Universal motors can run both on ac and dc supply. They are used in a large number of applications including vacuum cleaners, portable drilling machines, blenders, and juicers. The universal motor is actually a specially designed dc series motor. When connected to a dc source it utilizes rings and acts like a dc motor. A uni-current is achieved which produces unidirectional torque.
When it connects to ac, due to series connection the field and rotor coils are in series and same phase passes and electricity flow through left and right part of the coil is never unidirectional. The direction of magnetic flux changes (see video) which in turn results in that the torque action is always unidirectional. Irrespective of the speed of rotor and frequency torque is always in same direction and rotor continues to rotate.
An ordinary dc series motor can’t perform good and some modifications are required. Eddy current losses are large in the core, so the core layers should be laminated. Extra sparking is generated due to induction reaction at contacts so they should have high resistance.
The name universal motor is given to motors because they can run on both ac or dc.
Let’s understand the basic working of universal motors in this video:
In case of trouble watching the video please use the buttons below.