Autotransformer Starter | Auto Transformer Motor Starter Panel

In this article we will get the concept of autotransformer starter and also focus on the autotransformer starting circuit for induction motors. This starter is suitable for both star connected and delta connected induction motors. All three phase induction motors are generally self starting type, but the starting current is much higher, which is dangerous for the stator copper windings of the motor. That is why different kinds of starters are equipped with three phase induction motors for the sake of safety.

The autotransformer starter is particularly suitable for starting of three phase squirrel cage induction motors. The Direct On Line or DOL Starter is also used for the same purpose, but as it utilize the electric power directly from the line, so DOL starter is suitable for comparatively lower rating motors. The main objective of installing starter along with induction motor is to reduce the starting current into a safe value and protect the device from hazards. Autotransformer is a special type of transformer which uses only a common winding for both primary and secondary. There are several tappings available on the secondary side, which means the secondary output voltage is available in a wide range. The circuit is completed by connecting studs to the appropriate transformer tapping. In autotransformer starter the full line voltage is reduced into a safe value, and after that it is applied to the motor terminals. Therefore it is also called as reduced voltage starting of induction motor.

Theory of Auto Transformer Starting

Autotransformer-starterThe concept of this starter is taken from the principle of autotransformer. It provides additional flexibility as compared to other types of starting. In DOL starting method the motor is subjected to direct line voltage via some relay arrangement. In star delta starter, the starting voltage is reduced but it is fixed into a certain value. Whereas in autotransformer starter, the starting voltage is reduced and additionally the voltage is easily variable into a range. It is done by connecting the studs into different tappings. This feature provides versatility for the starter, which means by controlling the output voltage we can connect motors with different ratings, because the stator voltage is adjustable at each starting condition.

Suppose a 440 Volt supply is applied into the autotransformer and the output is 220 Volt (suppose). So the stator voltage is reduced 0.5 as compared to direct staring, or the transformation ratio is 0.5. It is found that the ratio of starting current with autotransformer starter versus starting current when direct switching is equals to the square of transformation ratio. And the same thing is happens for the starting torque.


So reduction of starting current and torque both are effectively done with autotransformer starter and the value of both of them are controllable by changing the tappings. A star delta starter is equivalent to an autotransformer starter if the transformation ratio is kept as 0.58.

Wiring Diagram & Operation of Autotransformer Starter

The main components of this type of starter are three phase autotransformer, contactor with movable rod and handle arrangement. The movable rod is arranged such a way that moving the operating handle will connect the line voltages at primary windings of autotransformer as well as also connects the secondary windings of autotransformer with motor terminals. Fuses are installed in series with each line for the primary safety. Generally circuit breakers or fuses are preinstalled with enclosed type starter panel, and power leads are mounted with proper insulation at outside casing. An under voltage relay and a over load protection relay is provided with RUN circuit. The cables which are feeding the input power to autotransformer must capable to carry the maximum input currents. Autotransformer taps are set at 65% transformation ratio during manufacturing. It can be adjusted with 50%, 65% and 80% voltage transformation ratio, during each starting. The tapings are varied as per the requirement of starting current and starting torque, because we already told that the ratio is depended up on the square of transformation ratio. The simplified wiring diagram is shown in the following figure.

Autotransformer Starter or autotransformer starting method

START Operation of Auto Transformer Starter

For the best performance and symmetrical voltage operation, all three motor terminals are connected with same tapping with their respective secondary coils. A double throwing switch is connected with each autotransformer and the assembly of all switches is controlled by the movement of the handle H. When the handle is moved, then all switches are connected with their respective contactors. In this way line voltages are applied to the primaries and the all secondary coils become connected with the motor terminals, and the motor starts running with reduced voltage.

RUN Operation of Auto Transformer Starter

The autotransformer reduced the supply voltage for limiting the high starting current. But after the safe starting with starter, the motor should feed with full load current, for the maximum efficiency and best performance. When the motor starts accelerating and pick up the speed of 80% of its rated value, then the handle H is moved to the RUN position instantly. In this way, the autotransformer is detached from the circuitry and full rated voltage is applied at the motor terminals. The over load relay senses the load voltage and current, which may be a thermal relay or electromagnetic relay. In modern autotransformer starters solid state relays are used for greater accuracy and reliability. The over load relay sense the high load current and trips the circuit by releasing the handle. The under voltage relay which is preinstalled with the starter, sense the rated voltage constantly. If anyhow the supply voltage is lowered than a predetermined safe value, then the relay coil releases the handle from RUN position and throws into OFF position. During the normal operating conditions, those two relays help to attract the handle H, and stack the contactors in RUN position.

Features and Specifications of Autotransformer Starter

The autotransformers starter is available to start induction motors with voltage rating 220 Volt, 380 Volt, 415 Volt and 500 Volt in 50 Hz operation and 200 Volt, 230 Volt, 460 Volt, 575 Volt for 60 Hz operation. Generally three types of tappings are widely available which includes transformation ratio of 50%, 65% and 80%, among which the 65% and 80% ratio is used mostly. With 65% transformation ratio, the line current and starting torque will be 42% of rated full load value and the starting current will also 42% of rated value. With 80% transformation ratio, the line current and starting torque both will be 64% of their corresponding full load rated value.

Advantages of Autotransformer Starter

The starting torque and starting current is reduced as well as adjustable by tap changing.
The construction is robust as only single winding autotransformer and contactors are used.
The operation is very easy, START and RUN mode is accessible by throwing the handle H.
Less wiring is needed, and the maintenance is quite hassle free.
Suitable for different induction motors with various voltage ratings.
It is the best way of soft starting of the induction motors.
The overload and under voltage protection is preinstalled for better safety of motor.

Applications of Autotransformer Starter

The autotransformer starters are mostly used for starting of three phase induction motors with large power rating. Because the direct line voltage would be dangerous for such applications, that is why the reduced voltage is applied by autotransformer. Sometime the starter co ordinate with braking devices and then the signal from RUN contact is transmitted to the controller of electric motor brake. The power factor correction capacitors are also co ordinate with this type of starters. The capacitors are attached in the line voltage side of autotransformer for such operation. Large HP motors are used in many industrial applications, and in conveyors. large pumps etc. The autotransformer starter is suitable for all those applications.