Several Practical Methods in Motor Repair

Various situations may be encountered during motor repair. If reasonable methods are adopted, detours can be avoided during repair, achieving twice the result with half the effort. Summarize the experience and experience in daily maintenance work into several practical methods for motor repair, mainly including methods for dealing with and repairing problems such as reduced insulation resistance of the motor, grounding point in the stator winding slot, inter turn short circuit of the winding, broken cage rotor bars, shooting of "parallel head covers" between motors, dismantling of old windings, wear and tear of rotor journals and end cover bearing chambers, for reference.
This article summarizes the common problems encountered in motor maintenance and introduces several practical methods in repair work.
1. Treatment for reducing the insulation resistance of the motor after installing the end cover
Before installing the covers at both ends during the repair of the cage motor, the insulation of the winding to the casing is good. After installation, the insulation resistance decreases. This phenomenon can determine that the fault occurred at both ends of the winding. The reason for the decrease in insulation resistance. It may be due to local damage to the paint film on the enameled wire or deviation of the winding mold, causing the end of the winding to protrude longer. Due to the fact that the inner wall of the end cover is close to or in contact with the fault point after being covered, the insulation resistance of the winding is reduced.
To eliminate this type of fault, the first step is to determine which end of the motor is at fault. The judgment method is to first install an end cover and use a 500V insulation resistance meter to measure the insulation resistance of the winding. If it is very low, it indicates that there is a fault in this end of the winding. Then, remove this end cover and install the other end cover to measure the insulation resistance of the winding. If it is also very low, it indicates that there is also a fault at the other end. During the process of removing and installing the end cover, the rotor may not be installed.
After determining the faulty end, it is generally sufficient to slightly reshape the winding end and apply paint. If the problem is not solved, a layer of thin film green shell paper can be placed on the outer side of the winding end where it is easy to contact the inner side of the end cover, and the fault can generally be eliminated.
2. Search and handling of grounding points in stator winding slots
Grounding in the stator winding slot usually occurs at the slot. Due to the deviation or damage of the slot insulation paper at the slot, or due to improper threading or marking at the slot, the enameled wire is placed under the slot insulation paper, or the slot insulation paper is squeezed when entering the slot wedge, resulting in a grounding fault.
For this type of fault, the first step is to determine whether the motor winding is damp. If it is damp, the motor should be baked. If the result is invalid, it is judged as a grounding fault.
There are several ways to find a grounding point:
(1) Carefully inspect the outgoing line and both ends of the slot, and it is found that the outgoing line is intact. The insulation of the slots on both ends of the slot is not damaged, and the inside and outside of the motor are clean, with no dust or oil stains in all places. This indicates that the grounding point is inside the slot.
(2) Unlock the Y-shaped connection point of the stator winding, measure the insulation resistance of each phase to ground, and identify the phase with ground fault.
(3) Use a single-phase voltage regulator, with the output phase line connected to one end of the grounding fault phase, and the zero line connected to the motor casing. Then, boost the voltage regulator from zero until smoke rises in the slot. At this point, immediately reduce the output voltage of the regulator to zero and cut off the power supply. The smoking point is the grounding point. Pay attention to safety throughout the operation, and place the motor on an insulating pad. Do not touch the motor before cutting off the power.
The treatment method is to carefully remove the slot wedge with a grounding slot, then place the motor in an oven with padding, dry the soft winding, and carefully remove the wire turns inside the slot to check the insulation of each wire. If the insulation is damaged, it should be painted and dried, and then the insulation paper should be placed again in the slot. The removed wire turns should be placed in the slot, inserted into the slot wedge, and the winding should be painted and dried.
If it is found that the enameled wire has been burnt out, two broken wire ends should be pulled to the end of the winding, and then a enameled wire with the same diameter should be placed in the slot. Pull wires at both ends of the winding, and the joints should be twisted, soldered, painted, dried, and then covered with insulating sleeves. When binding the ends, attention should be paid to binding the joints.
3. Judgment of stator winding inter turn short circuit fault
The stator winding turn short circuit fault is generally judged by measuring the DC resistance of the three-phase winding. However, for motors with larger capacity, the DC resistance of the stator winding is very small, and due to the influence of instrument accuracy and measurement errors, it is difficult to make a correct judgment. However, the following methods can be used to make the judgment.
Without disassembling the motor, use a single-phase self coupling voltage regulator with appropriate capacity to gradually increase the voltage from zero, and introduce low-voltage AC power to one of the phases. At the same time, use a clamp type ammeter to measure the current, so that the current rises to about one-third of the rated current of the motor. Then stop boosting and measure the induced voltage of other two phases with a multimeter. If one phase has turn to turn short circuit fault, its induced voltage will be lower than the other phase. Then switch one phase to power on and measure the induced voltage of other two phases in the same way. Based on whether the induced voltage is the same, it can be determined whether there is an inter turn short circuit fault. The stator inter turn short circuit fault usually requires replacing the motor winding during maintenance.
4. Judgment of broken squirrel cage bars
After the cage rotor is broken, it usually operates normally without load. However, with the addition of load, the motor speed decreases, the stator current increases, the motor vibrates, the temperature increases, and there is abnormal noise. The larger the load, the more broken the bars, and the more serious the above phenomenon becomes.
