Engineering School丨Understanding the principles of high-efficiency motors and their correct use

When engineers want to use electrical and electronic machines to do something in the real world, they will think about how to turn electrical signals into "force". The actuator that converts electrical signals into force is the motor. The motor can be regarded as "a component that converts electrical into mechanical force".

The most basic motor is a "DC motor (brush motor)". A coil is placed in a magnetic field. When current flows through it, the coil is repelled by the magnetic pole on one side and attracted by the magnetic pole on the other side, causing it to rotate continuously. During the rotation, the current flowing into the coil is reversed to keep it rotating. The motor has a part called a "commutator" that is powered by a "brush". The "brush" is located above the "steering gear" and moves continuously as it rotates. By changing the position of the brush, the direction of the current can be changed. The commutator and brushes are indispensable structures for the rotation of the DC motor (Figure 1).

Figure 1: Schematic diagram of the operation of a DC motor (brushless motor).

The commutator switches the flow of current in the coils, reversing the direction of the magnetic poles so that they always rotate to the right. The brushes supply power to the commutator, which rotates with the shaft.

We classify motors by power source and rotation principle (Figure 2). Let's take a quick look at the characteristics and uses of each type of motor.

Figure 2: Main types of electric motors

The DC motor (brush motor) with a simple structure and easy operation is usually used for the purpose of "opening and closing the CD tray" in home appliances. Or it is used for the purpose of "opening and closing the electric rearview mirror and steering control" in automobiles. Although it is cheap and can be used in many fields, it also has its drawbacks. Since the commutator contacts the brush, its life is very short, and the brush must be replaced regularly or repaired.

The stepper motor rotates according to the number of electric pulses sent to it. The amount of its movement depends on the number of electric pulses sent to it, so it is suitable for position adjustment. In the home, it is often used for "feeding paper in fax machines and printers", etc. Since the paper feeding steps of fax machines depend on the specifications (engraving, fineness), the stepper motor that rotates according to the number of electric pulses is very convenient to use. It is easy to solve the problem that the machine will stop temporarily when the signal stops.

Synchronous motors whose rotation speed changes with the power supply frequency are used in applications such as "rotating tables for microwave ovens". A gear reducer is included in the motor unit to obtain a rotation speed suitable for heating food. Induction motors are also affected by the power supply frequency, but the frequency and rotation speed are not consistent. Previously, this type of AC motor was used in fans or washing machines.

As can be seen, various types of motors are active in many fields. Among them, what characteristics do BLDC motors (brushless motors) have that make them so widely used?

The "BL" in BLDC motor means "brushless", which means the "brush" in DC motor (brush motor) is gone. The role of brushes in DC motor (brush motor) is to energize the coils in the rotor through the commutator. So how does the BLDC motor without brushes energize the coils in the rotor? It turns out that BLDC motors use permanent magnets as rotors, and there are no coils in the rotor. Since there are no coils in the rotor, there is no need for a commutator and brushes for energizing. Instead, there are coils as stators (Figure 3).

Figure 3: Schematic diagram of the operation of a BLDC motor.
A BLDC motor uses permanent magnets as the rotor. Since no power is required to the rotor, brushes and a commutator are not required. The power to the coils is controlled externally.

The magnetic field created by the fixed permanent magnet in the DC motor (brushless motor) does not move, and it rotates by controlling the magnetic field generated by the coil (rotor) inside it. The number of revolutions is changed by changing the voltage. The rotor of the BLDC motor is a permanent magnet, and the rotor rotates by changing the direction of the magnetic field generated by the surrounding coil. The rotation of the rotor is controlled by controlling the direction and size of the current flowing to the coil.

The BLDC motor has three coils on the stator, each with two wires, and a total of six lead wires. In fact, because it is internally wired, only three wires are usually required, but it is still one more than the DC motor (brushless motor) mentioned earlier. It will not move just by connecting the positive and negative poles of the battery. As for how to operate the BLDC motor, we will explain it in the second part of this series. This time we will focus on the advantages of the BLDC motor.

The first characteristic of BLDC motors is "high efficiency". Its gyroscopic force (torque) can be controlled to always maintain the maximum value. For DC motors (brush motors), the maximum torque can only be maintained for a moment during the rotation process and cannot always maintain the maximum value. If a DC motor (brush motor) wants to get the same torque as a BLDC motor, it can only increase its magnet. This is why a small BLDC motor can also produce powerful force.

The second characteristic is "good controllability", which is related to the first one. BLDC motors can get the torque and rotation speed you want without any error. BLDC motors can accurately feedback the target rotation speed, torque, etc. Through precise control, the heat generation and power consumption of the motor can be suppressed. If it is battery-driven, the driving time can be extended through careful control.

In addition, it is durable and has low electrical noise. The above two points are the advantages of brushless motors. However, DC motors (brushless motors) will wear out over time due to the contact between the brush and the commutator. Sparks will also be generated at the contacting parts. In particular, huge sparks and noise will occur when the gap of the commutator touches the brush. If you do not want to generate noise during use, you will consider using a BLDC motor.

Where are BLDC motors with high efficiency, various controls and long life generally used? They are often used in products that can take advantage of their high efficiency and long life and are used continuously. For example: home appliances. People have been using washing machines and air conditioners for a long time. Recently, BLDC motors have also been used in electric fans, and have successfully promoted a significant reduction in power consumption. It is precisely because of the high efficiency that the power consumption is reduced.

BLDC motors are also used in vacuum cleaners. In one case, the number of revolutions was significantly increased by changing the control system. This example demonstrates the good controllability of BLDC motors.

The hard disk, which is an important storage medium, also uses a BLDC motor for its rotating part. Since it is a motor that needs to run for a long time, durability is very important. Of course, it is also used to minimize power consumption. The high efficiency here is also related to low power consumption.

BLDC motors are expected to be used in a wider range of fields. BLDC motors will be widely used in small robots, especially in "service robots" that provide services in fields other than manufacturing. "Positioning is important for robots, shouldn't we use stepper motors that operate according to the number of electrical pulses?" Some people may think so. However, BLDC motors are more suitable for force control. In addition, if stepper motors are used, a considerable current will be required to fix structures such as robot wrists in a certain position. If BLDC motors are used, they can only provide the required power in accordance with external forces, thereby suppressing power consumption.

It can also be used in transportation. Electric vehicles for the elderly or golf carts have always used simple DC motors, but recently they have begun to use high-efficiency BLDC motors with good controllability. The battery life can be extended through subtle control. BLDC motors are also suitable for drones. In particular, drones with multi-axis frames control their flight posture by changing the number of rotations of the propellers, so BLDC motors that can precisely control rotation are very advantageous.

How is it? BLDC motors are high-efficiency, good controllability, and long-life high-quality motors. However, to maximize the power of BLDC motors, correct control is required. How to do it? Let's look forward to the next time.