ENERGY CONVERSION ONE (Course 25741)

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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

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### 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