Chapter 12 Induction Motor Drives: Speed Control
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12-1
Induction Motor Drives : Speed Control PPU
Speed control input
Induction motor
Sensors
Load
Controller
ùm
q Efficient speed control over a wide range - Reduced voltage control (inefficient) - Frequency control (efficient)
q PPU drives induction motor with variable frequency to maintain low slip q As frequency decreases, voltage must also decrease to avoid magnetic saturation © 2000 http://www.ece.umn.edu/groups/electricdrives
12-2
Rotor Losses Power crossing air gap to rotor: Pr = Tem ùsyn Power delivered through rotor to load: Pem = Tem ùm Power lost in rotor: Pr,loss = Pr – Pem = Tem (ùsyn-ùm) = Tem ùslip Therefore, to minimize rotor losses, ùslip should be small
© 2000 http://www.ece.umn.edu/groups/electricdrives
12-3
Minimizing ùslip For A Given TL and ùm ùslip1
ù ùsyn1
ùslip2
ùsyn2 ùsyn3
ùslip3
ˆ increasing B ms
ùm
TL
Tem
q Large flux density allows low slip ˆ ˆ as large as possible – maintain at B q Keep B ms,rated ms © 2000 http://www.ece.umn.edu/groups/electricdrives
12-4
Operating Characteristics with Bˆ ms = ( Bˆ ms ) rated wm
Load Torque (Constant)
wm
w syn , rated
w slip , rated
frated
w syn ,1
w syn ,1 w slip,1 {
f1
w slip,1
w slip ,2
f2 (a)
0
k
k
f3
w m , rated
f1
w m1
f2
w m2 (b)
Tem
frated
f3
0
q If flux is kept constant, slope will be the same at every frequency q Load torque and speed are met by adjusting frequency © 2000 http://www.ece.umn.edu/groups/electricdrives
Tem
12-5
ˆ Over Operating Maintaining B Frequencies and Current Levels by Adjusting Voltage ms,rated
Rs
jw Llr¢
j w Lls
+
I ra¢
+
Ia
Va
jwL m -
-
w
R¢ rw
I ma
Ema
Rs
j w Lls
+ Ia
syn
Ema
Va
slip
I ra¢
+
I ma
jwL m -
-
w Lsl Iˆra¢
Rs Iˆma , rated
Va I ma
© 2000 http://www.ece.umn.edu/groups/electricdrives
I ra¢
Ema
{{ reference w Lls Iˆma , rated Rs Iˆra¢
w
R¢ rw
syn
slip
12-6
ˆ Over Operating Maintaining B Frequencies and Current Levels by Adjusting Voltage (cont…) ms,rated
ˆ is equivalent to maintaining a q Maintaining constant B ms constant ˆI ma (magnetizing current) ˆ ˆ ˆ E E E ma ma , or ma should be kept constant q Since ˆI ma = ùL m ù f Va is a constant. q Ignoring Rs and Lls, this means that f As f decreases, so should Va. Constant volts per hertz. q This is a good first approximation © 2000 http://www.ece.umn.edu/groups/electricdrives
Adjusting Voltage – Stator Resistance Included ˆ = k× f ; k = q First approximation: V a
ˆ ˆ¢ (V a,rated - R s I ra,rated ) f rated
q Including voltage drop across Rs (leakage ˆ = k × f + R ˆI¢ inductance can be ignored): V a s ra Vˆa Vˆa,rated Vˆa at rated torque
voltage boost
¢ ,rated Rs Iˆra
{
0
Vˆa,rated Vˆa = constant = f frated
V at zeroTem slope a frated
f ( Hz)
q For large torques, considerable voltage boost is needed at low frequencies. This is the R s ˆI¢ra term.
