Lecture 15 Electro Mechanical System 1

Assignment 5

Page 95 : Problems 4-13, 4-14, 4-15 and 4.16

Due Date: Tuesday 8th March, 2011

Quiz No. Next WeekQuiz 3

Lecture 15 Electro Mechanical System 2

Circuit model development Armature circuit

Windings containing a set of identical coils and possessing a certain resistance, which can be modeled as a series resistance w.r.t. the terminals

Equivalent Circuit Model

Total armature resistance RO is measured between the terminals when the machine is at rest

Resistance is in series with the induced voltage, which is represented by a voltage source, EO

Field winding circuit Winding containing a set of identical coils in series Total field resistance Rf

Lecture 15 Electro Mechanical System 3

Consider the generator operating under constant speed and field excitation The exciting current is

controlled by a potentiometer The induced voltage EO is fixed

The voltage at the terminals E12

Equal to the induced voltage EO at no-load current condition, I = 0

Decreases as the load current increases E12 = EO – RO(I)

EO also decreases under load because of pole-tip saturation

Separately excited generator under load

Lecture 15 Electro Mechanical System 4

The terminal voltage of a self excited shunt generator falls off more sharply with increasing load than that of a separately excited generator Field current in a separately excited generator

remains constant under any load

Shunt Generator Under Load

The field current in a shunt generator is a function of the terminal voltage

Increased loading causes a drop in terminal voltage and consequently a drop in excitation current

For a self-excited shunt generator the voltage drop from no-load to full-load is 15% of the full-load voltage For separately excited generators, it is less than

10%

Lecture 15 Electro Mechanical System 5

The compound generator prevents the terminal voltage of a shunt generator from decreasing with increasing load A compound generator is similar

to a shunt generator except that it has additional field coils connected in series with the armature circuit

These series field coils are composed of a few turns of heavy gage wire for carrying the armature load current

The total resistance of the series coils is very small

Compound Generator

Lecture 15 Electro Mechanical System 6

At no-load, the current in the series coils is zero The shunt coils carry the

excitation current, IX to produce the field flux

As load increases Terminal voltage tends to drop,

but the load current IC now flows through the series field coils

The mmf developed by the series field coils acts in the same direction as the mmf of the shunt field coils

The flux increases under rising load

Over compound generator Terminal voltage increases

Compound Generator

Lecture 15 Electro Mechanical System 7

In a differential compound generator, the mmf of the series field acts opposite to the shunt field Under load, the terminal voltage falls drastically

with increasing load The series field circuit is reversed in polarity to

make a compound generator into a differential compound generator

Useful in welding applications Limits short-circuit currents

Differential Compound Generator

Lecture 15 Electro Mechanical System 8

loading characteristics of several generator types

Loading Characteristics

Lecture 15 Electro Mechanical System 9

Chapter 5:Chapter 5:Direct-Current Direct-Current

MotorsMotors

Lecture 15 Electro Mechanical System 10

DC motors transform electrical energy into mechanical energy DC motors are seldom used in ordinary industry but

are still found in many special applications Motors drive many types of loads from fans and

pumps to presses and conveyors Many loads have a definite torque-speed

characteristic Other loads have a highly variable torque-speed

characteristics Motors must be adapted to the type of loads driven

Motor types Shunt, series, and compound connections

Introduction

Lecture 15 Electro Mechanical System 11

DC motors are built the same way as generators Armature of a motor connected to a dc power supply Current flows through the armature winding Armature is within a magnetic field A force is exerted on the windings The force causes a torque on the shaft The shaft rotates

Motor Operation