Starters

Why starter is needed?  

Basically for a dc motor,

Current = (Applied Voltage - Back EMF) / (Armature resistance)

The back emf is directly proportional to speed and is ZERO at start when the speed is zero.

Hence for a 110 volt dc motor having typical armature resistance 1 ohm,

Starting current = 110/1 = 110 A

The rated current will normally only be around 20 A and hence the starting current is extremely large. This may damage the armature winding.

Hence starters are used to limit the starting current by providing high resistance at start and gradually removing them off as the speed increases.

DC motors are always self starting and they dont need external starting aid. So it's wrong to say dat a starter is used to start a motor

3 POINT STARTER

Generally 3 point starter is used to start shunt type of motor and compound type of motor in industries.

The reason for using starter is the during the starting of motor speed is zero, but the initial Armature resistance and starting resistance are very small so the motor draws high current initially at the time of starting is the voltage equal to rating of motor is applied at the time of starting. it could be around approximately 15 times than current then the current when the motor is working at full load. So this large amount of initial current may damage the other equipments.

The basic principle of starter is to resist the huge amount of the current to flow through the motor.

1)The point L is connected to armature terminal always connected to +ve DC supply and field terminal is connected to -ve DC supply.
2)Point F is connected to field winding and,
3)Point A is connect armature winding of motor through the resistance no. 10  i.e; the point at which motor is ON.

Initially at the time of starting the motor is kept off. The arm of starter is at maximum resistance so no current flows through it. When the arm is moved towards stud no. 1, the field circuit is directly connected to line and full starting resistance is applied in series with armature.the starting current drawn by armature is given by

Ia = E/(Ra+Rst).As the arm is moved towards stud no. 2,3,4,....the starting resistance goes on decreasing. When the arm reaches its maximum position i.e; at stud no. 10,  the resistance becomes nil, then the motor starts working.This is because back emf is developed with the speed to counter supply voltage and slow down armature current.

So question arises whether the handle remains at ON position or returns to OFF position?
There is soft iron piece on the handle.when the field current flows on No voltage coil its acts as electromagnet and holds the iron piece attached to the handle towards it. When the supply fails the electromagnetic properties of NVC disappears and  the handle is returned towards its OFF position due to strong spring connected to handle as shown in fig. When the supply is given the above process is repeated.

What happens when overloading of starter takes place?

This is the point when the Overload Coil comes into picture. When predetermined limit of current is crossed, then the current flowing through Overload coil will produce attractive force which will lift upwards the triangular iron piece. So now the No voltage Coil is shorted and voltage across it reduces to zero and handle returns to OFF position.

Drawback of 3 point started    

1)The NVC and field winding are connected in series. so current in NVC will be low if we reduce field current to apply flux control.
2)If current in below certain limit, the attractive force produced by NVC may not be able to hold in RUN position, thus the handle will return to OFF position and motor will be turned OFF.
3)It is unsuitable for use with variable speed motors. To overcome this 4 Point starter is used.

*4 Point starter will be posted soon

                                   

In Fig. is shown a, four-point starting rheostat, to which four connections are required. This type of rheostat is required by adjustable-speed motors in which the shunt-field current may be so greatly reduced at high speeds that a series holding coil would release the starter handle. In the four-point starter the holding-coil current is independent of motor operating conditions. The four-point starter is suitable for starting a series motor, whereas the three-point starter obviously is not.

In both the three-point starter and the four-point starter, the handle is returned to the "off" position by a spring if the line voltage becomes too low.

Some starters or controllers, like that illustrated in Fig., are intended for adjustable-speed duty and are designed so generously that the starting handle may be left on any control point indefinitely without causing overheating. Such controllers cost more than simple starters and, when such equipment is purchased, it is advisable to specify the type of duty - whether simple starting or combined starting and speed control. By permitting some of the starting resistance to remain in the circuit, it is possible to obtain motor speeds lower than those which may be had by shunt-field control. The efficiency of such control is poor because of the loss of power in the rheostat. Also, the speed regulation of a motor controlled by series resistance in the armature circuit is high (great), and this type of control is thus rendered unsuitable for many purposes.