Automatic Star/Delta 3 Phase Motor Starter

When you connect the windings of an appropriately designed motor in Star, the effective winding voltage is Phase to Neutral, so it is reduced by the square-root of three (1.732) and is thus 58% of the motor's design voltage. Breakdown torque and thus current draw is reduced by the square of the applied voltage, so .58 x .58 = .33. This means that the line current draw is reduced to 1/3 of what it would be if the motor was started Across The Line (Direct On Line), as is the torque. Hopefully the accelerating torque requirement is such that the motor can get to nearly full speed, at which time the starter reconfigures the winding connections to be Delta, giving the motor full rated voltage and thus full torque.

Chinned, the principles of opperation is just the way it sounds in your ears. First a Star and secondly a delta. In application, the motor is expected to start in star and after some predetermine time it is switched over to delta and the full supply voltage applied to the motor. The reason for this is to reduce the starting current of the motor by the increase resistance of the star circuit and using some predetermined time of the timer to allow the motor build up an opposing emf (usually known as Back EMF in electrical parlance) When the back emf is sufficiently build up, the motor is switched to delta. Note. In delta IP= 3^1/2.IL and in star IP=IL. In delta VP=VL and star VP=3^1/2.VL Also remember that V=IRcos@ So when the voltage is reduced, the current also reduces. Take care Chinedu.

There are two types.One simply changes over from star to delta connection with a surge in current during changeover. Other has a transition resistance in each phase to reduce the surge during changeover.(Korndorfer starter)