Page 636 - Basic Electrical Engineering

through the brass arc. The variable resistance, R can be used to control the
1
field current to control the speed of the motor. Current will also flow through

the NVR coil as supply will come through the starter arm, the brass arc, and
following the path abcde as has been shown. Thus, change in field current, I f

will not have any effect on the current flowing through the NVR coil circuit.
When the starter arm is brought to the RUN position, the armature will attain

full speed and remain connected to the supply via the starter arm. The field
circuit and the NVR circuit will get full voltage independently. The NVR will

keep the starter arm in the RUN position even after the hand is released. The
spring tension cannot bring back the arm to OFF position because of the
attractive force of the NVR. In case of overload, the NVR terminals will be

short circuited due to the attractive force of the OLR and the starter arm will
get released

7.12 TYPES AND CHARACTERISTICS OF DC MOTORS

DC machines are available as shunt, series, and compound machines. In

motoring mode of operation, they are called dc shunt motors, dc series
motors, and dc compound motors. The relationship between three variables,
namely torque, speed, and load (load current) are studied to find the

suitability of each type of motor for different applications. For example, if a
mechanical load has to be rotated at a constant speed, we would need a motor

as a drive whose speed will remain constant at all loads, i.e., there should not
be any variation of its speed from no-load condition upto full-load condition.

Again, if a set of dc motors are to drive an electric train, the starting torque
developed by the motors should be very high. The motors have to develop

sufficient torque so as to start the train from rest condition with a large
number of passengers and other loads inside the train.
The characteristics of all types of motors are drawn as follows.

7.12.1 Characteristics of DC Shunt Motors

The basic equations of a dc motor are