Motor fire hazard and preventive measures

First, the main cause of motor fire

Due to various faults, overcurrent will be overheated in the running motor winding, and the insulating material will burn after ignition. There are many reasons for the motor overheating and collapse. The common ones are as follows:

1. If the three-phase voltage is too high or too low, the motor will overheat. When the voltage is too high, the winding current of the motor increases, so that the temperature rise of the winding exceeds the allowable value and the insulation is ignited; if the voltage is too low, the speed of the motor and the impedance of the stator winding are both decreased and the current is increased. Fired after overheating the barbecued coke insulation material.

2. The three-phase voltage is not (balanced) symmetrical, which is usually caused by the power grid or the motor fault. If the three-phase voltage applied to the motor is asymmetrical, the various losses of the motor in operation will increase, which will cause additional heating of the motor. It is generally required that the difference between the three-phase voltages does not exceed 5%. Under such conditions, the motor can maintain long-term operation at rated power.

3. Phase loss operation Most of the three-phase power supply of the motor has one phase open circuit or one phase open circuit in the winding. If the phase loss occurs in the motor running, although it can continue to run, but the motor speed decreases, the current in the other two phases will increase by about 1.7-1.8 times than the current in normal operation, and it is easy to burn the winding, so the motor is not allowed for a long time. Phase loss operation.

4. The small horse-drawn cart is a mechanical overload operation, which easily causes the motor to burn the insulation material due to long-term over-current overheating and even cause a fire.

5. There is an error in the winding wiring. Generally, the external wiring is wrong, or there is one or several coils embedded or reversed in the pole phase group of the winding during maintenance. The motor will vibrate, abnormal sound and low speed, and the three-phase current is seriously unbalanced. And the winding is overheated and collapses.

6. Rotor winding end failure. If the rotor part of the motor has partial desoldering, the grade and size of the brush do not match, the grade and size of the brush do not match, the brush pressure is insufficient or too large, the brush and winding are in poor contact, and the long-term operation and foreign matter enters. Causes local overheating of the relevant part or sparks between the slip ring and the brush.

7. Various short circuits occur in the stator or rotor windings. If the motor windings are phase-to-phase short-circuited, the insulation near the short-circuit point is scorched and overheated due to overcurrent, causing the winding to burn. If the coil insulation of the stator winding is damaged and the conductors are in contact with each other, a turn-to-turn short circuit is formed. Because of the short circuit between the turns, the coil will flow through a large circulating current (2-10 times of the normal current), causing the coil to heat up severely. The three-phase current is unbalanced, the torque of the motor is reduced, and noise is generated.

8. If the insulation resistance is too low, the winding insulation of the motor in operation will be easily damaged and breakdown, causing various short circuits and collapse.

Second, the main measures to prevent motor fire

1. Whether the three-phase voltage is too high or too low, can be judged by using the multimeter AC voltage file to detect the bus voltage and the motor terminal voltage. For reasons of the power grid, it can be reflected to the power supply department, and it is required to adjust or use the adjustment switch of the transformer to adjust; if the branch voltage drop is too large, the wire area should be replaced and the distance between the motor and the busbar should be shortened; if the motor runs for a long time at around 340V It can be replaced with a motor that is 20% larger than the transmission machinery. However, it is not very economical and realistic to replace the motor in large quantities. It is best to add capacitor compensation on the grid. In addition, when the voltage is too low, the motor can be protected by a combination device such as an AC contactor or a three-phase thermal relay; when the voltage is too high, the three-phase thermal relay can be adjusted to a higher value.

2. Whether the three-phase voltage is symmetrical, can be judged by detecting the voltage and current values ​​of the three-phase busbar by using the multimeter AC voltage file and the clamp meter respectively. If a serious imbalance is found, it can be determined that there are too many single-phase high-power electric heaters and AC welding machines on the three-phase bus. In order to change this abnormal situation, the connection capacity of the three-phase busbar can be re-adjusted and reasonably distributed. Use the same method to check whether the voltage at the upper end of each motor is balanced with the load current. If a serious imbalance is found, first check the power to check whether the stator windings are short-circuited between phases or between turns, and if the stator windings are grounded, wait until the fault is found and repaired. Can be powered on test. In order to protect the safe operation of the motor, a combination of an automatic switch (circuit breaker), a three-phase thermal relay and an AC contactor can be used in the three-phase feeder.