The method to determine whether the bar is broken is as follows:
(1) Electrification method: Use a single-phase voltage regulator with appropriate capacity to apply low-voltage AC power to one phase winding of the motor. The voltage regulator slowly increases voltage from zero, while measuring the current with a clamp type ammeter. When the current rises to about half of the rated current of the motor, stop boosting, and then slowly rotate the motor shaft by hand to observe the pointer of the clamp type ammeter. If the pointer does not move or swings slightly, the rotor is intact. If the pointer oscillates periodically and the swing amplitude is large, it indicates that the rotor squirrel cage bar is broken.
(2) Substitution method: It is to find a motor rotor with the same frame number and iron core number as this motor, replace it with one installed on the faulty motor, test run with the same load, and all the above faults are eliminated, thus proving that there is indeed a problem with the rotor squirrel cage of this motor.
5. Repair after the "shooting of the head cover" in the motor room
After a short circuit is broken through between the head covers of a wound motor, a section of the wire head is usually burned off. When repairing, the method of lengthening can be used, and the specific method is as follows:
Use 1-2 250W infrared light bulbs to locally heat and soften the rotor end shot, remove copper foam, and then bend out the burnt (flat) copper wire slightly (for welding purposes). The extension wire can be welded with silver or tin. When welding, pay attention to protecting the winding to avoid burning out (wet asbestos cloth can be used for protection) and causing welding bumps and flat welding scars. After straightening the copper wire, wrap a layer of glass tape around it, apply insulation paint, and dry it with an infrared light bulb. When placing the wire in its original position, place a layer of thin film green shell paper underneath, then cut off the excess part of the copper wire, insert it into the joint cover, solder it, and finally paint the repair area and dry it. The head cover should be made of copper according to the original size, and the copper sheet should be coated with tin before production.
6. Removal of old motor windings
Generally, it is necessary to dry the soft winding before removing the wire, but for larger capacity motors, it is difficult to dry and remove the wire. Therefore, the following methods can be used to remove the old winding. Use a flat shovel to remove one end of the winding, with the notch flush with the slot. Cut open the other end of the winding with iron scissors, then use a copper rod of suitable thickness to punch out the enameled wire in the slot, and finally clean the slot. If the slot rod is suitable and the operation method is correct, the wires in each slot can be punched out together. When using a flat shovel, be careful not to damage the stator core.
When disassembling the wire using the above method, the drying and softening process before disassembly can be omitted, which saves time, electricity, and disassembly is relatively labor-saving. But this method is suitable for motors with a capacity of less than 7.5KW, as the casing of small motors or micro motors is small, making it inconvenient to operate with a flat shovel.
7. Grinding of new brushes
A DC motor or rotor is a wound motor that updates the brush. When the brush holder is perpendicular to the surface of the commutator, the method of grinding the brush is to lay a piece of sandpaper on the surface of the commutator, and hold a new brush of the same model to rub back and forth along the axis of the commutator. The worn surface of the brush will quickly be ground into an arc that matches the surface of the commutator. First use thicker sandpaper, then use fine sandpaper. When grinding, hold the brush in your hand and move it back and forth along the axis of the commutator without deviation. The reciprocating stroke should not be too long. Pay attention to preventing the brush powder from entering the armature and commutator grooves. After grinding, install the brush into the brush holder, and start the motor to run at no load for a few minutes before taking out each brush for inspection. The contact surface should generally account for more than 80% of the total area before proceeding. Otherwise, it should be re ground until qualified.
When the brush holder of the motor is not perpendicular to the surface of the commutator, the following method should be used: lay sandpaper on a flat surface, and hold the brush to grind an inclined surface according to the tilt angle of the brush holder. Then install the brush into the brush holder and rotate the motor rotor by hand, which will create bright spots on the inclined surface of the brush. Then, use sandpaper to polish the bright spots. After polishing, install the brush into the brush holder and rotate the motor rotor by hand, which will also cause bright spots and the area will be larger for the first time. By doing this several times, the contact surface of the brush will become larger and gradually form an arc. When polishing bright spots, do not polish too much each time, especially for the last time. Be careful, just use fine sandpaper to wipe them slightly, as excessive polishing can actually make them smaller. Practice has proven that using this method to grind tilted brushes is fast and effective.
8. Emergency repair of rotor journal and end cover bearing chamber wear
Specific methods and steps are given below:
(1) First, rinse repeatedly with ethanol or gasoline to clean the area that needs to be repainted.
(2) Heat the ternary nylon ethanol solution into a transparent liquid, and apply it thinly to the worn area with a small brush. For slightly more worn areas, extend the heating time appropriately to make the solution thicker before applying. Generally, it needs to be applied several times until the required thickness is reached. After the first application, it needs to be left for about 3 minutes, let it dry on its own, and when it is not sticky, it can be applied a second time.
(3) After coating, it can be dried up by heating (at a temperature not exceeding 80 degrees Celsius for 0.5 to 1 hour), or it can be allowed to dry naturally (at room temperature of 20, placed in a ventilated place for about 36 hours).
(4) If the wear amount is within 0.10mm, it can be used after being coated and dried. If the wear amount is greater than 0.10mm, in order to ensure the concentricity of the stator and rotor during motor assembly, turning processing is also required after coating. When turning (if both the outer and inner ring dimensions of the original bearing are within their tolerance range), the repair of the end cover bearing chamber can be processed according to P6. If the journal is repaired, it can be processed according to r6.
After the coated nylon solution is completely dried, it has sufficient adhesion and hardness to fully meet the requirements of motor assembly.
This method is only suitable for situations where the wear is not significant, only a few tenths of a millimeter.

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