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ENERGY CONVERSION ONE (Course 25741)PowerPoint Presentation

ENERGY CONVERSION ONE (Course 25741)

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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

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