Page 604 - Basic Electrical Engineering

7

DC Machines

TOPICS DISCUSSED

Working of a dc machine as a generator and as a motor
Basic principle of dc machines
Constructional details
Need of brush and commutator
Types of windings
EMF equation
Types of dc machines and their characteristics
Starting of a dc motor
Speed control
Losses and efficiency

7.1 INTRODUCTION AND PRINCIPLE OF WORKING

DC machines work either as a dc generator or as a dc motor. In a dc
generator, a set of conductors or coils placed on a rotating body, called

armature, are rotated continuously inside a magnetic field with the help of a
prime mover (prime mover is another machine, may be a diesel engine or a

turbine, which rotates the armature). The magnetic field is created by passing
dc current through the windings of a set of field magnets. When conductors
pass under alternate North and South poles, alternating EMF is induced in the

armature winding. The ac generated gets converted into dc when the voltage
is collected from the rotating armature through the brush and commutator

arrangement. The brush and commutator arrangement, therefore, works like a