What is Auto transformer Starter Working principle, diagram,advantages

Autotransformer  starter working pdf

The squirrel cage induction motor started on full line voltage using  D O L starter as a starting method draws a 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 is 6 to 8 times the rated current and this can cause a voltage dip in supply also motor and motor-driven equipment experience large torque surges which can damage the motor as well as connected equipment. Three phase autotransformer is used to reduce the high inrush starting current.

Single-phase tapped auto transformer
Single-phase tapped auto transformer

Many reduced voltage starting methods are used to start the induction motor. one of them is the autotransformer starter, The autotransformer starter is also a 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.

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

Theory of Auto transformer Starter

Auto-transformer starter (Variable Autotransformer) can be used with any squirrel cage induction motor, a motor supplied through taps of three-phase autotransformer starter. In the Auto-transformer Starter, the Motor is directly connected to the 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 purposes the line current is always less than the motor’s nominal current during start, because of the motor is connected to the secondary side of auto-transformer starter during acceleration.

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

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

Working Principle 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

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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 the Main contactor KM3, so that the motor is directly connected to the supply and KM2 is opened.

Advantages Auto transformer starter

  • The autotransformer starter limits significantly the inrush current.
  • 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.
  • 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.
  • The most effective ratio of the autotransformer is between 65-80%.
  • Highest torque per ampere of supply current.
  • Motor current larger than supply current.
  • This method is suitable for long starting periods.
  • Adjustment of starting voltage by the selection of proper tap on the auto transformer.

Drawbacks/disadvantages Auto transformer starter 

  • The circuit is quite complex and involves a relatively expensive autotransformer.
  • 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.
  • Low power factor
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Read Terms…

References // AUTO TRANSFORMER STARTER BCH electrical

What is the autotransformer starter?

Auto transformer starter is a reduced voltage starter that is used to start a large HP induction motor smoothly.

What is the working principle of auto transformer?

The initial high starting current is restricted by (using a three-phase autotransformer) reducing the initial stator applied voltage.


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