Autotransformers for starting of induction motors

Depending on power and type of induction motors their start-up can be conducted by:

  • direct connection to the supply network,
  • decrease of motor supply voltage (star/delta switch or autotransformer),
  • increase of rotor circuit resistance (starter),
  • change of the number of pole pairs (pole switch),
  • change of supply voltage frequency (frequency converter)  

An autotransformer can be used for current reduction at start-up of high power asynchronous squirrel-cage motors.
The autotransformer provides a lower voltage at start-up in a similar way that star-delta switching reduces current. The internal impedance of the autotransformer helps limit start-up current within the network, in a way which is not available with star-delta switching.

The advantage of this way of start-up is lowering of the following currents:

  • current flowing in the motor winding (IRS) – it is lower by autotransformer ratio:

  • current absorbed from supply network during the start-up (I1) – it is lower by autotransformer ratio square:

The disadvantage of this way of start-up is that the motor initial torque (MPR) is decreasing to the same degree as the current absorbed from network:

where: ϑ–autotransformer ratio , IP – initial start-up current when supplying the motor with full voltage, IRS – current flowing in winding motor, I2 – autotransformer secondary current, I1 –autotransformer primary current (absorbed from supply network), MPR –motor initial moment when supplying from autotransformer, MP – initial moment developed by the motor at full voltage

Therefore when selecting the ratio of start-up autotransformer it is always necessary to ensure that the torque developed by the motor at lowered voltage is greater than the torque of driven machine resistance.

Start-up system of the squirrel-cage induction motor with start-up autotransformer

The system presented on the drawing above is a commonly used solution for start-up of asynchronous motors. The start-up is conducted in two stages, without occurrence of voltage-free interruptions and it lasts for a period of 20 to 30 seconds. 
Initially the motor runs at a lowered voltage provided by the autotransformer (Q1 and Q2 contacts are closed and Q3 contact is open). At this time start-up current is limited by a suitable selection of autotransformer ratio. Next Q2 is opened, where the motor is supplied from the network via the series impedance of the autotransformer windings, which are connected in series. Thus the windings function as chokes limiting the start-up current. Finally, when the motor achieves an appropriate rotational speed, Q3 is closed, where the motor is supplied with full voltage directly from the network.
If necessary and after agreeing with Customer ELHAND can produce the start-up autotransformers with several taps. Remember that the value of start-up current is significantly greater than autotransformer rated current. Thus, in order to avoid the overheating of the autotransformer windings after multiple start-ups, the winding temperature is monitored using of temperature sensors installed inside the winding.

Start-up autotransformer ELHAND, type EA3R, with temperature sensors.