Question

Describe the effects of frequency and voltage change on an induction motor.

Effects of voltage change

At constant voltage if frequency is increased from 50Hz to 60Hz there is an increased Inductive resistance XL. As stator flux is reduced this effects the starting torque increasing starting current demand. Higher speed increases power output. If a centrifugal pump or fan the power increase is proportional to the speed cubed ( [60 / 50]3 = 1.728) giving a 73% increase in power demand.

At constant voltage if the frequency is decreased from 60Hz to 50Hz the stator flux is increased but the speed is reduced by a 83%. Unless the load is reduced the machine will run hotter than normal. Starters and contactors could be adversely affected. A 440v 60Hz system supplied from a 415v 50Hz shore supply runs at 83% speed, slightly hotter but should run without damage.

Effects of frequency change

At constant frequency if voltage is reduced this has little effect on speed (less than 5%) but increased current for same power. Torque is proportional to the square of the voltage therefore there is a corresponding and greater drop in available starting torque, this leads to longer run up times and the possibility of stalling.
As induction motors very really run at full load, a large voltage reduction would be required to cause a damaging current.

At constant frequency if voltage is increased gives a stronger stator flux depending on slot design and original flux density this could increase stator iron losses sufficiently to cause overheating.