The three-phase asynchronous motor speed is graded and is determined by the "number of poles" of the motor.
The "pole number" of a three-phase asynchronous motor is the number of magnetic poles that specify the sub-field. The stator windings are connected in different ways to form different pole numbers of the stator magnetic field. The number of poles for selecting the motor is determined by the speed required by the load. The number of poles of the motor directly affects the speed of the motor. Motor speed = 60x motor frequency / motor pole pairs. The current of the motor is only related to the voltage and power of the motor.
Classification
1. The number of poles reflects the synchronous speed of the motor. The 2-pole synchronous speed is 3000r/min, the 4-pole synchronous speed is 1500r/min, the 6-pole synchronous speed is 1000r/min, and the 8-pole synchronous speed is 750r/min.
One or one winding of the windings can form a loop, that is, the logarithm of the magnetic poles, which appear in pairs, and the poles are the meanings of the magnetic poles. When these windings pass current, a magnetic field is generated, and correspondingly there will be magnetic poles.
Each group of three-phase AC motors generates N and S poles, and the number of poles per phase of each motor is the number of poles. Since the magnetic poles appear in pairs, the motor has 2, 4, 6, 8 ... poles.
2. If the frequency of the three-phase alternating current is 50 Hz, the synchronous speed of the combined magnetic field is 50 r/s, that is, 3000 r/min. If the rotating magnetic field of the motor is more than a pair of magnetic poles, further analysis can also obtain the synchronous rotational speed n and the magnetic field magnetic pole pair. The relationship of the number p: n = 60f / pf is the frequency, the unit of Hz.n is r / min.
There is a strict relationship between ns and the frequency of the connected alternating current (f) and the number of magnetic pole pairs (P) of the motor ns=f/P.
In China, the power frequency is 50 Hz, so the synchronous speed of the two-pole motor is 3000 rpm, the synchronous speed of the four-pole motor is 1500 rpm, and so on. The speed of the rotor of an asynchronous machine is always lower or higher than the speed of its rotating magnetic field, and the name of asynchronous comes from this. The difference between the rotor speed of an asynchronous motor and the rotational magnetic field speed (called the slip) is usually within 10%. It can be seen that the speed of the AC motor (whether synchronous or asynchronous) is limited by the power frequency. Therefore, the speed regulation of the AC motor is more difficult. The best way is to change the frequency of the power supply. In the past, it was more complicated to change the power frequency.
3. The number of poles of the AC three-phase asynchronous motor is the total number of coil groups divided by three.
4. Synchronous motor speed = 60 * frequency / pole pairs.
Asynchronous motor speed = (60 * frequency / pole pair) × (1-s) s: slip, a physical quantity indicating the degree to which the rotor speed n is different from the magnetic field speed n0.
In addition, for motors of the same power, the higher the rotational speed, the smaller the output torque.
5. The number of poles of the synchronous motor
Large-capacity synchronous motors are all pole-type, that is, the rotor is a magnetic pole, which is generated by the excitation winding by direct current or by a permanent magnet on the rotor, and the pole pair of the synchronous machine is the logarithm of the rotor pole. The eight-pole motor is that the rotor has 8 magnetic poles, 2p=8, that is, the motor has 4 pairs of magnetic poles. Generally, steam turbine generators are mostly hidden pole motors. The number of pole pairs is very small, generally 1, 2 pairs, and n=60f/p, so his speed is very high, up to 3000 rpm (power frequency), and The number of poles of a hydro-generator is quite large, the structure of the rotor is salient, and the process is complicated. Because of its many poles, its speed is very low, and it may only be a few revolutions per second.
