ADVANCED ELECTRIC DRIVES-II EE6240

M. Tech. (Electrical Engg) Power System & drives 1. G. K. Dubey, “Power Semiconductor Controlled Drives”,

Prentice Hall International, Inc. 19892. B. K. Bose, “Modern Power Electronics and AC Drives”,

Pearson Education, 20023. R. Krishnan, “Electric Motor Drives”, Pearson Education

20014. M. H. Rashid, “Power Electronics”, Pearson Education

20045. P. C. Krause, “Analysis of Electrical Machinery”, Mc-

Graw Hill 1987 BHK1

ELECTRIC DRIVES

2

Power Semiconductor

ControllersMotor Load

Control Unit Sensing Unit

Power Supply

Command Signal

Feed Forward

Feedback

Drive (motor+controller) Equipment that initiates motion to a loadRatings or size: 1. Low: < 10 hp

2. Medium: 10-100 hp3. High: > 100 hp

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Steady State Operation: Constant speed operation Transient operation:

• Starting• Braking • Speed reversal• Speed change increase or decrease• Inching moving slowly and carefully• Jogging short time low speed operation, to

position the motor shaft

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DRIVE OPERATIONS

LOADS…General loads: Constant speed, operate in general environmt

e.g. Lathe machine, tube well, flour mill etcSpecial loads: Adjustable speed, operate in special environmt

• Electric propulsion (Traction)

• Pumps, fans, compressors• Spindles (a device to spine

fibers into threads) & servos

• Aerospace actuators• Robotic actuators• Rubber industry• Cement kiln• Steel mills (rolling mills)

• Paper and pulp mills • Textile mills• Automotive applications• Underwater excavators,

mining equipments• Conveyors, elevators,

escalators & lifts, hoist (crane)

• Power tools (machine tools), machine winders

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…LOAD

Some of the special requirements of drives:• Constant speed• Constant torque• Constant power• Frequent starting / stopping• Frequent overload• Inching and jogging

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ELECTRIC MOTORS…

DC Motors: Main features1. Can provide higher starting torque2. Speed control over wide range3. Methods of speed control are simple and less expensive4. Due to use of commutator:• dc motors are not suitable for very high speed

applications• Requires more maintenance• Not suitable for use in dirty and explosive environmentDue to theses reasons these motors are being replaced by ac motors

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…ELECTRIC MOTORS

AC Motors: main features1. AC Motors are light weight (20 to 40% lighter than equivalent

dc motors), inexpensive and have low maintenance, better reliability

2. Control of ac motor drives generally require complex control algorithm that can be performed by a powerful microprocessor (or DSP) and fast switching power converters

The advantages of ac motors overweigh the disadvantages and these are being preferred now-a-days. Particularly squirrel cage IMs are rugged, have lower cost, weight, volume, inertia and able to operate in dirty & explosive environments.

About 85% motors used in the field are of Induction type. BHK7

CLASSIFICATION OF AC MOTORS

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Induction Motor

Squirrel Cage typeWound rotor, slip ring or doubly fed typeRotating typeLinear type

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CLASSIFICATION OF AC MOTORS

Synchronous Motor

Wound fieldPermanent magnetRotating typeLinear type

ReluctanceConstant reluctanceVariable reluctance

Switched reluctanceStepper

Surface magnetBurried magnetSinusoidalTrapezoidalRadial air gap

Axial air gap

SKIP

FACTORS FOR SELECTION OF A MOTOR

For a particular application, often more than one type of motor can be used. The final selection will depend on cost / performance trade-off, where various factors to be considered are initial cost, maintenance, size & weight, efficiency, dynamic response, power factor, rotor inertia, reliability, need of position or speed sensing etc.

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INDUCTION MOTOR – BASICS…

When stator is supplied by balanced 3 ph voltage source of frequency ω rad/sec ( f Hz) a rotating magnetic field moves at synchronous speed in the air gap.

