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Chopper-Controlled DC Drives By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku Anisa, July 20081EEEB443 - Control & Drives

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Outline Introduction DC – DC Converter Fed Drives Step Down Class A Chopper Step Up Class B Chopper Two-quadrant Control Four-quadrant Control References Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives2

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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) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives3

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DC – DC Converter Fed Drives To obtain variable DC voltage from fixed 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 Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives4

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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 Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives5 T Q1 Q2 Q3Q4 V S D RaRa LaLa EaEa VaVa IaIa

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DC – DC Converter Fed Drives - Step Down Class A Chopper S is ON (0 t t on ) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives6 Motoring Duty Interval ( i a ) V a = V I a flows to motor |I a | increases V S D RaRa LaLa EaEa VaVa IaIa RaRa LaLa EaEa VaVa IaIa V

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DC – DC Converter Fed Drives - Step Down Class A Chopper S if OFF (t on t T) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives7 Motoring Freewheeling Interval ( i a ) V a = 0 I a freewheels through diode D F |I a | decreases RaRa LaLa EaEa VaVa IaIa IDID V S D RaRa LaLa EaEa VaVa IaIa

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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: Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives8 T Freewheeling Interval ( i a ) Duty Interval ( i a )

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DC – DC Converter Fed Drives - Step Up Class B Chopper Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives9 T Q1 Q2 Q3 Q4 Possible for speed above rated speed and down to nearly zero speed Application: Battery operated vehicles Regenerated power stored in battery V S D RaRa LaLa EaEa VaVa IaIa Regenerative Braking Provides positive output voltage and negative average output current Average power flows from load (motor) to source Switch (S) operated periodically with period T

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DC – DC Converter Fed Drives - Step Up Class B Chopper S is ON (0 t t on ) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives10 Regenerative Braking Energy Storage Interval ( i a ) V a = 0 (diode blocks V) i a increases due to E (since E > V a ) Mechanical energy converted to electrical (i.e. generator) Energy stored in L a Any remaining energy dissipated in R a and S V S D RaRa LaLa EaEa VaVa IaIa RaRa LaLa EaEa VaVa IaIa S

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DC – DC Converter Fed Drives - Step Up Class B Chopper S if OFF (t on t T) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives11 Regenerative Braking Duty Interval ( i a ) i a flows through diode D and source V i a decreases in negative direction Energy stored in L a & energy supplied by machine are fed to the source V S D RaRa LaLa EaEa VaVa IaIa RaRa LaLa EaEa VaVa IaIa V

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DC – DC Converter Fed Drives - Step Up Class B Chopper Regenerative Braking Duty cycle Under steady-state conditions Generator side: Chopper side, average armature voltage: Therefore, Hence, average armature current: Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives12 T Duty Interval ( i a ) Energy Storage Interval ( i a ) Negative because current flows from motor to source

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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) Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives13 D2 +Va-+Va- T1 D1 T2 D2 +V-+V- T Q1 Q2 Q3Q4 No Speed Reversal V a always +ve always +ve I a can be +ve or –ve Do not fire both switches together short circuit at supply

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DC – DC Converter Fed Drives - Two-quadrant Control Forward motoring Q1 - T1 and D2 (Class A) T1 conducting: V a = V (i a ) D2 conducting: V a = 0 (i a ) Dr. Ungku Anisa, July T1 T2 D1 +Va-+Va- D2 iaia +V+V T1 T2 D1 +Va-+Va- D2 iaia Average V a EaEa +V+V Average V a = 1 V, 1 = (t on T1 / T ), 2 = 0 EEEB443 - Control & Drives T2 always OFF T1 chopping ON & OFF Average V a positive Average V a made larger than back emf E a I a positive

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DC – DC Converter Fed Drives - Two-quadrant Control Forward braking Q2 – T2 and D1 (Class B) D1 conducting: V a = V (i a ) T2 conducting: V a = 0 (i a ) T1 T2 D1 +Va-+Va- D2 iaia T1 T2 D1 +Va-+Va- D2 iaia Average V a EaEa Average V a positive Average V a made smaller than back emf E a I a negative (motor acts as generator) +V+V +V+V Average V a =(1 - 2 )V, 1 = 0, 2 = (t on T2 / T ) 15 Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives T1 always OFF T2 chopping ON & OFF

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DC – DC Converter Fed Drives - Two-quadrant Control For fast transition from 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 ≤ t on If I a is positive (V a > E), current flows from supply to motor via T1 If I a is negative (E > V a ), current flows from motor to supply via D1 T1 is OFF and T2 is ON between t on < t ≤ T If I a is positive, current circulates via D2 If I a is negative, current circulates via T2 Duty ratio is given by: Average armature voltage is: Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives16 Average V a = V

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DC – DC Converter Fed Drives - Two-quadrant Control: Example Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives17

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DC – DC Converter Fed Drives - Four-quadrant Control Operation in all four quadrants V a and I a can be controlled in magnitude and polarity Power flow can be in either direction Speed and torque can be reversed + V a - T1 D1 T2 D2 D3 D4 T3 T4 T Q1 Q2 Q3Q4 iaia Note: Polarity of V a and direction of I a indicated are assumed positive. Dr. Ungku Anisa, July EEEB443 - Control & Drives

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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 i a 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 + V a - T1 D1 T2 D2 D3 D4 T3 T4 + V dc - Dr. Ungku Anisa, July EEEB443 - Control & Drives Leg A Leg B iaia

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DC – DC Converter Fed Drives - Four-quadrant Control Positive Current (I a > 0) V a = V dc when T1 and T2 are ON Current increases Q1 operation V a = 0 when current freewheels through T2 and D4 Current decreases V a = -V dc when D3 and D4 conducts current Current decreases Energy returned to supply Q4 operation + V a - T1 D1 T2 D2 D3 D4 T3 T4 T Q1 Q2 Q3Q4 + V dc - Dr. Ungku Anisa, July EEEB443 - Control & Drives iaia T3 and T4 off

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DC – DC Converter Fed Drives - Four-quadrant Control Negative Current (I a > 0) V a = -V dc when T3 and T4 are ON Current increases in negative direction Q3 operation V a = 0 when current freewheels through T4 and D2 Current decreases V a = V dc when D1 and D2 conducts current Current decreases Energy returned to supply Q2 operation T Q1 Q2 Q3Q4 + V dc - Dr. Ungku Anisa, July EEEB443 - Control & Drives iaia + V a - T1 D1 T2 D2 D3 D4 T3 T4 T1 and T2 off

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DC – DC Converter Fed Drives - Four-quadrant Control For both positive and negative current, output voltage can swing between: V dc and -V dc V dc and 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 V dc and -V dc or V dc and 0. Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives22

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DC – DC Converter Fed Drives Operation of DC motor drive depends on: Direction of I a (determined by torque, i.e. motoring or braking) Polarity of V a and E a (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 Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives23

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References Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3 rd ed., Pearson, New-Jersey, Dubey, G.K., Fundamentals of Electric Drives, 2 nd ed., Alpha Science Int. Ltd., UK, Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control, Prentice-Hall, New Jersey, Nik Idris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, Dr. Ungku Anisa, July EEEB443 - Control & Drives

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