Effect of Change in Supply Voltage on Torque and Speed
Obviously, torque at any speed is proportional to the square of the applied voltage. If stator voltage decreases by 10%, the torque decreases by 20%. Changes in supply voltage not only affect the starting torque Tst but torque under running conditions also. If V decreases, then T also decreases.
Hence, for maintaining the same torque, slip increases i.e. speed falls.
Effect of Changes in supply Frequency on Torque and Speed
Hardly any important changes in frequency take place on a large distribution system except during a major disturbance. However, large frequency changes often take place on isolated, low- power systems in which electric energy is generated by means of diesel engines or gas turbines. Examples of such systems are : emergency supply in a hospital and the electrical system on a ship etc.
The major effect of change in supply frequency is on motor speed. If frequency drops by 10%, then motor speed also drops by 10%. Machine tools and other motor-driven equipment meant for 50 Hz causes problem when connected to 60-Hz supply. Everything runs (60 – 50) ´ 100/50 = 20% faster than normal and this may not be acceptable in all applications. In that case, we have to use either gears to reduce motor speed or an expensive 50-Hz source.
A 50-Hz motor operates well on a 60-Hz line provided its terminal voltage is raised to 60/50 = 6/5 (i.e. 120%) of the name-plate rating. In that case, the new breakdown torque becomes equal to the original breakdown torque and the starting torque is only slightly reduced. However, power factor, efficiency and temperature rise remain satisfactory.
Similarly, a 60-Hz motor can operate satisfactorily on 50-Hz supply provided its terminal voltage is reduced to 5/6 (i.e. 80%) of its name-plate rating.
Full-load Torque and Maximum Torque
Let sf be the slip corresponding to full-load torque, then
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