Universal Motor


A Universal motor is a type of electrical motor which can run on DC supply or single phase AC supply of same voltage with similar characteristics are called as Universal motor.  Universal motors are series-wound (i.e. the armature and field windings are in series), due to which high torque is produced and they can run at high speed (about 3500rpm). These motors run at a higher speed on DC supply than they run on AC supply of the same voltage. This is due to the reactance voltage drop that is present only in AC and not in DC.

Connstruction of Universal Motor:

In Universal Motor, some modification are done which lets it run on AC supply as well as on DC supply. However, the construction is similar to that of Series Wound DC motor.

Construction of Universal Motor

It consists of a stator on which the field poles are mounted. Field coils are wound around the field poles. Both the stator field circuit and armature of a universal motor are laminated. Lamination is necessary to reduce the eddy currents that are produced 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. Hence, the brushes which are used in this motor have high resistance.

It is manufactured in two types:

  • Non-Compensated Type (Low H.P.)- The Non-compensated type has two poles, having entire magnetic path laminated. Armature is of Wound type.
  • Compensated Type (High H.P.)- The Compensated type has distributed field winding consisting of main field winding and compensating winding.
Compensated Type Universal Motor

Working of Universal Motor:

When Universal motor is fed with a DC supply, it works as a DC series motor. In this case, when the current flows in the field winding, it produces an electromagnetic field. The same current also flows in the armature conductors. When a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. This mechanical force causes the rotor to rotate. Fleming’s Left-hand rule gives us the direction of this force.

When Universal motor is fed with AC supply, a unidirectional torque is produced. This is because the armature winding and the field winding are connected in series and are in the same phase. Hence, when the polarity of the AC changes, the direction of the current in the armature and the field winding also changes. The direction of the magnetic field and the direction of armature current reverse so that the direction of force experienced by armature conductors remains the same. Hence, whether AC or DC supply is supplied, universal motors work on the same principle that DC series motors work on.

Properties of Universal Motor:

  • They run at high speed.
  • They have high starting torque.
  • They have Compact size and are lightweight.
  • They are noisy because of the commutator and brushes.
  • The efficiency of universal ranges from 30% for small motors to 75% for large motors.

Speed-Load Characteristic of Universal Motor:

Application of Universal Motor:

Universal motors are used in applications where high speed and good speed control is necessary. The various applications for which universal motor is used are:

  • Universal Motors are used in table fans, hairdryers and grinders.
  • They are used in portable drill machines.
  • They are used in polishers, blowers and kitchen appliances.