Autotransformer starter working principle,wiring and control diagram

  Autotransformer starter working, wiring and control diagram:

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

Auto transformer control diagram :–

Fig:auto transformer motor starter control diagram

The operation principle of auto transformer method is similar to the star delta starter method. The starting current is limited by (using a three phase auto transformer) reduce the initial stator applied voltage.
 
The auto transformer starter is more expensive, more complicated in operation and bulkier in construction when compared with the star – delta starter method.

But an auto transformer starter is suitable for both star and delta connected motors, and the starting current and torque can be adjusted to a desired value by taking the correct
tapping from the auto transformer. 

When the star delta method is considered, voltage can be adjusted only by
factor of . 1/√3

An autotransformer 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  auto transformer 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.