Motors

Well! You need to be explained the whole motor starting theory, indeed! At least are you aware that a motor's starting current is much more than its rated current, when started direct-on-line - typically 6 to 8 times its rated full load current? Now, how do you avoid this high starting current? In delta connected motors, when full line voltage is applied to the motor at the time of starting itself, this full line voltage is also the phase voltage (as it is delta connected) & this full line voltage is the one available to the motor's phase windings. As the applied voltage is high, so also will be the current. There could be problems to the motor as well as to the installation, due to this high starting current. One of the easier ways to reduce this high starting current is to reduce the voltage applied to the windings during starting. In star-delta starter, you do exactly the same thing. That is, when you initially connect the windings in star, the voltage seen by the phase winding of the motor is the phase voltage and not the line voltage. So, as the voltage is reduced so also will be the starting current. But, as the motor picks up its rated speed, it would start delivering Horse Power and please recall that HP or kW = Root3 x V x I x Cos Phi. Now, if you still kepp the motor windings connected in star itself, even after the motor has accelerated to its rated speed, then if the horse power demanded by the load is as per the rating, then as the voltage is only about 57%, going by the above formula, the motor would draw, 1.732 times more current to deliver the required HP. It would overheat & possibly fail. To avoid this, once the motor has picked up speed, the winding connections are changed to delta, wherein the voltage is the line voltage now and thus the 'running' current would be within limits.

Wait! Let me take a deep breath!