Phase-to-phase fault is an electrical fault unique to three-phase motor windings . From the statistics of faulty motors, it can be found that in terms of phase-to-phase faults, the problems of two-pole motors are relatively concentrated, and most of them occur at the ends of the windings.

From the distribution of the motor winding coils, the span of the two-pole motor winding coils is relatively large, and the end shaping is a big problem in the wire embedding process. Moreover, it is difficult to fix the phase-to-phase insulation and bind the windings, and phase-to-phase insulation displacement is prone to occur. question.

During the manufacturing process, the standardized motor manufacturers will check the phase-to-phase faults through the withstand voltage method, but the limit state of the breakdown may not be found during the winding performance inspection and no-load test. Such problems may occur Occurs when the motor is running under load.

The motor load test is a type test item, and only the no-load test is carried out during the factory test, which is one of the reasons for the motor to leave the factory with problems. However, from the perspective of manufacturing quality control, we should start with the standardization of the process, reduce and eliminate bad operations, and take necessary strengthening measures for different winding types.

Number of pole pairs of the motor

Each set of coils of a three-phase AC motor will generate N and S magnetic poles, and the number of magnetic poles contained in each phase of each motor is the number of poles. Since the magnetic poles appear in pairs, the motor has 2, 4, 6, 8… poles.

When there is only one coil in each phase winding of A, B, and C phases, which is evenly and symmetrically distributed on the circumference, the current changes once, and the rotating magnetic field turns around once, which is a pair of poles. If each phase of A, B, and C three-phase windings is composed of two coils in series, and the span of each coil is 1/4 circle, then the composite magnetic field established by the three-phase current is still a rotating magnetic field, and the current changes once , the rotating magnetic field only turns 1/2 turn, which is 2 pairs of poles. Similarly, if the windings are arranged according to certain rules, 3 pairs of poles, 4 pairs of poles or generally speaking, P pairs of poles can be obtained. P is the pole logarithm.

An eight-pole motor means that the rotor has 8 magnetic poles, 2p=8, that is, the motor has 4 pairs of magnetic poles. Generally, turbo generators are hidden pole motors, with few pole pairs, usually 1 or 2 pairs, and n=60f/p, so its speed is very high, up to 3000 revolutions (power frequency), and The number of poles of the hydroelectric generator is quite large, and the rotor structure is a salient pole type, and the process is relatively complicated. Because of its large number of poles, its speed is very low, maybe only a few revolutions per second.

Calculation of motor synchronous speed

The synchronous speed of the motor is calculated according to formula (1). Due to the slip factor of the asynchronous motor, there is a certain difference between the actual speed of the motor and the synchronous speed.

n=60f/p……………………(1)

 In formula (1):

n – motor speed;

60 – refers to the time, 60 seconds;

F——power frequency, the power frequency in my country is 50Hz, and the power frequency in foreign countries is 60 Hz;

P——the number of pole pairs of the motor, such as a 2-pole motor, P=1.

For example, for a 50Hz motor, the synchronous speed of a 2-pole (1 pair of poles) motor is 3000 rpm; the speed of a 4-pole (2 pairs of poles) motor is 60×50/2=1500 rpm.

In the case of constant output power, the more the number of pole pairs of the motor, the lower the speed of the motor, but the greater its torque. Therefore, when selecting a motor, consider how much starting torque the load requires.

The frequency of three-phase alternating current in our country is 50Hz. Therefore, the synchronous speed of a 2-pole motor is 3000r/min, the synchronous speed of a 4-pole motor is 1500r/min, the synchronous speed of a 6-pole motor is 1000r/min, and the synchronous speed of an 8-pole motor is 750r/min , The synchronous speed of the 10-pole motor is 600r/min, and the synchronous speed of the 12-pole motor is 500r/min.

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Post time: Apr-08-2023