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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 Speed Control of DC Motors By Dr. UngkuAnisaUngkuAmirulddin Department of Electrical Power Engineering College of Engineering EEEB283 – Electrical Machines & Drives
DC Drives Outline • Introduction to DC Drives • Separately Excited DC Motor • Speed Control Methods • Speed Control Strategy • Operating Modes • References EEEB283 – Electrical Machines & Drives
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
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
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
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
Separately Excited DC Motor – Steady State Condition • Time derivatives = 0. Therefore, • (1) • (2) • (3) • (4) • The developed power (5) EEEB283 – Electrical Machines & Drives
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
TL Va↓ Te Speed Control Methods – Va control Requires variable DC supply EEEB283 – Electrical Machines & Drives
Ra↑ Speed Control Methods – Ra control TL Simple control Losses in external resistor Rarely used. Te EEEB283 – Electrical Machines & Drives
If ↓ Speed Control Methods – control TL Not possible for PM motor Normally employed for speed above base speed Te EEEB283 – Electrical Machines & Drives
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 • > basespeed control by flux weakening (, i.e. If ↓) T base Vacontrol control EEEB283 – Electrical Machines & Drives
Va Ia 1.0 If, Te, Speed Control Strategy for Separately Excited DC Motor • = 0 to base speed control by Va • > basespeed 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
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
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
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
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
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
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
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