EEEB2833 Electrical Machines & Drives DC Drives By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives Dr. Ungku Anisa, July 2008

Outline Power Electronics Converters for DC Drives Controlled Rectifier Fed DC Drives Single Phase Three Phase DC – DC Converter Fed Drives Step Down Class A Chopper Step Up Class B Chopper Two-quadrant Control Four-quadrant Control Closed-loop Control (Brief overview) References Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Power Electronic Converters for DC Drives To obtain variable voltage Efficient Ideally lossless Depending on voltage source: AC voltage source  Controlled Rectifiers Fixed DC voltage source  DC-DC converters (switch mode converters) Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed DC Drives To obtain variable DC voltage from fixed AC source DC current flows in only 1 direction Example of a drive system Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed DC Drives Contains low frequency AC ripple To reduce ripple: extra inductance added in series with La Slow response Discontinuous current may occur if La not large enough Motor is lightly loaded Half-wave rectifier is used Effect of discontinuous current Rectifier output voltage increases  motor speed increases (poor speed regulation under open-loop operation) Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Single-phase DC Drives Q1 Q2 Q3 Q4  Two-quadrant drive Limited to applications up to 15 kW During regeneration, Ea can be reversed by reversing field excitation Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Single-phase DC Drives supply + Va  ia For continuous current: Armature voltage where Vm = peak voltage Armature current Field voltage 90o 180o  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Single-phase DC Drives Q1 Q2 Q3 Q4  Four-quadrant drive Converter 1 for operation in 1st and 4th quadrant Converter 2 for operation in 2nd and 3rd quadrant Limited to applications up to 15 kW Single-phase supply + Va  ia Converter 1 Converter 2 Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Single-phase DC Drives For continuous current: Armature voltage: If Converter 1 operates If Converter 2 operates where Vm = peak voltage Armature current Field voltage + Va  Converter 1 Converter 2 ia Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Three-phase DC Drives Q1 Q2 Q3 Q4  Two-quadrant drive Limited to applications up to 1500 kW During regeneration, Ea can be reversed by reversing field excitation Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Three-phase DC Drives supply + Va  ia For continuous current: Armature voltage where VL-L, m = peak line-to-line voltage Armature current Field voltage (assuming a three-phase supply is used for field excitation) 90o 180o  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Three-phase DC Drives Q1 Q2 Q3 Q4  Four-quadrant drive Converter 1 for operation in 1st and 4th quadrant Converter 2 for operation in 2nd and 3rd quadrant 3-phase supply + Va  ia Converter 1 Converter 2 Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Three-phase DC Drives Disadvantage: Circulating current Slow response For continuous current: Armature voltage: If Converter 1 operates If Converter 2 operates where VL-L, m = peak line-to-line voltage Armature current Field voltage + Va  Converter 1 Converter 2 ia Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Controlled Rectifier Fed – Three-phase DC Drives Q1 Q2 Q3 Q4  Four-quadrant drive One controlled rectifier with 2 pairs of contactors M1 and M2 closed for operation in 1st and 4th quadrant R1 and R2 closed for operation in 2nd and 3rd quadrant M1 M2 R1 R2 + Va - 3-phase supply Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & 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 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Q1 Q2 Q3 Q4  Motoring Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Motoring S is ON (0  t  ton) Duty Interval - ia  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Down Class A Chopper Motoring S if OFF (ton  t  T) Freewheeling Interval - ia  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed - Step Down Class A Chopper Motoring Duty cycle Under steady-state conditions Motor side: Chopper side: Hence, Duty Interval - ia  Freewheeling Interval - ia  average Va average Ia kT Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Up Class B Chopper Q1 Q2 Q3 Q4  Regenerative Braking Possible for speed above rated speed and down to nearly zero speed Application: Battery operated vehicles Regenerated power stored in battery Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Up Class B Chopper Regenerative Braking S is ON (0  t  ton) Va = 0 ia increases due to E Mechanical energy converted to electrical (i.e. generator) Energy stored in La Energy Storage Interval - ia  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

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 Energy stored in La & energy supplied by machine are fed to the source Duty Interval - ia  Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Step Up Class B Chopper Regenerative Braking Duty cycle Under steady-state conditions Generator side: Chopper side: Hence, Energy Storage Interval - ia  Duty Interval - ia  average Va average Ia  T Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Two-quadrant Control  Forward motoring Q1 - T1 and D2 Forward braking Q2 – T2 and D1 D2 + Va - T1 D1 T2 V No Speed Reversal Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Two-quadrant Control  Average Va positive Average Va made larger than back emf Eb Ia positive Forward motoring Q1 T1 conducting: Va = V T1 + V  D1 D2 conducting: Va = 0 ia + Va - D2 T2 T1 + V  D1 ia + Va - D2 T2 Va Eb Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Two-quadrant Control  Average Va positive Average Va made smaller than back emf Eb Ia negative Forward braking Q2 D1 conducting: Va = V T1 + V  D1 T2 conducting: Va = 0 ia + Va - D2 T2 T1 + Vdc  D1 ia + Va - D2 T2 Eb Va Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Four-quadrant Control  Operation in all quadrants Speed can be reversed + Va - T1 D1 T2 D2 D3 D4 T3 T4 Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Four-quadrant Control  T3 and T4 off Forward Motoring Q1 T1 and T2 on Va = V Ia increases Reverse Braking Q4 (Regeneration) T1 off but T2 still on Va = 0 Ia decays thru T2 and D4 T1 and T2 off Va = -V Ia decays thru D3 and D4 Energy returned to supply + Va - T1 D1 T2 D2 D3 D4 T3 T4 + V - Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

DC – DC Converter Fed Drives - Four-quadrant Control  T1 and T2 off Reverse Motoring Q3 T3 and T4 on Va = -V Ia increases in reverse direction Forward Braking Q2 (Regeneration) T3 off but T4 still on Va = 0 Ia decays thru T4 and D2 T3 and T4 off Va = V Ia decays thru D1 and D2 Energy returned to supply + Va - T1 D1 T2 D2 D3 D4 T3 T4 + V - Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Closed-loop Control Feedback loops may be provided to satisfy one or more of the following: Protection Enhancement of speed response Improve steady-state accuracy Variables to be controlled in drives: Torque – achieved by controlling current Speed Position Controllers are designed based on a linear averaged model Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Closed-loop Control Variables to be controlled in drives: Torque – achieved by controlling current Commonly employed current sensor: Current shunt – no electrical isolation, cheap Hall effect sensor – provides electrical isolation Speed is governed by torque: e.g. With phase-controlled rectifier firing circuit current controller controlled rectifier  + Va – vc iref - Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Closed-loop Control Variables to be controlled in drives: Speed – with or without current loop Commonly employed speed/position sensor: Tachogenerator – analog based Digital encoder – digital based, converts speed to pulses Torque is governed by speed demand: Without current loop: no limit on current – can be too high With current loop: current can be limited Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Power Electronic Converters Closed-loop Control Variables to be controlled in drives: Speed control without current loop: Simple implementation Current can be too high  may damage converter Speed controller Power Electronic Converters * + - va   vc Tacho Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

Power Electronic Converters Closed-loop Control Variables to be controlled in drives: Speed control with current loop: Two controllers required: speed and current Current limited by limiting ia* Speed controller Power Electronic Converters * + - va   vc Tacho Current ia* ia Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & 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 EEEB283 - Electrical Machines & Drives