Interphase fault is a unique electrical fault in the windings of three-phase motors. From the statistics of faulty motors, it can be found that in terms of phase to phase faults, the problems of bipolar motors are relatively concentrated, and most of them occur at the end of the winding.
From the distribution of motor winding coils, it can be seen that the span of the two pole motor winding coils is relatively large, and end shaping is a major challenge in the wire embedding process. Moreover, it is also difficult to fix the interphase insulation and bind the windings, which can easily lead to interphase insulation displacement.
In the production and manufacturing process, standardized motor manufacturers will inspect phase to phase faults through voltage withstand methods. However, the limit state of breakdown may not be detected during winding performance checks and no-load tests, and such problems may occur during the loaded operation of the motor.
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 why problematic motors leave the factory. However, from the perspective of manufacturing quality control, we should start with the standardization of the process, reduce and eliminate defective operations, and take necessary strengthening measures for different types of windings.
The number of pole pairs of the motor
Each coil of a three-phase AC motor generates N and S magnetic poles, and the number of magnetic poles per phase in each motor is the number of poles. Due to the fact that magnetic poles appear in pairs, motors are divided into poles of 2, 4, 6, 8.
When there is only one coil evenly and symmetrically distributed on the circumference of each phase winding of three-phase A, B, and C, the current changes once and the rotating magnetic field rotates once, which is a pair of poles. If the three-phase windings A, B, and C are composed of two coils connected in series in each phase, with a span of 1/4 of the circumference of each coil, then the composite 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 certain rules, 3 pairs of poles, 4 pairs of poles, or generally P pairs of poles can be obtained. P is the polar logarithm.
An eight pole motor has a rotor with 8 magnetic poles, 2p=8, which means the motor has 4 pairs of magnetic poles. Generally, steam turbine generators are mostly hidden pole motors with very few pole pairs, usually 1 or 2 pairs, and n=60f/p, so their speed is very high, up to 3000 revolutions per second (power frequency). However, water turbine generators have a considerable number of poles, and the rotor structure is convex pole, making the process more complex. Due to their large number of poles, their speed is very low, possibly only a few revolutions per second.
Calculation of synchronous motor speed
The synchronous speed of the motor is calculated according to equation (1). Due to the slip factor, there is a certain difference between the actual speed of an asynchronous motor and the synchronous speed.
N=60f/p (1)
In equation (1):
N – Motor speed;
60- refers to time, 60 seconds;
F – Power frequency. In China, the power frequency is 50Hz, while in foreign countries, there is a 60 Hz power frequency;
P – Number of motor poles, such as a 2-pole motor, P=1.
For example, a 50Hz motor with a 2-pole (1 pair of poles) motor has a synchronous speed of 3000 rpm; The speed of a 4-pole (2-pole) motor is 60 × 50/2=1500 rpm.
When the output power remains constant, the more pole pairs a motor has, the lower its speed will be, but the greater its torque will be. So when choosing a motor, consider how much starting torque the load requires.
When the output power remains constant, the more pole pairs a motor has, the lower its speed will be, but the greater its torque will be. So when choosing a motor, consider how much starting torque the load requires.