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EEEB443 Control & Drives

EEEB443 Control & Drives. Induction Motor – Scalar Control By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering. Outline. Introduction Speed Control of Induction Motors Pole Changing Variable-Voltage, Constant Frequency

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EEEB443 Control & Drives

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  1. EEEB443 Control & Drives Induction Motor – Scalar Control By Dr. UngkuAnisaUngkuAmirulddin Department of Electrical Power Engineering College of Engineering EEEB443 - Control & Drives

  2. Outline • Introduction • Speed Control of Induction Motors • Pole Changing • Variable-Voltage, Constant Frequency • Variable Frequency • Constant Volts/Hz (V/f) Control • Open-loop Implementation • Closed-loop Implementation • Constant Airgap Flux Control • References EEEB443 - Control & Drives

  3. Lls Is Llr’ Ir’ Rs + E1 – + Vs – Rr’/s Lm Im Introduction • Scalar Control - control of induction machine based on steady-state model (per phase SS equivalent circuit) EEEB443 - Control & Drives

  4. Intersection point (Te=TL) determines the steady –state speed Te TL rated rotor sm Introduction Te Pull out Torque (Tmax) Trated What if the load must be operated here? r s rotor’ s Requires speed control of motor 1 0 EEEB443 - Control & Drives

  5. Speed Control of IM • Given a load T– characteristic, the steady-state speed can be changed by altering the T– curve of the motor Varying voltage (amplitude) 2 Varying line frequency 3 Pole Changing 1 EEEB443 - Control & Drives

  6. Speed Control of IM Pole Changing • Machines must be specially manufactured (i.e. called pole changing motors or multi-speed motors) • Need special arrangement of stator windings • Only used with squirrel-cage motors • Because number of poles induced in squirrel cage rotor will follow number of stator poles • Two methods: • Multiple stator windings • stator has more than one set of 3-phase windings • only energize one set at a time • simple, expensive • Consequent poles • Discrete step change in speed EEEB443 - Control & Drives

  7. Speed Control of IM Pole Changing • Consequent poles • single winding divided into few coil groups • No. of poles changed by changing connections of coil groups • Change in pole number by factor of 2:1 only A two-pole stator winding for pole changing. Notice the very short pitch (60 to 90) of these windings. EEEB443 - Control & Drives

  8. Speed Control of IM Pole Changing • Consequent poles • Close up view of one phase of a pole changing winding. • In Figure (a): the 2-pole configuration, one coil is a north pole and the other is a south pole. • In Figure (b): when the connection on one of the two coils is reversed, they are both north poles, and the magnetic flux returns to the stator halfway between the two coils. The south poles are called consequent poles. Hence the winding is now 4-pole. EEEB443 - Control & Drives

  9. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency • Controlled using: • Transformer (rarely used) • Thyristor voltage controller • thyristors connected in anti-parallel motor can be star or delta connected • voltage control by firing angle control (gating signals are synchronized to phase voltages and are spaced at 60 intervals) • Only for operations in Quadrant 1 and Quadrant 3 (requires reversal of phase sequence) • also used for soft start of motors EEEB443 - Control & Drives

  10. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency • Voltage can only be reduced from rated Vs (i.e. 0 < Vs ≤ Vs,rated) • From torque equation, Te Vs2 • When Vs , Te and speed reduces. • If terminal voltage is reduced to bVs, (i.e. Vs =bVs,rated) : Note: b 1 EEEB443 - Control & Drives

  11. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency • Suitable for applications where torque demand reduces with speed (eg: fan and pump drives where TL m2) • Suitable for NEMA Class D (high-slip, high Rr’) type motors • High rotor copper loss, low efficiency motors • get appreciable speed range Practical speed range EEEB443 - Control & Drives

  12. Speed Control of IM Variable Voltage (amplitude), Constant Frequency • Disadvantages: • limited speed range  when applied to Class B (low-slip) motors • Excessive stator currents at low speeds  high copper losses • Distorted phase current in machine and line (harmonics introduced by thyristor switching) • Poor line power factor (power factor proportional to firing angle) • Hence, only used on low-power, appliance-type motors where efficiency is not important • e.g. small fan or pumps drives EEEB443 - Control & Drives

  13. Speed Control of IM Variable Frequency • Speed control above rated (base) speed • Requires the use of PWM inverters to control frequency of motor • Frequency increased (i.e. s increased) • Stator voltage held constant at rated value • Airgap flux and rotor current decreases • Developed torque decreases Te (1/s) • For control belowbase speed – use Constant Volts/Hz method EEEB443 - Control & Drives

