Autotransformer Starter Working Principle

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 starters are ideal for applications where the 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

How does an autotransformer starter work?

Autotransformer starting uses a step-down autotransformer (single-winding transformer) to reduce the line voltage. Autotransformer starters offer the greatest reduction of line current of any reduced-voltage starting method.

Multiple taps on the transformer permit the voltage, current, and torque to be adjusted to satisfy many different starting conditions. In closed transition starting, the motor is never disconnected from the line source during acceleration.

The diagram shows a typically closed transition autotransformer starting circuit.

The operation of an autotransformer starter can be summarized as follows:

• Closing the start button energizes the on-delay timer coil TR.
• Memory control contact TR1 closes to seal in and maintain timer coil TR.
• Contact TR2 closes to energize contactor coil C2.
• Normally open C2 auxiliary contact closes to energize contactor coil C3.
• Main power contacts of C2 and C3 close and the motor is connected through the transformer’s taps to
the power line.
• Normally closed C2 auxiliary contacts are opened at this point, providing an electrical interlock that prevents C1 and C2 from both being energized at the same time. A mechanical interlock is also provided between these two contactors as this circuit condition would overload the transformer. In addition, normally open control contact C3 closes to seal-in and maintain contactor coil C3.
• After a preset time, the on-delay timer times out.
• Normally closed timed TR4 contacts open to de-energize contactor coil C2 and return all C2 contacts
to their de-energized state.
• Normally open timed TR3 contact closes to energize contactor coil C1.
• Normally closed C1 auxiliary contact opens to deenergize contactor coil C3.
• The net result is the de-energizing of contactors C2 and C3 and the energizing of contactor C1, resulting
in the connection of the motor to full line voltage.
• During the transition from starting to full line voltage, the motor is never disconnected from the circuit, providing closed circuit transition.