ENERGY CONVERSION ONE (Course 25741)

ENERGY CONVERSION ONE(Course25741) 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 GENERATORSINTRODUCTION • 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 GENERATORSINTRODUCTION • 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 MOTOREQUIVALENT 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 MOTOREQUIVALENT CIRCUIT • A simplified equivalent circuit eliminating the brush voltage drop andcombining 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 MOTOREQUIVALENT CIRCUIT • The internal generated voltage is given by: EA = K φω • and the torque induced is ind = K φIA 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 MOTOREQUIVALENT 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 currentfor 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

ENERGY CONVERSION ONE (Course 25741)