Autotransformer starter: Working principle, Pros & Cons

What is autotransformer starter?

The autotransformer starter is a special type of motor starter that enables the start of squirrel-cage motors using a reduced starting current since the voltage is reduced during the start. With an autotransformer start, a device called a transformer is used.

With the use of the transformer, the voltage to the motor can be reduced and thus reducing the starting current and starting torque. It is possible to switch back to the nominal voltage whenever it’s desired.

Autotransformer starter working principle

The motor is connected to one tap of the autotransformer (magnetic only circuit-breaker closed, KA closed, KY closed) which reduces the network voltage by the factor “k” and which draws in the motor a current reduced by the same factor compared to the current that the motor would absorb when supplied directly at the full rated voltage. The current on the primary of the transformer, and consequently on the line, results to be reduced by k2. Consequently, to the voltage reduction by factor k, also the inrush torque is reduced by k2 compared to starting at full voltage.

When the motor has reached approximately 80% to 90% of its steady-state speed, the contactor KY is opened and the motor continues to be supplied at reduced voltage by the inductance of the auto-transformer windings. At this point, the contactor KL is closed and the contactor KA is opened so that the motor is supplied directly from the network.

Autotransformer starter circuit

Advantages of autotransformer starter

The benefits of autotransformer starter:

• It limits the inrush current significantly.
• It produces low power loss, less radiated heat.
• It has a good performance when starting large motors.
• It is suitable for long starting periods.
• Adjustment of starting voltage can be done by the selection of a proper tap on the autotransformer.
• The figure below shows a graph to compare starting current and starting torque for a transformer start at 80% voltage and direct-on-line start. The blue curve is for the direct online start and the red curve is for the transformer start. Dotted lines represent starting current and filled lines represent starting torque. As noticed, there is a lower starting current and starting torque with transformer start compared to the direct online start. This means that the transformer start can be more flexible compared to the star-delta starter.

Disadvantages of autotransformer starter

The drawbacks of autotransformer starter:

• Starting with auto-transformer is to be considered quite expensive. In lower power, it is the most expensive design. It is also not cost-friendly when it is applied to medium/high power squirrel-cage motors with high inertia.
• The control and power circuits are quite complex.
• Due to the physical size of the system, it might not be possible to apply an autotransformer starter to an application where space is limited.
• It has a low power factor.
• It has a limited duty cycle.
• Autotransformer starting has a distinct “jump” between the starting torque (lower value) and the normal running torque (high value). The mechanical and electrical transient produced by this switching might not be suitable for some loads.
• It is old technology.

Applications of autotransformer starter

Autotransformer starters are the most widely used electromechanical reduced voltage starter. An autotransformer starter may be used on any three-phase AC motor and provides the maximum starting torque with the minimum starting current.

Autotransformer starters are ideal for applications where available starting current is limited, but a minimal decrease in starting torque is required. Because the autotransformer provides the highest torque efficiency, it is also ideal for applications that may be run from generator power. Typical applications for autotransformer starters include:

• Centrifugal pumps.
• Fans.
• Compressors.
• Conveyors.

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Who invented the autotransformer starter?

The autotransformer starter was invented by Max Korndörfer in 1908. He was granted the patent US 1,096,922 in May 1914. Max Korndörfer assigned his patent to the General Electric Company.