EEEB283 Electrical Machines & Drives

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Presentation transcript:

EEEB283 Electrical Machines & Drives Induction Motor Drives – Scalar Control By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & 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 References Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Introduction Scalar Control - control of induction machine based on steady-state model (per phase steady-state equivalent circuit) Rr’/s + Vs – Rs Lls Llr’ E1 Is Ir’ Im Lm Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Introduction Te Pull out Torque (Tmax) rotor rated Trated r s 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 1 0 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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 For above base speed operation (f > frated): stator voltage maintained at rated value Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control Vs Vrated frated Linear offset Non-linear offset – varies with Is Boost Vs vs. f relation in Constant Volts/Hz drives f Dr. Ungku Anisa, July 2008 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) Dr. Ungku Anisa, July 2008 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. Dr. Ungku Anisa, July 2008 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. Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control Constant Torque Area Constant Power Area Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Constant Volts/Hz (V/f) Control – Open-loop Implementation PWM Voltage-Source Inverter (VSI) Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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. 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 EEEB283 - Electrical Machines & Drives