How Motors Work [How an AC and DC motor works]

As a college lecturer who teaches electrical science, I get asked on a daily basis by my students to give an in-depth explanation as to how electric motors work, and the principle of operation of an electric motor, both AC and DC. So rather than answering the same questions every day, I have decided to write this blog post to use as a study guide.

Motors work on the principle of electromagnetic induction, to convert electrical energy into mechanical energy. Motors consist of two main components, the rotor, and the stator. When current flows through the stator windings a magnetic field is produced, this provides the energy needed to create enough rotational torque to turn the rotor.

Understanding the intricate mechanisms that drive their operation is crucial for apprentices, engineers, scientists, and enthusiasts alike.

In this article, we will delve into the scientific principles behind electric motors, exploring the key components, the conversion of electrical energy into mechanical energy, and the fundamental concepts that govern their performance.

motor maintenance pdf

How Motors Work

Electric motors are the most commonly used electric appliance ever produced. Electric motors power everything from a huge array of machinery and household appliances to industrial equipment and electric vehicles.

Electric motors consist of two primary components: the stator and the rotor. The stator is the stationary part of the motor or the frame and the rotor is the shaft that turns the pulley you see on the outside.

induction motor disassembled

Motor Stator

The stator is made up of an arrangement of wire windings which are known as the stator windings, these windings are carefully arranged and very closely wound together as you can see from the image above.

The stator will usually either have two or three windings, depending on the type of motor being used. Each winding will be a continuous loop from one end to the other, meaning you will either have four or six end points.

The ends of the stator windings finish up inside the motor terminal box where they then get connected to the terminals usually marked “U V W”, along with the incoming supply cables.

Motor Rotor

The rotor, on the other hand, is the rotating part of the motor that is connected to the pulley, conveyor drive, gear, etc. that the motor is turning. The rotor is free to rotate and is held in place by bearings at each end, it is important to note the rotor has no electrical connection and works solely on the principle of electromagnetic induction.

The main purpose of the rotor is to interact with the magnetic field generated by the stator and produce an opposing force which would result in rotation.

A squirrel cage rotor is the most common type of rotor used in induction motors. It’s constructed using laminated iron cores stacked together to form a cylindrical shape. The laminations are made of a high-quality silicon steel, which helps to minimize eddy current losses.

The rotor core consists of evenly spaced slots where conductive bars are placed. These bars, typically made of aluminum or copper, are shorted at both ends by conducting end rings, giving the rotor its characteristic “squirrel cage” appearance.

These bars are positioned at a slightly skewed angle to produce a stronger electromagnetic torque and prevent flat spots, rocking, or difficulty starting due to the negative cycle of the sine wave.

Electromagnetic Induction

At the heart of electric motor operation lies the principle of electromagnetic induction. When an electric current flows through the wire windings of the stator, a magnetic field is produced around the windings which then interacts with the ferromagnetic rotor creating rotation.

This magnetic field is crucial for generating mechanical motion. As the magnetic field continuously changes with alternating current (AC) or remains constant with direct current (DC), it induces an electric current in the rotor.

demonstration of a magnetic field

The conductive bars and end rings of the rotor form a closed loop, which allows the flow of currents induced by the rotating magnetic field generated by the stator.

This design enables the rotor to produce torque or energy. This torque then propels the rotor into rotation, converting electrical energy into mechanical energy.

In AC motors, the rotating magnetic field produced by the stator induces currents in the rotor through electromagnetic induction. In DC motors, the rotor’s magnetic field aligns with the stator’s magnetic field, creating a torque that initiates rotation.

The materials used in a squirrel cage rotor are carefully selected to balance electrical conductivity, mechanical strength, and cost-effectiveness, ensuring reliable and efficient motor performance.

Ensuring these windings are connected in the correct order is crucial for the operation and life longevity of the motor.

In order to stop the motor windings shorting out when current is applied a special shellac varnish insulation is used to insulate the windings from themselves and the motor housing.

The speed and efficiency of electric motors can be controlled and optimized through various techniques. For DC motors, controlling the input voltage or employing pulse-width modulation (PWM) allows precise speed regulation.

