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EEEB283 Electrical Machines & Drives

EEEB283 Electrical Machines & Drives. Speed Control of DC Motors By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering. DC Drives Outline. Introduction to DC Drives Separately Excited DC Motor Speed Control Methods Speed Control Strategy

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EEEB283 Electrical Machines & Drives

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  1. EEEB283 Electrical Machines & Drives Speed Control of DC Motors By Dr. UngkuAnisaUngkuAmirulddin Department of Electrical Power Engineering College of Engineering EEEB283 – Electrical Machines & Drives

  2. DC Drives Outline • Introduction to DC Drives • Separately Excited DC Motor • Speed Control Methods • Speed Control Strategy • Operating Modes • References EEEB283 – Electrical Machines & Drives

  3. Introduction • DC Drives – Electric drives employing DC motors as prime movers • Dominated variable speed applications before introduction of Power Electronic converters • Still popular even after Power Electronics • Advantage: Precise torque and speed control without sophisticated electronics • Applications: Rolling mills, hoists, traction, cranes EEEB283 – Electrical Machines & Drives

  4. Introduction • Some limitations: • High maintenance (commutators & brushes) • Expensive • Speed limitations • Sparking • Commonly used DC motors • Separately excited • Series (mostly for traction applications) EEEB283 – Electrical Machines & Drives

  5. Ra Lf Rf La ia + ea _ + vt _ if + vf _ Electromagnetic torque Armature back e.m.f. Separately Excited DC Motor Kt = torque constant Kv = voltage constant (V/A-rad/s) Kv = Kt EEEB283 – Electrical Machines & Drives

  6. Separately Excited DC Motor • Motor is connected to a load. • Therefore, where TL= load torque J = load inertia (kg/m2) B = viscous friction coefficient (Nm/rad/s) EEEB283 – Electrical Machines & Drives

  7. Separately Excited DC Motor – Steady State Condition • Time derivatives = 0. Therefore, • (1) • (2) • (3) • (4) • The developed power (5) EEEB283 – Electrical Machines & Drives

  8. Speed Control Methods for Separately Excited DC Motor • From equation (3), • Three possible methods for speed control: • Armature voltage Va • Armature resistance Ra • Field current If (by changing field resistance Rf)  flux   Te EEEB283 – Electrical Machines & Drives

  9. TL Va↓ Te Speed Control Methods – Va control Requires variable DC supply EEEB283 – Electrical Machines & Drives

  10. Ra↑ Speed Control Methods – Ra control  TL Simple control Losses in external resistor  Rarely used. Te EEEB283 – Electrical Machines & Drives

  11. If ↓ Speed Control Methods –  control  TL Not possible for PM motor Normally employed for speed above base speed Te EEEB283 – Electrical Machines & Drives

  12. Speed Control Strategy for Separately Excited DC Motor • Base speed base = Speed at rated Va, If and Ia •  = 0 to base speed control by Va •  > basespeed control by flux weakening (, i.e. If ↓) T base Vacontrol  control EEEB283 – Electrical Machines & Drives

  13. Va Ia 1.0 If, Te,   Speed Control Strategy for Separately Excited DC Motor •  = 0 to base speed control by Va •  > basespeed control by flux weakening () • T  Ia For maximum torque capability, Ia= Ia max • Pd = EaIa = (KvIf)Ia = constant when  > base • in order to go beyond base,  If  (1/) Per unit quantities base Vacontrol  control EEEB283 – Electrical Machines & Drives

  14. Va Ia 1.0 P, T P If, Te,  Te  Speed Control Strategy Per unit quantities • Torque and power relations below and beyond base Vacontrol  control P =Te Te = KvIf Ia base constant torque constant power EEEB283 – Electrical Machines & Drives

  15. Operating Modes Motoring • Back EMF Ea < Va • Ia and If are positive • Motor develops torque to meet load demand (i.e. Te =TL) EEEB283 – Electrical Machines & Drives

  16. Operating Modes Regenerative Breaking • Motor acts as generator • Develops Ea > Va • Ia negative (flows back to source) • If positive • Machine slows down until Ea = Va • Used only when there are enough loads to absorb regenerated power EEEB283 – Electrical Machines & Drives

  17. Operating Modes Dynamic Breaking • Similar to regenerative breaking • But Va removed, replaced by Rb • Kinetic energy of motor is dissipated in Rb (i.e. machine works as generator) EEEB283 – Electrical Machines & Drives

  18. Operating Modes Plugging • Supply voltage Va is reversed • Va assists Eain forcing Ia in reverse direction • Rb connected in series to limit current EEEB283 – Electrical Machines & Drives

  19. Operating Modes - Four Quadrant Operation Q2 +Va , +Ea +  -Ia -T Power = -ve Q1 +Va , +Ea +  +Ia +T Power = +ve Q3 -Va , -Ea -  -Ia -T Power = +ve Q4 -Va , -Ea -  +Ia +T Power = -ve EEEB283 – Electrical Machines & Drives

  20. 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. • 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. EEEB283 – Electrical Machines & Drives

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