INDUCTION MOTOR Scalar Control (squirrel cage)

1. INDUCTION MOTORScalar Control(squirrel cage) MEP 1422 ELECTRIC DRIVES

2. Is Lls Llr’ Ir’ Rs + Eag – + Vs – Rr’/s Lm Im Scalar control of induction machine:Control of induction machine based on steady-state model (per phase SS equivalent circuit):

3. Intersection point (Te=TL) determines the steady –state speed Te TL rated rotor sm Scalar control of induction machine Te Pull out Torque (Tmax) Trated r s s

4. Variable voltage (amplitude), variable frequency Using power electronics converter Operated at low slip frequency Pole changing Synchronous speed change with no. of poles Discrete step change in speed Variable voltage (amplitude), frequency fixed E.g. using transformer or triac Slip becomes high as voltage reduced – low efficiency Scalar control of induction machine Given a load T– characteristic, the steady-state speed can be changed by altering the T– of the motor:

5. Variable voltage, fixed frequency e.g. 3–phase squirrel cage IM V = 460 V Rs= 0.25  Rr=0.2  Lr = Ls = 0.5/(2*pi*50) Lm=30/(2*pi*50) f = 50Hz p = 4 Lower speed  slip higher Low efficiency at low speed

6. Variable voltage, variable frequency Constant V/f Approximates constant air-gap flux when Eag is large Eag = k f ag = constant Speed is adjusted by varying f - maintaining V/f constant to avoid flux saturation

7. Variable voltage, variable frequency Constant V/f - assuming constant airgap flux

8. Variable voltage, variable frequency Constant V/f Vs Vrated frated f

9. Variable voltage, variable frequeny Constant V/f – open-loop Rectifier VSI 3-phase supply IM C f Pulse Width Modulator Ramp V + s*

10. Steady state T- Variable voltage, variable frequeny Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955,

11. Variable voltage, variable frequeny Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955, Steady state T- and transient T- characteristic – without ramp limitter

12. Variable voltage, variable frequeny Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955, Steady state T- and transient T- characteristic – with ramp limitter

13. 1 Variable voltage, variable frequency Constant V/f 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: Boost voltage at low speed Maintain Im constant – constant ag

14. Variable voltage, variable frequeny Constant V/f

15. Variable voltage, variable frequeny Constant V/f with compensation (Is,ratedRs) • Torque deteriorate at low frequency – hence compensation commonly performed at low frequency • In order to truly compensate need to measure stator current – seldom performed

16. Vrated Linear offset Non-linear offset – varies with Is Boost frated Variable voltage, variable frequeny Constant V/f with voltage boost at low frequency

17. Poor speed regulation 2 Variable voltage, variable frequeny Constant V/f Problems with open-loop constant V/f Solution: Compesate slip Closed-loop control

18. Rectifier VSI 3-phase supply IM C f Pulse Width Modulator Ramp + V + s* + + Vboost Slip speed calculator Vdc Idc Variable voltage, variable frequeny Constant V/f – open-loop with slip compensation and voltage boost

19. Is Lls Llr’ Ir’ Rs + Eag – + Vs – Rr’/s Lm Im Variable voltage, variable frequeny Constant air-gap flux A better solution : maintain ag constant. How? ag, constant → Eag/f , constant → Im, constant (rated) Controlled to maintain Im at rated maintain at rated

20. Variable voltage, variable frequeny Constant air-gap flux

21. Variable voltage, variable frequeny Constant air-gap flux • Current is controlled using current-controlled VSI • Dependent on rotor parameters – sensitive to parameter variation

22. Variable voltage, variable frequeny Constant air-gap flux VSI 3-phase supply Rectifier IM C Current controller slip |Is| * + PI - + s r +