In AC motors, frequency control and variable frequency drives (VFDs) are utilized to adjust the rotational speed. Efficiency is a crucial factor, and motors are designed to minimize losses due to friction, resistance, and other factors, thus maximizing the conversion of electrical energy into mechanical work.

As researchers and engineers continue to explore advancements in motor technology, the efficiency, performance, and versatility of electric motors will undoubtedly expand, driving innovation and powering the machinery of our modern world.

How AC Motors Work

Alternating Current (AC) motors operate based on the principle of electromagnetic induction and works by utilizing a single phase or three phase power supply. The stator, the stationary part of the motor, houses wire windings (stator windings) which are formed into a coil and arranged in a specific pattern.

When an AC current flows through these windings, it generates a rotating magnetic field. This magnetic field induces a current in the rotor, the rotating part of the motor, leading to an interaction with the stator’s magnetic field.

As a result, the rotor begins to rotate, initiating mechanical motion. AC motors come in various types, such as synchronous motors and induction motors, each with unique characteristics and applications.

How to change the speed of an AC motor

Changing the speed of an AC motor requires specific techniques that differ from those used for DC motors. AC motors are commonly used in various industrial, commercial, and residential applications, and having control over their speed is essential for optimizing performance. Here are some methods used to adjust the speed of an AC motor:

Frequency control

One the most commonly implemented ways to regulate the speed of an AC motor is by adjusting the frequency of the power supply. The speed of an AC motor is directly proportional to the frequency of the supplied current. By varying the frequency, the motor’s speed can be increased or decreased.

Frequency control is typically achieved using variable frequency drives (VFDs) or inverters. These devices allow precise control over the frequency and voltage supplied to the motor, enabling smooth and efficient speed regulation. VFDs also provide additional benefits such as soft-start capabilities, overload protection, and energy-saving features.

Change the number of poles

Some AC motors, such as induction motors, are designed with multiple sets of windings known as poles. By changing the number of active poles in the motor, the speed can be adjusted.

This method is commonly employed in applications where the motor requires a limited range of speed control, fine tuning is not possible by changing the number of poles in the motor.

By increasing the number of poles you decrease the speed of the motor, the most commonly used motor is a two pole motor due to its high speed.

Number of PolesSpeed Range (RPM)
23600-3000
41800-1500
61200-1000
8900-750
10720-600
12600-500
16450-375
20360-300
24300-250

As you can see from the table above the speed can vary depending on the motor type, load and temperature. It is also important to note, that when referring to poles, two poles are equal to one pair of poles which they are commonly referred to.

Pole changing can be achieved mechanically or electrically, depending on the motor’s design. Mechanical methods involve physically changing the connection of the windings, while electrical methods utilize switches or contacts to alter the winding configuration.

However, pole changing is not as commonly used as frequency control due to its limited speed adjustment range.

Variable Speed Drive Systems

Variable speed drive (VSD) systems provide advanced speed control capabilities for AC motors. These systems combine frequency control with sophisticated control algorithms to precisely regulate motor speed. VSDs utilize feedback sensors, such as encoders or tachometers, to monitor the motor’s actual speed and adjust the frequency and voltage accordingly to maintain the desired speed.

variable speed drive controllers

VSDs offer features like acceleration and deceleration ramps, torque control, and speed profiling, making them suitable for applications that require precise and dynamic speed regulation. They are commonly used in industries such as manufacturing, HVAC systems, pumps, and fans.

Regulate stator voltage

Another method to change the speed of an AC motor is by adjusting the voltage applied to the stator windings.

By varying the stator voltage, the magnetic field strength can be modified, which in turn affects the motor’s speed. However, it’s important to note that stator voltage control may also influence the motor’s torque characteristics.

Stator voltage control can be achieved using autotransformers, tapped windings, or electronic devices that regulate the voltage supplied to the motor. This method is often used in applications where speed adjustment is not critical or where the motor operates at a constant load.

Slip control

In induction motors, slip refers to the difference between the synchronous speed (the speed of the rotating magnetic field) and the rotor speed. By adjusting the slip, the speed of the induction motor can be controlled. Slip control is commonly achieved using rotor resistance control or by employing complex control algorithms in VSDs.

