Construction of Universal Motor:
The construction of universal motor is just similar to DC motor. It consists of a stator on which field poles are mounted. The Field coils are wound on the field poles. However, the whole magnetic path comprising stator field circuit and also rotor or armature is laminated. Lamination is necessary to minimize the eddy currents which induce while operating on AC. The rotary armature is of wound type having straight or skewed slots and commutator with brushes resting on it.
The commutation on AC is poorer than that for DC because of the current induced in the armature coils. For that reason brushes used are having high resistance.
Working of Universal Motor:
A universal motor works on either DC or single phase AC supply. When the universal motor is fed with a DC supply, the current flows in the field winding and it produces magnetic field in the air gap. The same current also flows through the armature conductors. According to basic motor principle, when a current carrying conductor is placed in the magnetic field, it experiences a mechanical force. Thus mechanical force is exerted on the current carrying armature conductors and torque is produced on rotor. Therefore the rotor starts to rotate.
When fed with AC supply, it still produces unidirectional torque. Because armature winding and field winding are connected in series, they carry same current. Hence, as polarity of AC voltage changes and current reverses its direction, the direction of current in armature conductors and magnetic field in the air-gap reverses at the same time. The direction of magnetic field and the direction of armature current reverse in such a way that the direction of force experienced by armature conductors remains same.
Thus unidirectional torque is produced and motor continues to run in the same direction. As motor works on AC or DC supply, it is referred as Universal motor.
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