EEEB283 Electrical Machines & Drives

EEEB283 Electrical Machines & Drives Induction Motor Drives – Scalar Control By Dr. UngkuAnisaUngkuAmirulddin Department of Electrical Power Engineering College of Engineering EEEB283 - Electrical Machines & Drives

Outline • Introduction • Speed Control of Induction Motors • Pole Changing • Variable-Voltage, Constant Frequency • Variable Frequency • Constant Volts/Hz (V/f) Control • References EEEB283 - Electrical Machines & Drives

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 steady-state equivalent circuit) EEEB283 - Electrical Machines & Drives

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 1 0 EEEB283 - Electrical Machines & 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 EEEB283 - Electrical Machines & Drives

Speed Control of IM Pole Changing • Machines must be specially manufactured • Only used with squirrel-cage motors • Two methods: • Multiple stator windings – simple, expensive • Consequent poles – single winding divided into few coil groups • Consequent poles: • No. of poles changed by changing connections of coil groups • Change in pole number by factor of 2:1 only • Discrete step change in speed EEEB283 - Electrical Machines & Drives

Speed Control of IM Variable-Voltage (amplitude), Constant Frequency • Controlled using: • AC Voltage Controllers (anti-parallel thyristors) • voltage control by firing angle control • also used for soft start of motors EEEB283 - Electrical Machines & Drives

Speed Control of IM Variable-Voltage (amplitude), Constant Frequency • From torque equation, Te Vs2 • When Vs , Te and speed reduces. • If terminal voltage is reduced to bVs,: Note: b 1 EEEB283 - Electrical Machines & 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 • Poor line power factor • Hence, only used on low-power, appliance-type motors where efficiency is not important • e.g. small fan or pumps drives EEEB283 - Electrical Machines & Drives

Speed Control of IM Variable Frequency • Speed control above rated (base) speed • Frequency increased • Stator voltage held constant at rated value • Airgap flux and rotor current decreases • Developed torque decreases • For control below base speed – use Constant Volts/Hz method EEEB283 - Electrical Machines & 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): - excessive stator currents flow - distortion of flux wave • Hence, keep ag=rated flux • stator voltage must be reduced proportionally Assuming small voltage drop across Rs and Lls EEEB283 - Electrical Machines & 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 • poor torque capability • stator voltage must be boosted – maintain constant ag • Forabove base speed operation (f > frated): • stator voltage maintained at rated value EEEB283 - Electrical Machines & Drives

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 f EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control • For operation at frequency  times rated frequency: • fs = fs,rated  s = s,rated (1) • Stator voltage: (2) • Voltage-to-frequency ratio = d = constant: (3) EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control • For operation at frequency  times rated frequency: • Hence, the torque produced: (4) where s and Vs are calculated from (1) and (2) respectively. EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control • For operation at frequency  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. EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control Constant Torque Area • Field Weakening Mode (f > frated) • Reduced flux • Torque reduces •  Constant Power Area EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control Constant Torque Area Constant Power Area EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control – Open-loop Implementation PWM Voltage-Source Inverter (VSI) EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control – Open-loop Implementation • Most popular speed control method • 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 • Boost voltage applied at low speeds • Constant voltage applied above base speed • Sinusoidal phase voltages (vabc*)generated from Vs* & s* • vabc* employed in PWM inverter connected to motor EEEB283 - Electrical Machines & 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. • NikIdris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. • Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008. EEEB283 - Electrical Machines & Drives

EEEB283 Electrical Machines & Drives