© 2000 http://www.ece.umn.edu/groups/electricdrives
12-7
12-8
Start-up Considerations f
ùm
ˆ = constant, rated B ms fsteadystate
ùsyn,start ùslip,start
{ 0
f start = f slip,rated fstart
50%
100%
ˆI'ra Tem , (%) Tem,rated I¢ra,rated © 2000 http://www.ece.umn.edu/groups/electricdrives
inertia
t
Capability Below and Above Rated Speed wm w m , rated Rated power capability 1 .0
Rated torque capability
0
1 .0
Tem Tem , rated
ˆ must be q Voltages limited to rated values, therefore B ms reduced at higher speeds (Flux Weakening) q Currents limited to rated values, therefore torque ˆ is limited B limited when ms © 2000 http://www.ece.umn.edu/groups/electricdrives
12-9
12-10
Braking in Induction Motor Drives speed of airgap field ùsyn
P P U
wm
ùm > ùsyn ùslip = negative Tem
Tem
ˆ = constant B m f0
Motoring mod e
R ¢r
Tem0
f1
ùm Generation mod e
0
ùsyn1
ùm
ùsyn 0 ùm
Tem1 < 0
f1 < f0
q To initiate braking, lower ùsyn to some value less than ùm q Braking torque can be adjusted by setting the negative slip frequency © 2000 http://www.ece.umn.edu/groups/electricdrives
Speed Control of Induction Motor Drives
12-11
ac input
Rectifier
Slip compensation
Voltage boost
id Vˆ
w m, ref
w
* m
+
å -
w syn
PWM controller
Inverter
Current limiter circuit
ùsyn = ùr, ref + Tem /K ùT 1424 3
; this is an estimate
ùslip
Vd limiter
-V + d
Motor
Measured Vd
Va = k f f + k vT Tem
q ùr,ref is passed through a rate limiter to avoid over driving the motor q This method does not give precise speed control © 2000 http://www.ece.umn.edu/groups/electricdrives
12-12
Pulse Width Modulated Power Processing Unit a +
b
Vd
c
-
a b
+
vcontrol,a (t)
qa (t) qb (t) qc (t)
Vd -
c
14243 acmotor
vcontrol,b (t)
vcontrol,c (t)
vtri (t)
© 2000 http://www.ece.umn.edu/groups/electricdrives
da (t)
db (t)
dc (t)
12-13
Harmonics in PPU vtri
vcontrol ,a
vcontrol ,b
vcontrol ,c
va f1
2fs - f1 2fs + f1
vb fs - f1
(a) vab = va - vb
fundamental, vab1
3fs - f1
fs + f1
fs
3fs - 2f1 2fs
3fs + f1 3fs + 2f1
3fs
t
q PPU with switching frequency of 800 Hz generating a fundamental sine wave of 50 Hz q Frequency spectrum shows large 50 Hz component and smaller components at higher frequencies due to switching q These higher frequency components add to the losses in the motor
© 2000 http://www.ece.umn.edu/groups/electricdrives
12-14
PPU – Supplied Induction Motor Ia1
R s jù1L ls
+
Va1
¢ Ira,1
Iah
jù1L¢lr
Ima,1 jù1L m
Fundamental Frequency Model
R s jhùL ls
jhùL¢lr
+
R ¢r R ¢r
ùm ùslip
Vah
jhùL m
R ¢r
ùsyn,h ùslip,h
Harmonic Frequency Model
q At harmonic frequencies R e q ; Rr¢ Magnetizing inductance can be ignored Harmonic currents controlled by leakage inductance ˆ V ah ˆI ; ah (X ls,h + X¢lr,h ) © 2000 http://www.ece.umn.edu/groups/electricdrives
; R ¢r
12-15
PPU – Supplied Induction Motor Model ia,1 (t ) R s
Lls
¢ ira ¢ ,1 (t ) Llr
+
Rr¢
im,1 (t )
+
Lm
va,1 (t )
¢ ,1 (t ) = Rr¢ var
-
-
ia (t ) Rs
Lls
¢ (t ) ira
Llr¢
Rr¢
wm i¢ w slip ra,1
¢ var ,1 (t )
+
-
Lm ib (t ) Rs
Lls
¢ (t ) irb
Llr¢
Rr¢
¢ ,1 (t ) vbr
+
-
Lm ic (t ) R s
Lls
¢ (t ) irc
v¢ (t ) Rr¢ + cr ,1 -
Llr¢ Lm
q Fundamental frequency drop across resistor replaced with AC voltage source q Harmonic currents produce voltage across ¢ Rr © 2000 http://www.ece.umn.edu/groups/electricdrives
12-16
Summary/Review q What are the applications of adjustable-speed drives? q Why are the thyristor-based, voltage reduction circuits for controlling induction-motor speed so inefficient? q In operating below the rated speed (and not considering the core losses), why is it most efficient to keep the flux-density peak in the air gap at the rated value? q Since an induction motor is operated at different values of frequency, hence different values of synchronous speed, how is the slip speed defined? q Supplying a load that demands a constant torque independent of speed, what is the slip speed at various values of the frequency f of the applied voltages? © 2000 http://www.ece.umn.edu/groups/electricdrives
12-17
Summary/Review q To keep the flux density peak in the air gap at the rated value, why do the voltage magnitudes, at a given frequency of operation, depend on the torque supplied by the motor? q At start-up, why should small-frequency voltages be applied initially? What determines the rate at which the frequency can be ramped up? q At speeds below the rated value, what is the limit on the torque that can be delivered, and why? q At speeds above the rated values, what is the limit on the power that can be delivered, and why? What does it mean for the torque that can be delivered above the rated speed?
© 2000 http://www.ece.umn.edu/groups/electricdrives