  14. Constant Volts/Hz (V/f) Control • Airgap flux in the motor is related to the induced stator voltage E1 : • For below base speed operation: • Frequency reduced at rated Vs - airgap flux saturates (f ,ag and enters saturation region oh B-H curve): - excessive stator currents flow - distortion of flux wave - increase in core losses and stator copper loss • Hence, keep ag=rated flux • stator voltage Vs must be reduced proportional to reduction in f (i.e. maintaining Vs / f ratio) Assuming small voltage drop across Rs and Lls EEEB443 - Control & Drives

  15. Constant Volts/Hz (V/f) Control • Max. torque remains almost constant • For low speed operation: • can’t ignore voltage drop across Rs and Lls (i.e. E1 Vs) • poor torque capability(i.e. torque decreased at low speeds shown by dotted lines) • stator voltage must be boosted – to compensate for voltage drop at Rs and Lls and maintain constant ag • Forabove base speed operation (f > frated): • stator voltage maintained at rated value • Same as Variable Frequency control (refer to slide 13) EEEB443 - Control & Drives

  16. Vrated Linear offset Non-linear offset – varies with Is Boost frated Constant Volts/Hz (V/f) Control Vs Vs vs. f relation in Constant Volts/Hz drives Boost - to compensate for voltage drop at Rs and Lls • Linear offset curve – • for high-starting torque loads • employed for most applications • Non-linear offset curve – • for low-starting torque loads f EEEB443 - Control & Drives

  17. Constant Volts/Hz (V/f) Control • For operation at frequency K times rated frequency: • fs = Kfs,rated  s = Ks,rated (1) (Note: in (1) , speed is given as mechanical speed) • Stator voltage: (2) • Voltage-to-frequency ratio = d = constant: (3) EEEB443 - Control & Drives

  18. Constant Volts/Hz (V/f) Control • For operation at frequency K times rated frequency: • Hence, the torque produced by the motor: (4) where s and Vs are calculated from (1) and (2) respectively. EEEB443 - Control & Drives

  19. Constant Volts/Hz (V/f) Control • For operation at frequency K times rated frequency: • The slip for maximum torque is: (5) • The maximum torque is then given by: (6) where s and Vs are calculated from (1) and (2) respectively. EEEB443 - Control & Drives

  20. Constant Volts/Hz (V/f) Control Constant Torque Area (below base speed) • Field Weakening Mode (f > frated) • Reduced flux (since Vs is constant) • Torque reduces • Constant Power Area • (above base speed) Rated (Base) frequency Note: Operation restricted between synchronous speed and Tmax for motoring and braking regions, i.e. in the linear region of the torque-speed curve. EEEB443 - Control & Drives

  21. Constant Volts/Hz (V/f) Control Constant Torque Area Constant Power Area EEEB443 - Control & Drives

  22. Example • A 4-pole, 3 phase, 400 V, 50 Hz, 1470 rpm induction motor has a rated torque of 30 Nm. The motor is used to drive a linear load with characteristic given by TL = K, such that the speed equals rated value at rated torque. If a constant Volts/Hz control method is employed, calculate: • The constant K in the TL - characteristic of the load. • Synchronous and motor speeds at 0.6 rated torque. • If a starting torque of 1.2 times rated torque is required, what should be the voltage and frequency applied at start-up? State any assumptions made for this calculation. • Answers: K = 0.195, synchronous speed = 899.47 rpm & motor speed = 881.47 rpm, At start up: frequency = 1.2 Hz, Voltage = 9.6 V EEEB443 - Control & Drives

  23. Constant Volts/Hz (V/f) Control – Open-loop Implementation PWM Voltage-Source Inverter (VSI) Note: e= s = synchronous speed EEEB443 - Control & Drives

  24. Constant Volts/Hz (V/f) Control – Open-loop Implementation • Most popular speed control method because it is easy to implement • Used in low-performance applications • where precise speed control unnecessary • Speed command s* - primary control variable • Phase voltage command Vs* generated from V/f relation(shown as the ‘G’ in slide 23) • Boost voltage Vo is added at low speeds • Constant voltage applied above base speed • Sinusoidal phase voltages (vabc*) is then generated from Vs* & s* where s* is obtained from the integral of s* • vabc* employed in PWM inverter connected to motor EEEB443 - Control & Drives

  25. Constant Volts/Hz (V/f) Control – Open-loop Implementation • Problems in open-loop drive operation: • Motor speed not controlled precisely • primary control variable is synchronous speed s • actual motor speed risless than s due to sl • sl depends on load connected to motor • slcannot be maintained since rnot measured • can lead to operation in unstable region of T- characteristic • stator currents can exceed rated value – endangering inverter-converter combination • Problems (to an extent) can be overcome by: • Open-loop Constant Volts/Hz Drive with Slip Compensation • Closed-loop implementation - having outer speed loop with slip regulation EEEB443 - Control & Drives

