How to understand the differences between various motors

Motor Categories

There are many types of motors, and the naming is also confusing. How do you understand the differences between the various motors? The following motors may seem a bit confusing, but can you understand each one? For example: AC asynchronous, permanent magnet synchronous, AC induction, reluctance motor, reactive stepper, permanent magnet stepper, hybrid stepper, coreless cup, DC brush, BLDC, PMSM, servo motor, two-phase stepper, three-phase stepper. But it doesn't matter if you don't understand. The purpose of this article is to hope that you will not be entangled with these after reading, but have a deeper understanding of the nature of motor operation.

How the motor turns

No matter how many tricks the motor has, it will eventually return to the basic principle, that is, opposite poles of magnets attract each other, and like poles repel each other. As shown in the figure below, the circular magnet in the middle and the square magnet on the side are in an unbalanced state when they are in such a position. The S pole of the circular magnet in the middle is attracted by the N pole of the square magnet, and the N pole is repelled by the N pole of the square magnet, so it rotates counterclockwise until the S pole and N pole are close and stationary.



If we rotate the square magnet along the dotted line, the circular magnet will also rotate. The key to the motor is how to produce two mutually attractive parts that rotate under the action of attraction. The rotating part is called the rotor. The rotor will reach balance after rotating by attraction. If you want the rotor to rotate continuously, you need to attract it in a new position. There is another

type of motor that does not rely on the attraction between the poles, but relies on another characteristic, that is, the magnetic lines of force always pass through the path with the least magnetic resistance. Just like the current always seeks the path with the least resistance. As shown on the right side of the figure below, the long axis of the rotor and the magnetic lines of force are in the same direction. In this state, the magnetic lines of force are subject to the least magnetic resistance. If the rotor rotates an angle as shown in the left figure, the path of the magnetic lines of force will become longer and the magnetic resistance will become larger. At this time, the magnetic lines of force will generate torque on the rotor, pushing it back to the original position with the minimum magnetic resistance.



The non-rotating part of the motor is called the stator, and the rotating part is called the rotor. The rotor is inside and the stator is outside. It can also be reversed, with the rotor outside and the middle part as the stator. The magnetic fields of the stator and rotor can be generated by current passing through the excitation coil, or one can be generated by current and the other by permanent magnets.

Let's talk about synchronization and asynchronous. A motor with a relatively long application history is the AC induction motor. Three-phase AC is excited in the stator of this motor, and the synthetic magnetic field is a rotating magnetic field that changes at the frequency of AC. The squirrel cage rotor in a stationary state begins to cut the magnetic lines of force and form a current. The current in turn generates a magnetic field, which interacts with the stator magnetic field and begins to rotate.

However, the speed of the rotor will always lag behind the rotation speed of the magnetic field in the stator, because once the speeds of the two are the same, the stator will no longer cut the magnetic lines of force and lose power. There is always a certain speed difference between the rotor and the stator, and they do not rotate synchronously, so it is called an asynchronous motor.

If the rotor uses permanent magnets, or uses excitation current to generate a magnetic field, then it can rotate along with the rotation of the stator magnetic field, and is called a synchronous motor. The permanent magnet synchronous motor PMSM (Permanent Magnet Synchronous Motor), which is widely used now, is a synchronous motor. Note that synchronization here does not mean that the rotor speed is synchronized with the stator magnetic field rotation speed, because the rotor speed is related to the number of pairs of magnetic poles it has.

If there is only one pair of magnetic poles, then the rotor speed is equal to the stator magnetic field rotation speed. If the stator has two pairs of magnetic poles, then its speed is half the stator magnetic field rotation speed.

Common DC Motors

In the embedded field, most of the motors used are DC motors. Let's disassemble some commonly used ones to get a more intuitive feeling. Brushed DC Motor. The stator is a permanent magnet and the rotor is a coil. After power is applied, the coil rotates under the action of the magnetic field. After rotating a certain angle, the energized electrode switches, and the rotor continues to rotate under the force of the new energized coil. Its control is very simple. It rotates when DC power is applied. After the polarity of the energized electrode is swapped, the rotation direction is also reversed, and the speed changes with the voltage.

In the figure below, we can see something called a carbon brush. One end of it is connected to the positive and negative poles of the power supply, and the other end is pressed on the commutator. The commutator rotates with the rotor to complete the switching of the energized coil. The disadvantages of carbon brushes are that they will generate noise, sparks, wear, and have a short life. Therefore, the current trend is to replace this mechanical commutation with electronic commutation, which is a brushless motor BLDC (Brushless DC Motor).



Let's look at a DC motor with a large number of electrodes. This is the motor we mentioned in another article about the principle of the soymilk machine. Although the soymilk machine is powered by 220 AC, the motor used in it is a DC motor. After rectifier, 220V AC becomes DC power, which is directly added to the coil of the motor through the carbon brush. It has many electrodes, so the torque is also large.

Hollow cup motor, it belongs to DC brush motor. Its rotor has no iron core, so the moment of inertia is small and the response is fast. Its volume can be made very small and is widely used in aircraft, focusing, instrumentation, etc. The picture below is a 716 hollow cup, that is, the diameter is 7mm and the length is 16mm.

Stepper motor, the picture below is a two-phase hybrid stepper motor. You can see that there are 8 windings in total, divided into A and B phases, 1 3 5 7 is one phase, and 2 4 6 8 is one phase. There are many small teeth evenly distributed on the stator and rotor. When the AB phase is energized, the small teeth of different polarities on the stator and rotor are staggered and aligned to drive the rotor to rotate. Its control is also very simple. By alternating the AB phase power pulses, the rotor can be driven to rotate step by step, with a step angle of 1.8°.

Brushless DC (BLDC) and Permanent Magnet Synchronous Motor (PMSM) Generally, we agree that the motor that uses Hall sensor to measure the rotor position, the back EMF is a trapezoidal wave, and is controlled by six-step commutation is called brushless DC. The motor that uses encoder to measure the rotor position, the back EMF is a sine wave, and is controlled by vector control (FOC) is called permanent magnet synchronous motor.

In fact, the back EMF of many BLDCs is not a standard square wave, but close to a sine wave, and can also be controlled by vector. Similarly, permanent magnet synchronous motors can also be controlled by six-step commutation. Moreover, permanent magnet synchronous motors are also powered by DC, and they are definitely brushless.

The permanent magnet synchronous motor with vector control has very small torque pulsation, rotates smoothly, and can output large torque at low speed. The brushless DC motor is simple to control and has lower cost. The

figure below shows a permanent magnet synchronous motor with a magnetic encoder. Most of the higher-end motors use photoelectric encoders, which can output the rotor position with higher accuracy, so that the controller can control the rotation of the rotor more accurately.






Servo motors, in fact, it is difficult to say which motor is or is not a servo motor. Servo itself refers to a system that can accurately and quickly control the position and speed of the rotor according to the input.

The following motor has a photoelectric encoder disk and an integrated motor drive controller. Its operation can be controlled from the outside by giving a pulse level signal.

Conclusion

There are so many types of motors. We hope that these examples will help you understand motors. What do you think of various motors? Leave a message to discuss!