What does “4 poles” mean in a 4-pole AC asynchronous motor?

4 poles refer to magnetic poles. If an AC asynchronous motor has 4 poles, then it has 2 pairs of poles. The formula is revolutions = 60f/p. F is the frequency of the alternating current, and P is the number of pole pairs. The rotation speed of a 4-pole motor is generally 1450 revolutions per minute.

They are the magnetic poles; the motor windings always have an even number of poles, with common values being 2, 4, 6, 8, 12

When three-phase alternating current is applied to the stator windings of a motor, a rotating magnetic field is generated, and the angle of change of the current is the same as that of the rotating magnetic field. As the current completes one cycle, the rotating magnetic field rotates through 360 degrees of electrical angle. When the number of magnetic pole pairs P=1, the electrical angle is equal to the mechanical angle, that is, one complete cycle of current variation. The magnetic field rotates through 360 degrees of mechanical angle, that is, one full rotation ； When the number of pole pairs is P, as the current changes by one cycle, the rotating magnetic field rotates through 360 degrees/P in mechanical angle, that is, through 1/P revolutions. Therefore, the rotational speed of the magnetic field per minute is related to the frequency of the stator winding current and the number of pole pairs P by the formula “n1=60f/p”, where n1 represents the rotational speed of the magnetic field per minute (synchronous speed) ； f is the current frequency of the stator winding ； P is the number of pole pairs in the motor. Since n1 maintains a constant relationship with f, it is called the synchronous speed. Since our country uses a power frequency of f=50Hz. Therefore, when P=1, n1 = 3000 rpm ； When p=2, n1=1500rpm ； When p=3, n1=1000rpm ； At p=4, n1=750rpm. The mechanical speed of the motor is always less than the synchronous speed, which is why it is called a “three-phase AC asynchronous motor”. An inverter can be used at a certain power level; all that is needed is to adjust the motor parameters within the inverter. However, the motor’s power cannot exceed the inverter’s rated power. Otherwise, the inverter may overheat and get damaged.

The number of poles can be up to 2; in the case of synchronous motors, the rotation speed is 3000 revolutions per minute, while asynchronous motors generally rotate at around 2900 revolutions per minute. Poles numbered 4, 6, 8, 12, etc., are all even numbers, and it’s possible to calculate that the rotation speed for a 4-pole motor is 1500 revolutions per minute; asynchronous motors have a lower rotation speed than this value.

It refers to the number of poles of the asynchronous motor. It is indicated on the motor nameplate.

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 coils 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 versions. 　　 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 reveals 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) in 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 (referred to as slip) is usually within 10%. It can be seen from this that the speed of AC motors (whether synchronous or asynchronous) is determined by the power supply frequency. Therefore, it is difficult to control 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 of the salient-pole type, with a very small number of pole pairs – usually 1 or 2 pairs. Since n=60f/p, their rotational 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 rotational speed is very low, possibly only a few revolutions per second!