Chapter 6 - AC Drives

UTM UNIVERSITI TEKNOLOGI MALAYSIA

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SEE 4433

Power Electronics

And Drives


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Chapter 6:

AC Drives


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Engineer/Motors_and_Drives/What_exactly_is_an_ac_drive_and_just_how_does_it_work%3F/21560/

http://www.engineerlive.com/Design-Engineer/Motors_and_Drives/What_exactly_is_an_ac_drive_and_just_how_does_it_work?/21560/




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Motor Drive


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• Why induction motor (IM)?

– Robust; No brushes. No contacts on rotor shaft

– High Power/Weight, Lower Cost/Power ratios

– Easy to manufacture

– Almost maintenance-free, except for bearing and other “external”

mechanical parts

• Disadvantages

– Essentially a “fixed-speed” machine

– Speed is determined by the supply frequency

– To vary its speed need a variable frequency supply

• Motivation for variable-speed AC drives

– Inverter configuration improved

– Fast switching, high power switches

– Sophisticated control strategy

– Microprocessor/DSP implementation

• Applications

– Conveyer line (belt) drives, Roller table, Paper mills, Traction, Electric

vehicles, Elevators, pulleys, Air-conditioning and any industrial process that

requires variable-speed operation.

• The state-of-the-art in IM drives is such that most of the DC drives will be

replaced with IM in very near future.

Induction Motor Drive


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Induction motors (IM) most widely used

IM (particularly squirrel-cage type) compared to DC motors

Rugged

Lower maintenance

More reliable

Lower cost, weight, volume

Higher efficiency

Able to operate in dirty and explosive environments

IM mainly used in applications requiring constant speed

Conventional speed control of IM expensive or highly

inefficient

IM drives replacing DC drives in a number of variable speed

applications due to improvement in power devices capabilities

Reduction in cost of power devices

Introduction


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Review on

3-phase IM

■ Balanced 3-phase winding on stator

■ Rotor of squirrel-cage (SC) type IM has conductors shorted by

end rings

■ By induction, same number of phases and poles produced by

rotor as in stator winding

■ When stator supplied by balanced 3-phase AC source (frequency

ω (rads/s) or f (Hz)) rotating field moving at synchronous speed, ωs

(rad/s) is produced


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Torque Production

• Only “squirrel-cage” IM (SCIM) is considered in this module

• Neglecting all harmonics, the stator establishes a spatially

distributed magnetic flux density in the air-gap that rotate at a

synchronous speed, ω1 or Ns

where ωe : supply frequency (in Hz);

p : pole pairs (p = 1 for 2 pole motor, p = 2 for 4 pole motor etc)

• If the rotor is initially stationary, its conductor is subjected to a

sweeping magnetic field, inducing rotor current at synchronous speed.

• If the rotor is rotating at synchronous speed (i.e. equals to f1), then

the rotor experience no induction. No current is induced in the rotor.


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• At any other rotor speed, say ωm, the speed differential ωi-ωm

creates slip. Per-unit slip is defined as:

Torque Production

• Slip frequency is defined as: ω2= ω1-ωm.

frequency rotor, f2 = sf1

• When rotor is rotating at ωm., rotor current at slip frequency

will be induced.

• The interaction between rotor current and air-gap flux

produces torque.


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Single-phase Equivalent Circuit (SPEC)


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Stator voltage equation:

Rotor voltage equation:


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SPEC, referred to stator

• From previous diagram, SPEC is a dual frequency circuit. On

the stator is ω1 and on the rotor ωm

• Difficult to do calculations.

• We can make the circuit a single frequency type, by referring

the quantities to the stator


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Rotor Current

If E is the back EMF in the stator phase, then the back EMF

in an equivalent rotor phase with the same effective turns

ratio will be E where :

Note that the quantities are now referred to the stator, but with

the rotor resistance alteration.


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Performance calculation

using SPEC


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Performance

calculation using SPEC


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Power and Torque

Power is transferred from stator to rotor via air–gap, known

as air gap power


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A single phase equivalent circuit of a 6-pole SCIM that operates

from a 220V line voltage at 60Hz is given below. Calculate the

stator current, output power, torque and efficiency at a slip of

2.5%. The fixed winding and friction losses is 350W. Neglect

the core loss.

Example :

Solution :


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Solution :


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Starting Current

For the previous example, Calculate the stating current when

motor is first switched on to rated applied voltage.

This is common for induction motors. Care should be taken

when starting induction motors.

Solution :


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Approximate SPEC

ω1=2πfe=electrical speed


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Single (Fixed Supply)

Frequency Characteristics


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Single Frequency Characteristics


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• As slip is increased from zero (synchronous), the torque

rapidly reaches the maximum. Then it decreases to standstill

when the slip is unity.

• At synchronous speed, torque is almost zero.

• At standstill, torque is not too high, but the current is very

high. Thus the VA requirement of the IM is several times than

the full load. Not economic to operate at this condition.

• Only at “low slip”, the motor current is low and efficiency and

power factor are high.

