Chuck Yung
EASA Technical Support Specialist
When repairing slip-ring machines, it is sometimes necessary to re-insulate the collector rings from the hub. In these cases, some procedural guidelines may be helpful. Specifics such as interference fit, type of insulating material, and type of ring material require careful attention.
Resistance varies
If the rings are damaged, or worn undersized, they should be replaced with the same material as the original. Resistance varies greatly among different grades of bronze. In the case of brass or bronze rings, avoid using bearing bronze to manufacture new rings. Bearing bronze is oil-saturated, and will prevent the rings from developing a film.
When a wound rotor motor is running, external resistance is applied across the rotor leads to vary the rotor resistance and therefore change the speed and torque characteristics. It follows, then, that a resistance change in the slip-rings would also affect motor performance. Changing the material will change the ring resistance and the operating characteristics of the machine.
Insulation material
The next consideration is the insulation material. Fiberglass banding material has long been used for this, with good results. Other options include 2-part epoxies applied with untreated fiberglass tape; epoxy potting compounds, or Scotchply (made by 3M). Scotchply is a semi-cured sheet material, and performs best when compressed to about 50% of its semi-cured thickness. Shrinkable Mylar can be used to accomplish this, in the same manner as with banding tape.
Interference fit
The interference fit between ring and insulated hub should be .001” per inch of diameter. If the interference fit is too loose, the ring is likely to slip during service. The operating temperature of slip-rings can be very high – they carry full rotor current, after all.
To keep the rings straight and parallel, try this trick: Machine the insulation with slight (1/16” to 1/8”) steps to locate the rings. The steps serve two functions: They position each ring and keep the rings parallel. Bore each ring to fit the corresponding step. Now the rings can be expanded and installed. Once installed, the rings will be concentric and parallel every time.
After the rings (and lead studs) are installed, the next step is to insulate between the rings. The added friction between the insulation and ring helps insure that the rings will not spin on the hub. It also seals the ring/lead insulation area to exclude carbon dust and other contaminants.
A final inspection should include a hipot test, both ring to ground and ring to ring. Use the same test voltage as for the rotor winding: (2x voltage) + 1,000 for AC; or 1.7 times that value if using a DC hipot. The end product should be a professional-looking slip-ring assembly.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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