1. Power Electronic Drives – Induction Motor & drives
EET421
Power Electronic Drives
– Induction Motor & drives
Abdul Rahim Abdul Razak
Systems Engineering
School of Electrical
ABD RAHIM 2008

2. Systems Engineering School of Electrical INDUCTION MOTORS (Rashid)
Per-phase equivalent circuit ….
(Rashid)
In real world:
Value of Xm is very large (Rm can be omitted)
Xm2 >> (Rs2 + Xs2)  Vs ≈Vm
Thus, for circuit simplification, the magnetizing reactance Xm can be moved-out to the stator winding.
Systems Engineering
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3. Systems Engineering School of Electrical INDUCTION MOTORS
The input impedance of the motor becomes:
Power factor angle:
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4. Systems Engineering School of Electrical INDUCTION MOTORS
Rotor current (rms):
Previously noted,
PAG or Pg=3Ir2Rr /s
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And developed torque τdev or,
τind =Pdev / ωm = PAG / ωsync
Thus , (15-18)
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5. Systems Engineering School of Electrical INDUCTION MOTORS
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6. Systems Engineering
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7. Systems Engineering School of Electrical
Extended Torque-speed characteristic ..
Pullout torque or
breakdown torque =
max possible
handling torque.
The curve is nearly linear
up to full load (Rr >> Xr,
thus Ir,Br and Tind rise
∞ increasing slip).
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Plugging = reversing of magnetic
field rotation by switching any two
stator phases. The reversed Tind
will stop the motor rapidly and
rotate it to reverse direction.
If speed driven
faster than ns the
Tind reverses and
motor becomes
generator.
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8. Systems Engineering School of Electrical
Pullout torque or
breakdown torque =
max possible
handling torque.
Extended Torque-speed characteristic ..
The curve is nearly linear
up to full load (Rr >> Xr,
thus Ir,Br and Tind rise
∞ increasing slip).
Systems Engineering
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If speed driven
faster than ns the
Tind reverses and
motor becomes
generator.
Plugging = reversing of magnetic
field rotation by switching any two
stator phases. The reversed Tind
will stop the motor rapidly and
rotate it to reverse direction.
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9. Speed & torque control strategies
INDUCTION MOTORS – DRIVES
Speed & torque control strategies
A) Change nsync
Pole changing
Line frequency control
B) Change slip, s
Rotor-resistance control
Rotor slip-energy recovery
Line/stator voltage control
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C) Parameters alteration
Rotor voltage control
Stator current control
Stator voltage and frequency control
Voltage, current and frequency control
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10. Systems Engineering School of Electrical INDUCTION MOTORS – DRIVES
With the existence & development of modern
solid-state variable-frequency motor drives.
The modern favorable speed and torque controller of induction motors :
Stator voltage control
Rotor voltage control
Frequency control
Stator voltage and frequency control
Stator current control
Voltage, current, and frequency control
To meet the torque-speed duty cycle of a drive, the voltage, current, and frequency control are normally used.
Systems Engineering
School of Electrical
ABD RAHIM 2008

11. INDUCTION MOTORS – Speed Control
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12. Systems Engineering School of Electrical 1. Stator voltage control
the torque is proportional to the square of the stator supply voltage,
a reduction in stator voltage will produce a reduction in speed.
If the terminal voltage is reduced to bVs, where b ≤ 1,Typical torque-speed characteristics for various values of b :
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13. Systems Engineering School of Electrical 1. Stator voltage control
In any magnetic circuit, the induced voltage is proportional to flux and frequency, and the rms air-gap flux can be expressed as
or (15-31)
Where,
Km is a constant and depends on the number of turns of the stator winding.
As the stator voltage is reduced, the air-gap flux and the torque are also reduced.
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14. Systems Engineering School of Electrical
The stator voltage can be varied by three-phase
AC voltage controllers,
voltage-fed variable DC-link inverters, or
pulse-width modulation (PWM) inverters.
However, due to limited speed range requirements, the AC voltage controllers are normally used to provide the voltage control.
The ac voltage controllers are very simple. However, the harmonic contents are high and the input PF of the controllers is low.
They are used mainly in low-power applications, such as fans, blowers, and centrifugal pumps, where the starting torque is low.
They are also used for starting high-power induction motors to limit the in-rush current.
