Out with the old, in with the new – this concept has perhaps become prevalent in consumption and even industrial processing. Of course, motor repair is no exception; new parts are generally better for ensuring motor performance. However, there are often times when repair and restoration become necessary.
Today, Ms. will briefly discuss slip rings. Slip rings are one of the essential components of a three-phase wound rotor motor, and they have a direct impact on motor performance. Today, we will discuss slip ring failures, cause analysis, and repair.
Common problems with slip rings
● The contact between the brush and the slip ring is a point contact or a line contact, rather than a relatively tight surface contact.
● The brush and slip ring were misaligned during installation.
● The brush pressure is too high or too low. Excessive pressure leads to increased mechanical wear; insufficient pressure results in excessive electrical losses.
● The surface of the collector ring is contaminated with oil or debris.
● When the motor is used under conditions of high vibration, the brushes may not make good contact with the slip ring due to vibration, which may cause arcing and increase losses. This will also raise the temperature of the slip ring.
● The brush brand is incorrect, and the current density is too high.
Causes and Repairs of Loose Slip Rings
Plastic-coated slip rings are made by die-casting a copper ring, pad, and connector together using 4330 plastic. They are relatively robust and reliable. Loosening in the fit between the slip ring and the shaft occurs because the interference fit between the shaft and the slip ring is insufficient. Especially during forward and reverse starting and braking operations, the plastic shrinks and cracks after a period of operation, creating a gap with the shaft. After several disassemblies, loosening will occur. In cases of severe loosening, the slip ring must be removed and replaced with one of the same specifications, or a layer of XH-2 fast-curing adhesive can be applied to the inside of the original slip ring and replaced along with a new key.
Loosening of sleeve-type slip rings (this type of structure is used for YZR280 frame sizes and above) mainly occurs between the copper ring and the sleeve. This is because the sleeve fit is insufficiently tight, and the loosening will be more serious if the copper ring heat-fitting process is not used. After the copper ring is heat-fitted at 150℃, it is not easy to loosen, and it also avoids loosening caused by aging of interlayer insulation varnish and insulation shrinkage. Loose copper rings should be repaired in time. First, remove the entire slip ring from the shaft, then remove the loose copper ring, clean the outer surface of the sleeve at the original copper ring location, and then refit a copper ring according to the outer diameter tolerance of the sleeve. The fit tolerance between the copper ring and the sleeve should be H8/n4. Heat the copper ring to less than 150℃ and heat-fit it onto the sleeve.
Damage to slip ring surface and repair methods
Over time, slip rings will develop spots, streaks, dents, grooves, marks, and abrasions on their working surfaces due to various reasons. If these faults are not addressed promptly, they can damage the slip rings or even the motor. Therefore, timely repair is essential.
●General Repair
The slip ring surface may have minor damage, such as spots, brush marks, or slight wear. First, use a fine flat file or oilstone to grind it while rotating. Be careful not to apply excessive pressure and ensure the file is applied evenly to avoid over-grinding or creating new unevenness. After the scratches are removed, polish with 00-grit sandpaper at high speed until the slip ring surface achieves a surface roughness of R6.3 to R3.2, at which point it can be reused.
●Refinement
When the slip ring surface is out of round, has grooves, burns, or severe unevenness, with groove depth reaching 1mm and the damaged area accounting for 20%~30% of the total area, the rotor should be precision-machined on a lathe. During machining, the cutting tool should be sharp, the feed rate approximately 0.2mm, and the surface linear velocity approximately 2m/s.
The eccentricity after cutting should not exceed 0.03~0.05mm. Then, polish with 00-grit sandpaper to achieve a surface roughness of R3.2~R.1.6.
4. Damage to slip ring insulation and repair methods
The slip ring structure of hoisting and metallurgical motors is relatively simple and reliable, with insulation damage being rare. However, in harsh operating conditions, such failures can occur. This is because the plastic parts of the slip ring become brittle and crack after prolonged use. When external coal dust, iron powder, other conductive dust, and oil contaminants penetrate, it can cause breakdown between the copper rings and to ground. Especially at lead connections, poor contact or solder joint failure can lead to localized overheating, causing the lead wire and its insulation to burn out. When these failures occur, careful and meticulous repairs are essential.
● Repair methods for burn marks on plastic surfaces caused by current collector ring voltage
First, use a sharp chisel to remove the burned part; then clean it, cleaning the entire current collector ring's plastic part to remove conductive dust. Next, use an insulation resistance meter to check for any short circuits. If it passes the test, mix equal parts of epoxy resin adhesive (6101) and hardener (650), apply the mixture to the removed part, smooth it out, and allow it to dry for 8 hours. Then, immerse it in 155-1 varnish before use.
● Burns to leads or connectors and their treatment
The insulation of lead wires or connectors often shows signs of burning. When repairing, the original burned insulation layer must be cleaned and then wrapped. First, wrap three layers of 0.05mm thick polyimide film (half-overlapping), then wrap one layer of 0.1mm thick alkali-free tape (half-overlapping), and apply quick-drying varnish. It can be used after the varnish dries.
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