1 / 21

EEEB283 Electrical Machines & Drives

EEEB283 Electrical Machines & Drives. Induction Motor Drives – 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

germaine
Télécharger la présentation

EEEB283 Electrical Machines & Drives

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. 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

  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 steady-state equivalent circuit) EEEB283 - Electrical Machines & 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 1 0 EEEB283 - Electrical Machines & 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 EEEB283 - Electrical Machines & Drives

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

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

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

  20. 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

  21. 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

More Related