3. To prevent the motor from igniting and causing a fire, take the following measures:

(1) Three small power indicator lights are connected to the output end of the three-phase thermal relay, which can be judged that the motor in operation is one-phase power-off and one-phase disconnection of the stator winding. If the power supply is disconnected from the power supply, the indicator light of the phase should be off or darkened. The power should be checked first to check whether the three-phase feeder is broken, the fuse is blown, the AC contactor or the main contact of the circuit breaker is in poor contact. If the joint is loose, wait until the fault point is found and repaired, then the test can be powered on; if the indicator light is on, the three-phase current can be checked by the clamp meter to determine whether the stator winding of the motor runs out of phase. When the circuit is broken, immediately stop the power and disassemble the terminal part of the motor winding, find the fault point and reconnect the welding fast. After the insulation is applied, apply the insulating paint before installing the test.

(2) When the capacity is 1.7-20 kW, the Y-connection method can be used to connect a low-voltage relay of about 10-40V between the neutral point of the Y-shape and the ground (zero), and the normally-closed contact of the relay When the series is connected to the AC contactor coil circuit, if the power supply or the stator winding has a phase disconnection, the motor power can be automatically cut off.

4. When the small horse-drawn cart is found, the following fire prevention measures and protection methods should be adopted:

(1) If there is a machine marked on the nameplate, the motor can be selected according to the power on the nameplate; if there is no nameplate, first try to reduce the mechanical load so that the load current of the motor is not greater than the rated current.

(2) If the mechanical load cannot be reduced, only a larger capacity motor can be selected to adapt, but the load current of the motor should not be greater than the rated current.

(3) A combination of three-phase thermal relay, AC contactor and automatic switch (circuit breaker) can be installed in the motor feeder as the overload protection of the motor.

5. In order to prevent the winding wiring from being wrong, pay attention to the repair or modification of the motor:

(1) Do not reverse the coil of part of the winding, or do not reverse the beginning of the phase winding of the three phases, or do not wrap the winding turns too little.

(2) If the nameplate indicates 380V/220V, Y/â–³ connecting motor, when the power supply voltage is 380V, it must not be connected to â–³ shape; when the power supply voltage is 220V, it should be connected to â–³ shape, but sometimes it can be as needed Connected to a Y shape; if the nameplate indicates a 660V/380V motor, it should be connected to a delta shape when used in a system with a line voltage of 380V.

(3) When wiring the stator winding, do not connect the delta shape to the Y shape and cause the motor to collapse.

(4) When the motor is started by Y-â–³, do not mistake or misconnect the number of the six lead wires when wiring.

(5) If the wiring of the winding is to be checked correctly, a circular silicon steel sheet can be used, which is drilled in the middle and sleeved on the copper strip as a rotor. The silicon steel sheet is placed along the center of the inner surface of the stator. When the stator winding is connected to the three phases 30 -50% of the rated voltage, whether the pole or phase or group or a coil wiring is wrong, the silicon steel sheet does not rotate; if the pole or phase or group of the winding or a coil is wired correctly, the silicon steel sheet should be rotated.

6. For the end fault of the rotor winding, check the winding one-phase open circuit or desoldering with a check lamp or a multimeter.

(1) If the brush and the slip ring are in poor contact, the brush pressure can be adjusted and the contact area between the brush and the slip ring can be improved;

(2) If the wire breakage or partial desoldering is found, the soldering joint should be reconnected, and the insulation and the insulating paint should be applied to the test;

(3) If it is found that the spark between the brush and the slip ring is too large, the brush number and the size may not match, and the appropriate brush is replaced;

(4) If the brush pressure is insufficient or too large, the brush pressure can be adjusted;

(5) If the brush is rolled in the brush holder, the brush can be ground;

(6) If there is dirt and debris on the surface of the slip ring, it can be polished with No. 0 sand cloth and cleaned with clean cotton yarn;

(7) The slip ring is not round or has deep marks. It can be ground with No. 0 abrasive cloth or a slippery car.

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