Like star-delta starting, the use of an auto-transformer is a reduced voltage starting method. Unlike star-delta, which forces a fixed reduction in voltage, the use of an auto-transformer enables almost any range of voltage to be considered during the starting period. Given tapping's on the transformer, the voltage can be varied through the starting period.

The ability to vary the level of voltage applied during stating not only allows the starting current to be constrained within limits, but also allows for closer matching of the starting torque to the mechanical requirements. The major downside (at least compared to star-delta) is the increased cost due to the auto-transformer.

Power and Control Circuiting

The illustration (click for a larger image) shows typical power and control circuits for a auto-transformer starter.

Auto-Transformer Starter

Power Circuiting

Auto-transformer starting is typically implemented in three stages:

1. the transformer is connected in star (-KM1) and the line contactor (-KM2) closed. This applies a reduced voltage for the initial starting of the motor.
2. when the motor has reached full speed the auto-transformer star contactor (-KM1) is opened. This momentarily places the transformer in-line with the motor (as an inductance), until step 3 is complete.
3. the line contractor (-KM3) is closed putting the line voltage onto the motor and the auto-transformer isolated by contactor (-KM2) opening.

Step 2 ensures that the motor is never disconnected from the supply and forms a closed transition start. It should be noted that -KM1 and -KM3 are normally mechanically interlocked, thereby preventing both being closed at the same time.

Other control schemes do exist whereby the motor is momentarily disconnected from the supply while the auto-transformer is removed from the circuit. This type of open transition starting can cause large transient spikes and is not recommended.

Control Circuiting

If you now have a good grasp of control circuits from the previous two articles, then while slightly more complicated the control circuit for auto-transformer should be understandable.

Pressing button -S2 will close -KM1, followed by -KM2 and the sequence timer -KA1. -KM3 is mechanically interlocked and will not close. Latching contacts (13,14) keep the relays energised and the motor is not started.

After a time delay (-KA1), -KM1 will open and -KM3 will close. -KM3 closing will de-energise -KA1, opening -KM2.

Pressing the stop button, -S1 will remove power from all devices and stop the motor.

-F2 is a thermal sensing device used to determine how long the auto-transformer has been energised and prevent overheating. It effectively limits the number of starts per hour.

Auto-Transformer Characteristics

Available starting current: 40 /65 /80%

Peak starting current: 1.7 to 4 In

Peak starting torque: 40/ 65/ 80%

So far we have looked at full voltage starting and reduced voltage (both star-delta and auto-transformer). In the next two articles, I’ll cover techniques reliant on the use of resistors during the starting period to limit inrush currents. The first article will look at the introduction of resistors into the primary winding, with the follow on article concerned with resistors in the rotor winding.

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