Starters for DC Motors

At starting the motor takes large amount of current which is nearly 25 x full load current. This large amount of current can not be allowed to flow in a motor even for a short period of time. This exclusive starting current has to be prevented because

• It cause sudden depression of voltage of supply(large voltage drop occurs) system causing disturbances to other loads connected in the system.
• It would cause heavy sparking at the brushes which may destroy the commutator and brush gear.
• Due to heavy inrush of current at start there is possibility of damage of the motor winding.

Preventing large starting current

A resistance is introduced in series with the armature for very start duration of starting period only, which limits the starting current to a very safe value. This starting resistance is gradually cut out as the motor gains speed and develops the back emf which then regulates the speed of the motor.

Types of starters for DC motor

• Two point Starter
• Three point Starter
• Four point Starter

Two point starter

Two point starter is used in series motors because in case of series motor, the armature winding and field winding are connected in series. Therefore series motor achieves dangerous high speed. So series motor should not be started without any load. Two point starter construction is very much similar to the combination of rheostat with a tap changing . The starting resistance and no load release coil is connected in series with the armature of a series motor. When a series motor is given a supply, the handle is moved from OFF position stud no.1 ie) full resistance is given at starting. Therefore inrush of high starting current to the series motor is reduced. Then starting resistance is gradually cut down and the motor gathers speed, which will then develop back emf.

Purpose of no load release coil

The load current first flow through the no load release coil as they are connected in series with the armature of the series motor. As speed of the dc series motor is inversely proportional to the load current, the motor should not be allowed to start at no load. If the dc series motor is stared with no load , then no load release coil will detect it and it de-energized itself to release the handle to OFF position

Three point starter

Three point starter is used in shunt wound DC motor or compound wound DC motor. To start the motor, the starter arm is moved to stud 1 in the clockwise direction ie) whole of the resistance is added to the armature winding of the dc motor. The field winding will also get full supply through the coil of the No-Volt-Release (NVR). The motor will develop a torque and start rotating with full starting resistance in the armature winding. Once the dc motor starts rotating, the starter arm is moved in clockwise direction and will be brought to the RUN position ie) zero resistance to the armature winding. In RUN position, the soft iron piece fixed on the starter arm will face the NVR magnet piece and remain attracted. The starter arm, therefore, will stay in the RUN position against the spring tension, and the engineer can remove his/her hand from the arm. In case of supply failure, the NVR electromagnet will get demagnetized, and the starter arm will automatically return to OFF position due to the spring pressure in the starter arm. In case of dc motor is over loaded, the armature winding will draw excessive current and there is possibility of damage of armature winding.. Due to this reason, the Over-Load-Release (OLR) coil will short circuited thereby demagnetizing the NVR electromagnet. Thus, the starter arm will eventually return to OFF position stopping the DC motor.

Purpose of three point starter

Three point starter is more preferable for armature speed control of DC motor (below rated speed).

Disadvantages of three point starter

When a large value of resistance is connected in the field circuit to increase the speed of a Dc motor, the field current get reduced. Since the field winding followed by NVR are connected in series, the current flowing through the NVR coil will also get reduced. This reduced current in NVR coil may not be sufficient to hold the starter arm at some point. ie) Under normal operation of a dc motor, when current flowing through the NVR coil get reduced, then starter will move to OFF position. The effect of this will be that the motor will stop, which is undesirable.

Four point starter

Disadvantage in three point starter is overcomed in four point starter, Here the NVR coil is connected independently across the supply voltage instead of connecting it in series with the motor field winding. In a four point starter there will be three parallel circuits connected across the supply voltage . When the starter arm is brought to stud 1 position, current will flow through the armature winding through the starter resistance. This will reduce the high inrush of starting current as a whole resistance is added to the armature winding. At the same time, the field winding will also get full supply through the brass arc. The speed of a dc motor is get control, by varying a field winding current with the variable resistance R1 As in four point starter, NVR coil and field winding are isolated from each of it. Thus any change in field winding current ( I_f ) will not affect NVR coil unlike three point starter. When started arm is brought to the RUN position, the armature of the dc motor will run at its full speed and remain connected to the supply through the starter arm. Thus the field winding and NVR coil will get full voltage independently. Now its possible for an engineer to release his/her hand from the starter as NVR coil is supplied independently, which means the spring tension cannot bring back the starter arm to OFF position because of the attractive force of the NVR. In case of overload, the OLR terminal will be short circuited to the NVR coil to release the starter arm to the OFF position.

Purpose of four point starter

Four point starter is more preferable for field speed control of dc motor (above rated speed).