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Universal Motor

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Universal Motor: An Overview

The electric appliances like the washing machines we use in your houses contain a universal motor. A universal motor can run on either DC or single-phase AC power. Also, it is known as a Commutator motor. The speed of the universal motor is over 3500 RPM. DC motors run at higher speeds on a DC supply. However, AC motors run at lower speeds on a DC supply due to the reactance voltage drop present in AC but not in DC.

Construction of Universal Motor

Universal motors are built very similarly to DC machines. A stator is mounted on which field poles are attached. Coils are wound on the field poles. The entire magnetic path is laminated. When operating on AC, lamination is required to minimise eddy currents.

The brushes used in the universal motor have high resistance because there is a commutator with brushes resting on a wound rotary armature that has straight or skewed slots. As a result of the current induced in the coils of the armature, AC commutation is poorer than DC commutation.

Brushes: These are the part of the stator that helps in passing the current to the rotor or moving part of the universal motor. Graphite is commonly used for making brushes.

Commutator: It works like a switch that reverses the direction of current between the rotating part and external circuit during the motion of the universal motor.

Universal Motor Diagram

A Universal Motor

A Universal Motor

This is the diagram of a Universal motor where supply is provided by an AC or DC source to the rotor through magnets.

Working of Universal Motors

There are two types of universal motors: DC motors and single-phase AC motors.

When We Provide DC Supply

Field windings produce electromagnetic fields when current flows through them. The same current is flowing through the armature conductors. Mechanical force is experienced by the conductor when it is placed in an electromagnetic field. The rotor starts to rote due to mechanical force or torque. We use Fleming's left-hand rule to find the direction of the force.

When We Provide an AC Supply

The effect of AC supply on the universal motor is that unidirectional torque is produced. Because the armature winding and field winding are connected in series, so they are in the same phase. Hence, AC changes its polarity periodically, and the direction of current for both armature and field winding gets reversed at the same time.

Therefore, the direction of the magnetic field and of the armature current reverse in a way that the direction of force experienced by armature conductors remains the same. It does not depend on whether we use AC or DC supply, a universal motor works on the same principle as that of a DC series motor.

Speed Load Characteristics of Universal Motor

The speed of the universal motor is low at full load and high at no load conditions.

Graph Showing Speed Load Characteristics of Universal Motor

Graph Showing Speed Load Characteristics of Universal Motor

Speed Control of Universal Motor

These motors run at a very high speed to avoid overheating and damage to the motor. We use some methods to reduce the speed of the motor, so they can run at a safe limit.

Method of Controlling the Speed of Universal Motor

  • The use of rheostat in the line circuit due to this variable voltage appearing across the motor terminals results in reducing the speed of the motor.

  • Governor controlling method. A governor consists of an assembly of spring-loaded contacts which is mounted on the shaft of a universal motor.

  • Using a gear mechanism, the gear train is used in this method with different gear sizes according to the requirement to get the desired speed.

Interesting Facts

  • Due to their cheap running capability, these are used in kitchen and house appliances like a blender, washing machines, electric shavers, hair dryers, vacuum cleaners, etc.

  • The typical operating speed range for universal motors is between 4000 and 20,000 revolutions per minute (RPM).

Solved Questions

1. What is the principle of the universal motor?

Solution: The armature current and magnetic field directions are reversed, keeping the armature conductors' sense of force in the same direction. Therefore, universal motors operate on the same principle as DC series motors, whether they are powered by an AC or DC supply.

2. What are the two types of the universal motor?

Solution: There are two kinds of universal motors in use: compensated and non-compensated. Typically, focused or conspicuous poles are used to construct the non-compensated motor. The compensated motor, on the other hand, has scattered field windings (main field and compensating winding).


You are now aware of what a universal motor is and its many varieties. This article discussed this motor as a unique type of electric motor whose power is dependent on both AC and DC. Additionally, this motor has strong starting torque and variable speed characteristics. In addition, we discussed how to regulate the universal motor's speed.

Last updated date: 18th Sep 2023
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FAQs on Universal Motor

1. What is the purpose of the brushes in a universal motor?

After passing via brushes and the commutator to reach the stator winding (also known as the field coils), electricity then travels to the armature coils. Electricity flows through each of the brushes in a single direction while they are all concurrently attached to various commutator segments. The conduction of current between the stator and rotor is done by the brushes. They are a very small part of the universal motor. It is also known as a motor brush. That is the only reason for using brushes in the motor.

2. Are universal motors AC or DC?

A unique kind of motor called a universal motor is made to operate on either a single phase of AC power or a DC source. These motors often have series-wound armatures and field windings, which results in strong starting torque. Due to this, universal motors have been pre-installed in the machinery they are designed to power. The majority of universal motors are built to function at speeds more than 3500 RPM. They can be used in small machines like drills, etc.

3. Why are universal motors so loud?

When the motor runs the brushes rub on the slotted armature, this produces very high noise during operation as most universal motors are manufactured for use at speeds up to 12000 rpm and can go up to 20,000 rpm. In some motors, we use a speed regulator to control the speed; otherwise, it can also make the motor hot if running for long hours. Due to the high operating speed, these motors are much smaller in size compared to other motors. For example, Electric drills and Vacuum cleaners.