Page 625 - Basic Electrical Engineering

7.6 APPLICATIONS OF DC GENERATORS

Shunt generators are used in applications, such as for battery charging, dc
excitation in ac generators, lighting applications etc. A series generator’s load

characteristic is a rising one. That is, as the load increases, voltage increases
due to increase in field current. Such generators cannot be used for lighting

applications as voltage variation will effect illumination level.
However, dc series generators can be used to boost up voltage of an

existing system to compensate for the voltage drop in the system. Series
generators are also used as welding generators and in arc lamps. The

characteristics of dc compound generators can be modified by the use of
series winding either in circumlative or in differential mode. That is, either
the series field flux will be aiding the main field flux or opposing it. The

operation of a dc machine as a dc motor will now be dealt with.

7.7 OPERATION OF A DC MACHINE AS A MOTOR

Like dc generators, dc motors are also constructed to work as dc shunt, series,
and compound motors. In all such motors, electrical power input, i.e., V × I

gets converted into mechanical output. The mechanical output is in the form
of torque developed which enables the motor shaft to carry some mechanical
load on it. For example, a dc motor will rotate the wheels of an electric train

or a trolley bus. The constructional details of a dc motor is similar to that of a
dc generator except for certain minor changes in the cooling system.

7.7.1 Working Principle of a DC Motor

A dc motor works on the basic principle that when a current-carrying
conductor is placed in a magnetic field it experiences a force. In a dc motor,

the armature carries a number of conductors placed in slots and the armature
is placed inside the magnetic field created by field magnets as shown in Fig.

7.15. The field magnets are excited by field current passing through them.
The field windings may either be excited from a separate dc source or could
be excited from the source of supply provided to the armature. The armature

is fed from a dc supply source of V Volts as shown. The direction of current