You must have noticed that we use a starter to run motors. But have you ever thought why starter is used for running a motor?? Lets dig in to the cause.
As we know, the current drawn by a motor armature is given by the relation: Ia=(V-Eb)/Ra
In this equation,
Eb is the back emf, and
Ra is the armature resistance.
In the OFF condition of the motor, there is no back e.m.f available in the motor armature. At this stage, if we apply full supply to the still armature, it will draw a considerable high amount of current because of the minute armature resistance.
For example – Lets take an example of a 415v motor with 10HP(7.46 kW) power rating. Assume the motor’s armature resistance is 0.5Ω and the full load rating of 100 A. Now, if we start the motor with direct supply, then its starting current will be :
830/100=8.3 times its full load value.
This ‘too much’ current will immediately blow out the fuses. Do not take it lightly as this is not the only problem. Before blowing fuses, it will damage the brushes, commutator and other parts also.
In order to prevent such situations to happen, an external resistance is placed with the armature in series. This external resistance restricts this ‘too much’ starting current to a safe limit. Necessary Arrangements are done so that the external resistance is applied for a small duration of starting time only. The starting resistance is progressively dished out as the motor picks up speed and a back e.m.f. is generated in it. This back e.m.f then controls the motor’s speed.
If we consider the case of small motors, they do not need any starting arrangement to keep the motor safe. It is because :
As they are small in size, their moment of inertia is very small. It is why they pick their speed in quick time.
The large starting current drawn by such motors for a very small time frame is inefficient to generate a considerable change in the supply lines voltage.
the starting current of such motors less because their armature resistance is high as compared to big motors.
Types of Starter
There are basically two types of starters depending on type of motor being used. They are:
(a) Shunt motor Starter
(b) Series Motor Starter.
Both types of starters are described separately.