What is Motor Starter

What is Motor Starter The primary function of a motor starter is to start and stop the motor to which it is connected. These are specially designed electromechanical switches similar to relays. The main difference between a relay and a starter is that a starter contains overload protection for the motor.

So the purpose of the starter is twofold, i.e., to switch the power automatically or manually to a motor and at the same time protect the motor from overload or faults.

Motor starters are available in different ratings and sizes depending on the motor (AC motor) rating and size. These staters safely switch the necessary power to the motor and also prevent the motor from drawing heavy currents.

In this article we are going to deal only with AC motor starters, as they are much interested in industries and commercial applications.

Why We Need to Connect a Starter to Induction Motor?

A stator is needed for an induction motor (three phase type) in order to limit the starting current. In a three phase induction motor, 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 standstill condition (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. Therefore, a starter is necessary to limit this starting current to avoid damage to the motor as well as to other adjoining equipment.

A starter is a device that reduces initial high current of the motor by reducing the supply voltage applied to the motor. Such reduction is applied for very short duration and once the motor accelerates, slip value decreases and hence a normal voltage is then applied.

In addition to the starting current protection, motor starter also provides the protection against overload, single phasing and low voltage protection.

The overload protection is necessary because motor draws more current during overload condition and it causes to produce excessive heat in the windings. This extra heat reduces the motor’s life and may cause burning of windings and hence fire.

All starter devices are provided with some overheating protection element to limit high current during overload. Most of these devices work on timed overload concept in which overload current is allowed for a short time (very few seconds) and then stops the motor if the current exists for beyond that time.

Most starters are equipped with bimetallic strips to achieve this operation.

Some motors that are rated below 5 HP are directly connected (using DOL starter) without reducing the supply voltage (at initial condition), but they are provided with overload, low voltage protection and single phasing protection. This is because such motors can withstand high starting current for short duration.

How a Motor Starter Operates?

Basically, a starter is a switching device that consists of electrical contacts (both incoming and outgoing).

Based on the operation, starters are primarily divided into hand operated and electrically operated devices.

Hand operated starter consists of a lever on the side of it that can be turned on or off. Usually these are used for smaller motors as they are incapable of operating remotely.

This type of motor starters causes the motors to restart immediately after a power interruption. This instant operation of the motor after power failure may leads to flow of dangerous currents into the motor and hence the motor will be damaged. This is the reason why most of the starters are equipped with electrical switches.

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