ENERGY CONVERSION ONE (Course 25741) CHAPTER NINE ….continued DC MOTORS AND GENERATORS

DC MOTORS AND GENERATORS Summary 1. The Equivalent Circuit of a DC Motor 2. The Magnetization Curve of a DC Machine 3. Separately Excited and Shunt DC Motors - The Terminal Characteristics of a Shunt DC Motor - Nonlinear Analysis of a Shunt DC Motor - Speed Control of Shunt DC Motors - The Effect of an Open Field Circuit 4. The Permanent-Magnet DC Motor 5. The Series DC Motor - Induced Torque in a Series DC Motor - The Terminal Characteristic of a Series DC Motor - Speed Control of Series DC Motors. 6. DC Motor Starters - DC Motor Problems on Starting - DC Motor Starting Circuits 7. Introduction to DC generators 8. Separately Excited Generator - Terminal Characteristic of a separately Excited DC Generator - Control of Terminal Voltage - Nonlinear Analysis of a Separately Excited DC generator

DC MOTORS AND GENERATORS INTRODUCTION The same physical dc machine can operate as either motor or a generator & it depends on direction of power flow Introduction to DC motors: dc motors have a significant fraction of machinery purchased each year through 1960s Reasons: existence of dc power system in cars, trucks and aircraft Another application: when wide variations in speed are needed Before widespread use of power electronic rectifier-inverters, dc motors were dominant means of speed control Even without a dc power source, solid-state rectifier & chopper circuits used to create necessary dc power & dc motors used to provide speed control Today induction motors with solid-state drive packages are preferred choice over dc motors for most speed control applications, while still in some applications dc motors preferred

DC MOTORS AND GENERATORS INTRODUCTION DC motors are compared by their speed regulation: SR= [ωnl-ωfl]/ωfl x 100% It is a rough measure of shape of motor’s torque-speed characteristic A positive regulation means speed drops with increasing load & a negative speed regulation means speed increases with increasing load Magnitude of S.R. approximately show how steep is the slope of torque-speed Dc motors driven from a dc power supply (unless specified) and input voltage assumed constant) Five major types of dc motor: 1- separately excited dc motor 2-shunt dc motor 3-permnent-magnet dc motor 4- series dc motor 5-compounded dc motor

DC MOTOR EQUIVALENT CIRCUIT Figure below shows a dc motor equivalent cct. Armature cct. represented by an ideal voltage source EA & a resistor RA This is thevenin equivalent of entire rotor, including coils, interpoles & compensating windings Brush voltage drop represented by a small battery Vbrush opposing direction of current flow

DC MOTOR EQUIVALENT CIRCUIT A simplified equivalent circuit eliminating the brush voltage drop and combining Radj with the field resistance shown in (b) Some of the few variations and simplifications: 1- brush drop voltage is often only a very tiny fraction of generated voltage in the machine. where it is not too critical, brush drop voltage may be left out or included in the RA. 2- internal resistance of field coils is sometimes lumped together with variable resistor and total is called RF 3- Some generators have more than one field coil, all of which appear on the equivalent circuit

DC MOTOR EQUIVALENT CIRCUIT The internal generated voltage is given by: EA = K φω and the torque induced is ind = K φ IA The Magnetization Curve of a DC Machine - EA is directly proportional to flux and the speed of rotation of the machine EA is therefore related to the field current field current in a dc machine produces a field mmf given by mmf=NFIF

DC MOTOR EQUIVALENT CIRCUIT mmf produces a flux in the machine in accordance with its magnetization curve

DC MOTOR:EQUIVALENT CCT Since If is proportional to mmf & since EA is proportional to flux, magnetization curve can represented as a plot of EA vs field current for a given speed ω0

SEPARATELY EXCITED AND SHUNT DC MOTORS Equivalent cct. of separately excited dc motor shown below

SEPARATELY EXCITED AND SHUNT DC MOTORS separately excited dc motor is a motor whose field cct. is supplied by another constant-voltage supply shunt dc motor is a motor whose field circuit gets its power directly from armature terminals of motor When supply voltage to a motor assumed constant, there is no practical difference in behavior between these two machines Kirchhoff’s voltage law KVL equation for armature cct. of these motors is: VT=EA+IARA

TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR Terminal characteristic of a motor is a plot of output torque versus speed If load on shaft of a shunt motor is increased, then load torque Tload exceed induced torque Tind & motor will start to slow down & Its internal generated voltage EA=Kφω decrease Then IA= (VT-EA)/ RA increases consequently Tind=KφIA increases & finally Tind will equal Tload at a lower mechanical speed

TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR O/P characteristic of shunt dc motor can be derived using Tind, EA equations & KVl Combing these three equations: VT=EA+IARA  VT=Kφω+IARA & IA = Tind /(Kφ)  VT=Kφω+ Tind /(Kφ) RA  ω = VT / (Kφ) - Tind/(Kφ)^2 RA This equation is a straight line with a negative slope

TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR Torque – speed characteristic of a shunt or separately excited dc motor

TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR Armature reaction affect the torque speed characteristic As shown in last slide, as load increase, flux weakening effect reduce the flux in shunt motor And according to speed equation, reduction in flux will increase speed If a motor has compensating winding, then there would be no flux weakening & flux remain constant