5.7 Necessity of D.C. Motor Starter
At starting, when the motor is stationary, there is no back e.m.f. in the armature.
Consequently, if the motor is directly switched on to the mains, the armature
will draw a heavy current (Ia = V/Ra) because of small armature resistance. As
an example, 5 H.P., 220 V shunt motor has a full-load current of 20 A and an
armature resistance of about 0.5 . If this motor is directly switched on to
supply, it would take an armature current of 220/0.5 = 440 A which is 22 times
the full-load current. This high starting current may result in:
(i) burning of armature due to excessive heating effect,
(ii) damaging the commutator and brushes due to heavy ...view middle of the document...
For this purpose, we use no-volt
(ii) when the motor becomes overloaded or develops a fault causing the
motor to take an excessive current. For this purpose, we use overload
There are two principal types of d.c. motor starters viz., three-point starter and
four-point starter. As we shall see, the two types of starters differ only in the
manner in which the no-volt release coil is connected.
5.9 Three-Point Starter
This type of starter is widely used for starting shunt and compound motors.
Fig. (5.16) shows the schematic diagram of a three-point starter for a shunt
motor with protective devices. It is so called because it has three terminals L, Z
and A. The starter consists of starting resistance divided into several sections
and connected in series with the armature. The tapping points of the starting
resistance are brought out to a number of studs. The three terminals L, Z and A
of the starter are connected respectively to the positive line terminal, shunt field
terminal and armature terminal. The other terminals of the armature and shunt
field windings are connected to the negative terminal of the supply. The no-volt
release coil is connected in the shunt field circuit. One end of the handle is
connected to the terminal L through the over-load release coil. The other end of
the handle moves against a spiral spring and makes contact with each stud
during starting operation, cutting out more and more starting resistance as it
passes over each stud in clockwise direction.
(i) To start with, the d.c. supply is switched on with handle in the OFF
(ii) The handle is now moved clockwise to the first stud. As soon as it comes in
contact with the first stud, the shunt field winding is directly connected
across the supply, while the whole starting resistance is inserted in series
with the armature circuit.
(iii) As the handle is gradually moved over to the final stud, the starting
resistance is cut out of the armature circuit in steps. The handle is now held
magnetically by the no-volt release coil which is energized by shunt field
(iv) If the supply...