Starters

1. Expt.no:1
STUDY OF STARTERS
AIM
To study 3-point and 4-point starters for DC shunt motor.

STARTERS
Need for starters
When a motor is stationary, there is no back E.M.F. and the armature behaves as a
low resistance circuit. If the motor is switched on across the supply, it draws a
heavy current from the mains due to the armature resistance being very small [by the equation ].
This may result in
1. damage to the armature winding and insulation due to overheating.
2. detrimental sparking at commutator.
3. large dips in the supply voltage.
4. high starting torque and a very rapid acceleration with possibility of damage to the rotating
parts of the D.C. machine and thus load connected to the shaft.
To limit the starting current, a starter is used which reduces the voltage applied to the armature during the starting
period.
The Face plate box type starters used for starting shunt and compound motors of ordinary industrial capacity, are
of two kinds, known as three-point and four-point starters respectively.

Three-point starter

The three terminals of starter box are marked as L, F and A. The point L is connected to one line from the supply,
point F to the field terminal and point A to one of the armature terminals. The second terminal of the armature
and field are tied together and connected to the other end of the supply. The point L is also connected to the
starting arm through the over-load release (OLR).

2. When main switch is closed, the starting arm is slowly moved to the right. As soon as the arm makes contact with
the first stud, the field circuit is directly connected across the line and at the same time, full starting resistance
is placed in series with the armature. The starting current drawn by the armature = V/().
As the arm is further moved, the starting resistance is gradually cut out till the arm reaches the ON position, when
all the resistance is cut out. The arm moves over various studs against a strong spring force which tend to restore it
to OFF position. There is a soft iron piece S attached to the arm which is attracted and held by an electromagnet E
energized by the shunt field current. This is called ‘hold-on coil’. The supply to the field has to travel back through
all resistance studs cut by the arm, but as the value of starting resistance is very small as compared to the shunt
field resistance, so this slight decrease in is negligible. This defect can be remedied by using a brass arc
connected to the first stud; the field circuit is completed through this arc so that now the current does not have to
pass through the starting resistance.
The OLR is connected in series with the armature. It carries the full load current. When the machine is overloaded,
it draws a heavy current which flows through the coil of OLR and strengthen the electromagnet. This causes D to
be lifted, short-circuiting the terminals of the hold-on coil, thus demagnetizing it. Thus the arm gets released and
the motor gets disconnected from the supply.
Often motors are protected by thermal overload relays in which a bimetallic strip is heated by a motor current at
the same rate at which the motor itself is heating up. Above a certain temperature, this relay trips and opens the
line contractor thereby isolating the motor from the supply.
If it is desired to control the speed of the motor, then a field rheostat is connected in the field circuit. The motor
speed can be increased by weakening the flux (since N α 1/ø). One of the limitations of the three-point starter is
that if too much resistance is included in the field, then field current is reduced thus weakening the electromagnet.
Hence arm is pulled back to OFF position unnecessarily. Hence this type of starter is unsuitable for use with
variable speed motors.

Four-point starter

3. One of the important changes that have been made is that the hold-on coil has been taken out of the shunt field
circuit and has been connected directly across the line through a protecting resistance. When the arm touches the
first stud, then the line current divides into 3 parts, one part passing through starting resistance, second part
passing through the hold-on coil and current protecting resistance R. The current in the shunt field circuit does not
at all affect the current passing through the hold-on coil because two circuits are independent of each other. This
means that the electromagnetic pull exerted by the hold-on coil will always be sufficient and will prevent the
spring from restoring the starting arm to the OFF position, no matter how the field rheostat or field is adjusted.

RESULT
Familiarized with the three-point and four-point starters used for DC motors.