All About Universal Motor

The universal motor is a special type of motor that can run on both AC and DC.  These motors find applications in various household appliances such as vacuum cleaners, drink and food mixers, domestic sewing machines, etc. In this article, we will acquaint you with the basics of universal motor and the basic principles on which they work.

What is a Universal Motor?

A universal motor is a special type of motor that runs on either an AC or DC power supplies. The armature and field windings in a universal motor are in series. This series connection allows them to generate high torque. As universal motors generate high torques, they are normally built into the device that they are meant to drive.

How does a Universal Motor work?

There must be two magnetic fields for an electric motor to work. These magnetic fields must be associated with the stator and rotor. At least one of these magnetic fields must be moving in order to generate a rotary mechanical rotation. However, in a universal motor, the situation is different. A universal motor, as we know, contains a stator winding and rotor winding connected in series, as shown in the circuit diagram below. This series connection creates a voltage difference between them so that the rotor can pursue the stator’s rotating magnetic field.

Theoretically, universal motors can operate both on AC and DC. However, a universal motor may not efficiently operate on AC. To perform efficiently on the AC power supply, the motor needs some modification. A compensating winding is added with the pole pieces laminated to prevent wasteful circulating current for better performance. As compared to the universal motor intended for DC operation, there are more coils and plates in the armature with fewer windings in each coil of a universal motor operated in AC. The AC operated universal motors are commonly used in small appliances that require high power for their operation such as blenders, vacuum cleaners and sewing machines.

Universal Motor Speed Control

The speed of the universal motor is controlled to prevent it from running at dangerously high speed. Following are some of the ways by which the speed of the universal motor is controlled:

  • Centrifugal Mechanism
  • Field Tapping method of Speed Control
  • Resistance Method of Speed Control
  • Using a gear train to reduce the actual load speed

How can a Universal Motor be reversed?

The direction of the universal motor can be reversed by one of the following methods:

  • By using two field windings that are wound in opposite directions such that the one connected in series with the armature gives clockwise rotation. In contrast, the other in series with the armature gives counterclockwise rotation.
  • Reversing the field connection with respect to the armature

Speed Load Characteristics of Universal Motor

The universal motor’s speed is highest at the no-load condition and the lowest at the full load condition. Gear trains are used to get the required speed at the required load. The speed-load characteristics of universal motors for both AC and DC are shown in the figure below.