What is Auto-transformer Starter,Working principle,diagram,advantages

Autotransformer  starter working pdf

Squirrel cage induction motor started on full line voltage using  D O L starter as a starting method draw very large starting current which can damage motor winding and also create a current surge on the power system. Hence another starting method is used to start three phase induction motor called an autotransformer starter. Auto transformer Starter can be used for both star/delta connected induction motors.

The starting current 6 to 8 times the rated current and this can cause voltage dip in supply and also motor and motor-driven equipment experience large torque surge which can damage the motor as well as connected equipment. Three phase autotransformer is used to reduced high inrush starting current.
Single-phase tapped autotransformer
Single-phase tapped auto transformer

Many reduce voltage starting methods are used to start induction motor. one of them is autotransformer starter, autotransformer starter is also reduced voltage starter particularly used for higher rating motors greater than 10HP.

The auto transformer starter is the most popular of the electromechanical reduced-voltage starters.
Its popularity is attributed to the fact this type of starter can reduce the inrush current in the distribution system to the lowest level of all the different electromechanical types of starters. 

In addition, the taps on the autotransformer permit adjustment of voltage and motor torque. It is ideally suited for starting most industrial loads.

In this article, we will discuss auto transformer start working principle, theory, construction, circuit diagram, control, power diagram and it's an application, uses advantages, disadvantages.

Theory of Auto transformer Starter

Auto-transformer starter (Variable Autotransformer) can be used with any squirrel cage induction motor, motor supplied through taps of three-phase auto transformer starter.In Auto-transformer Starter, Motor is directly connected on secondary of autotransformer.

The taps provided on auto-transformer limit the starting voltage and supply motor in steps of 50% 65% or 80% of nominal voltage. Using auto transformer for starting purpose the line current is always less than motor nominal current during start, because of motor connected to the secondary side of auto-transformer starter during acceleration.

If motor connected to 50% tap of auto transformer, the motor current would be reduced up to 50% of nominal starting current. But the line current will be only 25% of nominal starting current. The difference between line current and motor current due to auto transformer is in the circuit.

Due to the lower line, current auto transformer starter is a very popular type of reduce voltage starter. Since the motor starting current is greater than the line current with autotransformer starter, the starter produced more torque per ampere of line current than any other type of reduce voltage starter

Working of Auto transformer starter:

auto transformer starter circuit diagram
autotransformer starter diagram

Stage 1 - At starting auto-transformer is star-connected by activating Star contactor KM1, and then the Run contactor KM2 is closed.
This arrangement start an induction motor with reduced voltage and the value of voltage depends upon the ratio of tap selected. The standard taps are 50%, 65% and 80% of the full line voltage

Stage 2 - After the preset time delay, the Star contactor KM1 is opened, and the auto-transformer acts as an inductor connected in series with the motor.

The supply to the motor is, thus, maintained during the transition. This transition is normally timed to occur when the motor speed has stabilized at the end of the run-up period.

Stage 3 - The transformer is shunted completely by energizing Main contactor KM3, so that the motor is directly connected to the supply and KM2 is opened.

Advantages Auto transformer 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 an 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%.

5) Highest torque per ampere of supply current.

6) Motor current larger than supply current. 

7) This method is suitable for long starting periods. 

8) Adjustment of starting voltage by selection of proper tap on the auto transformer.

Drawbacks/disadvantages Auto transformer 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.

3) Low power factor


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