Induction Motor Drives – Scalar ControlByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering
Dr. Ungku Anisa, July 2008 1EEEB283 - Electrical Machines & Drives
OutlineIntroductionSpeed Control of Induction Motors
Pole ChangingVariable-Voltage, Constant FrequencyVariable Frequency
Constant Volts/Hz (V/f) ControlReferences
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 2
IntroductionScalar Control - control of induction machine
based on steady-state model (per phase steady-state equivalent circuit)
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Rr’/s+
Vs
–
RsLls Llr’
+
E1
–
Is Ir’
Im
Lm
Introduction
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 4
r
s
Trated
Pull out Torque(Tmax)
Te
ssm ratedrotor
TL
Te
Intersection point (Te=TL) determines the steady –state speed
1 0
What if the load must be operated here?
rotor’
Speed Control of IMGiven a load T– characteristic, the steady-state speed can be
changed by altering the T– curve of the motor
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 5
fPPs 42
2
2'
2'3
lrlsr
s
s
s
re
XXsR
R
V
s
RT
Pole Changing
Varying line frequency
Varying voltage (amplitude)2
3
1
Speed Control of IMPole ChangingMachines must be specially manufacturedOnly used with squirrel-cage motorsTwo methods:
Multiple stator windings – simple, expensiveConsequent poles – single winding divided into few coil groups
Consequent poles:No. of poles changed by changing connections of coil groupsChange in pole number by factor of 2:1 only
Discrete step change in speed
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Speed Control of IMVariable-Voltage (amplitude), Constant FrequencyControlled using:
AC Voltage Controllers (anti-parallel thyristors) voltage control by firing angle
control also used for soft start of
motors
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 7
Speed Control of IMVariable-Voltage (amplitude), Constant FrequencyFrom torque equation, Te Vs
2
When Vs , Te and speed reduces.If terminal voltage is reduced to bVs,:
Note: b 1
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 8
2
2'
2'3
lrlsr
s
s
s
re
XXsR
R
bV
s
RT
Speed Control of IMVariable Voltage (amplitude),
Constant FrequencyDisadvantages:
limited speed range when applied to Class B (low-slip) motors
Excessive stator currents at low speeds high copper losses
Distorted phase current in machine and line
Poor line power factorHence, only used on low-power,
appliance-type motors where efficiency is not importante.g. small fan or pumps drives
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Speed Control of IMVariable FrequencySpeed control above rated
(base) speed Frequency increasedStator voltage held constant at
rated valueAirgap flux and rotor current
decreases Developed torque decreases
For control below base speed – use Constant Volts/Hz method
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 10
Constant Volts/Hz (V/f) ControlAirgap flux in the motor is related to the induced stator
voltage E1 :
For below base speed operation:Frequency reduced at rated Vs - airgap flux saturates
(f ,ag ):- excessive stator currents flow- distortion of flux wave
Hence, keep ag = rated fluxstator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 11
f
Eag
1f
Vs Assuming small voltage drop across Rs and Lls
Constant Volts/Hz (V/f) ControlMax. torque remains almost
constantFor low speed operation:
can’t ignore voltage drop across Rs and Lls
poor torque capabilitystator voltage must be
boosted – maintain constant ag
For above base speed operation (f > frated):stator voltage maintained at
rated value
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 12
s
sVT
2
max
Constant Volts/Hz (V/f) Control
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Vrated
frated
Linear offset
Non-linear offset – varies with IsBoost
Vs vs. f relation in Constant Volts/Hz drives
Vs
f
Constant Volts/Hz (V/f) Control For operation at frequency times rated frequency:
fs = fs,rated s = s,rated (1)
Stator voltage: (2)
Voltage-to-frequency ratio = d = constant:
(3)
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 14
rated,
rated,
s
sVd
rated,rated,
rated,rated,
when,
when,
sss
ssss ffV
ffVV
Constant Volts/Hz (V/f) Control For operation at frequency times rated frequency:
Hence, the torque produced:
(4)
where s and Vs are calculated from (1) and (2) respectively.
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22
2'
2'3
lrlsr
s
s
s
re
XXsR
R
V
s
RT
Constant Volts/Hz (V/f) Control For operation at frequency times rated frequency:
The slip for maximum torque is:
(5)
The maximum torque is then given by:
(6)
where s and Vs are calculated from (1) and (2) respectively.
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222
'
max
lrlss
r
XXR
Rs
222
2
max 2
3
lrlsss
s
s XXRR
VT
Constant Volts/Hz (V/f) Control
Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 17
Field Weakening Mode (f > frated)• Reduced flux• Torque reduces
Constant Power Area
Constant Torque Area
Constant Volts/Hz (V/f) Control
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Constant Power Area
Constant Torque Area
Constant Volts/Hz (V/f) Control – Open-loop Implementation
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PWM Voltage-Source
Inverter (VSI)
Constant Volts/Hz (V/f) Control – Open-loop ImplementationMost popular speed control methodUsed in low-performance applications
where precise speed control unnecessarySpeed command s* - primary control variable Phase voltage command Vs* generated from V/f relation
Boost voltage applied at low speedsConstant voltage applied above base speed
Sinusoidal phase voltages (vabc*)generated from Vs* & s*
vabc* employed in PWM inverter connected to motorDr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 20
ReferencesKrishnan, R., Electric Motor Drives: Modeling, Analysis and Control,
Prentice-Hall, New Jersey, 2001.Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
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