FUNCTION OF STARTER IN MOTOR

NEED OF STARTER

Armature Current in DC Motor is given by

Iₐ = (V - Eb)/Rₐ.......eq1

where, V = Supply Voltage

            E_b = Back emf

            Rₐ = Armature Resistance

When the motor is at rest (i.e, speed = 0), then the back emf is also zero (because Eb ∝ Speed). So, the initial armature current at the time of starting is

Iₐ = (V - 0)/Rₐ = V/Rₐ......eq2

Since the armature is designed in such a way that its resistance is low.So as from eq2, the armature will draw a very high inrush current which is 5 to 7 times of the full load current. This high inrush current may heat up the armature and damage the armature winding. Excessive current can also damage the brushes and cause sparking on the commutator.

So to avoid the damages which may cause due to high inrush starting current, an extra variable resistance is connected in series with the armature called starting resistance at the time of starting. Once the motor starts to pick up the speed , back emf also starts to rise and the difference (V-Eb) starts decreasing. This results in the decrease of armature current , therefore the starting resistance is gradually cutout in the steps as the motor gains speed.

FUNCTION OF STARTER

Function of motor starter are:

1. It is used to limit the excessive starting inrush current.

2. It also protects the motor from overload and no voltage conditions.

Conclusively, the function of starter is to limit the high starting current and when the motor attains the speed, back emf starts to build up and the resistance is gradually cutout from the motor circuit.

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