EE 8601 - SOLID STATE
DRIVES
Unit -III
INDUCTION MOTOR DRIVES
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Contents
Advantageous features of converter Fed induction motor in
comparison with line fed induction motor
Speed control of induction
Speed control by Variable Voltage method
speed control by rotor resistance variation
Slip Energy Recovery Schemes
Speed Control of IM Using Variable Frequency
Features of VSI Fed IM Drives
Features of PWM Fed IM Drives
Features of CSI Fed IM Drives
Slip controlled Drives
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Advantageous features of converter Fed induction
motor in comparison with line fed induction motor
Smooth Speed variation with VVVF(Variable Voltage Variable
Frequency)
Assured smooth Start up
Soft Starting and acceleration at constant current and torque
are possible.
No switching surge currents with Direct Switching on even for
Higher Ratings
High Moments of Inertia can be accelerated without the need
for over dimensioning the motor
Speed control of IM by changing slip frequency.
Speed control of IM by changing stator frequency which can
change the Synchronous speed of the motor
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Speed control of induction motor
Three simple means of limited speed control for an
induction motor are:
1) Reduced applied voltage magnitude
2) Adjusting rotor circuit resistance
(suitable for a wound rotor machine
and discussed earlier)
3) Adjusting stator voltage and frequency
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Speed control by Variable Voltage method
Controller Circuit and Characteristics of Induction Motor
With Variable Voltage
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Slip for maximum torque
S m= r 21
((r12+(r21/s)2+(x1+x21)2 )1/2
•Slip at max torque does not depends on Applied
Voltage and it can be changed by changing the
rotor resistance.
•In slip Ring IM it is possible.
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Characteristics with Rotor Resistance Control
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Td= m1* x2 * Vr2(r21/s)
(2*pi*ns)((r12+(r21/s)2+(x1+x21)2
Torque is Proportional to Square of the Voltage
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Conclusion from the above characteristics
Linear portion of torque curve meets the locus of
the breakdown torque point.
Sm increases with increase in r21
Maximum torque is independent of r21
If Slip increases rotor copper loss increases
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speed control by rotor resistance variation
Block Diagram for Rotor Resistance Control
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R*=R2(1-)
=1 R*=Zero
=0 R*= R2
0<R* <R2
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Features
Speed Range
Braking
Harmonics
Torque Pulsations
Good pf
Poor Efficiency
Reasonable Cost
General
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Draw backs of Stator Voltage Control and Rotor
Resistance Control
Poor Efficiency at low speed.
Limited range of Speed Control
Slip power is wasted in the Motor
Resistances in Stator Control and in Rotor
Resistance in Rotor Resistance control
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Slip Energy Recovery Schemes
Block Diagram for Slip Energy Recovery
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Dc voltage of the diode rectifier
Vd=1.35(sE 20)
Corresponding to no voltage condition
Vd0=1.35(sE 20)
For Stator to Rotor turns Ratio ‘a’
Vd0=1.35(sVL/a)
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Vdi= 1.35(VL cos )
Vd0= -Vdi
1.35(VL cos )=1.35(sVL/a)
s= -a cos
Rotor Copper loss =sPg (Pg -Air Gap Power)
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sPg =VdId
Torque Developed=Td=Pg /2*pi*ns
= VdId/s*2*pi*ns
Put Vd= 1.35(sVL/a)
Td= 1.35*VLId/a*2*pi*ns
Td= KtId
Where Kt =1.35*VL/a*2*pi*ns
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Features of Slip Power Recovery
Power Factor is Improved
Slip Power Can be recovered to the mains instead
of wasting the same in the resistances of the motor
itself.
Converter group handles Slip power only.
Therefore it’s rating can be low if speed control is in
a limited range.
Contd..
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For achieving Zero Speed Converter rating should
be equal to the Motor rating.
Improved efficiency
Maximum power factor attained is 0.7.Still the pf
can be improved by designing the inverter if the
converter operates at 180 Degree firing angle
Contd..
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For Achieving Super synchronous Speed ,Power
should flow to the rotor circuit Via the converter
Cascade.
This can be achieved by
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Achievement of Super Synchronous Speed
Replacing Diode rectifier by Phase Controlled
rectifier operating as rectifier.
By replacing converter cascade by a
Cycloconverter.This is known as Scherbius Drives
Rotor Currents are non sinusoidal and it causes
network reactions and torque pulsations.
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Scherbius systems
Power Circuit Diagram for Scherbius Systems
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Speed Control of IM Using Variable Frequency
f= pns
Iffrequency varies Saturation Problems Will occur
◦ To avoid this V/f has to maintained at a constant
value
◦ To avoid Impedance drop at low frequency
compensation is necessary (i.e E/f Control)
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V/f control circuit for IM (open loop control)
Open loop V/f control
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V/f control circuit for IM (closed loop control)
Closed loop V/f control
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Features of v/f control
Bestpossible utilisation of available current
capability
Generate Highest possible Torque per Ampere of
Stator Current.
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Features of VSI Fed IM Drives
Can be used for Multi motor Drives
Load independent Commutation of the Inverter
Devices.
Inverter Frequencies can go up to 1500 Hz.
Suitable for high speed operation
Capacity upto 100 KVA
At very low speed Commutation voltage is also very
low. Up to 10% of the Speed is not realisable.
Speed Range 1:20
Not suitable for acceleration on Load and Sudden Load
Changes
Dynamic braking can be realised by an additional
converter at the line side.
Low cost with simple control circuit.
Efficiency is very poor.
Contd..
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Features of PWM Based VSI Fed IM Drives
Speed range: Up to zero speed
Nearly Sinusoidal voltage and current.
Minimized torque pulsations.
Line pf is closer to Unity.
High converter cost.
Inverter has constant dc link voltage and employs
PWM principle for both voltage control and
Harmonic neutralisation.
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Improved Output voltage wave form.
Uninterrupted operation is possible when buffer
battery is used.
Control is complicated.
Four quadrant operation is possible.
Smooth change over of voltage and frequency
values at zero crossing for speed reversal.
Contd..
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Operating frequency is limited at 150 Hz.
Speed Control range 1:10.
The inverter and motor need not be matched. The
converter operates as source to which the motor can
be plugged.
Size of the harmonic filter decreases.
Good dynamic response.
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Features of CSI Fed IM Drives
Simple Configuration.
Feed back diodes are absent. Blocking diodes
needed.
Load dependent commutation.
Multi motor operation is not possible.
Four quadrant operation is straight forward.
Inverter is force commutated to provide variable
frequency.
Contd..
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Finds application in medium to high power drive.
Torque pulsations at low speed can be eliminated
by PWM operations.
Both constant torque and constant power
operations are possible.
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Slip controlled drives
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Slip Controlled
Drive
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Features of Slip Controlled Drives
Highly Efficient
Precise and accurate control of torque is possible in the
complete speed range.
The slip frequency can be any value up to the value
corresponding to break down torque from no load slip.
Stable operation with good pf.
Drive efficiency is comparable to a thyristorized dc
drive.
High power to Weight ratio, least maintenance, low
inertia, no limitations on power and speed ranges.
Selective harmonic elimination is possible.
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References
BimalK. Bose. ‘Modern Power Electronics and AC
Drives’, Pearson Education, 2002.
G.K. Dubey, ‘Power semi-conductor controlled
drives’, prentice hall of india, 1989.
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