DC motors have very small armature resistance around one to ten ohms, so when a DC motor is started, a very large current flows through its armature winding. This large amount of current is usually more than 20 times the full load current. Even a small duration of such a high current can easily damage armature winding, commutators, and carbon brushes.
Hence, there should be some means to prevent such a high current flow at the starting of a motor in order to prevent possible damage to the motor windings. Furthermore, this sudden high current flow can cause disturbances to other loads connected with the same system. This spike in current also produces spikes on brushes of motors causing their damage.
The solution of the above problems could be to add a high resistance in series with the armature winding during the starting of a motor to prevent very high current flow through the armature winding, and then that resistance will then be removed, once the motor has started running. For this purpose, the DC motor is been started with the help of starter circuits. There are multiple starter circuits available for different types of motors and different applications. One of those starter circuits is a 2 point starter used for series DC motors.
Uses of 2 Point Starter
2 point starter is usually used in series DC motors. In series DC motors, armature coil is been connected in series with the field winding. Therefore, at the starting of the motor, in absence of load, due to low resistance of armature winding, a large current will flow and the motor can gain very high speed up to the destruction level of the motor. Thus, Series motor should never be started under the no-load condition and load should always be there before starting the motor. The two-point starter is been shown above in figure 1. Its construction is quite similar to a potentiometer or rheostat having a tap to change the value of resistances. When DC motor is started, the starting resistance and the no-load release coil is connected in series with the armature winding in order to increase the starting resistance to avoid surge current due to low winding resistance. When power is been applied from mains, the start arm is moved from off to on position resulting in full resistance available in a two-point starter circuit to be automatically added in series with the armature winding. Latter after the starting of the motor, this series resistance gradually decreased and ultimately completely removed as motor gains speed and obtains its steady state. This is possible by the gradual movement of tap resulting in the decrease of resistance attached in series with the armature winding.
The no-load release coil in 2 point starter circuit is for the purpose of detecting the no-load condition and stopping the motor from running under the no-load condition to avoid the damage due to very high speed under no load. As the starting resistance and no-load release, the coil is been connected in series with the armature coil, so the current will first flow through the no-load release coil and then will reach the armature winding. As the load current is inversely proportional to the speed of the motor, so the no-load release coil detects the no-load condition by sensing the value of current, and de-energizes itself to break the flow of the current to the winding, by moving the start arm to an off state, in order to stop the motor, and avoid chances of any damage due to high speed. In normal speed conditions, no-load release coil holds the connection by staying energized and current flow to the armature winding and the motor keeps on running in normal state. As soon as the load decreases below a certain level, no load release coil de-energizes and releases the connection resulting in stopping of the motor under the no-load condition.
Conclusion of 2 point starter
So 2 point starter circuit provides protection against high starting torque due to low armature resistance and also provides protection for the no-load condition where the motor can damage itself by running at very high speed. As discussed previously, starting current is reduced by the introduction of high series resistance in series with the armature winding, which is gradually removed as motor picks the speed and reaches constant speed. While no-load protection is possible by the use of a no-load release coil, which releases the connection when it detects the no-load condition and holds the connection under normal load operation.