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

2. 2 School of Electrical Systems Engineering ABD RAHIM 2008 Per-phase equivalent circuit …. INDUCTION MOTORS (Rashid) In real world: Value of X m is very large (R m can be omitted) X m 2 >> (R s 2 + X s 2 )  Vs ≈Vm Thus, for circuit simplification, the magnetizing reactance X m can be moved-out to the stator winding.

3. 3 School of Electrical Systems Engineering ABD RAHIM 2008 INDUCTION MOTORS The input impedance of the motor becomes: Power factor angle:

4. 4 School of Electrical Systems Engineering ABD RAHIM 2008 INDUCTION MOTORS Rotor current (rms): P AG or P g =3I r 2 R r /s Previously noted, And developed torque τ dev or, τ ind =P dev / ω m = P AG / ω sync Thus, --(15-18)

5. 5 School of Electrical Systems Engineering ABD RAHIM 2008 INDUCTION MOTORS

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7. 7 School of Electrical Systems Engineering ABD RAHIM 2008 Extended Torque-speed characteristic.. The curve is nearly linear up to full load (Rr >> Xr, thus Ir,Br and Tind rise ∞ increasing slip). Pullout torque or breakdown torque = max possible handling torque. 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. s

8. 8 School of Electrical Systems Engineering ABD RAHIM 2008 Extended Torque-speed characteristic.. The curve is nearly linear up to full load (Rr >> Xr, thus Ir,Br and Tind rise ∞ increasing slip). Pullout torque or breakdown torque = max possible handling torque. 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.

9. 9 School of Electrical Systems Engineering ABD RAHIM 2008 INDUCTION MOTORS – DRIVES Speed & torque control strategies A) Change n sync 1.Pole changing 2.Line frequency control B) Change slip, s 3.Rotor-resistance control 4.Rotor slip-energy recovery 5.Line/stator voltage control C) Parameters alteration 6.Rotor voltage control 7.Stator current control 8.Stator voltage and frequency control 9.Voltage, current and frequency control

10. 10 School of Electrical Systems Engineering ABD RAHIM 2008 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 : 1.Stator voltage control 2.Rotor voltage control 3.Frequency control 4.Stator voltage and frequency control 5.Stator current control 6.Voltage, current, and frequency control To meet the torque-speed duty cycle of a drive, the voltage, current, and frequency control are normally used.

11. 11 ABD RAHIM 2008 INDUCTION MOTORS – Speed Control

12. 12 School of Electrical Systems Engineering ABD RAHIM 2008 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 bV s, where b ≤ 1 r%, Eq. (15-18) gives the developed  toroue Typical torque-speed characteristics for various values of b.

13. 13 School of Electrical Systems Engineering ABD RAHIM 2008 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 bV s, where b ≤ 1,Typical torque- speed characteristics for various values of b :

14. 14 School of Electrical Systems Engineering ABD RAHIM 2008 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 Where, K m 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.

15. 15 School of Electrical Systems Engineering ABD RAHIM 2008  The stator voltage can be varied by three-phase 1.AC voltage controllers, 2.voltage-fed variable DC-link inverters, or 3.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.

16. 16 School of Electrical Systems Engineering ABD RAHIM 2008 Advantages : Reduced starting current Reduced starting torque Less mechanical stress Improved control of acceleration and deceleration (to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period) 1) AC voltage controller & soft start

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18. 18 School of Electrical Systems Engineering ABD RAHIM 2008 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, P no-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, τ L b) The rotor current, I R c) Stator supply voltage, V a d) Motor input current, I i e) Motor input power, P i f) The slip for maximum current,s a g) Maximum rotor current, I r(max) h) Speed at maximum rotor current, ω a i) Torque at maximum current, τ a

19. 19 School of Electrical Systems Engineering ABD RAHIM 2008 Breakdown torque peak is shifted to zero speed by increasing rotor resistance. Wound rotor induction motor. 2. Rotor voltage control (Rotor resistance control)

20. 20 School of Electrical Systems Engineering ABD RAHIM 2008 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).

21. 21 School of Electrical Systems Engineering ABD RAHIM 2008 -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.

22. 22 School of Electrical Systems Engineering ABD RAHIM 2008 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 L d acts as a current source I d and the dc converter varies the effective resistance, which can be found from Eq. (14.45): R e = 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. 2. Rotor voltage control (Rotor resistance control)

23. 23 School of Electrical Systems Engineering ABD RAHIM 2008 2. Rotor voltage control (Rotor resistance control)

24. 24 School of Electrical Systems Engineering ABD RAHIM 2008 2. Rotor voltage control (Rotor resistance control)

25. 25 School of Electrical Systems Engineering ABD RAHIM 2008 2. Rotor voltage control (Rotor resistance control) Example 15.3a: Rs = 0.05Ω, Rr =0.055Ω, Xs = 0.31Ω, Xr = 0.35Ω and Xm = 7.1Ω

26. 26 School of Electrical Systems Engineering ABD RAHIM 2008 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.

27. 27 School of Electrical Systems Engineering ABD RAHIM 2008 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 s, the synchronous speed at any other frequency becomes, thus, and a, : Far Bot- a^. @n - B.t - Bat ( r5-56) Thet orquee xpressioinn Eq. (15-18b)e comes 3R,Vz sFcoa[(R+, R,/s)2+ (BX" + BX,)z] (l 5-57) The typical torque-speedc haracteristicsa re shown in Fig. l5-9 for various values of B. The three-phase inv€rter in Fig. l0-5a can vary the frequency at a fixed voltage. If R" is negligible, Eq. (15-26) gives the maximum torque at the base Torque \ -/TmF = Tnb = conslanl l5 a- 9L Figure l5-9 Torque characteristics with frequency control, Sec.1 5-2 lnductionM otorD rives 559

28. 28 School of Electrical Systems Engineering ABD RAHIM 2008 3.Frequency control

29. 29 School of Electrical Systems Engineering ABD RAHIM 2008 3.Frequency control

30. 30 School of Electrical Systems Engineering ABD RAHIM 2008 3.Frequency control - Rotation of magnetic fields n sync will change in direct proportion to change in electrical frequency, f e. 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”

31. 31 School of Electrical Systems Engineering ABD RAHIM 2008 INDUCTION MOTORS – Speed Control

32. 32 School of Electrical Systems Engineering ABD RAHIM 2008 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.

33. 33 School of Electrical Systems Engineering ABD RAHIM 2008 Line frequency Control

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