative Drives vs. DC Dynamic Braking • DC regen drives provide constant torque deceleration and stopping. • DC dynamic braking power diminishes with speed reduction. • Both require full field power / neither will work in power outage. • DC regen requires drive to be fully operational (no faults) • DB can be used in conjunction with a regen drive, for certain stopping conditions • DC regen added benefits include full 4-quadrant torque control. • DB may require an additional or, if the manufacturer uses an AC input or.

DC DRIVE MARKET MARKETPLACE FOR THE DC THYRISTOR DRIVE • Most widely used drive in heavy industry • for 40% of total variable speed drive market (much higher percentage in process industries). • Estimate 0 - 5 % growth / annually to 2000 • Very established mature product with continuing development.

AC DRIVE MARKET MARKETPLACE FOR THE V/Hz AC PWM DRIVE z

z z

s for 60% of total variable speed drive market (much lower percentage in process industries) Estimated 5 - 10% growth / annual to 2000 Mature product but due to limited performance used generally only on peripheral rather than process drives.

AC VECTOR DRIVE MARKET MARKETPLACE FOR THE AC FLUX VECTOR DRIVE

z z z

z

Introduced during last eight years “Sensorless” introduced during last three years Growing use in most process industries (very strong growth in elevators & hoists etc) Only AC drives currently available with similar or equivalent performance to DC

Measuring Bandwidth Response AC Vector Drive Speed Loop

“TEST”

Speed Error Speed Ref 1.0

Torque Ref.

.7071 Speed

PWM

Torque Regulator

Speed Regulator

Actual Torque

45 degrees

Encoder

Torque Loop

Torque Calculator

Freq. & Voltage Reference

Firing

Frequency

Speed

• A sine-wave signal generator is applied to the reference input • is monitored as reference frequency is increased. • When lags reference by 45 degrees, and amplitude is reduced to 71% of the input signal, this is defined as the “BANDWIDTH RESPONSE”.

Drive Performance Comparison Speed Regulation

Speed Loop Response

Torque Accuracy

Torque Response

DC open loop

2-3%

.5 - 2 Hz

5 - 10%

10 - 20 Hz

DC closed loop

.01 - 1%

10 - 20 Hz

2 - 5%

20 - 100 Hz

AC V/Hz

1 - 5%

1 - 2 Hz

10 - 20%

5 - 10 Hz

AC Sensorless Vector

.1- .5%

15 - 25 Hz

2 - 10%

75 - 200 Hz

.01 -.05%

20 - 100 Hz

.5 - 1%

200 - 1000 Hz

AC Flux Vector

Performance varies widely, between drive manufacturers • Speed regulation is dependent upon speed device used. • Open loop regulation is motor-dependent • Response rates are rarely published & can be misleading.

Common Drive Formulas for AC & DC TORQUE AND HORSEPOWER

HP =

Torque x RPM 5252

For a 4-pole (1800 RPM) motor:

Torque (lb-ft) = 3 x HP For a 6-pole (1200 RPM) motor:

Torque =

HP x 5252 RPM

Torque (lb-ft) = 4.5 x HP For a 2-pole (3600 RPM) motor:

Torque (lb-ft) = 1.5 x HP

Accelerating / Decelerating an inertial load: Wk2 x ∆ RPM Torque = 308 x ∆ tsec ∆ tsec =

Wk2 x ∆ RPM 308 x Torque

*(Wk2 is inertia in lb-ft2)

Common Electrical Formulas for AC & DC Drives AC line current and armature current (DC Drives)

IDC = IAC / .83 AC line voltage and DC bus voltage (AC Drives)

VDC = VL-L x 1.41 Horsepower and Kilowatts

HP = KW / .746 KW = HP x .746 Three phase Power

KVA =

VL-L x I x 1.732 1000

KVA = KW / P.F.

Pout % Efficiency = X 100 Pin

Power losses in AC & DC controllers (5 - 100 HP; excluding motor; full speed & load)

DC:

AC to DC

98%

EFFICIENCY

SCR losses = 1% Fixed losses = 500 -1000W CONTROL & FANS

AC: AC to DC

Cap losses = .5%

DC to AC

96%

EFFICIENCY

SCR / Diode losses = 1%

IGBT losses = 1.5% CONTROL & FANS Fixed losses = 800 -1500W

Power Factor on AC and DC Drives .96

On AC Drives, input displacement power factor remains nearly constant with speed & load.

AC

.85

On DC Drives, power factor varies directly with SCR phase-firing angle, peaking near .85 .

POWER FACTOR

DC Since power increases linearly with speed, the effects of low power factor at low speed are negligible.

.30

20%

SPEED

100%

DC Drive Advantages over AC • Simple Controller Design- only one power conversion stage, no power storage elements. • Higher Controller Efficiency- 98%+ electrically efficient • Simple, 4-quadrant line regeneration - with 6 reverse SCRs • Efficient, inherent Torque control - Field & Armature flux always positioned optimally. • Retrofit to existing DC motors - previously power by M-G set or older drive types. • Most cost-effective drive package above 100HP • High Controller reliability - Low maintenance due to simple power module design

…more DC Drive Advantages over AC • Lower power line harmonic contribution - less than 50% of AC • Smaller line reactors- less costly • More compact controller size per equivalent HP • More robust power semiconductors - SCRs have better overload and peak voltage characteristics, vs. IGBTs. • Low motor acoustical noise: no “carrier” noise. • Fewer motor lead-length issues: no capacitive coupling, dV/dT or standing wave problems. • Easier troubleshooting & serviceability

AC Drive Advantages over DC • Simple, low-maintenance motor - no brushes or commutator. • High dynamic performance - low rotor inertia, compared with DC armature. • Motors are inexpensive & readily available • Motors suitable for harsh, rugged environments : some explosionproof ratings available. • Better open-loop speed regulation - with Sensorless Vector & slip compensation. • Higher torque response bandwidth - on Vector-type; not limited by AC line frequency. • More cost-effective drive package below 100HP • Multi-motor & inherent load sharing on single controller • Line-by option - permits single-speed motor operation during controller maintenance

…more AC Drive Advantages over DC

• No separate motor field - no field loss sensing required • Wider speed ranges - motors available through 6000 RPM & higher. • or-free dynamic braking - linear braking power to zero speed. • Retrofit onto existing single-speed AC applications • Smaller motor frame sizes than equivalent DC. • Longer power-dip ride-through capabilities • Near unity power factor regardless of speed and load