What is a Brushless Motor? With an Explanation of the Number of Poles and the Number of Slots in a Motor, Mechanical Angles and Electrical Angles, and Hall Elements (Hall ICs)

Contents of Episode 2:

• ・What is a Motor?
• ・Principle of Motor Rotation
• ・What is a Brushless Motor?
• ・The Number of Poles and Number of Slots in a Motor
• ・Mechanical Angle and Electrical Angle of a Motor
• ・IC Hall Elements (Hall ICs) in Brushless Motors
• ・Construction of Brushless Motors
• ・Applications of Brushless Motors

What is a Brushless Motor?

From here, I will be talking about brushless motors.

The motor with the construction described in “Principle of Rotation of Motors” in the previous article is a motor with brushes, and therefore is called a brushed motor, a motor with brushes, or a commutator motor. To explain brushed motors with improvements added using the diagram below, commutators (conducting plates), which rotate together with a rotor, can be said to play the role of mechanical switches which repeatedly switch contact with the brushes on and off, so that the poles of the rotor change automatically and the rotor can continue to rotate.

The motor continues to rotate automatically, but this construction has the problem of a short lifetime due to the wear that occurs because the brushes and the commutator rub against each other (the brush material is often targeted in motor development efforts). Hence improvement of the service lifetime of this mechanical switch component has been studied, and as a result the brushless motor emerged as a completely new switching mechanism.

The switch component of a brushless motor uses transistor semiconductor devices that have been developed so as to be capable of practical performance as a switching mechanism. In place of the commutator and brush components of a brushed motor, six transistors (in the case of three-phase full-wave switching; in the diagram, MOSFETs are used) are employed in a circuit like that shown below to drive a brushless motor.

When such a configuration was adopted, new technologies were needed. First of all, where the above-mentioned transistors were concerned, switching operation is essential, and knowledge relating to use is necessary. Signals for transistor on/off control are needed in place of the operation to switch the wiring that was performed automatically when mechanical switches were used. Because an integrated circuit (IC) called a controller was developed to create such signals, driving control has become a theme of R&D. And in order to create these control signals, magnet positions must be detected; hence technology for magnet position detection has also come to be studied. A motor driver incorporates all these technologies and more.

Up to this point, I have explained the differences between brushed and brushless designs from the standpoint of the electrical circuits, but there are also differences in the mechanical structures. In a brushed motor, permanent magnets are fixed in place, and one or more electromagnets rotate. There is a reason for this: if the arrangement were reversed, brushes would rotate, and there would be the problem of the wires connected to the brushes being twisted. For this reason, as a rule it is the electromagnets that rotate.

In a brushless motor, however, this configuration is reversed. The windings remain connected to the circuitry, and consequently the electromagnets cannot be rotated. Hence electromagnets form the stator, and permanent magnets form the rotor.

To sum up, a brushless motor can be called a motor constructed so that transistors are used to switch the magnetic poles of electromagnets, such that the electromagnets rotate a rotor built from permanent magnets, with the signals that switch the electromagnets generated by a controller based on rotor position information.

This is how a brushless motor works. But when actually designing such a motor, one may run into strange or uncommon concepts and part names, making things difficult.

So here I will give simple explanations of some basic terms that are needed for motor design: the “number of poles and number of slots”, “mechanical angles and electrical angles”, and “Hall elements (Hall ICs)”.

The Number of Poles and Number of Slots in a Motor

I’ll start with the number of poles and number of slots, which are numbers that characterize a motor; I’ll also explain the related concepts of mechanical angle and electrical angle.

A motor has a certain number of poles and a certain number of slots; these are numerical values that indicate characteristics of the motor. The number of poles indicates the number of magnetic poles of the rotor. Strictly speaking, the number of slots specifies the number of spaces, called slots, shown in the figure below; but in the case of a motor with what is called concentrated winding, in which one winding (coil) is wound around one tooth as in the figure, the number of slots is treated as equivalent to the number of coils. While not shown below, the winding method in which windings span multiple teeth is called distributed winding.

