According to the investigation and feedback, motor burning is mostly caused by two-phase operation. The factors leading to two-phase operation of the motor are as follows:

1、 In terms of power consumption: due to the concentration of power consumption in daily work, the fuse at the low-voltage outlet of the transformer and the main power supply knife switch fuse in the distribution room are most likely to fuse;

2、 In terms of motor control knife switch: the knife switch used to control the motor is of poor quality. In addition, it has been used for a long time and has not been repaired in time, this kind of fault can also occur. According to statistics, it is the most common that the screw holes in the knife switch cannot compress the wires (heads) and fuses entering and leaving the knife switch due to the long service life, which makes it easy for the wire heads to come out of the screw holes. Other reasons include loose fastening screws on the back of the front end of the knife, or the elastic clamping force of the contact clamp is lost, and poor contact with the blade. If such problems are found, replace them with new ones in time;

3、 Line: The disconnection described in the low-voltage power supply line is also a factor that causes two-phase operation of the motor. Especially in the remote sections (such as ravines, canyons, etc.) where the low-voltage line mileage is relatively long after a storm, a comprehensive inspection should be carried out in advance for the wires in these sections;

4、 Motor itself: if the internal or connector of one phase winding of the motor is broken, the motor shall be comprehensively inspected in advance, and the motor with such faults shall be repaired before use.

Only one of the above factors exists. It will cause the motor to run in two phases. Whether there are preventive measures is required. Theoretically, two-phase power supply cannot start the motor, but there are also accidents. As early as 1973, I experienced the event that a two-phase power supply started the pump motor, which is still fresh in my memory. At that time, I had little experience. I started it three times in a row, and the fourth time was successful. However, I felt that the motor sounded abnormally. Fortunately, the power was cut off in time, and the motor survived. After the power failure, the three-phase fuse is in good condition, the power line is detected with an electric pen, and one phase is powered off. So the inspection was carried out along the way, and it was finally found in the power distribution room that the fuse on the main switch blade had blown. During the operation of the motor, if two-phase operation occurs due to one phase power failure, the motor will not stop running, but an abnormal dull buzz will occur. At this time, the current of one phase is zero, the current of the other two phases increases greatly, the motor heats up sharply, and sometimes the smell of paint can be smelled. In this case, power off immediately. However, special personnel shall be left on duty in front of the motor.

With the development of the electronic industry, a class of products of motor phase loss protection have come out one after another. This undoubtedly brings great convenience to the prevention of motor two-phase operation. However, this product does not protect against faults caused by short circuit, so it is necessary to fuse the motor when it is working; However, sometimes we are in urgent need and do not have such products on hand, so we can temporarily adopt the method of double knife switch control.

It shall be selected according to the rated current of the motor or slightly less than the rated current (when the load is light). If possible, the actual current of stator winding can be measured to select the fuse of "operating knife switch". After this point is discussed in detail, after checking that the wiring is correct, first close the "starting knife switch", when the motor starts and enters into normal operation, then close the "operating knife switch", at the same time open the "starting knife switch", and the "operating knife switch" will work independently. When the motor runs in two phases, the "operating knife switch" uses a fuse with the same rated current as the motor. When the current increases to the fusing current of the fuse, the fuse quickly blows, cutting off the power supply and protecting the motor.

We know why the motor fuse for internal phase operation will not blow. The fuse is normally selected within the range of 1.5-2.5 times the rated current of the motor, depending on the load during startup. Light load and small multiple; The load is heavy and the multiple is large. This is mainly due to the large current during startup. The fusing current of the fuse is 1.3-2.1 times of its rated current (for example, we choose lead tin alloy wire with a rated current of 2A, and the minimum fusing current is 2.6A, while the actual value is 3A). Therefore, the minimum fusing current of the fuse should be at least 1.3 * 1.5=1.95 times of the rated current of the motor. If the fuse materials are different, this multiple should be increased. When the motor operates in two phases, the winding current is 1,73 times of its rated current. Therefore, when the motor operates in two phases, the fuse normally selected will not blow.

"Running knife switch" fuse is selected in the preventive measures of double knife switch. The "operating knife switch" undertakes the dual task of ensuring the normal operation of the motor and protecting the motor in case of two-phase operation. Therefore, the selection of "operating knife switch" fuse is more strict. Let's take a 4.5kw (2900rpm) water pump motor as an example for a more detailed discussion. The rated current of 4.5kw motor when wiring with 380V line voltage is 9.1A, then the minimum rated current of the "starting knife switch" fuse should be 9.1A * 1.5=13.65A, and the minimum fusing current should be 13.65A * 1.3=17.75A, to ensure that the motor completes the starting process. In case of short circuit failure, the fuse can quickly blow. The rated current of the fuse selected for "operating knife switch" should be the rated current of the motor (it can also be measured and selected), or slightly less (for light load). The minimum fusing current of the fuse is 9.1. A * 1.3=11.83A; To ensure that the fuse can quickly blow when the winding current increases to 9.1A * 1.73=15.74A when the motor runs in two phases.

We can choose according to the fusing current of the fuse. It can be used within the range of two-phase running winding current less than 15.74A and fuse fusing current greater than 11.83A. In this example, we can choose copper fuse with fusing current of 11A and 13.5A (wire diameter of 0.27 and 0.32mm). thus. Although the rated current of the fuse we selected is small, the fusing current is just within the range we need. Therefore, as long as the motor starts and enters into normal operation, the fuse's fusing current is greater than the rated current of the motor, the fuse will not be blown; When the motor runs in two phases, the current increases sharply. Because the fuse fusing current we selected is less than the current increasing at this time, the fuse will quickly fuse and cut off the power supply. Practice has proved that this method is simple, economical and reliable as long as the fuse selection is accurate.