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EEEB283 Electrical Machines & Drives

EEEB283 Electrical Machines & Drives. Chopper-Controlled DC Drives By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering. Outline. Introduction DC – DC Converter Fed Drives Step Down Class A Chopper Step Up Class B Chopper

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EEEB283 Electrical Machines & Drives

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  1. EEEB283 Electrical Machines & Drives Chopper-Controlled DC Drives By Dr. UngkuAnisaUngkuAmirulddin Department of Electrical Power Engineering College of Engineering EEEB443 - Control & Drives

  2. Outline • Introduction • DC – DC Converter Fed Drives • Step Down Class A Chopper • Step Up Class B Chopper • Two-quadrant Control • Four-quadrant Control • References EEEB443 - Control & Drives

  3. Power Electronic Converters for DC Drives • Power electronics converters are used to obtain variable voltage • Highly efficient • Ideally lossless • Type of converter used is depending on voltage source : • AC voltage source  Controlled Rectifiers • Fixed DC voltage source  DC-DC converters (switch mode converters) EEEB443 - Control & Drives

  4. DC – DC Converter Fed Drives • To obtain variable DC voltage fromfixed DC source • Self-commutated devices preferred (MOSFETs, IGBTs, GTOs) over thyristors • Commutated by lower power control signal • Commutation circuit not needed • Can be switched at higher frequency for same rating • Improved motor performance (less ripple, no discontinuous currents, increased control bandwidth) • Suitable for high performance applications • Regenerative braking possible up to very low speeds even when fed from fixed DC voltage source EEEB443 - Control & Drives

  5. Q1 Q2 Q3 Q4 T DC – DC Converter Fed Drives- Step Down Class A Chopper Motoring • Provides positive output voltage and current • Average power flows from source to load (motor) • Switch (S) operated periodically with period T Ia S Ra Va La V D Ea EEEB443 - Control & Drives

  6. DC – DC Converter Fed Drives- Step Down Class A Chopper Motoring S is ON (0  t  ton) Ia Ia S Ra Ra V Va Va La La V D • Va = V • Ia flows to motor • |Ia| increases Duty Interval ( ia ) Ea Ea EEEB443 - Control & Drives

  7. DC – DC Converter Fed Drives- Step Down Class A Chopper Motoring S if OFF (ton t  T) Ia Ia S Ra Ra Va Va ID La La V D • Va= 0 • Ia freewheels through diode DF • |Ia| decreases Freewheeling Interval ( ia ) Ea Ea EEEB443 - Control & Drives

  8. DC – DC Converter Fed- Step Down Class A Chopper Motoring • Duty cycle • Under steady-state conditions: Motor side: Chopper side, average armature voltage: Therefore, • Hence, average armature current: Duty Interval ( ia ) Freewheeling Interval ( ia )  T EEEB443 - Control & Drives

  9. Q1 Q2 Q4 Q3 T DC – DC Converter Fed Drives- Step Up Class B Chopper Regenerative Braking • Provides positive output voltage and negative average output current • Average power flows from load (motor) to source Ia D • Possible for speed above rated speed and down to nearly zero speed • Application: • Battery operated vehicles • Regenerated power stored in battery Ra Va S V La Switch (S) operated periodically with period T Ea EEEB443 - Control & Drives

  10. DC – DC Converter Fed Drives- Step Up Class B Chopper Regenerative Braking S is ON (0  t  ton) • Va = 0 (diode blocks V) • ia increases due to E (since E >Va) • Mechanical energy converted to electrical (i.e. generator) • Energy stored in La • Any remaining energy dissipated in Ra and S Ia Ia D Ra Ra Va Va S S V La La Energy Storage Interval ( ia ) Ea Ea EEEB443 - Control & Drives

  11. DC – DC Converter Fed Drives- Step Up Class B Chopper Regenerative Braking S if OFF (ton t  T) • ia flows through diode D and source V • ia decreasesin negative direction • Energy stored in La & energy supplied by machine are fed to the source Ia Ia D Ra Ra Va Va V S V La La Duty Interval ( ia ) Ea Ea EEEB443 - Control & Drives

  12. DC – DC Converter Fed Drives- Step Up Class B Chopper Negative because current flows from motor to source Regenerative Braking • Duty cycle • Under steady-state conditions Generator side: Chopper side, average armature voltage: • Therefore, • Hence, average armature current: Energy Storage Interval ( ia ) Duty Interval ( ia )  T EEEB443 - Control & Drives

  13. T1 D1 Q1 Q2 Q3 Q4 T + Va - T2 D2 No Speed Reversal DC – DC Converter Fed Drives- Two-quadrant Control • Combination of Class A & B choppers • Forward motoring Q1 - T1 and D2 (Class A) • Forward braking Q2 – T2 and D1 (Class B) + V - • Vaalways +ve   always +ve • Iacan be +ve or –ve • Do not fire both switches • together  short circuit at • supply D2 EEEB443 - Control & Drives

