Autotransformer starter working, wiring and control diagram

  Autotransformer starter working, wiring and control diagram:

  There are various  starter used to start induction motor like Direct online starter_DOL starter, Soft starter,Star delta starter,auto transformer starter,auto transformer starter is used for large HP motor.

Auto transformer :–

Auto transformer starter

 

An auto transformer starter reduces inrush current by using a transformer in the line just ahead of the motor to step down the voltage applied to the motor terminals. 

By reducing the voltage, the current drawn from the line is reduced during start-up. When the setting time on the timer has expired, the auto transformer is bypassed. The 1S contactor drops out, the run contactor closes, and the 2S contactor opens, proving full voltage to the motor. 

The  autotransformer starter is a closed transition type, meaning that the motor remains connected to the line during the entire acceleration period. 

The transformer has three taps which provide 50%, 65% and 80% of full line voltage. 

At delivery, the transformer is connected to the 65% tap; the inrush current will be reduced to 42% of normal; and the starting torque will be reduced to 42%. 

The autotransformer starter can be used for any squirrel-cage motor.

Advantages Auto transformar starter:

1) The autotransformer starter limits significantly the inrush current. 

2)It is used for large motors, in which start by direct connection to the network is not possible. For large motors also the star-delta starter cannot be used, especially if they are started with a significant load.
3)The circuit has advantage over starting with a regular autotransformer, which needs to be at some point completely disconnected during the start inducing high voltage impulses, which can damage the electrical insulation of the stator.
4)The most effective ratio of the autotransformer is between 65-80%.

Drawbacks Auto transformar motor starter:

1) The circuit is quite complex and involves relatively expensive autotransformer. 

2) Due to the physical size of the whole device it might not be possible to add the Korndorfer starter to an existing machine if space is scarce.