Auto transformer starters are used generally with AC squirrel cage motors having a power rating of more than 20 HP. Motors of these power ratings draw excessive starting current with the use of Star/Delta starter.
 
This may cause excessive strain on the motor and creates an intense voltage dip. This can prove harmful for the motor in the long run and may disturb the machines connected to the same line. Therefore it is advised to use an autotransformer starter with the motors over 20 HP.
 
Also, they provide the highest starting torque with the lowest starting line current compared to other reduced voltage starters, so they are  used widely for starting of large squirrel cage induction motors.
 
Moreover, high voltage AC squirrel cage motors are connected in star to reduce the voltage strain on the winding insulation. So we cannot use the star-delta starters with them and the autotransformer starter is the only choice.
 
However, if low starting torque is permitted and circumstances allow, one can also go with Star/Delta starters because they are cheap alternatives.
 
When a Star/Delta starter changes the motor connections from Star to Delta a high transient current is generated that may cause false tripping of the other machines connected to the same line. It also gives a mechanical jerk to the machine which may result in high wear and tear of the machine in some cases. An autotransformer starter provides a solution to both these problems.

Auto transformer Starter Working

Such starters consist of an auto-transformer, with necessary arrangements. At the time of starting, a reduced voltage is applied across the motor through the tappings of the autotransformer. The motor gets a lower starting voltage according to the selection of the tapping. Also, the motor draws less starting current and develops less starting torque accordingly.
 
When the motor attains 80% of its normal speed, auto-transformers are cut out and the full supply voltage is applied to the motor. The switch making these changes from ‘start’ to ‘run’ may be air-break (for small motors) or maybe oil-immersed (for large motors) to reduce sparking.
 
Most of the auto-starters are provided with 3 sets of taps, to reduce the voltage to 80, 65, or 50 percent of the line voltage, to suit the local conditions of supply.
Most squirrel cage induction motors can be successfully started at 65 percent of line voltage. Where this fraction of line voltage does not provide sufficient starting torque, the 80 percent tap can be used. Where 50 percent tap creates excessive voltage dip, the 65 percent taps can be used.

Auto Transformer Starter Diagram

Autotransformer Starter Control Circuit Diagram

The working of the starter is as under:

• When the start button is pressed a circuit is completed to the coil of main contactor M, timer TR and shorting contactor S. And all these get energized, the motor gets supply through the tappings of the autotransformer.
• The normally closed S contact connected in series with R coil opens to prevent both S and R contactors from being energized at the same time.
• When the time sequence for TR timer is completed, it operates. Its normally closed TR contact opens. This disconnects contactor S from the line and opens the shorting of the autotransformer. In this state, winding of autotransformer is connected is series of motor like a reactor and keeps on running smoothly.
• After a very short time delay, normally open contact of timer TR closes that closes the run contactor R and connects the mains supply directly to the motor. After that motor reaches full speed and is then in normal operating mode.
• The normally open contact of the timer operates after its normally closed contact with a very short time delay due to its special time delay design. This time delay is very essential for the smooth operation of the motor.
• When the stop button is pressed, the main contactor M is de-energizes. This disconnects all other control components from the power line and the circuit returns to its normal position.

Advantages of Auto-transformer Starter

• Due to the availability of different tappings, the autotransformer starter is a more flexible option than the other starters. Therefore, the user can select the best tapping according to the need of starting torque and local conditions like voltage and voltage dip during the commissioning of the starter.
• These starters provide a continuous power supply to the machine during the change over from low voltage to full voltage. This eliminates the high transients switching current, false tripping of other machines, and unneeded mechanical jerks on the machine.
• They produce the highest starting torque per ampere of line current drawn. So, are the best suited for the applications where high torque is needed to overcome the inertia of the load, and at the same time starting current is to be limited to the minimum.

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