Starting of asynchronous motors

Starting of asynchronous motors

Star delta starters are specially suited to machines which do not present a high load torque at start up or which normally start off-loads. The relatively high peak current during the star to delta transition, which is characteristic of these starters, means that where large motors above a certain rating are to be used, some form of current limiting techniques may be necessary.
Introducing a delay of one to two seconds during the star to delta transition. This will effectively reduce the level of peak transient current. It should be noted however, that this solution may only be applied to low inertia loads, to avoid too large a drop in speed during the transition.
Three stage starting, star – delta plus resistance stage in star. The transient peak current still exits but its value is limited by the resistance being left in circuit for about three seconds after the star delta transition.
Star-delta starting plus resistance stage and a closed transition. The resistance bank is put into the circuit just before the star contactor opens. In this way, the motor current is never actuarially broken and so the transient peak is eliminated. The adoption of the above techniques will obviously mean that more components are needed and so for a given starter, the cost could increase quite considerably.
For difficult applications the use of a suitable electronic “soft start” such as the telemecanique “altistart” for example, could well be the best solution.