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15. Systems Engineering School of Electrical
1) AC voltage controller & soft start
(to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period)
Advantages :
Reduced starting current
Reduced starting torque
Less mechanical stress
Improved control of acceleration and deceleration
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The soft start is designed to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period. The acceleration time being dependent on the application and desired characteristics. The added advantage of this method of reduced voltage control is that the motor can also be stopped gradually by slowly reducing the output voltage to the .‘Soft Stop’ feature offers a smooth stop in many process industries such as pumps, where fast stops can result in ‘water hammer’ and mechanical damage.
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16. Systems Engineering
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17. Systems Engineering School of Electrical Example 15.2a:
A 3-phase 460V, 4-pole Y-connected induction motor has below parameters:
Rs = 1.5ohm, Rr = 0.75ohm, Xs = 1.5ohm, Xr = 2.25ohm and Xm = 65ohm.
The no-load loss, Pno-load is negligible. The load torque is proportional to the speed squared, is 45Nm at 1740rpm. If the motor speed is 1550rpm, determine :
a) Load torque, τL
b) The rotor current, IR
c) Stator supply voltage , Va
d) Motor input current, Ii
e) Motor input power , Pi
f) The slip for maximum current ,sa
g) Maximum rotor current , Ir(max)
h) Speed at maximum rotor current, ωa
i) Torque at maximum current, τ a
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18. Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
Wound rotor induction motor.
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Breakdown torque peak is shifted to zero speed by increasing rotor resistance.
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19. Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
applicable for a wound-rotor motor type only.
an external three-phase resistor may be connected thru slip rings as shown in figure:
The method may increase starting torque while limiting starting current.
However, this is an inefficient method and there would be imbalance
voltages and currents if the resistances in the rotor circuit are not equal.
The increase in resistance does not affect the value of maximum torque but increases the slip at maximum torque.
The wound-rotor motors are widely used in application which need frequent starting and braking with large motor torque (hoist, crane).
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20. Systems Engineering School of Electrical
Applicable for wound-rotor induction motors only
The shape of torque-speed curve is altered by inserting extra
resistances to the rotor circuit
Inserting extra resistances, will seriously reduce efficiency of the motor
Usually applied for short period only.
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21. Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
The three-phase resistor may be replaced by a three-phase diode rectifier and a dc converter, as shown in Figure, where the gate-turn-off thyristor (GTO) or an insulated-gate bipolar transistor (IGBT) operates as a dc converter switch.
The inductor Ld acts as a current source Id and the dc converter varies the effective resistance, which can be found from Eq. (14.45):
Re = R (l - k) (15.41)
where k is the duty cycle of the dc converter and the motor speed can be controlled by varying the duty cycle.
The portion of the air-gap power, which is not converted into mechanical power, is called slip power. The slip power is dissipated in R.
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22. Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
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23. Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
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24. Neglected Systems Engineering School of Electrical
2. Rotor voltage control (Rotor resistance control)
Neglected
Example 15.3a:
Rs = 0.05Ω, Rr =0.055Ω, Xs = 0.31Ω, Xr = 0.35Ω and Xm = 7.1Ω
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25. Systems Engineering School of Electrical 3. Frequency control
The torque and speed of induction motors can be controlled by changing the supply frequency.
We can notice from Eq. (15.31) that at the rated voltage and rated frequency, the flux is the rated value.
If the voltage is maintained fixed at its rated value while the frequency is reduced below its rated value, the flux increases.
This would cause saturation of the air-gap flux, and the motor parameters would not be valid in determining the torque-speed characteristics.
At low frequency, the reactances decrease and the motor current may be too high. This type of frequency control is not normally used.
Systems Engineering
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26. Systems Engineering School of Electrical 3. Frequency control
If the frequency is increased above its rated value, the flux and torque would decrease. If the synchronous speed corresponding to the rated frequency is called the base speed ω b, the synchronous speed at any other frequency becomes,
thus,
and
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27. Systems Engineering School of Electrical 3. Frequency control
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28. Systems Engineering School of Electrical 3. Frequency control
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29. Systems Engineering School of Electrical 3. Frequency control
- Rotation of magnetic fields nsync will change in direct proportion to
change in electrical frequency, fe. so does the no-load point on the torque-speed characteristic curve.
The base speed will change with the same pattern.
Controlling under base speed, terminal voltage level and maximum operating torque MUST be derated accordingly.
Beyond based speed (up to 2p.u), voltage should be maintained to take care on its insulation.
but the maximum operating torque will automatically decreasing.
