Why do low-pole-number motors often have phase-to-phase faul_Wolong Nanyang Explosion-proof Motor, China Motor Manufacturer

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Why do low-pole-number motors often have phase-to-phase faul

source：未知 time：2024-11-14 10:00nbsp; click：

Phase-to-phase fault is a type of electrical fault that is 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 large, and end shaping is a major problem in the wire embedding process. In addition, it is also difficult to fix the phase-to-phase insulation and tie the windings, which is prone to phase-to-phase insulation displacement problems.
During the manufacturing process, standard motor manufacturers will check for phase-to-phase faults by means of voltage withstand, but the limit state of breakdown may not be discovered during winding performance inspections and no-load tests. Such problems may occur during the motor's loaded operation.

The motor load test is a type test item. During the factory test, only no-load tests are performed, which is one of the reasons why motors with problems are shipped. 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 produce N and S poles. The number of poles contained in each phase of each motor is the number of poles. Since the poles appear in pairs, the motor has 2, 4, 6, 8... poles. When there is only one coil evenly and symmetrically distributed on the circumference of each phase winding of A, B, and C, the current changes once and the rotating magnetic field rotates one circle, which is a pair of poles. If each phase of the A, B, and C three-phase winding is composed of two coils in series, and the span of each coil is 1/4 of the circumference, then the synthetic magnetic field established by the three-phase current is still a rotating magnetic field, and the current changes once, and the rotating magnetic field only rotates 1/2 turn, which is 2 pairs of poles. Similarly, if the windings are arranged according to a certain rule, we can get 3 pairs of poles, 4 pairs of poles, or generally P pairs of poles. P is the number of pole pairs.

An eight-pole motor has eight magnetic poles on its rotor, 2p=8, which means that this motor has four pairs of magnetic poles. Generally, steam turbine generators are mostly non-salient pole motors with very few pole pairs, usually 1 or 2 pairs, and n=60f/p, so its speed is very high, up to 3000 rpm (operating frequency), while the number of poles of a hydro-turbine generator is quite large, and the rotor structure is salient pole type, which is more complicated to process. Due to its large number of poles, its speed is very low, which may be only a few revolutions per second.

Calculation of synchronous speed of the motor The synchronous speed of the motor is calculated according to formula (1). Due to the slip factor of asynchronous motors, the actual speed of the motor is somewhat different from the synchronous speed. n=60f/p………………………………(1) In formula (1): n——motor speed; 60——refers to time, 60 seconds; F——power supply frequency, my country's power supply frequency is 50Hz, and foreign countries have 60 Hz power supply frequency; P——number of motor pole pairs, such as 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.

When the output power remains unchanged, the more pole pairs the motor has, the lower the motor speed will be, but the greater its torque will be. Therefore, when selecting a motor, consider how much starting torque the load requires.

The frequency of three-phase AC in our country is 50Hz, so 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, the synchronous speed of an 8-pole motor is 750r/min, the synchronous speed of a 10-pole motor is 600r/min, and the synchronous speed of a 12-pole motor is 500r/min.
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