Universal Motor: Construction, Application, Characteristics, Types and Working

Universal Motor Overview

A universal motor is a type of motor that can operate on both AC (Alternating Current) and DC (Direct Current); this motor is therefore called a Universal Motor. The electric motor, which is a mechanism capable of turning electrical energy into mechanical energy, was created by Michael Faraday and Henry Joseph. The motor generates a rotational motion by utilizing the magnetic fields generated by the rotor and stator. Universal motors can even run on all frequencies, making them widely adaptable.

What is Universal Motor?

Universal Motor can work with nearly the same features on both an AC and a DC power source. It is referred to as 'Universal' because of its ability to work at all power levels.

Most universal motors are designed to run at rates of up to 3500 RPM. These motors operate faster on a DC source than they do on an AC supply of the same voltage. This is owing to the reactance voltage loss, which exists only in AC and not in DC.

Features of Universal Motor

1. Dual AC and DC operation: The universal motor is capable of operating on both AC and DC power, making it versatile and adaptable to a wide range of applications.
2. High starting torque: The universal motor has a high starting torque, which allows it to quickly reach full speed and handle heavy loads.
3. Compact and lightweight: The universal motor is typically compact and lightweight, making it ideal for use in portable devices and applications where space is limited.
4. Variable speed control: The speed of the universal motor can be easily controlled by adjusting the voltage applied to the motor, making it well-suited for applications that require variable speed control.
5. Relatively inexpensive: The universal motor is relatively inexpensive to manufacture, making it a cost-effective option for many applications.
6. Simple construction: The universal motor has a simple construction, consisting of a stator and a rotor, which makes it easy to manufacture and maintain.
7. High power density: The universal motor has a high-power density, meaning it can produce a relatively high amount of power in a small size, making it ideal for applications where space is limited.
8. Suitable for a wide range of applications: The universal motor is widely used in a variety of applications, including power tools, vacuum cleaners, food mixers, hairdryers, and many others.

Universal Motor Construction 

The construction of a universal motor is relatively simple and consists of two main parts: the stator and the rotor.

The stator is the stationary part of the motor and is made up of a series of electromagnets. These electromagnets are typically arranged in a circular pattern around the inside of the motor housing. The number of electromagnets can vary depending on the size and power rating of the motor. The stator is usually made of laminated steel to reduce eddy current losses.

The rotor is the rotating part of the motor and is typically made up of a series of conducting bars that are arranged in a cylindrical shape. These bars are usually made of copper or aluminium and are connected by two end rings that complete the circuit. The rotor is mounted on a shaft that is supported by bearings and is free to rotate within the stator.

The rotor and stator are connected by a set of brushes that contact the commutator. The commutator is a segmented ring that is mounted on the rotor shaft and is connected to the rotor bars. As the rotor spins, the brushes contact different segments of the commutator, which changes the direction of the current flow through the rotor windings.

Universal Motor Working Principle

When a universal motor is supplied with a DC power source, it operates similarly to a DC series motor with brushes. The current flows from the field coils on the stator to the commutator through the brushes and reaches the armature coils or rotor winding. Each segment of the commutator conducts current in the same direction, which produces an electromagnetic flux. As a result of the magnetic field around it, the current-carrying armature conductor experiences a mechanical force or torque, causing the rotor to spin. The motor operates with high efficiency in this configuration.

On the other hand, when the universal motor is subjected to an AC power source, the current flows in different directions in the opposite coils. To address this, a series connection is used, which causes the polarity change and magnetic flux in the winding to reverse simultaneously. As a result of the unidirectional magnetic field and torque action, the rotor rotates in the same direction despite the alternating current. However, the efficiency of the motor is much lower when using AC power.

Connecting a DC series motor to an AC power source results in significant losses due to eddy currents and a fluctuating magnetic field. Therefore, to prevent this, the stator windings and armature are laminated. To change the direction of the motor's rotation, either the stator or the rotor winding connections need to be reversed.

Universal Motor Characteristics

1. Universal motors can operate on both AC and DC power sources.
2. They have a high starting torque, making them ideal for applications that require a strong initial boost.
3. Universal motors have a high power-to-weight ratio, making them lightweight and compact.
4. They have a simple and inexpensive construction, making them cost-effective.
5. Universal motors are highly efficient when operated on DC power, but their efficiency decreases when operated on AC power.
6. They can operate at high speeds, making them suitable for applications that require rapid motion.
7. Universal motors produce high levels of noise and vibration due to the rapid changes in magnetic fields and the use of brushes.
8. They require periodic maintenance due to the wear and tear of brushes and commutators.
9. Universal motors are commonly used in household appliances, power tools, and small machinery.

Universal Motor Types

The two types of Universal Motor are:

• Non-Compensated Motor

This motor has concentrated poles and is used for appliances that require minimal power. High resistance brushes are used in the motor's construction to enhance commutation. 

• Compensated Motor

A distributed field winding is used in this form of motor. The compensating winding is an additional winding in addition to the main field winding. When the motor is supplied with alternating current, the additional winding reduces the high voltage reactance in the armature caused by the alternating flux. It is found in high-powered equipment. 

The compensating winding is incorporated into the pole faces and linked to the armature in sequence. The compensating windings magnetic field cancels out the armature's cross-magnetic flow.

Application of Universal Motor 

1. Power Tools: Universal motors are widely used in power tools like drills, sanders, grinders, and circular saws due to their high torque, speed control, and compact size.
2. Household Appliances: Universal motors are used in various household appliances like vacuum cleaners, blenders, mixers, washing machines, and fans due to their low cost, high torque, and compact size.
3. Automotive Industry: Universal motors are used in electric vehicles, electric bicycles, and other automotive applications due to their high power-to-weight ratio, high starting torque, and easy speed control.
4. Industrial Machinery: Universal motors are used in various industrial machinery like textile machines, conveyor systems, and printing presses due to their high starting torque, speed control, and compact size.
5. Entertainment Industry: Universal motors are used in entertainment industry equipment like audio mixers, DJ turntables, and lighting systems due to their high-speed capabilities, torque, and easy control.
6. Medical Equipment: Universal motors are used in medical equipment like dental drills, centrifuges, and other surgical instruments due to their high torque, low vibration, and compact size.
7. Aerospace Industry: Universal motors are used in various aerospace applications like robotic arms, drilling machines, and other machinery due to their high starting torque, speed control, and lightweight design.

Overall, the versatility, efficiency, and ease of control of universal motors make them an excellent choice for various industrial, commercial, and domestic applications.

Universal Motor: Things to Remember

• Universal motors operate at a very high speed when there is no weight, which is very risky.
• It is a double-exciting series-wound motor.
• It has a variable speed and strong starting torque. 
• When the motor is powered by an AC source, compensating winding is used to reduce the reactance voltage in the armature.
• Due to its high speed, a universal motor generates more electricity than any other A.C. motor.
• A universal motor's construction is a little different from a DC series motor, despite the fact that their operating principles are similar. High resistance brushes and compensating windings are used to minimize armature reaction and achieve commutation when using an AC supply.