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Malfunction and protection of motor two-phase operation
1, three-phase asynchronous motor two-phase operation reasons: three-phase asynchronous motor two-phase operation is caused by motor damage is a common cause. Why the motor is equipped with fuse protection, magnetic starter additional thermal relay protection, circuit breaker over-current protection, are not well on the motor two-phase operation from the effective protection? 1) First of all, according to the principle of electromotor, if it is connected to two-phase power supply, its stator winding can not generate rotating magnetic field, the rotational torque is zero, and the motor only vibrates without rotating. Motor into the two-phase power start, in fact, is in short-circuit state, the short-circuit current is 0.866 times the start-up current of three-phase start, and the general induction motor starting current is 4 to 7 times the rated current, so the motor into the two-phase When the power is started, the current corresponding to the two-phase short circuit is 3.464 to 6.062 times of the rated current, so the above current is smaller than the starting current and much larger than the rated current of the motor. Motor two-phase start, the motor is not running, the operator will immediately find, and the fuse will be blown, because the fuse fuse current is generally selected according to the following two principles: For the number of start-up less and start-up time shorter motor, According IH = IZ / 2.5 selected; for the motor to start and accelerate slowly, press IH = IZ / (1.6 ~ 2) selected. In the above two formulas, IH is the rated current of the fuse; IZ is the three-phase motor starting current. For a motor in operation, a sudden phase off the power after the mechanical inertia, under certain conditions can still be slow rotation. Since the motor over-current multiples and the actual motor load and the motor itself, the maximum torque multiples K. When the maximum torque multiplier is greater than 2, the motor will maintain two-phase operation, but the speed is greatly reduced, the larger the K, the greater the overload of the motor two-phase operation. When the maximum torque multiplier K is equal to 2, the motor with a rated load and two-phase operation occurs, the motor overcurrent is about 3.5 times the rated current, then the motor if the fuse according to the provisions of the protection for the protection, the fuse can be blown , And play a protective role. 2) When the motor with only 50% of the rated load, the two-phase running current roughly equal to the rated current. When the motor load is above 50% of the rated load, and the two-phase operation below the rated load, the fuse can not reliably play a protective role. Normal motor starting current for the motor rated current of 4 to 7 times. It can be seen from the fuse can not be reliably protected motor two-phase operation. 3) The third situation is the motor maximum torque multiplier K is less than 2, the motor will decelerate to stop until the fuse blown. In addition to fuse protection, three-phase low voltage motor protection, thermal relay is also used for motor overload protection. The operating current is generally selected 1.1 times the rated current, consider the spare margin, in order to prevent the motor voltage changes and ambient temperature changes and mistakenly cut the motor, usually 1.2 ~ 1.3 times the rated current selection of thermal components, relying on thermal inertia generated by thermal inertia Delay, avoid starting current. Therefore, the overload protection by the thermal components, it is impossible to reliably protect the motor two-phase operation. The same circuit breaker overcurrent protection, generally by avoiding the motor starting current setting, obviously, according to this setting value can not correctly protect the motor two-phase operation. 2, on the motor two-phase protection of the operation: In recent years many proposals put forward by various places, basically can be summarized into two categories: one is to install the motor fuse signal indication, and the other is the use of transistor negative sequence protection . The use of these methods, but also a certain effect, but still not perfect, so the promotion is not universal. This can be used double fuse, constitute a relatively simple and reliable motor two-phase protection. Method is to use 6 fuses, each consisting of two three-phase fuse in parallel protection, two fuses in each phase, one according to the motor 1.2 ~ 1.3 times the rated current setting, the other according to the fuse rated current formula selected . When the motor starts up, close the three fuses to make the motor start normally. After starting, close the first three fuses and then open the latter 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 motor running, only to strengthen the monitoring, sum up experience, attention to the abnormal phenomenon of missing phase operation, timely removal of two-phase motor running to ensure the safe and reliable operation of the motor.