Autotransformer Starter Diagram, Parts, Working Principle

There are various types of starting methods and starters for the starting of induction motors. The starting method or starter are used according to the size of the motor. For example, DOL starter or Direct On Line Starter is used for the starting of low size or low power motors. The main disadvantage of this starter is, it cannot limit the current at the starting. So, if we connect a big-size induction motor with a DOL Starter, then it will draw a very high starting current, so the coil of the motor may be damaged. So, another very popular type of starter is used for the starting of the induction motor, that is the Autotransformer starter.

Autotransformer starters can limit or reduce the high current during the starting of the induction motor. We know that the main function of a transformer is to step up or step down the voltage. So, in this starter also, the autotransformer steps down the voltage so the current also be decreased. First of all, let's see the basic construction of the autotransformer start, so we can easily understand how an autotransformer starter works.

Autotransformer Starter Diagram and Parts

Here, you can see the basic diagram of Autotransformer starter.

Remember that the practical starter consists of so many other components. But the main important parts of the autotransformer starter  on which the total working principle is dependent are,

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

Main Switch

The main switch helps to turn on or turn off the main incoming power supply to the starter circuit. It may be an MCCB(Moulded Case Circuit Breaker) or any other circuit breaker or fuse box. The main incoming power supply is to be connected to the input of this main switch. And the output of the main switch is to be connected to the starter input terminal. When this main switched is turned off all the system including motor and starter will disconnect from the main power supply.

Contactor 1 or Shorting Contactor

It is connected to one side input terminals of the autotransformer. The main function of this contactor is to make a short between all the three coils of the autotransformer. That is why it is called Shorting Contactor. When this contactor is turned off the coils of the Autotransformer will be disconnected from each other and when this contactor is turned on the coils of the Autotransformer will be connected to each other.

Contactor 2 or Transformer Contactor

It is connected between the main input terminals of the autotransformer and incoming power supply terminals from the main switch. When this contactor is closed the autotransformer will be connected to the main power supply. And when this contactor is open or turned off the Autotransformer will be disconnected from the input power. It is known as a Transformer contactor.

Contactor 3 or Running Contactor

It is connected between the incoming power supply terminals from the main switch and motor terminals. It makes a direct connection between motor and input power supply. During the running condition of the motor, it remains in the closed condition and the motor will run at the full input voltage, that is why it is called as a running contactor.

Autotransformer

An autotransformer is used to reduce the voltage at the starting time of the motor. It has a total of 9 terminals. Among those three terminals are the main input terminals that are connected to the incoming power supply through the transformer contactor. Another three input terminals are connected with the shorting contactor. The other three terminals are brought out from the tapping of the winding and they are connected to the Induction Motor.

Working Principle of Autotransformer Starter

The autotransformer starter can be used for the both star and delta connected induction motor. It works in four different stages.

Stage 1

At the first stage, contactor 1 or shorting contactor is in closed condition and contactor 2 gets turned on. Once contactor 2 gets turned on the motor will get the supply voltage and starts running. This initial supply voltage depends upon the number of tapping selected on the autotransformer. Generally, the initial voltage is used are 50% or 60% or 70% of the full voltage. This shorting contactor connects the transformer coils in star configuration.

Stage 2

After some time(prest time set in the timer), the shorting contactor gets opened and the coils of the autotransformer act as series inductors with the induction motor.

Stage 3

Once the motor gets a stable speed the contactor 3 or running contactor gets closed. It is also set by the timer. In this condition, the autotransformer work as a shunt with the induction motor.

Stage 4

The contactor 2 or transformer contactor is opened and the autotransformer got insulated from the circuit and the motor running with the full supply voltage. At the running condition, only contactor 3 or running contactor will be in closed condition, remaining all the contactors will be in open condition.

Read Also: 

• [Explained] Star Delta Starter Diagram | Control and Power Circuit
• DOL Starter Connection and Wiring Diagram with OLR
• Motor Control Circuit Forward Reverse | Wiring and Connection
• Electric Motor Control Procedure, Methods, Devices, Circuits
• All types of Motor Starter: Applications and Advantages Explanation

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