[Episode 2] Sugiken appears! The first step to becoming a super engineer

2025.06.02

table of contents

・What is a Motor?
・Principle of Motor Rotation

In episode 2, I explain the basics of motors. Just what is a motor? What is the principle of motor rotation? After using a basic brushed motor as an example in explaining these things, I’ll go on to the main topic, which is brushless motors.

Contents of Episode 2:

What is a Motor?

The word “motor” is widely used to mean motive force or power, as when speaking of “motor sports” and the like. Here, however, “motor” refers to an electrical machine that converts electrical energy into mechanical energy.

Well then, how does a motor, as an electrical machine, convert electrical energy into mechanical energy?

The answer is that an electromagnet is utilized. That is, the electrical energy is used to create an electromagnet, and magnet attractive and repulsive forces are used to generate mechanical energy; this is also mentioned in “The Role of the Motor Driver IC” and in “The Relationship between the Motor Driver IC and the Motor Device.”

As another answer, one may consider that a motor uses the force that occurs when a current is passed in a conductor that has been placed in a magnetic field. This explanation may be more widely known, using what is called Fleming’s left-hand rule. This can be explained using a schematic diagram of a rotating mechanism like that shown below, but because the structure is somewhat different from the structure of actual motors that I will be explaining, here I want to use the first answer to the question, which is that a motor runs by using an electromagnet.

Simple rotating mechanism

Use of an electromagnet

An electromagnet is formed by winding an electrical conductor about a magnetic material such as iron. As such, nothing happens, but when an electrical current is passed in the conductor, the magnetic material takes on the characteristics of a magnet, and attracts other magnetic materials (such as iron). Because the current direction can be changed to change the magnet polarity, when another magnet is positioned facing the electromagnet, the force imparted to the magnet can be switched between attraction and repulsion (see the figure below).

Use of an electromagnet

A motor employs these characteristics of an electromagnet to use electrical energy to cause rotational motion (there are also “linear motors” and other motors that cause linear motion).

Well then, we see that mechanical energy is obtained by attracting or repulsing a magnet; but how do we achieve continuous rotational motion? This can easily be understood by taking a look at the construction of the motors used in radio-controlled cars and machine tools.

Principle of Motor Rotation

I’ll explain the principle of motor rotation, taking as an example a brushed motor that will serve as our starting point.

Construction of motor

The mechanical construction of this motor consists of three coils and conducting plates, positioned at 120 degree intervals, two plates called brushes that are positioned so as to surround the conducting plates on opposite sides, and a permanent magnet. Because this motor has brushes, it is called a brushed motor or a motor with brushes, or a commutator motor.

Three electromagnets

Why does the motor have this construction, with components repeated every 120 degrees?

Motors with one electromagnet

One electromagnet

In the above figure, there are two brushes in addition to the single electromagnet; one of the brushes is connected to a positive electrode, the other to a negative electrode. When the conducting plates are enclosed between the brushes as shown above, a voltage appears across the coil connected to the plates, and a current flows. The conducting plates are affixed to the shaft of the motor, so that they rotate together with the coil (electromagnet).

Given this basic construction, suppose that the conducting plates are positioned such that when for example the coil is at position <c> in the figure, a current flows so as to create the electromagnet with the polarity shown in the figure. Then a counterclockwise force on the coil appears, and the coil rotates.

As a result, the coil arrives at position <d>; if at this position the electromagnet polarity remains unchanged, the coil stops rotating. Therefore, at this position the polarity of the electromagnet is changed. The positions of the conducting plates are precisely adjusted such that, exactly at this position, the connections of the brushes and the conducting plates are reversed. As a result, the motor continues to rotate.

However, there are problems with this construction. In actuality it is difficult to switch the electrodes at position <d> with the right timing; and if the electrodes are not switched properly, the motor will stop. Moreover, if one of the conducting plates comes into contact with both brushes, the positive and negative electrodes are short-circuited, possibly causing damage.

As one method to resolve these problems, a construction was devised in which three coils are used, positioning electromagnets at 120 degree intervals.

Motors with three electromagnets

A motor with such a construction is called a three-phase winding motor; the motor I’m using here as an example is of this type. A motor with a single electromagnet is called a single-phase motor.

As was shown in the above figure of the motor with three electromagnets, in a three-phase motor construction there are three coils, with one end of each of the coils connected to a conducting plate and the other ends connected together. With this wiring, the brushes are used to apply a voltage to two of the conducting plates to magnetize two of the coils. By doing so, at position <a> the A and B coils are magnetized, whereas at position <b> the B coil, which had been magnetized as the N pole at position <a>, is no longer magnetized, and the C coil is magnetized as the N pole. Through this operation, a situation like that of d in the figure for a single electromagnet does not occur. Moreover, when the conducting plates are positioned in this way, short-circuiting of the positive and negative electrodes does not occur.

Adjustment of conducting plate positions

In this way, a three-phase winding motor continues to rotate due to switching of electromagnet polarities with appropriate timing.

Next, I will move on to the main subject, three-phase brushless motors.

Key points of this article

・The word “motor” is widely used to mean a power source, but these articles explain that motors are electrical machines that convert electrical energy into mechanical energy.

・A motor can be called a device that uses electrical energy to create an electromagnet in order to produce mechanical energy by utilizing the force of the electromagnet that attracts or repels magnets.

・A motor is made to rotate by switching the polarity of the electromagnet with appropriate timing.

・Problems occur in a single-phase motor that uses one electromagnet. Reliable rotation can be obtained by for example using a three-phase motor structure that employs three electromagnets.

・The problems surrounding the timing with which polarities are switched in a single-phase motor using a single electromagnet can be resolved by adopting a three-phase motor construction with three electromagnets, so that reliable rotation is achieved.

Next page: What is a Brushless Motor?

Teacher Sugiken’s Motor Library

Teacher Sugiken’s Motor Driver Dojo

An Introduction to Motors

An Introduction to Motors

Brushless Mortor Driver

What Is a Brushless Motor Driver?

Motor Q&A