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A

B

C

ωms

Air gap flux

ωm

Rotor

G. K. DubeyCh. 6, P 203

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…INDUCTION MOTOR – BASICS…

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…INDUCTION MOTOR – BASICS…

ωr

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Rotor surface

ωr

Rotor surface

Rotor surfaceδ = π/2 + θr r

Air gap flux density, Bm

Rotor conductor induced voltage wave

Rotor conductor induced current wave

Rotor mmf (Fr) wave

Bm

Bm

Ιr

Ιr

0

θr

θr

ωe

ωe

ωe

ωe

ωr

ωr

θr Rotor pf angle

ωe

Developed diag. of rotor

ωr

Rotor (2 Pole m/c)

0 Bm

δr Torque angle or phase angle between air gap flux

density and rotor mmf

EQUIVALENT CIRCUIT OF IM…

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RrrsXlrr

IrrsE/aT1EV

Rs Xls

Is

StatorFrequency = f

RotorFrequency = sf

Rotor

Fig. (a)

…EQUIVALENT CIRCUIT OF IM…

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Rrr/sXlrr

IrrE/aT1EV

Rs Xls

Is

Stator Frequency = f Rotor Frequency = f

Fig. (b) Stator & rotor ckts have same frequecny

IoImIc

XmRc

aT1 : 1

Rrr , Xlrr are resistance and leakage reactance referred to rotorRr , Xlr are resistance and leakage reactance referred to stator

Xm is magnetizing reactance

…EQUIVALENT CIRCUIT OF IM…

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Rr/sXlr

IrEV

Rs Xls

Is

Fig. (c) Exact equivalent circuit referred to stator, neglecting core losses

Im

Xm

A

B

Air gap

Rr = aT12. Rrr and Xlr = aT12.Xlrr … … (7)

…EQUIVALENT CIRCUIT OF IM…

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In the exact equivalent circuit of Fig. (c) the portion on the left hand side of line AB can be replaced by its Thevnin’sequivalent as shown in Fig. (c´) below: (steps are shown in the next slide)

Rr/sXlr

IrEVth

Rth Xth A

B

Air gapθth__

Fig. (c’) Exact eqt ckt, Thevnin’s version

C

D

…EQUIVALENT CIRCUIT OF IM…

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Rs Xls

Xm Vth θth__

A

B

V 0o__

Rs Xls

Xm =>

Rth XthA A

B B

Thevnin’s circuit parameters may be given as:

…EQUIVALENT CIRCUIT OF IM

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Rr/sXlr

IrEV

Rs XlsIs

Fig. (d) Approximate equivalent circuit referred to stator, neglecting core losses

Im

Xm

A

B

Air gap

C

D

Active power supplied to rotor

BHK21Rotor side

Stator side

Air gap flux linkage per poleψm

ψm

θφ

δr

θr

θr

Ir

- Ir

IrIs

ImIc

Io

EIs Rs

Is XlsVs

E

Ir Xlr

Ir Rr /s

E = Xm Im = ω Lm Im = ω ψmψm = Lm Im = E / ω

Phasor diagram of IM Based on exact eqt circuit

STEADY STATE PERFORMANCES…

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…STEADY STATE PERFORMANCES…

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…STEADY STATE PERFORMANCES…

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…STEADY STATE PERFORMANCES…

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…STEADY STATE PERFORMANCES…

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…STEADY STATE PERFORMANCES

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If the approximate eqt ckt of Fig (d) is used, the expressions for Ir, T, Tmax and sm can be obtained when Vth, Rth, and Xth are replaced by V, Rs, and Xls respectively.

…STEADY STATE PERFORMANCES

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SPEED-TORQUE CHARACTERISTICS

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TIr

ωm

ωms

IrstTst0s=1

s=0sm

Tmax

T

Ir

POWER FLOW DIAGRAM

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Pin Input power

Pcs Stator

copper loss (= Pcr Rotor copper loss)

sPg Slip power

Pg Air-gap Power

Pm=(1-s)Pg Mechanical power

developed

Windage & frictional loss

PshaftShaft power

FOUR QUADRANT OPERATIONS

Braking operation:1. Regenerative Brk2. Plugging3. Dynamic Brk

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T

ωm

ωm Τωm Τ

ωm Τ ωm Τ

Forward motoring

Reverse motoring

Forward braking

Reverse braking

REGENERATIVE BRAKING…

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ωm

ωms

Tst10s=1

s=0

sm

Tmax1Tmax2

- sm

Supe

r syn

chro

nous

sp

eed

regi

on

Forward regenerative

braking

Forward motoring

…REGENERATIVE BRAKING…

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At speeds above synchronous speed an IM enters regenerative braking operation. In regenerative braking, the motor runs as Induction Generator and feeds power back to the source. The magnetizing current required to produce flux is obtained from the source. Thus the machine can not regenerate unless it is connected to a source. The mechanical energy is supplied from the stored KE of the motor and load. Thus it slows down.

