What is a Motor Starter?
A motor starter is an electrical device that is used to start & stop a motor safely. Similar to a relay, the motor starter switches the power ON/OFF & unlike a relay, it also provides a low voltage & overcurrent protection.
The main function of a motor starter is;
To safely start a motor
To safely stop a motor
To reverse the direction of a motor
To protect the motor from low voltage & overcurrent.
Direct Online (DOL) Starter
DOL aka Direct Online Starter is the simplest form of motor starter that connects the motor directly to the power supply. It consists of a magnetic contactor that connects the motor with a supply line & an overload relay for protection against overcurrent. There is no voltage reduction for safe starting a motor. Therefore the motor used with such starters has below 5 hp rating. It has two simple push buttons that start & stop the motor.
Star Delta Starter
This is another common starting method used in industries for large motors. The windings of 3 phase induction motor are switched between star and delta connection to start the motor.
To start the induction motor, it is connected in star using a triple pole double throw relay. The phase voltage in star connection is reduced by the factor 1/√3 & it reduces the starting current as well as the starting torque by 1/3 of the normal rated value.
When the motor accelerates, a timer relay switches the star connection of the stator windings into the delta connection, allowing the full voltage across each winding. The motor runs at rated speed.
Soft Starter
The soft starter also uses the voltage reduction technique. It uses the semiconductor switches like TRIAC to control the voltage as well as the starting current supplied to the induction motor.
A phase-controlled TRIAC is used to provide variable voltage. The voltage is varied by varying the conduction angle or firing angle of the TRIAC. The conduction angle is kept at minimum to provide reduced voltage. The voltage is increased gradually by increasing the conduction angle. At maximum conduction angle, the full line voltage is applied to the induction motor & it runs at rated speed.
It provides a gradual & smooth increase in the starting voltage, current as well as the torque. Thus there is no mechanical jerk & provide a smooth operation that increases the life span of the machine.
Variable Frequency Drive (VFD)
Just like the soft starter, a Variable frequency drive (VFD) can vary the voltage as well as the frequency of the supplying current. It is mainly used for controlling the speed of the induction motor as it depends on the supply frequency.
The AC from the supply line is converted into DC using rectifiers. The pure DC is converted into AC with adjustable frequency & voltage using pulse width modulation technique through power transistor like IGBTs.
It provides full control over the motor speed from 0 to rated speed. The speed adjust option with the variable voltage provides a better starting current & acceleration.
I am Diwaker Bishnoi, CEO & Managing Director of Vishal Technopower Private Limited, and I have an experience of more than 12 years+ of executing various electrical projects on turnkey and non turnkey basis PAN India with various clients and for various electrical requirements.
I am on a mission to educate people about electrical engineering so that they can form a positive, safe and healthy relationship with electricity.
In this channel you can learn about electrical basic and engineering in the most simple language and without the use of much technical words.
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24 ноя 2023
