The motor is powered at reduced voltage via an autotransformer which is bypassed when the starting process is completed.
The starting process is in three steps:
– in the first place, the autotransformer is star-connected, then the motor is connected to the mains via part of the autotransformer windings.
The process is run at a reduced voltage which depends on the transformation ratio. The autotransformer is usually tapped to select this ratio to find the most suitable voltage reduction value,
– the star connection is opened before going onto full voltage. The fraction of coil connected to the mains then acts as an inductance in series with the motor. This operation takes place when the speed balances out at the end of the first step,
– full voltage connection is made after the second step which usually only lasts a fraction of a second. The piece of autotransformer winding in series with the motor is short-circuited and the autotransformer is switched off.
The current and the starting torque vary in the same proportions. They are divided by (mains V/reduced V2).
The values obtained are:
SC = 1.7 to 4 RC
ST = 0.5 to 0.85 RT
The starting process runs with no break in the current in the motor, so transient phenomena due to breaks do not occur.
However, if a number of precautions are not taken, similar transient phenomena can appear on full voltage connection because the value of the inductance in series with the motor is high compared to the motor’s after the star arrangement is open. This leads to a steep drop in voltage
which causes a high transient current peak on full voltage connection.
To overcome this drawback, the magnetic circuit in the autotransformer has an air gap which helps to lower the inductance value. This value is calculated to prevent any voltage variation at the motor terminals when the star arrangement opens in the second step.
The air gap causes an increase in the magnetising current in the autotransformer. This current increases the inrush current in the mains supply when the autotransformer is energised.
This starting system is usually used in LV for motors powered at over 150kW.
It does however make equipment rather expensive because of the high cost of the autotransformer.