We know that the absence of a phase of a three-phase motor is not possible, but a three-phase motor can still operate without a phase in operation, which is called phase loss operation. The most common fault of a three-phase asynchronous motor during operation is phase loss operation, such as breaking a live wire or breaking a phase winding. At this time, if the load on the shaft does not change, the motor is in a severe overload state, the stator current will greatly exceed the rated value, and the motor will burn out after a long time. Due to the lack of phase operation, the accident of burning is more in the production, causing greater economic losses to the enterprise. The following is a brief analysis of the reasons and countermeasures for the phase loss operation of the three-phase asynchronous motor.
For three-phase asynchronous motors, three-phase power supply must be used in normal operation, and phase loss is a taboo for normal operation of the motor. In the absence of phase, the motor that was originally stopped will not start, and a “beep†sound will be emitted. At this time, if the motor rotor shaft is turned by hand, it may rotate slowly; when the original rotating motor is out of phase, the rotation speed decreases. And slower, the current increases significantly, the motor temperature rises, the outer casing is hot, and an abnormal sound is emitted. If the phase loss operation for a long time will inevitably cause the motor to overheat and burn.
Causes of phase loss:
The cause of motor phase loss is usually divided into external causes and internal causes. The external cause is mainly the quality problem of the external network power supply. One is the lack of phase of the power supply. Due to the failure of the power supply line, the power supply has lost one phase or two phases before reaching the motor protection circuit, causing the motor to fail to start or start abnormal operation; Second, the phase loss of the motor on the high-side or low-side side of the distribution transformer causes the motor to run out of phase. In this case, all motors powered by the transformer will run out of phase.
Internal causes mainly include loss of phase due to control switches, contactors, relay contact oxidation, burns, looseness, and poor contact in the protection circuit. The melt contact of a phase fuse is poor, or the fuse is too tight and almost broken, or the melt current is selected too small, causing the passing current to be slightly larger and fusing. In particular, under the impact of the motor starting current, melt non-fracture blowing is more likely to occur. Sometimes the motor load line is broken, which is usually caused by improper installation. Especially when the single-core wire is loosened, the small ring kinks and joint damage may cause the wire to break during operation. Due to the long-term use of the motor, the internal joints or leads of the windings are loosened or partially overheated to blow the windings, resulting in a phase loss operation of the motor.
Why is the phase loss operation of the three-phase motor burned? For example, when a fuse of a phase of a delta connection motor is blown, the three-phase stator winding is divided into two paths, and only one phase winding is used in one path. Only the current in the one-phase winding loop is greatly increased compared to the three-phase operation, so the phase winding is inevitably burned first. Turn on the motor end check, if you see one phase winding burnt and the other two phase windings are good, it is the result of the phase loss operation of the delta connection motor. Another example is that after the star-connected motor is powered off, the other two-phase windings are connected in series, the applied voltage is 380 volts, and the voltage of each phase winding is reduced from 220 volts to 190 volts. If the load power of the motor is constant, then The current of the two-phase stator winding is beyond the normal value. Under the rated load, the current of the motor in the phase loss operation is about 1.732 times larger than the rated current. This current will overheat the stator winding and cause burnt. Open the motor end cover inspection. If the two-phase winding is burnt, one phase is intact, which is an accident caused by the lack of phase operation. The reason for causing the motor to run out of phase is generally: a, the fuse of the motor breaks one phase; b, the low voltage line is broken; c, the switch is in poor contact, one phase is not connected; d, the joint of one wire is loose or broken Wait.
Corresponding countermeasures: Prevent the motor from running out of phase. First, strengthen the monitoring and inspection during operation. Second, some appropriate automatic protection measures can be taken. Regardless of whether the motor is static or dynamic, the direct hazard of phase loss operation is that the motor's one-phase or two-phase windings are overheated or even burned out. At the same time, the insulation aging is accelerated due to the overcurrent operation of the power cable. Especially in the static state, the phase loss will generate several times the stall current of the rated current in the motor winding. The speed at which the windings burn out is faster and more severe than the sudden phase loss in operation. Therefore, while we perform routine maintenance and overhaul of the motor, we must carry out comprehensive inspection and testing of the corresponding MCC function unit of the motor. In particular, it is necessary to carefully check the reliability of the load switch, power line, and static contact. Eliminate the lack of phase operation.
----- Editor-in-Chief: Dalan Oil Pump Motor 02-Procurement Consultant
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