Classification of motors

The speed of a three-phase asynchronous motor is stepped, and it is determined by the motor’s \"number of poles\". The \"pole count\" of a three-phase asynchronous motor refers to the number of magnetic poles in the stator field. Different connection methods of the stator windings can result in different pole numbers for the stator magnetic field. Common types include: 2-pole motors, with 1 pair of stator magnetic poles, a synchronous speed of 3000 revolutions per minute, and a rated speed of around 2880 revolutions per minute. 4-pole motor, with 2 pairs of stator field poles; synchronous speed of 1500 revolutions per minute, rated speed of approximately 1440 revolutions per minute. 6-pole motor, with 3 pairs of stator field poles; synchronous speed of 1000 revolutions per minute, rated speed of approximately 720 revolutions per minute. The more poles there are, the lower the rotational speed; at the same power level, the size increases and the cost rises, while the efficiency decreases. Therefore, motors with more than 8 poles are rarely used. n=60f/p —— Rotational speed = 60*frequency/pair of poles

1. The number of poles indicates the synchronous speed of the motor; the synchronous speed for 2 poles is 3000 r/min, 1500 r/min for 4 poles, 1000 r/min for 6 poles, and 750 r/min for 8 poles. 　　The back-and-forth movement of the windings forms a circuit; the number of pole pairs is even, as poles appear in pairs. A pole refers to a magnetic pole, and when current flows through these windings, a magnetic field is generated, resulting in the formation of poles. 　　In a three-phase AC motor, each set of windings generates N and S poles, and the number of poles per phase in each motor constitutes the pole count. Since magnetic poles appear in pairs, motors come in 2, 4, 6, 8… pole configurations. 　　2. If the frequency of the three-phase alternating current is 50 Hz, then the synchronous speed of the resulting magnetic field is 50 r/s, which is equivalent to 3000 r/min. If the rotating magnetic field of the motor consists of more than one pair of poles, further analysis shows that there is a relationship between the synchronous speed n and the number of pole pairs p: n = 60f/p, where f is the frequency in Hz, and n is expressed in r/min. There is a strict relationship between ns, the synchronous speed, and the frequency of the alternating current (f) as well as the number of pole pairs (P) of the motor: ns = f/P. In China, the power supply frequency is 50 Hz; therefore, the synchronous speed of a two-pole motor is 3000 revolutions per minute, while that of a four-pole motor is 1500 revolutions per minute, and so on. The speed of the rotor in an asynchronous motor is always lower or higher than the speed of its rotating magnetic field, which is why it is called asynchronous. The difference between the rotor speed of an asynchronous motor and the speed of the rotating magnetic field (known as slip) is usually within 10%. It can be seen that the speed of AC motors (whether synchronous or asynchronous) is determined by the power supply frequency. Therefore, it is difficult to adjust the speed of an AC motor; the best approach is to change the frequency of the power supply, although changing the power supply frequency used to be quite complicated. Therefore, before the 1970s, DC motors were commonly used in applications that required speed control. With the development of power electronics technology, variable-frequency speed control technology for AC motors has begun to be put into practical use. 　　3. The speed of a synchronous motor = number of pole pairs × frequency (the industrial frequency in China is 50 Hz). The speed of an asynchronous motor = number of pole pairs × frequency – number of pole pairs × frequency × slip rate. Additionally, for motors with the same power, the higher the speed, the lower the output torque. 　　4. Number of poles in synchronous motors High-capacity synchronous motors are all of the pole-type design, meaning that the rotor consists of magnetic poles generated by direct current flowing through excitation windings; the number of pole pairs in such motors corresponds to the number of magnetic poles on the rotor. An eight-pole motor has 8 magnetic poles on its rotor; 2p=8, which means this motor has 4 pairs of magnetic poles. Typical steam turbine generators are permanent-magnet motors with a very small number of pole pairs, usually 1 or 2 pairs. Since n=60f/p, their speed is quite high, reaching up to 3000 revolutions per minute at the power frequency. In contrast, hydro turbine generators have a large number of pole pairs; their rotors are of the salient-pole type, and their manufacturing process is more complex. Due to this large number of poles, their speed is very low, possibly only a few revolutions per second!

The number of pole pairs in the motor, available in 1, 2, 3, and 4 levels； When referring to levels alone, one can only talk about level 2, level 4, level 6, level 8, and so on.