The motor is now operating in so called “field weakening mode”
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30. INDUCTION MOTORS – Speed Control
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31. Systems Engineering School of Electrical Line frequency Control
The important of derating…
the stator voltage is not decreased nearly with decreasing of stator frequency, the steel in the core of the induction motor will saturate and excessive magnetization currents will flow in the machine.
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32. Systems Engineering School of Electrical Line frequency Control
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33. Systems Engineering
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34. Systems Engineering School of Electrical
Voltage and frequency control (volt/hertz control )
From previous equation , flux is direct proportional to stator voltage and inverse proportional with frequency :
thus, if the ratio is kept constant, the flux value will remain constant.
the maximum torque (τm) can be maintained constant as it only depends on base speed (ωb) and β.
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- Once ωb ↓, β ↓ thus Va need to be ↓ as well in order to maintain the same Tm value.
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35. Systems Engineering School of Electrical
Voltage and frequency control (volt/hertz control )
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As the frequency reduced, β decreases, thus the slip at maximum torque increases.
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36. Systems Engineering School of Electrical
Voltage and frequency control (volt/hertz control )
Volt/hertz control usually applied to maintain torque while speed is varied.
voltage supply are obtained from 3-phase inverter or cycloconverters
as for very large power applications (locomotives , cement mills) cycloconverters are used.
As the frequency reduced, β decreases, thus the slip at maximum torque increases.
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37. Systems Engineering School of Electrical
3 possible circuit arrangement for obtaining variable voltage and frequency :
- regeneration is not possible and inverter would generate harmonics into the AC supply.
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- harmonics into the AC supply is reduced.
- input power factor is low
- Regeneration is possible
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38. Systems Engineering School of Electrical 5. Current control
torque of induction motor can be decreased by reducing rotor current.
but only the accessible input
current is varied.
at fixed input current, the rotor
current depends on the relative
values of magnetizing and rotor
circuit impedance.
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39. Systems Engineering School of Electrical 5. Current control
maximum torque depends on squared of current.
starting torque is low due to Xm is large compared to Xs an Xr
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40. Systems Engineering School of Electrical 5. Current control
as the speed increase, slip decrease, stator voltage rises and the torque increase.
Torque is increased due to increasing flux. Magnetizing current also increased and saturation will happen if the input current is not limited.
the constant current can be supplied by 3-phases CSI.
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41. Systems Engineering School of Electrical
2 possible CSI circuit for Current control :
Inductor act as current source
controlled rectifier controlling the current source
input power factor is very low.
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the chopper controlling the current source
input power factor is higher.
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42. Systems Engineering School of Electrical
6. Voltage, current and frequency control
In some application or requirement, all the 3 parameters might have to be adjusted.
1st region: speed is varied by voltage (or current) control at constant torque.
2nd region: motor operated at constant current and slip is varied.
3rd region: speed is controlled by frequency at reduced stator current.
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43. Systems Engineering School of Electrical
6. Voltage, current and frequency control
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44. Systems Engineering
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45. Systems Engineering School of Electrical electric drives control :
closed loop : means that some sort of feedback device attached to the motor feed speed information back to the drive for use in correcting any speed discrepancies.
Open loop: like most AC Drives, means no such feedback exists, and the drive assumes that what it told the motor to do is actually being done.
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46. Systems Engineering School of Electrical
Modern electric drives (With closed-loop control )
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closed-loop control is required to satisfy the steady-state and transient performance specs of AC drives
control strategies:
1) scalar control: control variable are DC quantities and only magnitude are controlled.
2) Vector control : both magnitude and phase of control variables.
3) Adaptive control: parameters are continuously varied to adapt the output variables variations.
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47. Systems Engineering School of Electrical
Modern electric drives (With closed-loop control )
Input
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Output
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48. Systems Engineering School of Electrical
Modern electric drives (With closed-loop control )
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49. Systems Engineering School of Electrical Choice of Electrical Drives
Choice of an electrical drive depends on a number of factors. Some of the important factors are :
Steady state operation requirement : nature of speed-torque characteristics, speed regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuations if any, ratings.
Transient operation requirement: values of acceleration and deceleration, starting, braking and reversing performance.
Requirement related to the source : type of source, and its capacity, magnitude of voltage, voltage fluctuations, power factor, harmonics and their effect on other load, ability to accept regenerated power.
Capital and running cost, maintenance needs, lifespan expectation.
Space and weight restrictions if any.
Environment and location. 7. Reliability.
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