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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