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Facts about Universal Motor Running on AC Universal Motor Circuit

Some single phase motors can be run not only with DC circuit but also with AC universal motor circuit and for that reason they are known as Universal Motors. These motors are also known as commutation type motors as they continue to run in the same direction even if the polarity of the line terminals of their DC circuit is reversed.

AC universal motor circuit

The polarity of both the fields and the direction of current in the armature can help in determining the direction of the running motor because armature current and fluctuation are proportional to torque. If, a DC motor is connected to AC universal motor circuit the reversal of flow of AC current from negative to positive will affect direction of current in armature and flux polarity but not the direction of the motor.

The change in AC universal motor circuit will not change the direction of rotation of the motor if the direction of the torque developed is positive. But the frequency of the torque will be twice the frequency of the line because torque is pulsating by nature.

In this way a universal motor works with AC universal motor circuit as well as DC circuit with equal efficiency. But a motor that is quite specifically designed to work on DC can show following issues if connected to single phase AC supply.

  • Low efficiency due to lots of current loss

  • Excessive sparking at the bushes

  • These drawbacks can be defeated by making following modifications in DC circuit to work in unison with AC circuit.

  • Making field core with the materials with low current loss like laminated materials

  • Reducing density of flux by increasing area of the filed poles as less use of iron will reduce drop in voltage

  • Increase the number of conductors in the armature to get required torque

  • Use compensating winding in the stator slots to improve the commutation process and reduce the reaction of armature

  • Commutation problem can also be reduced by increasing the area high resistance brushes

  • Losses of eddy current can also be reduced yoke and stator core can be laminated

  • Solid state devices can be used to control the speed of universal motors

  • In this way, a universal motor can be run with DC currents as well as AC universal motor circuit with equal efficiency by making few modifications. 

Keywords in the article: AC universal motor circuit, AC universal motor, Power Motor