As its name suggests – the main principle of a motor starter is to start and stop the motor to which it is attached. These are uniquely designed electromechanical switches similar to relays. The primary difference between a relay and a starter is that a starter generally includes overload protection for the motor.
It can hence be stated that the purpose of the starter is dual, i.e., to switch the power automatically or manually to a motor and at the same time shield the motor from the damages caused by overload or faults. Motor starters can be found in several ratings and sizes depending on the motor (AC motor) rating and size. These staters securely switch the necessary power to the motor and also limit the motor from drawing heavy currents.
Why Does an Induction Motor Need A Starter?
A stator is required for an induction motor (three-phase type) in order to restrict the starting current. In a three-phase induction motor, the rotor induced EMF is proportionate to the slip (which is the relative speed between stator and rotor) of the induction motor. This rotor emf propels the current through the rotor.
When the motor is at standstill condition (in the beginning), the speed of the motor is zero and hence, slip is at maximum. This induces a very high EMF in the rotor at starting condition and hence 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. The huge current at the starting of a motor can degrade the motor windings and this current can also result in a large voltage drop in the line. These voltage spikes can even negatively impact other appliances connected to the same line. Therefore, a starter is essential in order to limit this starting current to minimise damage to the motor as well as to other linked equipment. Generally, starters are furnished with push-button switches.
Types of starters include –
- DOL starters
- Contactors (Electric Relays)
- Soft Starters
A starter is a device that lessens the initial high current of the motor by lowering the supply voltage applied to the motor. Such reduction is applied for a short duration and once the motor hastens, slip value decreases and therefor a normal voltage is then applied. Along with the starting current protection, motor starter also offers protection against overload, single phasing, and even low voltage protection. The overload protection is required since a motor draws excessive current during overload condition that more often than not results in extra heat in the windings. This extra heat diminishes the motor’s life and may result in damage in the form of fire. #SchneiderElectricIndia is a leading motor starter manufacturer.