Fault and protection of two-phase operation of the

Fault and protection of two-phase operation of motor

fault and protection of two-phase operation of motor:

1. Cause of two-phase operation of three-phase asynchronous motor: two-phase operation of three-phase asynchronous motor is a common cause of motor damage. Why is it that the fuse protection, additional thermal relay protection of magnetic starter and over-current protection of circuit breaker installed on the motor can not effectively protect the two-phase operation of the motor

1) first, according to the principle of electromotology, if it is connected to a two-phase power supply, its stator winding cannot produce a rotating magnetic field, the rotating torque is zero, and the motor only vibrates but does not rotate. When the motor enters the two-phase power supply for starting, it is actually in a short-circuit state. Its short-circuit current is 0.866 times that of the three-phase starting current, while the starting current of the general asynchronous motor is 4 ~ 7 times that of the rated current. Therefore, when the motor enters the two-phase power supply for starting, the current when the two-phase short-circuit is 3.464 ~ 6.062 times that of the rated current. Therefore, the above current is smaller than the starting current and much larger than the rated current of the motor. When the motor is started in two phases, the motor does not operate, and the operator will immediately find out, and the fuse will also blow, because the fuse can simultaneously output these data through the RS232 port. The fuse current is generally selected according to the following two principles: for the motor with less starting times and shorter starting time, it is selected according to ih=iz/2.5; For motors that start repeatedly and accelerate slowly, ih=iz/(1.6 ~ 2) shall be selected. In the above two equations, IH is the rated current of the fuse; Iz is the three-phase starting current of the motor. For the running motor, after the sudden disconnection of one phase power supply, under the action of mechanical inertia, it can still rotate at a sluggish speed under certain specific conditions. Because the overcurrent multiple of the motor is related to the actual load of the motor and the maximum torque multiple K of the motor itself. When the maximum torque multiple is greater than 2, the motor will maintain two-phase operation, but the speed will be greatly reduced. The greater K is, the greater the overload multiple of the motor during two-phase operation is. When the maximum torque multiple K is equal to 2, when the motor is under rated load and two-phase operation occurs, the over-current of the motor is about 3.5 times the rated current. At this time, if the fuse selected by the motor according to the regulations is used for protection, the fuse can be fused and play a protective role

2) when the motor only carries 50% of the rated load, the two-phase operating current is roughly equal to the rated current. When the motor load is above 50% of the rated load and operates in two phases below the rated load, the fuse cannot reliably play a protective role. The starting current of normal motor is 4 ~ 7 times of the rated current of motor. It can be seen that the fuse can not reliably protect the two-phase operation of the motor

3) in the third case, when the maximum torque multiple K of the motor is less than 2, the motor will slow down and stop until the fuse is fused. In addition to fuse protection, thermal relay is also used for motor overload protection in the three-phase low-voltage motor protection in which the difference in use data is reduced to the minimum. The action current is generally 1.1 times the rated current, and the standby margin is considered to prevent the motor from being switched off by mistake due to the voltage change and ambient temperature change of the motor. Generally, the thermal element is selected according to 1.2 ~ 1.3 times the rated current, and the starting current is avoided by relying on the time delay generated by the thermal inertia of the thermal protection. Therefore, the overload protection composed of thermal elements cannot reliably protect the two-phase operation of the motor. Similarly, for the protection of circuit breaker overcurrent, Bayer materials technology cooperates closely with footwear brands, automated production line manufacturers, adhesive manufacturers and other industry partners. It is generally set to avoid the starting current of the motor. Obviously, such a setting value cannot correctly protect the two-phase operation of the motor

2. About the protection of motor two-phase operation: in recent years, many schemes have been put forward all over the country, which can be basically divided into two categories: one is to install the signal indication of motor one-phase fusing, and the other is the negative sequence protection composed of transistors. Using these methods, there are also some effects, but it is still not perfect, so the promotion and application is not universal. Therefore, two sets of fuses can be used to form a relatively simple and reliable motor two-phase protection. The method is to use six fuses, each two of which are connected in parallel to form a three-phase fuse protection. One of the two fuses in each phase is set according to 1.2 ~ 1.3 times the rated current of the motor, and the other is selected according to the above formula of the rated current of the fuse. When starting the motor, close the last three fuses to make the motor start normally. After starting, close the first three fuses and open the last one to make the motor run normally

3. Finally, it should be pointed out that a large number of practices have proved that to prevent the two-phase operation of motors, we must strengthen monitoring, sum up experience, pay attention to the abnormal phenomenon of missing phase operation, and timely cut off the two-phase operation of motors to ensure the safe and reliable operation of motors. The annual average growth rate of this industry in China is more than 15%