What Is Starter ? Why Do We Need Starter?

    The purpose of a starter is not to just start the motor, but it performs the two main functions. They are as follows.
 > To reduce the heavy starting current
 > To provide overload and under voltage protection.

Why Do We Need Starter? 
In Simple terms,: considering induction motor
 > Torque is proportional to square of applied voltage
> Voltage is proportional to current
 So, by reducing the voltage during starting, both the starting torque and starting current can be controlled. Starters do this job control the starting current by reducing voltage

Detailed Explanation
  We will discuss why starters required for induction motors which is commonly used in ship
> Starter is required to limit the starting current. 
>  The rotor induced emf is proportional to the slip (it is the relative speed between stator and rotor) of the induction motor. This rotor emf drives the current through the rotor.
> When the motor is at the start, the speed of the motor is zero and hence slip is at maximum. This induces very high emf in the rotor at starting condition and thereby a very high current flows through the rotor.
> As the rotor needs a high current, stator winding draws a very high current from the supply. 
> This initial drawing current can be of the order of 5-8 times the full load current of the motor.
> This huge current at the starting of a motor can damage the motor windings and also this current can cause a large voltage drop in the line.
> These voltage spikes may affect the other appliances connected to the same line.    
So, starter is necessary to reduce the starting current to avoid the damage to the motor as well as the adjoining equipment.

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