EEEB283 Electrical Machines & Drives Speed Control of DC Motors 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

DC Drives Outline Introduction to DC Drives Separately Excited DC Motor Speed Control Methods Speed Control Strategy Operating Modes References Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Introduction Some limitations: Commonly used DC motors High maintenance (commutators & brushes) Expensive Speed limitations Sparking Commonly used DC motors Separately excited Series (mostly for traction applications) Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Separately Excited DC Motor Lf Rf if + ea _ La Ra ia vt vf Electromagnetic torque Kt = torque constant Kv = voltage constant (V/A-rad/s) Kv = Kt Armature back e.m.f. Dr. Ungku Anisa, July 2008 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) Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Separately Excited DC Motor – Steady State Condition Time derivatives = 0. Therefore, (1) (2) (3) (4) The developed power (5) Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Speed Control Methods for Separately Excited DC Motor  Te From equation (3), Three possible methods for speed control: Armature voltage Va Armature resistance Ra Field current If (by changing field resistance Rf)  flux  Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Speed Control Methods – Va control  TL Va↓ Requires variable DC supply Te Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Speed Control Methods – Ra control  TL Simple control Losses in external resistor  Rarely used. Ra ↑ Te Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Speed Control Methods –  control  If ↓ TL Not possible for PM motor Normally employed for speed above base speed Te Dr. Ungku Anisa, July 2008 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  > base  speed control by flux weakening (, i.e. If ↓) T base  Va control  control Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

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 Va Ia 1.0 If, Te,   Va control  control Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Speed Control Strategy  Per unit quantities Va 1.0 Ia If, Te,  Va control  control base Torque and power relations below and beyond base P, T P Te P =Te Te = KvIf Ia constant torque constant power Dr. Ungku Anisa, July 2008 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 ) Dr. Ungku Anisa, July 2008 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 Dr. Ungku Anisa, July 2008 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) Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Operating Modes Plugging Supply voltage Va is reversed Va assists Ea in forcing Ia in reverse direction Rb connected in series to limit current Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives

Operating Modes - Four Quadrant Operation +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 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. 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