AC Drives (Oct09) lecture slides (as of 16 Nov09)

8/3/2019 AC Drives (Oct09) lecture slides (as of 16 Nov09)

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Chapter 4: Induction Motor Drives


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Induction Motor Drives

LEARNING OUTCOMESLEARNING OUTCOMESTo understand the characteristics of

various induction motor drives


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Describe various types of speed control

Understand variable voltage operationDescribe various configurations of phase-controlled convertersUnderstand variable frequency operationDescribe various configurations of voltage-fed inverters


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Describe various configurations of current-

fed invertersDescribe merits and demerits of voltage-fedand current-fed invertersExplain effect of harmonics on motorperformance

Describe Scalar Control and Vector Control. e L e a

r n i n g # 3 ( w

e e k 1 2 )

e L e a

r n i n g # 3 (

w e e k 1 2

)


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Induction Motor Drives - eLearning

eLearning#3 (week 12)eLearning#3 (week 12)Self learning on the selected sections and atthe end of the session attend Self-

Assessment MCQs of 10 MCQs per quiz(multiple attempts) to gauge on the progressof learning. Formal quiz (single attempt) onthe chapter will be conducted on week17/18 (for more details pls refer to Module

Map)


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Basic model

Rotatingflux


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Basic model

When three-phase voltage is applied, itproduces rotating magnetic field and the speedat which it rotates is denoted by N sCurrent is induced in the rotor conductorsForce is developed on the rotor conductors,

Hence motor starts rotating, at a speed NrInduction motor can be viewed as atransformer with short-circuited secondary

which is rotating


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Basic model

Speed Ns is always higher than N r, because ifthe two speeds become equal, there will be noflux cutting and hence no induced current and

torque. Hence motor operation will not bepossible


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Basic Speed-torque characteristics

rpm

Ns


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Various types of speedcontrol

see Fig 6.3 (N-T curves)For different designs ofinduction motor(a) high slip , (b) double

cage , (c) deep barSpeed control designdependent

T

(a)(b)

(c)

Generalpurpose


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Fig 6.1a (Equivalent circuit)


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Fig 6.1b (Equivalent circuit)


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Various types of speed control

see 6.8 (Speed Control)Pole Changing (6.9)Stator Voltage Control (6.11)Variable Voltage Variable Frequency (6.12)CSI (6.13.1)

Rotor resistance (6.15)Slip power recovery (6.16)

Squirrel-cage

Wound-rotor

Squirrel-cage

Wound-rotor


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Induction Motor DrivesVarious types of speed control


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Various types of speed control

Pole Changing (6.9)Fig 6.24 and 6.25

Nr = (1-s) N s p

f N s

=120


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Stator Voltage Control(6.11)

Fig 6.31Tmax is proportional to V 2

Torque is proportional tovoltage squared

Hence as V is reduced toreduce speed

2b

2a

b

a

V

V

T

T= TT11TT22


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Stator Voltage Control

Can be controlled by1-phase triac voltage controller (Fig 6.32a)3-phase thyristor voltage controller (Fig 6.32b)

Vary by firing angleT1 (V1-ok, V 2-not ok); T 2 (V1-not ok, V 2-ok)

Not suitable for constant load torqueSuitable for applications require low startingtorque, narrow speed range with low slip (eg.domestic fan, pump drives)


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Variable frequency operationsee 6.12

Ns (controlled by frequency)Nr = (1 - s) N sTmax (Eqn. 6.69)

21

2'2

2

2

max

)(4

)(

T

++

=

r sss L L

f R

f R

f V

K

p f N s = 120


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Variable frequency operationStator voltage (induced), V = 4.44 k f N To avoid saturation & minimise losses, maintain

ratio constant at rated value

Tmax (Eqn. 6.69)

(6.69)212'2

2

2

max

)(4

)(T

++

=

r sss L L

f R

f R

f V

K

f

V

constantMotor inductances

At higher f , 2 (Ls+Lr) >> (R s /f) hence T max becomes Eqn. 6.70


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Variable frequency operation

From the eqn.6.70

This suggests that:With constant (V/ f ) ratio, motor develops constant maxtorque

Except at low speeds (or frequencies)Hence motor operates in constant torque mode seeFig 6.33a

When V reaches rated value at base speed, it cannotbe increased with f to maintain (V/ f ) ratio

[ ])(2)(

T '

2

maxr s L L

f

V K

+=


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Variable frequency operationFig 6.33a


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Variable frequency operationbeyond base speed

V/f ratio constant

V at rated value; f beyond rated;V/f ratio decreases; T decreases


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Various configurations of voltage-fedinverters (VSI)

see Fig 6.36Types

SquareSquare --wave inverterwave inverterPWM inverterPWM inverter


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VSIvariable frequency supply can be obtained from

dc supplycan be operated as Square-wave or PWM inverter

VSI circuit Fig 6.35a


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SquareSquare --wave inverterwave inverter

Line

current(IL)

Line

voltage(VLL)

DCDC ACAC

switch in their sequence numberwith phase difference of 60 o ( /3) Each is kept on for 180 o () Waveforms: V LL & IL Frequency is varied bychanging duration bet. turn-onof transistors Output voltage : by dc input

2

T2 T3 T4T5

T6

T1

/3


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SquareSquare --wave inverterwave inverter

Inverter output voltages given as in Eqn.6.71Harmonics of odd order (ie. 3 rd, 5 th, 7 th, 11 th,etc.)

Motor develops pulsating torque whichcauses jerky motion of rotor at low speed

Better performance with PWM inverter


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PWM inverterPWM inverter Fig 6.35

DCDCACAC

operates as the same as Stepped Wave inverter switches are turned on & off many times in a cycle

technique sinusoidal PWM Triangular carrier wave is compared withfundamental sine modulating wave (50Hz)


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Induction Motor Drives PWM inverterPWM inverter

The intersection points decide the switching angles Line & phase voltages (V LL & Vph ) & current (I L)


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Induction Motor Drives PWM inverterPWM inverter

Line voltage (V LL)

Phase voltage

Line current (I L)

Fig 6.35 The pulsewidths are

proportional to sinevalues of the angle atwhich the pulse appears


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PWM inverterPWM inverterInverter output voltages:

harmonics are reduced ascompared with Square-waveinverter (see Table 1)

Smooth motion at low speedsFig 6.36a: regeneration possible,

low input p.f. of the dual converterat high

Fig 6.36b: chopper - dc i/p;inverter frequency; harmonicin ection into ac su l reduced

d


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PWM inverterPWM inverter Example

see

e L eae L eaI d i M D i


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e a r n i n g # 3

e a r n i n g # 3

( w e e k 1 2 )

( w e e k 1 2 )


Current-fed inverters (CSI)(CSI)

ControlledRectifier-fed

Chopper-fed

I d i M D ie L eae L ea


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Rotor resistance (6.15)

Slip power recovery (6.16)Applicable to wound-rotor induction motor

e a r n i n g # 3

e a r n i n g # 3

( w e e k 1 2 )

( w e e k 1 2 )

Wound-rotor

I d ti M t D ie L eae L e a


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Scalar Control and Vector Control

e a r n i n g # 3

a r n i n g # 3

( w e e k 1 2 )

( w e e k 1 2 )

I d ti M t D i


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I d ti M t D i


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AC Drives (Oct09) lecture slides (as of 16 Nov09)

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