EEEB283 Electrical Machines & Drives Chopper-Controlled DC Drives By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives Dr. Ungku Anisa, July 2008

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 2008 EEEB443 - Control & Drives

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 2008 EEEB443 - Control & Drives

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 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Q1 Q2 Q3 Q4  Motoring Provides positive output voltage and current Average power flows from source to load (motor) Switch (S) operated periodically with period T V S D Ra La Ea Va Ia Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Ra La Ea Va Ia Motoring S is ON (0  t  ton) Ra La Ea Va Ia V Va = V Ia flows to motor |Ia| increases Duty Interval ( ia  ) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Ra La Ea Va Ia Motoring S if OFF (ton  t  T) Ra La Ea Va Ia ID Va = 0 Ia freewheels through diode DF |Ia| decreases Freewheeling Interval ( ia  ) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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

DC – DC Converter Fed Drives - Step Up Class B Chopper Ra La Ea Va Ia 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 Ra La Ea Va Ia S Energy Storage Interval ( ia  ) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Step Up Class B Chopper Ra La Ea Va Ia Regenerative Braking S if OFF (ton  t  T) ia flows through diode D and source V ia decreases in negative direction Energy stored in La & energy supplied by machine are fed to the source Ra La Ea Va Ia V Duty Interval ( ia  ) Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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: Negative because current flows from motor to source Energy Storage Interval ( ia  ) Duty Interval ( ia  )  T Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Two-quadrant Control No Speed Reversal Combination of Class A & B choppers Forward motoring Q1 - T1 and D2 (Class A) Forward braking Q2 – T2 and D1 (Class B) T Q1 Q2 Q3 Q4  D2 + Va - T1 D1 T2 V Va always +ve   always +ve Ia can be +ve or –ve Do not fire both switches together  short circuit at supply Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 Average Va Ea T1 chopping ON & OFF T2 always OFF Average Va positive Average Va made larger than back emf Ea Ia positive EEEB443 - Control & Drives Dr. Ungku Anisa, July 2008

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 ) Ea Average Va T2 chopping ON & OFF Average Va positive Average Va made smaller than back emf Ea Ia negative (motor acts as generator) T1 always OFF Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 ≤ 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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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

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 T Q1 Q2 Q3 Q4  + Va - T1 D1 T2 D2 D3 D4 T3 T4 ia Note: Polarity of Va and direction of Ia indicated are assumed positive. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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 + Va - T1 D1 T2 D2 D3 D4 T3 T4 + Vdc - ia Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives Leg A

DC – DC Converter Fed Drives - Four-quadrant Control  Positive Current (Ia > 0) Va = Vdc when T1 and T2 are ON Current increases Q1 operation Va = 0 when current freewheels through T2 and D4 Current decreases Va = -Vdc when D3 and D4 conducts current Energy returned to supply Q4 operation + Va - T1 D1 T2 D2 D3 D4 T3 T4 + Vdc - ia T3 and T4 off Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

DC – DC Converter Fed Drives - Four-quadrant Control  Negative Current (Ia > 0) Va = -Vdc when T3 and T4 are ON Current increases in negative direction Q3 operation Va = 0 when current freewheels through T4 and D2 Current decreases Va = Vdc when D1 and D2 conducts current Energy returned to supply Q2 operation + Va - T1 D1 T2 D2 D3 D4 T3 T4 + Vdc - ia T1 and T2 off Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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

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 Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives

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. Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives