Motor Starter & Its types
Introduction:
A motor Starter is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.
There are we discuses popular four types of starters.
1. Direct On Line (DOL) starter.
2. star delta starter.
3. Auto transformer starter.
4. Electronic Soft starter.
Direct on Line DOL starter:
A directly online or across-the-line starter applies the full line voltage to the motor terminals, the starters or cubicle locations can usually be found on an ELO drawing. This is the simplest type of motor starter.
A DOL motor starter also contains protection devices, and in some cases, condition monitoring. The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consists of an MCCB or Circuit Breaker, Contactor, and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
Star delta starter:
The Star Delta Starter is a very common type of starter and is used extensively as compared to the other type of starting methods of the induction motor. A star-delta is used for a cage motor designed to run normally on the delta-connected stator winding. This is the reduced voltage starting method.
Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated. During starting the motor windings are connected in a star configuration and this reduces the voltage across each winding. After some time the winding is reconfigured as delta and the motor runs normally.
Star Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
Autotransformer starter:
An Autotransformer Starter is suitable for both star and delta-connected motors. In this method, the starting current is limited by using a three-phase autotransformer to reduce the initial stator applied voltage. The operation principle of the autotransformer method is similar to the star delta starter method. The starting current is limited by reducing the initial stator applied voltage.
The autotransformer starter is more expensive, more complicated in operation, and bulkier in construction when compared with the star delta starter method. But an autotransformer starter is suitable for both star and delta connected motors, and the starting current and torque can be adjusted to the desired value by taking the correct tapping from the autotransformer.
Electronic Soft starter:
A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the power train and electric current surge of the motor during start-up. A start voltage is applied with time, irrespective of the current drawn or the speed of the motor.
For each phase, two SCRs are connected back to back and the SCRs are conducted initially at a delay of 180 degrees during the respective half wave cycles. This delay is reduced gradually with time until the applied voltage ramps up to the full supply voltage. This is also known as Time Voltage Ramp System. This method is not relevant as it doesn't control the motor acceleration.