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Induction Motor – Constant Flux Control

EE 4237 Lecture-3. Induction Motor – Constant Flux Control. 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

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Induction Motor – Constant Flux Control

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  1. EE 4237 Lecture-3 Induction Motor – Constant Flux Control EE 4237 - Special Machines and AC Drives

  2. 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 EE 4237 - Special Machines and AC Drives

  3. 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 EE 4237 - Special Machines and AC Drives

  4. 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 EE 4237 - Special Machines and AC Drives

  5. 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 EE 4237 - Special Machines and AC Drives

  6. Constant Airgap Flux Control • Constant airgap flux requires control of magnetising current Imwhich is not accessible • From equivalent circuit: • From equation (1), plot Is againstslwhen Imis kept at rated value. (1) EE 4237 - Special Machines and AC Drives

  7. 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) EE 4237 - Special Machines and AC Drives

  8. 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 (1) (from slide 6) + r EE 4237 - Special Machines and AC Drives

  9. + i*a Voltage Source Inverter (VSI) i*b + i*c + Motor Current-Controlled VSI Implementation • Hysteresis Controller EE 4237 - Special Machines and AC Drives

  10. 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 EE 4237 - Special Machines and AC Drives

  11. 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 EE 4237 - Special Machines and AC Drives

  12. 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. Courtesy: http://kaliasgoldmedal.yolasite.com/resources/SSD/Induction%20Motor%20-%20Scalar%20Control.ppt EE 4237 - Special Machines and AC Drives

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