What is the Structure of a Motor? | The Principle and Mechanism of Rotation, and Reasons for Widespread Use

table of contents

• ・Basic Structure of a Motor
• ・Principle of Motor Rotation
• ・Applications of Motors and Reasons for Widespread Use
• ・Summary

A motor converts electrical energy into mechanical energy. Motors are used in a variety of fields, from familiar products such as household appliances, automobiles and communication devices to industrial equipment. What structures and mechanisms do these motors have? And what are the reasons for the widespread use of motors among the various power sources that are available?

There are many different types of motors, but this article focuses on brushed DC motors and explains their structure, principle of rotation and fields of application.

Basic Structure of a Motor

There are many types of motors, including direct current (DC) and alternating current (AC) motors. Of these, the brushed DC motor, which is a direct current motor, is the most basic motor. Motor structure is explained below, using a brushed DC motor as an example.

Brushed DC motors consist of a rotor, coil, stator, permanent magnets, commutator and brushes. The roles of each of these are shown below.

Rotor

The rotor is the part of the motor that rotates. The rotor is coiled (wound). The rotation of the rotor is what powers the machine.

Coil (windings)

A coil is an electrical wire (conductor). It is mounted on the periphery of the rotor and is able to conduct an electric current. Copper and aluminum are common materials. Both ends of the coil are connected to the commutator, described below.

Stator

The stator is the component that generates the force that rotates the rotor, and is installed around the outside of the rotor. The stator incorporates permanent magnets.

Permanent magnets

Permanent magnets are built into the stator and are placed so that the N and S poles face each other. This means that the coil and rotor are located in the magnetic field of the N and S poles. When a DC current is applied to the coil, an electromagnetic force acts, causing rotation.

Commutator

The commutator is an element connected to the two ends of the coil. It changes the direction of the current every half revolution and is responsible for keeping the coil and rotor rotating. The commutator is also in contact with the brushes.

Brushes

Brushes are electrodes that make contact with the commutator. The brushes and commutator are not always in contact with each other, but repeatedly make and break contact during rotation. When the brushes and commutator are in contact, current flows through the coil and the rotor rotates. When there is no contact, no current flows, but inertia causes the rotor to rotate and the brushes and commutator then make contact again.

[Supplement] Brushless DC motors have no commutator and no brushes

Brushless DC motors are similar to brushed DC motors, but without a commutator or brushes. Instead, the motor is driven by an electrical circuit comprising transistors and other elements. Without a commutator and brushes, there is no wear due to contact between them. As a result, such motors are quieter and more durable than brushed DC motors.

Other Types of Motors

There are various types of motors other than DC motors.

【AC motors (alternating current motors)】
Motors that use alternating voltage (positive and negative voltages switching in regular cycles). Depending on the design, they can be divided into commutator-type motors, synchronous motors and induction motors.

【PM motors (permanent magnet motors, magnet synchronous motors)】
Motors that use permanent magnets in the rotor. Easy to miniaturize.

【Stepping motors (pulse motors)】
Motors that operate in synchronization with pulse signals. The higher the frequency of the pulse signal, the faster the rotation.

【Induction motors】
Motors that are asynchronous and can be used by connecting directly to an AC power source. They can be manufactured at relatively low cost.

【Ultrasonic motors】
Unlike electromagnetic motors, ultrasonic motors use vibrations in the ultrasonic range (20 kHz or higher) to drive the rotor section.

【Servomotors】
Motors that can automatically control the position and speed in a servomechanism.
(This does not refer to a specific motor type, but only to a motor used in a servomechanism.)

【In-wheel motors】
Motors that are driven inside the wheels of an automobile.

Principle of Motor Rotation

Motors rotate by means of the following four stages. Let’s look at this mechanism, using a brushed DC motor as an example.

• 1. Generation of a magnetic field
• 2. Generation of electromagnetic force
• 3. Rotation induced by the electromagnetic force
• 4. Continuation of the rotation by a commutator

1. Generation of a magnetic field

The stator has N and S pole permanent magnets facing each other, which generate a magnetic field. The direction of the magnetic field is from the N pole towards the S pole. The coil (and rotor) is placed in the center of the magnetic field.

2. Generation of electromagnetic force

The ends of the coil are connected to the commutator, which is in contact with the brushes, which are electrodes. When an electric current flows, it flows in order from the brush positive side to the, commutator, then to the coil, then to the commutator negative side, then to the brush. When current flows through the coil in a magnetic field, an electromagnetic force (the force that makes the motor rotate) is created.

3. Rotation induced by electromagnetic force

The electromagnetic force generated works in the direction in accordance with Fleming’s left-hand rule. Fleming’s left-hand rule is a law that determines the directions of the current, magnetic field and force (electromagnetic force). Applying Fleming’s left-hand rule to the directions of the current and magnetic field shows that the coil on the N pole side acts upwards and the coil on the S pole side acts downwards. This means that the coil rotates clockwise.

When the coil rotates 90°, the commutator leaves the brushes. No current flows and the electromagnetic force is lost, but it continues to rotate due to inertia. When the commutator contacts the brushes again, current flows and an electromagnetic force is generated again.

4. Continuation of rotation by the commutator

When the coil makes a half turn (180°), the commutator reverses the direction of the current in the coil, maintaining the direction in which the electromagnetic force operates and allowing the coil to continue rotating in the same direction. Without the commutator, the direction of the current in the coil would not change, so the coil would rotate counterclockwise after half a turn. Since the clockwise and counterclockwise rotation would be repeated endlessly, a commutator is required to keep the current flowing in the necessary direction.

Applications of Motors and Reasons for Widespread Use

Motors are used in a wide range of applications and are indispensable components.

Sector	Product
Industrial equipment	Elevators, automatic doors, conveyor belts, electric forklifts, robots, generators
Automotive	Hybrid systems, power steering, power windows, power seats, fuel pumps, windscreen wipers, regenerative braking for EVs
Medical	CT scanners, MRI, electronic blood pressure monitors, blood transfusion equipment, electric wheelchairs
Telecoms	Smartphones, tablets, PCs, mobile phone base stations
Home appliances	Air conditioners, hairdryers, mixers, washing machines, fans, vacuum cleaners

Reasons why motors are widely used

The following are four reasons why motors are used in many sectors.

1. Excellent energy conversion efficiency
Compared to other sources of power, motors have excellent energy conversion efficiency. Generally, the energy conversion efficiency of motors is 80-90%. In contrast, the energy conversion efficiency of gasoline engines is 30% and that of diesel engines is 40%. Motors have smaller energy losses than engines and obtain energy more efficiently.

2. Ability to generate electricity
The structure of a motor has so far been described as a mechanism for converting electrical energy into kinetic energy, but motors can also generate electrical energy from kinetic energy. For example, electric vehicles, wind turbines and bullet trains use motors to generate electrical energy from kinetic energy. Because motors can generate electricity in addition to producing power for driving, they are used in a wide range of applications.

3. Ease of control
Compared with other power sources, motors have a simple structure and are easy to control. Engines have many parts and require electronic control using sensors. Motors, on the other hand, can be miniaturized due to their simple structure and can therefore be easily integrated into small components.

4. Eco-friendly power sources
Motors are an environmentally friendly power source. Engines use petrol and emit greenhouse gases such as carbon dioxide. In contrast, motors generate energy from electricity. The motor itself does not emit greenhouse gases and is therefore an environmentally clean device.

Summary

Motors, which comprise a rotor, coils and permanent magnets, are used in many fields such as automobiles, household appliances and medical equipment because of their simple structure and ease of handling. Motors are important components of our society today.