At starting when a rated voltage is applied across the stationary armature terminals of the dc motor, it draws more current comparatively greater than the rated current. When such heavy currents pass through the armature windings, it gets overheated and will damage the commutator and brushes.
Hence to reduce these heavy inrush currents a resistance must be connected in series with the armature winding. Therefore a starter that consists of set resistances is connected to limit this starting current.
Starting of DC Motors :
When the supply is connected to the armature terminals, motor draws huge currents more than its rated current. This is because the resistance of the armature circuit is relatively small. This can be understood from the expression of armature current Ia of the dc motor.
Where,- V = Supply voltage
- Ra = Armature resistance
- Eb = Back emf
From the above equation, we can see that armature current Ia is the ratio of voltage V to armature resistance Ra. Initially, when a dc motor is started the bach emf Eb induced in the armature will be zero (since E = K Φ N and N = 0). By substituting Eb = 0, the expression for armature current at starting is given as,
Generally, the armature resistance of a dc motor is kept very low (fraction of an ohm), and therefore armature draws current many times the full-load rated current (order of hundreds of amperes). These heavy currents when circulated through the armature winding can cause damage to the winding.
However once the motor starts rotating the back emf in the motor builds, thus it gradually decreases the starting current (since the value V - Eb decreases) as the speed goes on increasing. At a moment the value of this current becomes less than the rated value.
Hence the current should be limited at starting period only in order to prevent damage to the winding. To limit these starting currents flowing in armature winding, resistance is introduced in series with armature winding a shown in the figure below.
As the motor gains speed the value of this resistance gradually cuts off. Therefore the expression for armature current when extra resistance R is connected is shown below.
When the extra resistance R is added to the armature resistance Ra, the value of armature current Ia reduces. As the speed of the motor increases, the Eb increases, which in turn decreases the difference (V - Eb) i.e., the numerator is reduced. Here we can see that the need for extra resistance is also reduced with respect to speed (i.e., with respect to the increase in numerator value).
At a value of back emf Eb i.e., at a certain speed, the current can be limited without the need for extra resistance R. If the extra resistance remains continuous in the armature circuit, power is dissipated in the form of Ia2 R loss, thereby decreasing the motor efficiency.
From the diagram, we can see that field and armature windings are connected across the supply. If the field winding is connected in parallel with the armature winding, due to power loss in resistor R, the voltage available across the armature terminals decreases. Therefore the field current If decreases and the field excitation cannot develop the required torque on the armature to rotate the rotor.
Necessity of DC Motor Starter :
In practice, a device is used at starting to bring extra resistance with the armature winding know as 'Starter'. It consists of a set of resistors placed in an enclosure. The value of resistance can be increased or decreased by varying the number of resistors connected to the armature circuit.
This can be done by a movable handle (starter arm) that moves on resistors tappings in such a way that, at starting position no resistance is connected and resistance increases as the handle moves. The starter is provided with protective devices to disconnect the motor (by moving the starter handle to the off position) in the event of supply failure or in an overload condition.
Suppose if the starter fails to disconnect the armature from the supply, the armature will get directly connected across the supply when it is restored. Also in case of overload condition motor draws more current, therefore overloaded protection will be also equipped in the starter to disconnect the motor.