When explaining such basic matters as the concept of a three-phase motor, often a two-pole, three-slot diagram is used in which a pair of N and S poles and three windings, for the U phase, V phase, and W phase, are depicted (the windings may also be called the A phase, B phase, and C phase). However, actual motors frequently use four-pole, six-slot configurations, or six-pole, nine-slot configurations, or other multiples of this 2:3 design. There are also motors that use ratios other than 2:3, such as eight-pole, nine-slot motors and ten-pole, 12-slot motors.

These combinations of numbers of poles and numbers of slots have their own advantages and disadvantages; the motor manufacturers each decide on the numbers of poles and slots according to the motor applications. When designing a motor driver, it is essential that the design of the motor with which the driver is to be used be known.

Mechanical Angle and Electrical Angle of a Motor

From the differences due to the different numbers of poles described above, there emerge the concepts of mechanical angle and electrical angle (or electrical cycle). As its name implies, a mechanical angle is an angle in one mechanical rotation of a motor. There are 360 degrees from the position at which the rotor shaft begins to rotate until it returns to its original position. On the other hand, the electrical angle is an angle in one switching cycle (details to come) of a switch that applies voltages to the windings (coils), and one cycle is 360 degrees.

In a little more detail, for example if there are two poles and three slots, the mechanical angle and the electrical angle coincide (see the figure below). If however there are four poles and six slots, over the 360 degrees of the mechanical angle, there are two cycles of the electrical angle (at a mechanical angle of 180°, the electrical angle has reached 360°; or put another way, when one electrical cycle has elapsed, the rotor has only made half a revolution).

This difference in the mechanical angle and the electrical angle must be kept firmly in mind. For example, the rotation rate of a motor is frequently expressed using the unit r/min (an abbreviation of rotations per minute; also written rpm or min-1. However, “r” may also be expressed as “round”, “roll”, or “revolutions”), indicating the number of times the motor rotates in one minute; in addition, angles are used to indicate the position of the rotor or the placement of a position detector. These quantities are all based on mechanical angles. On the other hand, electrical signals output by a motor driver are repeated in each cycle of switching, and so are based on electrical angles.

In a switching diagram like that shown above, it may be difficult to understand the importance of electrical angles. But with advances in brushless motor driving technology, the concept of electrical images has become extremely important. The relations described in the above diagram should be committed to memory.

Hall Elements (Hall ICs) in Brushless Motors

Position detection is necessary in order to drive a brushless motor. One method for doing so uses Hall elements (Hall ICs).

The magnetic poles of the electromagnets in a motor must be changed according to the rotor position. In a brushed motor, the brushes and commutator play this role; in a brushless motor, which does not have these components, some other method is needed. Here I will explain Hall elements, which are used as one such method for detecting the position of a rotor.

A Hall element is a magnetic detection element that makes use of the Hall effect (discovered by Dr. Edwin Hall). In the Hall effect, when a magnetic field is applied perpendicularly to a current flowing in an object, the resulting Lorentz force causes electrons to move, and an electromotive force (emf) appears in the direction perpendicular to both the current and the magnetic field. At present, materials such as InSb (indium antimonide), GaAs (gallium arsenide), and Si (silicon) are used in Hall elements due to their satisfactory characteristics.

The emf that appears across the terminals of a Hall element is roughly proportional to the magnitudes of the magnetic flux density and the current, and the electrical polarity changes if the magnetic polarity changes. Hence by measuring the voltage appearing across a Hall element, the position of the rotor can be detected with reference to the Hall element position. In addition to analog output that is proportional to the magnitude of the magnetic force, some Hall element output signals are digitized.

When used in a motor, a Hall element is placed at a position on a board where it can detect the magnetic poles of the rotor. Assuming that the switch pattern is changed at every 60° of the electrical angle I explained earlier, a total of three Hall elements are installed at electrical angle positions separated by 120°. By installing Hall elements every 120°, six positions can be detected from the signals of the three sensors, that is, the rotor position can be detected every 60° (the details will be explained later).

Of late, there has also appeared a driving method called sensorless driving that does not use Hall elements, as well as a method that uses just a single Hall element. When you get the chance, you should also study the various characteristics of Hall elements (electrical and temperature characteristics etc.), since they are of fundamental importance for position detection in motors.

Next time, I will introduce an example of the construction of a brushless motor.

Next page: Construction and Applications of Brushless Motors