Slip control methods allow for efficient speed control in applications where high torque is required at low speeds, such as in conveyor systems, cranes, and elevators.

How DC Motors Work

Direct Current (DC) motors also work on the principle of electromagnetic induction but differ slightly from AC motor, the main difference being, DC motors create motion by using electromagnetism and rely on the power of a direct current (DC) electricity supply to generate mechanical motion.

Check out this article to find out what are the 5 types of single-phase motors and how exactly how they differ from three-phase motors.

DC motors find extensive use in diverse applications, ranging from electric vehicles and robotics to household appliances.

A typical DC motors consists of four essential components: the stator, rotor, commutator, and brushes. The stator will either contain permanent magnets or electromagnetic windings as we seen with the AC motor, while the rotor typically features coils wound around a laminated iron core.

Below is an excellent Youtube video by the Engineering Mindset covering how DC motors work with visual representations.

How a commutator works

The commutator is a segmented cylindrical structure which plays a crucial role in the conversion of electrical energy in DC motors. A commutator makes electrical contact with the carbon brushes which connects the electrical supply to the windings.

This then allows the current to flow through the windings of a DC motor. The commutator is an essential part of a DC motor and refers to the process of reversing the direction of current flow in the rotor winding of a DC motor.

By reversing the direction of current flow in the rotor winding, the torque produced by the interaction of magnetic fields between the stator and rotor remains constant. This enables the rotor to keep spinning, providing the necessary mechanical motion.

Why brushes are used in a DC motor

The brushes are small, typically rectangular blocks made of carbon or graphite, that are in contact with the commutator. The commutator is divided into multiple segments, usually made of copper or copper alloys, that are insulated from each other.

The main purpose of brushes in a DC motor is to provide electrical contact between the stationary part of the motor, known as the stator, and the rotating part, which is the rotor. The brushes are responsible for transferring electrical current from the power source to the rotor windings, enabling the generation of the magnetic field that drives the motor’s rotation.

open view of carbon brushes making electrical contact with a dc commutator

The key functions of brushes in a DC motor:

Current Transfer: Brushes act as conductive elements that bridge the electrical gap between the stationary stator windings and the rotating rotor windings. They establish a continuous electrical connection, allowing the flow of current from the power source to the rotor.

Commutation: The commutation process in a DC motor involves the reversal of the current direction in the rotor windings as the rotor rotates. This reversal ensures that the magnetic field produced by the rotor aligns with the magnetic field of the stator, enabling smooth and continuous rotation. Brushes facilitate this process by making contact with different segments of the commutator as it rotates, ensuring the appropriate current flow through the rotor windings.

Wear Compensation: Due to the mechanical contact between the brushes and the commutator, some wear and friction occur. Over time, the brushes can wear down and become shorter. However, the brushes are designed with a specific spring mechanism or pressure system to compensate for wear. This ensures consistent and adequate contact between the brushes and the commutator, maintaining efficient electrical conductivity.

Brush Seating: When a DC motor is initially started or after maintenance, the brushes need to seat properly against the commutator surface. Proper seating ensures optimal contact and reduces the risk of sparking and brush instability. This process may involve running the motor at low speeds for a short period to allow the brushes to conform to the shape of the commutator.

It is worth noting that the choice of brush material is crucial, as it affects various aspects of motor performance.

Carbon and graphite brushes are commonly used due to their good electrical conductivity, self-lubricating properties, and ability to handle high temperatures along with having a temperature coefficient, meaning the hotter they get the less resistance they have.

Most Other factors considered when selecting brushes include the motor’s speed, load, and desired operating conditions.

Because AC motors use alternating current a commutator and brushes isn’t needed due to the alternating magnetic field.

Effective commutation is essential to maintain smooth motor operation, prevent arcing and sparking, and ensure optimal performance. Proper commutation ensures that the current flow through the rotor coils aligns with the position of the magnetic field, enabling efficient

How to control the speed of a DC motor

Voltage Control

One straightforward method to change the speed of a DC motor is by adjusting the applied voltage. By increasing or decreasing the voltage supplied to the motor, the speed can be modified accordingly.