When motor is fed from a fixed frequency source, regenerative braking is possible only for speeds above synchronous speeds. When fed from a variable frequency source, the source frequency can be adjusted such that motor

speed is always maintained above synchronous speed.

…REGENERATIVE BRAKING

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As seen from eqn (20) the developed maximum braking torque, Tmax2 is always more than maximum motoring torque, Tmax1 due to change of sign in the denominator.

Actual braking torque available at the shaft is higher than developed braking torque as it has to supply power to overcome friction, windage and core losses also.

Since braking speeds are also higher than motoring speeds, the regenerative power is much higher then motoring power.

PLUGGING…

To initiate plugging braking the 3 ph supply terminals to the motor are opened and reconnected by reversing the phase sequence. A rotating magnetic field that rotates at synchronous speed in a direction opposite to that of direction of rotation of rotor is produced. Therefore the slip is greater than 1.

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T

ωm

s=1

s=0

s=2s=0

s=1

s=2

AA΄ss΄

+ive phase sequence

ABC

-ive phase sequence

ACB

Forward motoring

Reverse motoring

Reverse braking

Forward braking

PLUGGING…

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When the motor is operating at point A in forward direction, it can be braked by changing the phase sequence of the stator supply (interchanging any two terminals). The operating point jumps to A´. The developed motor torque reverses and its speed starts decreasing. The braking torque does not decrease to zero at zero speed. When braked for stopping, the motor should be disconnected from the supply at or near zero speed. An additional device will be required for detecting zero speed and disconnecting the motor from the supply. Therefore, plugging is not suitable for stopping. It is, however, quite suitable for reversing the motor.

As the plugging operation is initiated the slip changes from close to zero to close to 2. The resistance Rr/s decreases to a very low value and power loss in the rotor is excessive.

Example 6.1 / P222, G K DubeyThis is a numerical example on regen braking & pluggingA 3-ph, Y-connected, 6-pole, 60 Hz, IM has the following constants:Vth = 231 V, Rth = Rr =1 ohm, Xth = Xlr = 2 ohm1. If the motor is used for regenerative braking,

(a) determine the range of active load torque it can hold and the corresponding range of speed.

(b) calculate the speed and current for an active load torque of 150 Nm

2. If the motor is used for plugging , determine the braking torque and current for a speed of 1200 rpm.

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END

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ADVANCED ELECTRIC DRIVES-II EE6240ELECTRIC DRIVESSlide Number 3LOADS……LOADELECTRIC MOTORS……ELECTRIC MOTORSCLASSIFICATION OF AC MOTORSSlide Number 9FACTORS FOR SELECTION OF A MOTORINDUCTION MOTOR – BASICS……INDUCTION MOTOR – BASICS……INDUCTION MOTOR – BASICS…Slide Number 14EQUIVALENT CIRCUIT OF IM……EQUIVALENT CIRCUIT OF IM……EQUIVALENT CIRCUIT OF IM……EQUIVALENT CIRCUIT OF IM……EQUIVALENT CIRCUIT OF IM……EQUIVALENT CIRCUIT OF IMSlide Number 21STEADY STATE PERFORMANCES……STEADY STATE PERFORMANCES……STEADY STATE PERFORMANCES……STEADY STATE PERFORMANCES……STEADY STATE PERFORMANCES……STEADY STATE PERFORMANCES…STEADY STATE PERFORMANCESSPEED-TORQUE CHARACTERISTICSPOWER FLOW DIAGRAMFOUR QUADRANT OPERATIONSREGENERATIVE BRAKING……REGENERATIVE BRAKING……REGENERATIVE BRAKINGPLUGGING…PLUGGING…Slide Number 37Slide Number 38Slide Number 39