Induction Motor Drives

A part of the course "Solid State Drives"

1. Unit-3 Induction Motor Drives

2. Speed Control Methods of Induction Motor D rives 2

3. 3 Stator voltage control using AC voltage controllers

4. 4 Stator voltage control using AC voltage controllers

5. 5 Stator Frequency Control (Or) Field Weakening Method Of Speed Control

6. 6 Stator Frequency Control (Or) Field Weakening Method Of Speed Control

7. V/F Control Refer your Class notes 7

8. Closed Loop Speed Control of Induction Motor Fed From Voltage Source Inverter (VSI)

9. 9 Closed Loop Speed Control of Induction Motor Fed From Voltage Source Inverter (VSI)

10. 10 Closed Loop Speed Control of Induction Motor Fed From Voltage Source Inverter (VSI)

11. 11 Closed Loop Speed Control of Induction Motor Fed From Current Source Inverter (CSI)

12. 12 Closed Loop Speed Control of Induction Motor Fed From Current Source Inverter (CSI)

13. 13 Closed Loop Speed Control of Induction Motor Fed From Current Source Inverter (CSI)

14. 14

15. SLIP POWER RECOVERY SCHEME In rotor resistance control method of speed control, the slip power is wasted in the external resistance and hence the efficiency reduces. However instead of wasting the slip power in external resistor, it can be recovered and supplied back in order to improve the overall efficiency. This scheme of recovering the power is called slip power recovery scheme and this is done by connecting an external source of emf of slip frequency to the rotor circuit. The injected emf can either oppose the rotor induced emf or aids the rotor induced emf . If it opposes the rotor induced emf , the total rotor resistance increases and hence speed decreases. If the injected emf aids the main rotor emf , the total resistance decreases and hence speed increases . Therefore by injecting induced emf in rotor circuit, the speed can be easily controlled. 15

16. Types of Slip Power Recovery Scheme The slip power recovery system can be classified into two types, 1 . Kramer system: Conventional Kramer system. Static Kramer system. 2 . Scherbius system Conventional Scherbius system. Static Scherbius system. DC Link static Scherbius drive Cycloconverter static Scherbius drive 16

17. Static Kramer system In rotor resistance control method, slip power is wasted in rotor circuit resistance. Instead of wasting slip power in rotor circuit resistance, it can be converted to 50 Hz ac and pumped back to the line. Here , slip power can flow only in one direction. This method of drive is called static Kramer drive. It offers speed control for sub-synchronous speed only i.e., speed can be controlled only less than the synchronous speed . 17

18. 18 Static Kramer system

19. Static Scherbius System- I. DC link static Scherbius drive This system consists of SRIM, 2 number of phase controlled bridges, smoothing inductor and step up transformer. This system is used for both sub-synchronous speed and super-synchronous speed operation . 19

20. Static Scherbius System- I. DC link static Scherbius drive 20 a . Sub-synchronous speed operation:

21. Sub-synchronous speed operation: Slip power is removed from rotor ckt . and pumped back into ac supply. When m/c is operated at sub-synchronous speed, phase controlled bridge 1 operates in rectifier mode and bridge 2 operates in inverter mode. In other words, bridge 1 has firing angle less than 90 whereas bridge 2 has firing angle more than 90 . The slip power flows from rotor circuit to bridge 1, bridge 2, transformer and returned to supply. 21 Static Scherbius System- I. DC link static Scherbius drive

22. 22 Static Scherbius System- I. DC link static Scherbius drive b . Super-synchronous speed operation:

23. Static Scherbius System- I. DC link static Scherbius drive b. Super synchronous speed operation: A dditional power is fed into the rotor circuit at slip frequency. When machine is operated at super synchronous speed, phase controlled bridge 2 should operate in rectifier mode and bridge 1 in inverter mode. In other words, bridge 2 has firing angle less than 90 whereas bridge 1 has firing angle more than 90 . The slip power flows from the supply to transformer, bridge 2 ( rectifier), bridge 1 (line commutated inverter ) and to the rotor circuit . Near synchronous speed, the rotor voltage is low and forced commutation must be employed in inverter , which makes the scheme less attractive . 23

