DC Motor Starter

Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing external resistance to the motor, which is connected in series to the motor’s armature winding and restricts the current to an acceptable level. Then the removal of this resistance is done in steps as the motor accelerates.

It is very important and desirable to provide the starter with protective devices to enable the starter arm to return to the OFF position for the following conditions.

1. when the supply fails, thus preventing the armature from being directly across the mains when this voltage is restored. For this purpose, we use a no-volt release coil.
2. when the motor becomes overloaded or develops a fault causing the motor to take an excessive current. For this purpose, we use an overload-release coil.

Types of Starter

1. Two-point starter
2. Three-point starter
3. Four-point starter

• Three-point starter:

1. In a three-point starter, the no-volt release coil is connected in series with the
2. Shunt field circuit so that it carries the shunt field current.
3. While exercising speed control through the field regulator, the field current may be weakened to such an extent that the no-volt release coil may not be able to keep the starter arm in the ON position.
4. This may disconnect the motor from the supply when it is not desired. This drawback is overcome in the four-point starter.

• Four-point starter:

1.  In a four-point starter, the no-volt release coil is connected directly across the supply line through a protective resistance R.
2. Now the no-volt release coil circuit is independent of the shunt field circuit. Therefore, proper speed control can be exercised without affecting the operation of the no-volt release coil
3. Note that the only difference between a three-point starter and a four-point starter is the method in which a no-volt release coil is connected. However, the working of the two starters is the same.
4. It may be noted that the three-point starter also provides protection against an open-field Circuit. This protection is not provided by the four-point starter.

 

 

• Two-point starter:

1. This starter is only for D.C. series motors. The basic construction of a two-point starter is similar to that of a three-point starter except for the fact that it has only two terminals namely line (L) and field (F).
2. The F terminal is one end of the series combination of field and the armature winding. The action of the starter is similar to that of a three-phase starter.
3. The main problem in the case of d.c. the series motor is its speeding action when the load is less.
4. This can be prevented using two-point starters. The no-volt coil is connected in series with the motor so both currents are equal.
5. In a no-load situation load current drawn by the motor decreases causing the no-volt coil losses its required magnetism and release the handle to the OFF position.

Reference:
[1] https://www.scribd.com/doc/38037500/d-c-Motor-Starter
[2] https://www.javatpoint.com/starting-of-dc-motors