A single-phase universal motor is operated with AC source. The torque of the motor during the negative half cycle of the input current:

Concept:

Universal motor:

• A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque.
• They run at lower speed on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is present in AC and not in DC.

Construction:

• Construction of a universal motor is very similar to the construction of a DC machine.
•  It consists of a stator on which field poles are mounted. Field coils are wound on the field poles.
  However, the whole magnetic path (stator field circuit and also 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:

• When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in same phase.
•  Hence, as polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time.
• Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same.
• Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works.