Update:20 Mar 2020

Level 4 motor refers to the motor with 4 stator magnetic poles. The actual synchronous speed of 1430r / min is 1500 rpm motor. According to the speed formula: speed = time (60s) × frequency (50Hz) divided by the number of poles, one pole pair is two poles, and 3000 ÷ 1500 = two pole pairs is four pole motor.

"Pole number" of three-phase asynchronous motor refers to the number of stator magnetic poles. Different connection ways of stator winding can form different pole numbers of stator magnetic field. The number of poles of 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. The speed of the motor = 60x motor frequency / motor pole pairs.

The current of the motor is only related to the voltage and power of the motor. Only one or two windings can form a circuit, that is, the number of poles, which appears in pairs. Poles are the meaning of magnetic poles. When these windings pass through the current, they will generate a magnetic field, and the corresponding poles will exist.

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. Asynchronous motor rotor speed is always lower than or higher than the speed of its rotating magnetic field, asynchronous name comes from this. The difference between the rotor speed and the rotating magnetic field speed of asynchronous motor (referred to as the slip) is usually within 10%.

Therefore, the speed of AC motor (synchronous or asynchronous) is restricted by the power frequency. Therefore, it is difficult to adjust the speed of AC motor. The best way is to change the frequency of power supply, which is more complicated in the past. Therefore, before the 1970s, in the occasion of speed regulation, DC motor was widely used.

With the development of power electronic technology, the frequency control technology of AC motor has been applied.

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