autotransformer starter

Autotransformer Starter: Advantages, Disadvantages, Applications And All You Want To Know

by abdul waheed

Last Updated on April 29, 2024 by Jawad Ali

Autotransformer starter

  1. Both star & delta-connected motors can benefit from an Autotransformer Starter. The starting current is controlled by lowering the first stator applied voltage with a three-phase autotransformer.
  2. It comes with a variety of toppings. The starter is linked to a single tapping to acquire the best starting voltage. To link the autotransformer inside the circuit for starting, a dual switch S is used. 
  3. The handle H is quickly pushed to the RUN position when the engine accelerates to around 80% of its rated speed.
  4. As a result, the autotransformer is removed from the circuit. The motor is connected directly to the line, allowing it to reach its maximum rated voltage. An under voltage relay keeps the handle in the RUN position.
  5. The handle gets released and returned to the OFF position if the supply voltage fails and falls below a specific amount. When overload protection is necessary, thermal overload relays are used.

What kind of autotransformer is this, exactly?

To create an autotransformer, two wires of a two-winding transformers can be connected together in series. Each of the primary and secondary windings of two windings is employed as the secondary. As a result, the secondary coil employed in the transformers will determine its voltage ratio & output.

Parts for an Autotransformerd

Keep in mind that the practical starter is made up of numerous other components. However, the following are the major components of the switching device starter on which the overall operation is based: 

  • Main Switch
  • Contactor 1 or Shorting Contactor 
  • Contactor 2 as well as Transformer Contactor 
  • Contactor 3 or Running Contactor 
  • Autotransformer

Main On/Off Switch

The main switch turns on or off the starter circuit’s primary incoming power source. The primary incoming power supply should be linked to this main switch’s input. The main switch’s output should be linked to the starting input port.

There are two contactors in the autotransformer starter.

Contactor 1 (also known as the Shorting Contactor)

It is connected to the autotransformer’s one-side input terminals. The primary function of this Contactor is to create a short between the autotransformer’s three coils. That’s why it’s known as a Shorting Contactor.

Contactor 2 (also known as a running contactor) is a type of Contactor.

It connects the motor terminals to the incoming power distribution terminals from the main switch. The running Contactor remains closed while the motor operates, which is why it is a running contactor.

Autotransformer Starter Theory

  1. Any squirrel cage induction motor fed through the taps of a three-phase autotransformer can be utilized with an auto-transformer starter (Variable Autotransformer).
  2. The motor is immediately connected to the secondary of the autotransformer is an auto-transformer starter.
  3. The auto-taps Transformer limits the starting voltage & supply motor to 50 percent, 65 percent, or 80 percent of nominal voltage. 
  4. However, the line current will only be 25% of the nominal starting current. The circuit contains the difference between current & motor current owing to the autotransformer.
  5. Current auto transformer starters are standard reduction voltage starters because of the lower line. 

Autotransformer starter: Principle of Operation

Because of k2, the current just on Transformer’s primary, and hence on the line, is lowered. As a result of the factor k reduction in voltage, the inrush torque was reduced by factor k2 compared to starting at full voltage.

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Circuit for starting an autotransformer

Benefits of using an autotransformer starter

The advantages of using an autotransformer starter are as follows:

  • It considerably reduces inrush current.
  • It has a low power loss and creates less heat radiation.
  • It performs admirably when starting huge motors.
  • It is appropriate for extensive startup periods.
  • The selection of a correct tap just on the autotransformer can be used to adjust the starting voltage.

A graph comparing beginning current and torque for just a transformer beginning at 80 percent voltage and a direct-on-line start. The online ticket start is in blue, whereas the transformer start is in red. Initial current is shown by dotted lines while starting torque is represented by complete lines.

  • Compared to a straight online start, there is a lower starting current and torque with a transformer start. In comparison to the star-delta starter, this means that the transformer start could be more flexible.

Autotransformer starter disadvantages

The disadvantages of using an autotransformer as a starter:

  • Using an autotransformer as a starter is extremely expensive. This is the most costly design at lesser power.
  • The system and control circuits are highly complex.
  • Due to the physical size of the network, an autotransformer starter would not be able to be used in a space-constrained application.
  • It has a low power factor and a short duty cycle.
  • Autotransformer beginning has a noticeable “jump” between the initiating torque (lower value) as well as the typical running torque (higher value) (high value). Some loads may not be suitable for the electrical and mechanical transients produced by this switching.
  • The technology is outdated.

Autotransformer applications

  • It’s used as a starter to supply up to 50% of between 60% of full voltage to the stator of a shunt motor during startup.
  • It’s used to raise the voltage of a distribution cable to correct a voltage drop.
  • It also functions as a voltage regulator.
  • This material is in power transmission & distribution networks and audio systems for trains.
  • In distribution systems, voltage fluctuations are adjusted by raising the supply voltage.
  • An autotransformer is used as a variac in a laboratory or if a continuous variable over a broad range is required.


What is the purpose of an autotransformer starter?

An autotransformer starter is utilized during the initial stages of a motor’s operation to reduce the applied voltage. When large motors are utilized in a circuit for an extended period, a potential difference can occur, causing damage to electronic systems connected to the same supply line.

Where do autotransformer starters come into play?

Both star and delta interconnected induction motors can be started with an autotransformer starter. The beginning current is 6 to 8 times the average current, resulting in a voltage dip in the supply and strong torque surges in the motor and engine equipment and which could also destroy the motor and linked equipment.

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