INDUCTION MOTOR Scalar Control (squirrel cage)

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INDUCTION MOTORScalar Control(squirrel cage)

• MEP 1523
• ELECTRIC DRIVES

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Scalar control of induction machineControl of
induction machine based on steady-state model
(per phase SS equivalent circuit)
Is
Ir
Lls
Llr
Rs
Vs
Eag
Rr/s
Lm
Im
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Scalar control of induction machine
Te
Pull out Torque (Tmax)
Trated
?r
?s
s
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Scalar control of induction machine
Given a load T? characteristic, the steady-state
speed can be changed by altering the T? of the
motor
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Variable voltage, fixed frequency
e.g. 3phase squirrel cage IM V 460 V Rs
0.25 ? Rr0.2 ? Lr Ls 0.5/(2pi50)
Lm30/(2pi50) f 50Hz p 4
Lower speed ? slip higher
Low efficiency at low speed
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Variable voltage, variable frequency
Constant V/f operation
At low slip
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Variable voltage, variable frequency Constant
V/f
If Fag is constant ? Te a slip frequency
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Variable voltage, variable frequency Constant
V/f
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Variable voltage, variable frequency Constant
V/f
Characteristic with constant
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Variable voltage, variable frequency
Constant ? constant V/f
Vs
Vrated
Constant slope
frated
f
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Variable voltage, variable frequency
Constant V/f open-loop
Rectifier
VSI
3-phase supply
IM
C
f
Pulse Width Modulator
Ramp
V

?s
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Variable voltage, variable frequency
Constant V/f open-loop
Simulation example 415V, 50Hz, 4 pole, Rs
0.25?, Rr 0.2?, LrLs 0.0971 H, Lm 0.0955, J
0.046 kgm2 , Load k?2
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Variable voltage, variable frequency
Constant V/f open-loop
Simulation example 415V, 50Hz, 4 pole, Rs
0.25?, Rr 0.2?, LrLs 0.0971 H, Lm 0.0955, J
0.046 kgm2 , Load k?2
constant_vhz_withoutBoost/Signal Builder Group 1
Signal 1
50
40
30
20
10
0
0
0.5
1
1.5
2
2.5
3
3.5
Time (sec)
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Variable voltage, variable frequency
Constant V/f open-loop
Simulation example 415V, 50Hz, 4 pole, Rs
0.25?, Rr 0.2?, LrLs 0.0971 H, Lm 0.0955, J
0.046 kgm2 , Load k?2
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Variable voltage, variable frequency
Constant V/f open-loop
Simulation example 415V, 50Hz, 4 pole, Rs
0.25?, Rr 0.2?, LrLs 0.0971 H, Lm 0.0955, J
0.046 kgm2 , Load k?2
With almost no rate limiter
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Variable voltage, variable frequency
Constant V/f open-loop
Simulation example 415V, 50Hz, 4 pole, Rs
0.25?, Rr 0.2?, LrLs 0.0971 H, Lm 0.0955, J
0.046 kgm2 , Load k?2
With 628 rad/s2
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Variable voltage, variable frequency
Constant V/f open-loop low speed problems
Problems with open-loop constant V/f
At low speed, voltage drop across stator
impedance is significant compared to airgap
voltage - poor torque capability at low speed
Solution (i) Voltage boost at low
frequency (ii) Maintain Im constant ? stator
current control
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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
voltage boost

• Torque deteriorate at low frequency hence
  compensation commonly performed at low frequency
• In order to truly compensate need to measure
  stator current seldom performed

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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
voltage boost
With voltage boost of IratedRs

• Torque deteriorate at low frequency hence
  compensation commonly performed at low frequency
• In order to truly compensate need to measure
  stator current seldom performed

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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
voltage boost
Voltage boost at low frequency
Vrated
frated
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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
voltage boost
Idc
Rectifier
Vdc -
VSI
3-phase supply
IM
C
f
Pulse Width Modulator
Ramp

V
?s

Vboost
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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
Constant Im
?ag, constant ? Eag/f , constant ? Im,
constant (rated)
Controlled to maintain Im at rated
Is
Lls
Llr
Ir
Rs
Vs
Eag
Lm
Rr/s
maintain at rated
Im
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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
Constant Im
From per-phase equivalent circuit,

• Current is controlled using current-controlled
  VSI
• The problem of stator impedance drop is solved
• Dependent on rotor parameters sensitive to
  parameter variation

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Variable voltage, variable frequency
Constant V/f open-loop low speed problems (i)
Constant Im
VSI
3-phase supply
Rectifier
IM
C
Current controller
Tacho
?slip
Is
?

PI
-

?s
?r
Current reference generator

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Variable voltage, variable frequency
Constant V/f
Problems with open-loop constant V/f
Poor speed regulation
Solution (i) Slip compensation (ii)
Closed-loop control
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Variable voltage, variable frequency
Constant V/f poor speed regulation (i) slip
compensation
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Variable voltage, variable frequency
Constant V/f poor speed regulation (i) slip
compensation
Idc
Rectifier
Vdc -
VSI
3-phase supply
IM
C
f
Pulse Width Modulator
Ramp

V

?s


Vboost
Slip speed calculator
Vdc
Idc
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Variable voltage, variable frequency
Constant V/f poor speed regulation (i) slip
compensation
How is the slip frequency calculated ?
Pmotor,in Pdc Pinv,losses
Pdc VdcIdc
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Variable voltage, variable frequency
Constant V/f poor speed regulation (i) slip
compensation
How is the slip frequency calculated ?
Pair-gapc Te??syn
Te ? Pair-gap/?syn
For constant V/f control,
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Variable voltage, variable frequency
Constant V/f poor speed regulation (i)
closed-loop speed

• Require speed encoder
• Increase complexity