Voltage control in DC motors refers to the method of adjusting the applied voltage to regulate the motor’s speed. This control method directly influences the electromotive force (EMF) generated in the motor’s armature winding, which, in turn, affects the speed at which the motor rotates.

A higher voltage results in increased motor speed, while a lower voltage reduces the speed. This method is simple and effective, particularly for basic speed control applications. However, it has limitations, as changing the voltage directly affects the torque characteristics of the motor, which may impact overall performance.

dc voltage controller

In a DC motor, the armature winding is connected to a power supply, usually in the form of a DC voltage source. The voltage level determines the strength of the electric field generated within the motor, which influences the motor’s ability to produce torque and achieve different speeds. By manipulating the applied voltage, the motor’s speed can be increased or decreased.

When a higher voltage is applied, the armature experiences a stronger electric field, resulting in increased motor speed. Conversely, reducing the voltage decreases the electric field strength, causing the motor to rotate at a slower speed. This voltage control method is relatively straightforward and can be implemented through manual adjustment or with the help of electronic circuits and control systems.

It’s important to note that while voltage control can effectively change the speed of a DC motor, it may also have an impact on the motor’s torque characteristics. Altering the applied voltage directly affects the strength of the magnetic field in the motor, which in turn affects the motor’s ability to produce torque. Therefore, careful consideration must be given to balancing the desired speed control with the motor’s torque requirements for optimal performance in different applications.

Voltage control is commonly employed in various industries and applications that require adjustable speed control of DC motors.

Resistance control

Another method to control the speed of a DC motor is by adding external resistors in series with the motor windings. These resistors limit the current flowing through the motor, thus reducing the speed. By changing the resistance value, the speed can be adjusted.

This method is commonly used in simple DC motor speed control applications, but it has drawbacks such as power dissipation and reduced efficiency due to the energy loss in the resistors.

Pulse Width Modulation (PWM)

Pulse Width Modulation is a widely used technique for controlling the speed of DC motors. It involves rapidly switching the motor’s power supply on and off with a variable duty cycle. The average voltage applied to the motor is adjusted by changing the ratio of the on-time to the off-time.

A higher duty cycle corresponds to a higher average voltage, resulting in increased motor speed, while a lower duty cycle decreases the speed. PWM offers precise speed control, higher efficiency compared to resistance control, and reduced power dissipation. It is commonly implemented using microcontrollers or dedicated PWM controllers.

Current control

Controlling the current flowing through the motor windings can also influence the motor speed. By adjusting the current, the torque produced by the motor changes, which affects the rotational speed.

Current control can be achieved using various techniques such as chopper circuits, which regulate the current by rapidly switching the motor supply voltage. By modulating the switching frequency or duty cycle, the current and consequently the speed can be adjusted.

Field control

In some DC motors, particularly those with separately excited or shunt-wound field windings, adjusting the field current can vary the motor speed. By changing the field current, the magnetic field strength is altered, influencing the motor’s torque-speed characteristics. This method offers fine speed control and is commonly used in applications where precise speed regulation is required, such as in industrial machinery and robotics.

Electronic Speed Controllers (ESC)

Electronic Speed Controllers are widely used in DC motor speed control applications, particularly in RC (radio-controlled) vehicles. ESCs use advanced electronic circuits and microcontrollers to regulate the motor speed.

They employ techniques such as PWM or current control to adjust the motor speed based on user input or control signals. ESCs offer precise control, acceleration and deceleration profiles, and often include additional features like motor protection and reverse capabilities.

Conclusion

In summary, understanding how motors work reveals the fascinating interplay between electrical energy and mechanical motion. We explored the fundamental principles of AC and DC motors, delving into electromagnetic induction and the generation of rotating magnetic fields. Torque, speed control, and efficiency were discussed as crucial aspects of motor operation.

As we continue to advance, motors remain an integral part of technological progress, symbolizing the synergy between science, engineering, and human imagination.

Gavin

I'm Gavin and Iv been teaching electrical science to apprentice electricians in a local technological university since 2022. I hold an Electrical Level 6 QQI Qualification along with several NZEB Certifications.

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