Several practical methods in motor repair (One)

23/10/2019

Now I will summarize the problems often encountered in motor maintenance and introduce several practical methods in the repair work for your reference.

1 After the end cover is installed, the motor insulation resistance is reduced.

The winding is well insulated from the casing before the cage motor is repaired without the two ends. After mounting, the insulation resistance is reduced. This phenomenon can be judged that the fault occurs at both ends of the winding. The reason for the decrease in insulation resistance may be that the paint film on the enamel wire has a partial break or the winding mode is too large to extend the end of the winding. After the end cap is covered, the inner wall of the end cap approaches or contacts the fault point, so that the winding insulation resistance is lowered.

To eliminate this type of fault, first determine which end of the motor the fault is on.

The method of judging is: first install an end cap, and measure the insulation resistance of the winding with a 500V insulation resistance meter. If it is very low, it indicates that there is a fault in the end winding. Then, remove the end cap and attach the other end cap to measure the insulation resistance of the winding. If it is also low, the other end is also faulty. The rotor may not be installed during the process of disassembling and installing the end cap.
After determining the faulty end, generally shape the end of the winding slightly and apply some insulating paint. If the problem is not solved, a layer of film green paper can be easily placed on the outside of the end of the winding. Generally can be eliminated.

2The grounding point in the stator winding slot is searched and processed

Grounding in the stator winding slot generally occurs in the slot. Because the slot insulation paper is biased or damaged in the slot, or the indentation is inadvertently placed in the slot, the enameled wire is placed under the slot insulation paper, or the slot insulation paper is crushed when the slot wedge is driven. Causes a ground fault.

For such a fault, first determine whether the motor winding is damp. If it is wet, the motor is baked. If the result is invalid, it is judged to have a ground fault.

The methods for finding the grounding point are as follows:

1. Carefully check the lead wires and the two ends of the slot. It is found that the lead wires are intact, and the slots at both ends of the slot are not damaged. The inside and outside of the motor are clean and there is no dust or oil everywhere. This means that the grounding point is in the slot.

2. Untie the Y-connection point of the stator winding, measure the relative insulation resistance of each stator, and find the phase with ground fault.

3. With a single-phase voltage regulator, the phase line at the output end is connected to the one end lead line with the ground fault phase, the zero line is connected to the motor casing, and then the voltage regulator is boosted from zero to the tank. Tobacco, this is to immediately turn the regulator output voltage to zero and cut off the power supply, the smoke is the grounding point. Pay attention to safety throughout the operation. Place the motor on an insulating mat. The human body cannot touch the motor before the power is cut off.

The treatment method is as follows: carefully remove the wedge with the grounding groove, then heat the motor into the oven, bake the soft winding, and carefully remove the inner wire of the groove to check the insulation of each wire. If the insulation is damaged, paint and dry it, then place the groove insulation paper in the groove again, then put the removed wire into the groove, punch into the groove wedge, and dip the paint and dry the winding.

If it is found that the enameled wire has been blown, pull the two broken wires to the end of the winding, and then find a wire with the same wire diameter into the groove and wire at both ends of the winding. The joint should be stranded, painted, dried, and then covered with an insulating sleeve. When strapping the ends, pay attention to the binding joints.

3 Judgment of the inter-turn short circuit fault of stator winding

The short circuit fault of the stator winding is generally determined by measuring the DC resistance of the three-phase winding. However, the DC resistance of the stator winding of the motor with a large capacity is small, and the influence of the relationship between the accuracy of the meter and the measurement error is difficult to obtain a correct judgment, but the following method can be used to judge.

Without disassembling the motor, use a single-phase auto-coupled voltage regulator of appropriate capacity to gradually step up from zero and pass low-voltage alternating current to one of the phases. At the same time, the current is measured with a clamp-type ammeter to increase the current to about 1/3 of the rated current of the motor. Then stop the boost, use the multimeter to measure the other two-phase induced voltage separately. If one phase has a turn-to-turn short circuit fault, its induced voltage is lower than the other phase. Then, one phase is energized, and the induced voltages of the other two phases are measured in the same manner. According to whether the induced voltage is the same, it can be judged whether there is a turn-to-turn short circuit fault. Stator inter-turn short circuit fault, generally need to replace the motor winding during maintenance.

4 judgment of squirrel cage broken strip

After the cage rotor is broken, it is generally normal for no-load operation, but with the load, the motor speed is reduced, the stator current is increased, the motor is vibrated, the temperature is increased, and abnormal noise is accompanied. The greater the load, the more broken bars, the more serious the above phenomenon.

The method for judging whether a broken bar is as follows:

1. Power-on method: Use a single-phase voltage regulator with appropriate capacity to connect low-voltage AC to the motor's one-phase winding. The regulator gradually boosts from zero, and the current is measured with a clamp-type ammeter to stop the boost when the current rises to about 1/2 of the rated current of the motor. Then slowly turn the shaft of the motor by hand and observe the pointer of the clamp ammeter. If the pointer does not move or oscillate slightly, it indicates that the rotor is intact. If the pointer is periodically oscillated and the swing amplitude is large, it indicates that the rotor squirrel cage has a broken strip.

2, the alternative method: is to find a motor rotor with the same chassis number, the same as the iron core, replaced by the faulty motor installed. With the same load test, all the above faults are eliminated. This proves that there is a problem with the rotor cage of this motor.