  26. Constant Volts/Hz (V/f) Control – Open-loop Implementation Open-loop Constant Volts/Hz Drive with Slip Compensation - Slip speed is estimated and added to the reference speed r* Vdc = Vd Idc Slip Compensator sl r* Note: e= s = synchronous speed EEEB443 - Control & Drives

  27. Constant Volts/Hz (V/f) Control – Open-loop Implementation Open-loop Constant Volts/Hz Drive with Slip Compensation • How is sl estimated in the Slip Compensator? • Using T- curve, sl Te • sl can be estimated by estimating torque where: (8) (9) (7) Note: In the figure, slip= sl = slip speed syn= s = synchronous speed EEEB443 - Control & Drives

  28. Constant Volts/Hz (V/f) Control – Closed-loop Implementation Open-loop system (as in slide 23) Slip Controller Note: e= s = synchronous speed EEEB443 - Control & Drives

  29. Constant Volts/Hz (V/f) Control – Closed-loop Implementation • Reference motor speed r* is compared to the actual speed r to obtain the speed loop error • Speed loop error generates slip command sl* from PI controller and limiter • Limiter ensures that the sl* is kept within the allowable slip speed of the motor (i.e. sl*  slip speed for maximum torque) • sl* is then added to the actual motor speed rto generate synchronous speed command s* (or frequency command) • s* generates voltage command Vs* from V/f relation • Boost voltage is added at low speeds • Constant voltage applied above base speed • Scheme can be considered open loop torque control (since T  s) within speed control loop EEEB443 - Control & Drives

  30. Constant Airgap Flux Control • Constant V/f control employs the use of variable frequency voltage source inverters (VSI) • Constant Airgap Flux control employs variable frequency current source inverters or current-controlled VSI • Provides better performance compared to Constant V/f control with Slip Compensation • airgap flux is maintained at rated value through stator current control • Speed response similar to equivalent separately-excited dc motor drive but torque and flux channels still coupled • Fast torque response means: • High-performance drive obtained • Suitable for demanding applications • Able to replace separately-excited dc motor drives • Above only true is airgap flux remains constant at rated value EEEB443 - Control & Drives

  31. Lls Is Llr’ Ir’ Rs + E1 Vs – + Vs – Rr’/s Lm Im Constant Airgap Flux Control Assuming small voltage drop across Rs and Lls • Constant airgap flux in the motor means: • For agto be kept constant at rated value, the magnetising current Im must remain constant at rated value • Hence, in this control scheme stator current Isis controlled to maintain Im at rated value Controlled to maintain Im at rated maintain at rated EEEB443 - Control & Drives

  32. Constant Airgap Flux Control • From torque equation (with agkept constant at rated value), since ss = sland ignoring Rs and Lls, • By rearranging the equation: Te sl  sl can be varied instantly  instantaneous (fast) Teresponse EEEB443 - Control & Drives

  33. Constant Airgap Flux Control • Constant airgap flux requires control of magnetising current Imwhich is not accessible • From equivalent circuit (on slide 31): • From equation (10), plot Is againstslwhen Imis kept at rated value. • Drive is operated to maintain Is againstslrelationship when frequency is changed to control speed. • Hence, control is achieved by controlling stator current Is and stator frequency: • Iscontrolled using current-controlled VSI • Control scheme sensitive to parameter variation (due to Tr and r) (10) EEEB443 - Control & Drives

  34. Constant Airgap Flux Control - Implementation Current Controlled VSI Voltage Source Inverter (VSI) 3-phase supply Rectifier IM C • Current controller options: • Hysteresis Controller • PI controller + PWM Current controller slip |Is| r* + PI i*a - i*b + s r i*c Equation (10) (from slide 33) + r EEEB443 - Control & Drives

  35. + i*a Voltage Source Inverter (VSI) i*b + i*c + Motor Current-Controlled VSI Implementation • Hysteresis Controller EEEB443 - Control & Drives

  36. PWM PWM PI PI PI PWM PWM PWM PWM Current-Controlled VSI Implementation + • PI Controller + Sinusoidal PWM i*a Voltage Source Inverter (VSI) i*b + i*c + • Due to interactions between phases • (assuming balanced conditions) •  actually only require 2 controllers Motor EEEB443 - Control & Drives

  37. dq abc PI PI abcdq Current-Controlled VSI Implementation • PI Controller + Sinusoidal PWM (2 phase) i*a abcdq id* PWM Voltage Source Inverter (VSI) i*b iq* i*c iq id Motor EEEB443 - Control & Drives

  38. References • Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control, Prentice-Hall, New Jersey, 2001. • Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall, New Jersey, 2002. • Trzynadlowski, A. M., Control of Induction Motors, Academic Press, San Diego, 2001. • Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd ed., Pearson, New-Jersey, 2004. • NikIdris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. • Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. EEEB443 - Control & Drives

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