24. 24 Static Scherbius System- II. Cycloconverter static Scherbius drive

25. Mode 1: Sub-synchronous motoring This mode, shown in fig. 5.43(a) is similar to that of the static Kramer system. The stator input or air gap power P ag remains constant and the slip power sP ag , which is proportional to slip (which is + ve ), returned back to line through cycloconverter .  , line supplies net mechanical power P m = (1 – s) P ag consumed by shaft . The slip fr. power in rotor creates a rotating field in the same direction as in stator and rotor speed  r corresponds to diff. ( s -  sl ) b/w these two frequencies. At slip equal to zero, cycloconverter supplies dc excitation to rotor and the m/c behave like a standard syn. motor . 25 Static Scherbius System- II. Cycloconverter static Scherbius drive

26. Mode 2: Super-synchronous motoring In this mode, (fig. 5.43b ), shaft speed  beyond syn. speed, slip becomes - ve and slip power is absorbed by rotor. Slip power sP ag supplements air gap power P ag for total mechanical power output (1 + s) P ag . The line  supplies slip power in addition to stator i/p power. During this condition, slip voltage is reversed, so that slip frequency-induced rotating magnetic field is opposite to that of stator. 26 Static Scherbius System- II. Cycloconverter static Scherbius drive

27. Mode 3: Sub-synchronous regeneration: In regenerative braking condition, (fig. 5.43c ), shaft is driven by load and mech. energy is converted into electrical energy. With constant - ve shaft torque, mech. power i/p to shaft P m =(1-s) P ag  with speed and this equals electrical power fed to the line. In subsynchronous speed range, slip s is + ve and air gap power P ag is - ve . The slip power sP ag is fed to rotor from cycloconverter so that total air gap power is constant. Slip voltage has a + ve phase sequence. At synchronous speed, cycloconverter supplies dc excitation current to rotor circuit and m/c behaves as a syn. generator. Main application is a variable – speed wind generating system . 27 Static Scherbius System- II. Cycloconverter static Scherbius drive

28. 28 Static Scherbius System- II. Cycloconverter static Scherbius drive

29. Mode 4: Super- synchronous regeneration . Super-synchronous regeneration is indicated (fig. 5.43d ). Here , stator o/p power remains constant, but addl. Mech. power i/p is reflected as slip power o/p. Now rotor field rotates in opposite direction because cycloconverter phase sequence is reversed . Power distribution as a function of slip in subsynchronous and supersynchronous speed ranged is summarized for all four modes in figure 5.44, where the operating speed range of 50 percent about the synchronous speed is indicated. 29 Static Scherbius System- II. Cycloconverter static Scherbius drive

30. 30 Static Scherbius System- II. Cycloconverter static Scherbius drive

31. Advantages: 1. Prob. of commutation near syn. speed disappears . 2. Cycloconverter can easily operate as a phase-controlled rectifier, supplying dc ct. in rotor and permitting true syn. m/c operation . 3. Near-sinusoidal ct. waves in the rotor, which reduce harmonic loss, and m/c over excitation capacity that permits leading PF operation on stator side so the line’s PF is unity . 4 . Cycloconverter is to be controlled so that its o/p fr. tracks precisely with slip fr. Disadvantages: 1. Cycloconverter cost increases. 2 . Control of Scherbius drive is some what complex. Applications 1. Multi – MW, Variable - speed pumps / generators. 2. Flywheel energy storage systems. 31 Static Scherbius System- II. Cycloconverter static Scherbius drive

32. Vector Control of Induction Motor Drives 32

33. 33 Vector Control

34. 34 Vector Control

35. 35 Vector Control