  14. Average Va Ea DC – DC Converter Fed Drives- Two-quadrant Control • Forward motoring Q1 - T1 and D2 (Class A) • T1 conducting: Va = V (ia) • D2 conducting:Va = 0 (ia) + V  + V  T1 T1 D1 D1 ia ia + Va - + Va - D2 D2 T2 T2 Average Va= 1V, 1 = (ton T1 / T ), 2= 0 T1 chopping ON & OFF • AverageVa positive • AverageVa made larger than back emfEa • Ia positive T2 always OFF EEEB443 - Control & Drives

  15. Average Va Ea DC – DC Converter Fed Drives- Two-quadrant Control • Forward braking Q2 – T2 and D1 (Class B) • D1 conducting: Va = V (ia) • T2 conducting: Va = 0 (ia) + V  + V  T1 T1 D1 D1 ia ia + Va - + Va - D2 D2 T2 T2 Average Va=(1 - 2)V, 1 = 0, 2 = (ton T2 / T ) T2 chopping ON & OFF • AverageVa positive • AverageVa made smallerthan back emfEa • Ia negative (motor acts as generator) T1 always OFF EEEB443 - Control & Drives

  16. DC – DC Converter Fed Drives- Two-quadrant Control • For fast transitionfrom motoring (Q1) to braking (Q2) and vice versa, both T1 and T2 are controlled simultaneously, i.e. within a period T: • T1 in ON and T2 is OFF between time 0 < t ≤ ton • If Ia is positive (Va > E), current flows from supply to motor via T1 • If Ia is negative (E > Va), current flows from motor to supply via D1 • T1 is OFF and T2 is ON between ton < t ≤ T • If Ia is positive, current circulates via D2 • If Ia is negative, current circulates via T2 • Duty ratio is given by: • Average armature voltage is: Average Va=V EEEB443 - Control & Drives

  17. DC – DC Converter Fed Drives - Two-quadrant Control: Example EEEB443 - Control & Drives

  18. D1 D3 T1 T3 Q1 Q2 + Va- Q3 Q4 T T4 T2 D2 D4 DC – DC Converter Fed Drives- Four-quadrant Control • Operation in all four quadrants • Va and Ia can be controlled in magnitude and polarity • Power flow can be in either direction • Speed and torque can be reversed ia Note: Polarity of Va and direction of Iaindicated are assumed positive. EEEB443 - Control & Drives

  19. D1 D3 T1 T3 + Va- T4 T2 D2 D4 DC – DC Converter Fed Drives- Four-quadrant Control • When a switch is on (i.e. ‘ON state’) it may or may not conduct current depending on the direction of ia • If a switch conducts current, it is in a conducting state • Converter has two legs (Leg A & Leg B) • Both switches in each leg, are alternately switched • If T1 = ON, T4 = OFF • If T4 = ON, T1 = OFF Leg B + Vdc - ia EEEB443 - Control & Drives Leg A

  20. D1 D3 T1 T3 Q1 Q2 + Va- Q3 Q4 T T4 T2 D2 D4 DC – DC Converter Fed Drives- Four-quadrant Control • Positive Current (Ia > 0) • Va = VdcwhenT1 and T2 are ON • Current increases • Q1 operation • Va = 0 when current freewheels through T2 and D4 • Current decreases • Va = -VdcwhenD3 and D4 conducts current • Current decreases • Energy returned to supply • Q4 operation + Vdc - ia T3 and T4 off EEEB443 - Control & Drives

  21. D1 D3 T1 T3 Q1 Q2 + Va- Q3 Q4 T T4 T2 D2 D4 DC – DC Converter Fed Drives- Four-quadrant Control • Negative Current (Ia > 0) • Va = -VdcwhenT3 and T4 are ON • Current increases in negative direction • Q3 operation • Va = 0 when current freewheels through T4 and D2 • Current decreases • Va = VdcwhenD1 and D2 conducts current • Current decreases • Energy returned to supply • Q2 operation + Vdc - ia T1 and T2 off EEEB443 - Control & Drives

  22. DC – DC Converter Fed Drives- Four-quadrant Control • For both positive and negative current, output voltage can swing between: • Vdcand-Vdc • Vdcand 0 • Four quadrant chopper has two legs, so it requires two switching signals (one for each leg) • Depending on relationship between the two switching signals, 4-quadrant chopper has two switching schemes: • Bipolar switching • Unipolar switching • Switching scheme determines output voltage swing between Vdcand-VdcorVdcand 0. EEEB443 - Control & Drives

  23. DC – DC Converter Fed Drives • Operation of DC motor drive depends on: • Direction of Ia(determined by torque, i.e. motoring or braking) • Polarity of Va and Ea(determined by speed, i.e. forward or reverse) • the duty cycle of the DC-DC Converter (either two-quadrant or four-quadrant) • Open loop control is achieved by changing the duty cycle manually as and when required EEEB443 - Control & Drives

  24. References • Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd ed., Pearson, New-Jersey, 2004. • Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha Science Int. Ltd., UK, 2001. • Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control, Prentice-Hall, New Jersey, 2001. • 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. EEEB443 - Control & Drives

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