Want to learn about soft starters


Thread Starter

Cary Kuntz

Hi, I would like to find information about what I believe is a Star Delta type starter. If anyone is familiar with this style, please steer me in a direction to learn about it. Thanks
Jeremy Kuntz
Star Delta.

This is probably the most commonly used reduced current starter as it is the most economical. It comprises a line contactor, a 'star' contactor, 'delta' contactor , timing relay, overload relay and push buttons. This type of starting requires that all six leads of the motor be brought out to the terminal box.

This method of starting avoids the initial high inrush current and also avoids the high torque at approx 80% speed, which in certain cases can be
undesirable. When switched in 'star', a normally 'delta' connected motor has 57.7% of its nominal full 'delta' voltage impressed on the stator, and since a 'star' connected winding draws 57.7% of the current of a 'delta' connected winding, the nett result is a 33 1/3 % normal starting current.
Unfortunately, this reduced current is accompanied by a proportional reduction in starting torque.

If, for instance, a motor had locked rotor figures of 625% full load current and 125% full load torque respectively then with a star delta starter these would be reduced to approx 200% and 42% respectively. During acceleration the current falls, while torque increases to slightly higher than full load torque at approx 80% speed and then falls rapidly. When the motor reaches
approximately 90% speed the timer operates to de-energise the "star" contactor and energise the "delta" contactor to enable the motor to reach
full speed. At this point the current and torque will peak to the normal value on the DOL speed torque curve.

With this type of starter, it is important to match the speed/ torque curve of the load to the speed/torque curve of the motor under star/delta
starting, to ensure that at no point does the load curve cross the motor curve. If it does the motor will stall or else will draw excessive currents until full speed is reached. This type of starter is generally used for centrifugal fans and pumps where the load varies as the square of the speed, and a low starting torque is therefore adequate.
The Star-Delta starter is not strictly a soft starter.

It attempts to reduce the start up current of the motor by wiring a three phase motor in Star (or Wye) configuration to get the motor moving,
switching to Delta configuration once the motor is under load.

This starter is produced by hardwiring a number of contactors and a timer into a specific configuration. Just about all of the big names in contactors will be able to sell you a pre-configured Star-Delta starter (AB, Siemens,

For more information read a good electrotechnology text, I think Hughes has
it. Better still the Newnes Electrical Engineering Pocket Book.

A soft starter, is an electronic device, which controls the start of the motor by chopping the AC waveform with thyristors. For further information see Allen Bradley's website, http://www.ab.com, look for SMC, SMCPlus. I
have also used, and can recomend AOIP