Single Frequency Characteristics


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Scalar

Control Of

Induction

Machine


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Variable Line Voltage,

Fixed Frequency


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Typical IM Drive System :

Variable voltage, variable frequency


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V V V F – Variable Speed Characteristics

• For variable speed operation, the supply is an inverter.

• The frequency of the fundamental AC voltage will

determine the speed of IM. To vary the speed of IM, the

inverter fundamental frequency need to be changed.

• The inverter output frequency must be kept close to the

required motor speed. This is necessary as the IM

operates under low slip conditions.

• To maintain constant torque, the slip frequency has to be

maintained over the range of supply frequencies.


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Variable Voltage, Variable Frequency

( V V V F ) Control Scheme

In order for maximum torque production, motor flux (air gap)

should be maintained at its rated value.

(assume negligible volt drop across stator impedance)

Therefore, in order to

maintain the motor flux,

the ( E1 / f1 ) ratio has to be

kept constant.

This is popularly

known as the

constant Volt / Hertz

operation


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Variable Voltage, Variable

Frequency Constant V / f

T- ω characteristic

of the motor shifted

horizontally

Small slip

frequency operation

at all operating

frequency

(high efficiency)

50 Hz

30 Hz

10 Hz


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At low speed , the volt drop at Rs and Xls quite significant

compared to supply Vs, therefore to compensate the drop, Vs

normally boosted at low frequency


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Three

phase

power

supply

Voltage

Source

Inverter

(VSI)

Rectifier

Induction

Motor

IM


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Problems with open-loop constant V/f

At low speed, voltage drop across stator impedance is

significant compared to air gap voltage - poor torque

capability at low speed

Solution:

Boost voltage at low speed Maintain Im constant – constant

Φag

•Torque deteriorate at low frequency – hence compensation

commonly performed at low frequency

• In order to truly compensate need to measure stator current

– seldom performed

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with voltage boost at low frequency


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Poor Speed Regulation

Solution:

Compensate slip closed-loop control

TL

Slip ωr ωsyn

Motor

Characteristic after

compensation

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Constant V/f

– open-loop with slip compensation and voltage boost



Three

phase

power

supply

Voltage

Source

Inverter

(VSI)

Rectifier

Induction

Motor

IM

Slip Speed

Calculator


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Constant Torque Region

• Hence for VVVF operation, there is a need to control the

fundamental voltage of the inverter if its frequency (and

therefore the frequency of the IM) need to be varied.

• To vary the fundamental component of the inverter, the

MODULATION INDEX, MI can be changed.

• The rated supply frequency is normally used as the base

speed

• At frequencies below the base speed, the supply magnitude

need to be reduced so as to maintain a constant Volt / Hertz.

• The motor is operated at rated slip at all supply frequencies.

Hence a “constant torque” region is obtained.


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Rated Slip


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• Above base speed, the stator voltage reaches the rated

value and the motor enters a constant power region.

• In this region, the air-gap flux decreases. This is due to

increase in frequency frequency while maintaining fixed

voltage.

• However, the stator current is maintained constant by

increasing the slip. This is equivalent to field weakening

mode of a separately excited DC motor.

Constant Power Region


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“Field Weakening” Constant Torque

Base Speed


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VVVF Summary

Constant Power Constant Torque


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Idealized

Motor

Characteristic

DC Motor Induction Motor


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Example :

Motor is controlled by a voltage source inverter with

constant V/f

Calculate:

a) Speed for frequency of 30 Hz and 80% of full load

b) Frequency for a speed of 1000 rpm and full load

c) Torque for a frequency of 40 Hz and speed of 1100 rpm

Answer: 796 rpm, 37.67 Hz, 31.07 Nm


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A three-phase 4-pole, 10 horsepower, 460 Vrms/60 Hz (line-to

line) runs at full-load speed of 1746 rpm. The motor is fed from

an inverter. The flux is made to be constant.

Plot the torque-speed graphs for the following frequency: 60

Hz, 45 Hz, 30 Hz,15 Hz.

A three-phase induction motor is using a three-phase VSI for

VVVF operation. The IM has the following rated parameters:

• voltage : 415 V (RMS)

• frequency : 50 Hz

• slip : (p.u) 5%

• pole pair : 2

– If the inverter gives 415V (RMS) with modulation index of 0.8,

calculate the required modulation index if the motor need to be

operated at rotor mechanical speed of 10 Hz.

Example :


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a) Briefly discuss the circuit topologies for variable

voltage variable frequency induction motor speed

Drives

b) A three-phase 4-pole, 10 kW, 415 Vrms/50 Hz (line-to

line) runs at full-load speed of 1320 rpm. The motor is

fed from an inverter. The flux is made to be constant.

Plot the torque-speed (Hz) graphs for the following

supply inverter frequency: 50 Hz, 30 Hz, 20 Hz.

Also plot T-ω when f = 60 Hz and 100 Hz.

Example :


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The End

of Chapter 6

Thank You

Chapter 6 - AC Drives

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Transcript of Chapter 6 - AC Drives