Effect of Change in Supply Frequency on Torque and Speed
The change in supply frequency hardly occurs in large distribution systems used on land. If there are some major disturbances or very heavy load fluctuating continuously, then there might be a minimal frequency variation. But large frequency variations are possible on electrical systems used on board ships and emergency supply systems for factories and hospitals. Such large frequency variations are possible on low power systems where diesel engines and gas turbines are used as prime movers.
As already mentioned, the relation between the speed of the motor and its frequency is given by the expression N = 120f/P.
From this expression, it is evident that the speed of the motor is directly proportional to the supply frequency. Thus any decrease or increase in frequency will affect the speed of the motor. Let us now analyze what exactly happens when a motor of 50Hz made to run with 60Hz supply and vice-versa.
When a 50 Hz motor is made to run on 60 Hz supply:
It is general practice in several countries to have all house-hold items and equipments rated for 50 Hz supply. So when such small domestic devices are connected to a 60 Hz supply, they cause a severe problem. For better understanding, let us visualize this small calculation:
[(60Hz – 50 Hz)/ 50 Hz] * 100 = 20 %. Thus all such equipments run 20 % faster than their normal rated speed. This is not safe for the equipment as the insulations may be rated for lesser capacity and windings may burn-out. To run safely, we either require a reduction gear or an expensive 50 Hz source.
Also this 50 Hz motor will operate perfectly on a 60 Hz supply provided its supply voltage is stepped-up.
60 Hz/ 50 Hz = 6/5 * 100 = 120 %.
60 Hz motor connected to 50 Hz supply:
It is same as the above, but instead of stepping-up the supply voltage, it is necessary to step-down the supply voltage.
50Hz/ 60 Hz = 5/6 * 100 = 80 %.