EEEB443 Control & Drives Induction Motor – Scalar Control By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives Dr. Ungku Anisa, July 2008

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Introduction Scalar Control - control of induction machine based on steady-state model (per phase SS equivalent circuit) Rr’/s + Vs – Rs Lls Llr’ E1 Is Ir’ Im Lm Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Requires speed control of motor Introduction Te Pull out Torque (Tmax) rotor TL Te Intersection point (Te=TL) determines the steady –state speed sm rated Trated What if the load must be operated here? r s rotor’ s Requires speed control of motor 1 0 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 below base speed – use Constant Volts/Hz method Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 For above base speed operation (f > frated): stator voltage maintained at rated value Same as Variable Frequency control (refer to slide 13) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Vrated frated Linear offset Non-linear offset – varies with Is Boost Linear offset curve – for high-starting torque loads employed for most applications Non-linear offset curve – for low-starting torque loads f Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Volts/Hz (V/f) Control Constant Torque Area Constant Power Area Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Volts/Hz (V/f) Control – Open-loop Implementation PWM Voltage-Source Inverter (VSI) Note: e= s = synchronous speed Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 r is less than s due to sl sl depends on load connected to motor sl cannot be maintained since r not 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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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* Slip Compensator Idc Vdc = Vd sl r* Note: e= s = synchronous speed Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Volts/Hz (V/f) Control – Closed-loop Implementation Open-loop system (as in slide 23) Slip Controller Note: e= s = synchronous speed Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 r to 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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Airgap Flux Control Constant airgap flux in the motor means: For ag to be kept constant at rated value, the magnetising current Im must remain constant at rated value Hence, in this control scheme stator current Is is controlled to maintain Im at rated value Assuming small voltage drop across Rs and Lls Controlled to maintain Im at rated Rr’/s + Vs – Rs Lls Llr’ E1  Vs Is Ir’ Im Lm maintain at rated Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Airgap Flux Control From torque equation (with ag kept constant at rated value), since ss = sl and ignoring Rs and Lls, By rearranging the equation: Te  sl  sl can be varied instantly  instantaneous (fast) Te response Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Airgap Flux Control Constant airgap flux requires control of magnetising current Im which is not accessible From equivalent circuit (on slide 31): From equation (10), plot Is against sl when Im is kept at rated value. Drive is operated to maintain Is against sl relationship when frequency is changed to control speed. Hence, control is achieved by controlling stator current Is and stator frequency: Is controlled using current-controlled VSI Control scheme sensitive to parameter variation (due to Tr and r) (10) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Constant Airgap Flux Control - Implementation Current Controlled VSI Voltage Source Inverter (VSI) Rectifier 3-phase supply IM r* + |Is| slip C Current controller s PI r - Current controller options: Hysteresis Controller PI controller + PWM i*a i*b i*c Equation (10) (from slide 33) r Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Current-Controlled VSI Implementation Hysteresis Controller Motor + i*a i*b i*c Voltage Source Inverter (VSI) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Current-Controlled VSI Implementation PI Controller + Sinusoidal PWM Motor + i*a i*b i*c PWM PI Voltage Source Inverter (VSI) Due to interactions between phases (assuming balanced conditions)  actually only require 2 controllers Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

Current-Controlled VSI Implementation PI Controller + Sinusoidal PWM (2 phase) i*a abcdq dq abc id* PI PWM Voltage Source Inverter (VSI) i*b iq* i*c iq abcdq id Motor Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Nik Idris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives