(1) The power supply voltage is too low. When the motor load is constant, if the power supply voltage is lowered, the current will increase and the motor temperature rise will be too high. For example, for a motor with a rated voltage of 380V, the power supply voltage is lower than 300V, and the motor will be forced to stop or run at a low speed. At this time, the current will increase rapidly and the motor will be burnt quickly.
The reason for the low power supply voltage is:
1 The voltage of the high voltage power supply itself is low;
2 The transmission line connected to the motor is too far, and the cross-sectional area of ​​the wire is too small, causing the line voltage drop to be too large;
3 line load is too large.
(2) Connection error. For example, the motor nameplate stipulates that the 380V adopts the "Y" connection method. At this time, the voltage of each phase of the motor is 220V. If it is connected to the "â–³" connection method, the voltage of each phase will be 380V, which is greater than the rated voltage, and the motor is quickly Burned; otherwise, the "â–³" connection is staggered into a "Y" shape [see figure], the voltage of each phase is only 220V, the motor runs under a lot of rated voltage, and the input current exceeds each phase when fully loaded. The rated current, the same motor will also burn out.
(3) Mechanical failure. There are many types of mechanical faults, and the situation is complicated. Commonly, there are bearing damage, bearing oil shortage and so on. The shaft power of some mechanical equipment is proportional to the square of the multiple of the speed change, such as the fan, the speed is doubled, and the power needs to be increased by three times. Therefore, improper use of machinery can also cause motor overload.
(4) The manufacturing quality is unqualified. Common situations are:
1 fixed, the rotor air gap is too large;
2 The iron core is of poor quality, the burr is large, and the stacking pressure is uneven;
3 squirrel cage rotor cast aluminum quality is poor;
4 The coil data is too different from the original design. This is due to multiple repairs, and the coil turns and wire diameter specifications are wrong.
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