When should you dip a commutator in varnish?

There are several good reasons to make sure that you never subject a v-ring commutator to varnish or VPI, all of which are critical to the unit's operation.

Commutators are designed with gaps throughout (see Fig. 1). This allows for differential expansion and contraction of the various materials in operation, and results in successful operation over many years. If varnish is introduced into these gaps, the commutator can no longer perform as designed and the varnish can cause three distinct problems:

Overheating: When dipped with varnish, these gaps are filled, which inhibits cooling and can often result in overheating in specific areas of the commutator.

Imbalance: Uneven distribution of the varnish may result in imbalance of the armature. For example, if the unit is dried horizontally, the varnish will pool to one side, and within the confines of the commutator, it may never entirely cure. This material can then result in the overheating noted above, but also in imbalance in operation.

Shorting: In addition to the problems noted above,exposing the commutator to any impurities in the varnish can also result in failure due to shorting bars. Though most repair facilities keep their varnish as clean as possible, minimal impurities which would not affect coils, will potentially bridge the small spaces between commutator bars.

What should you do if you receive a commutator that has been dipped?
Depending on the severity, the comm may indeed need to be refilled. However, if after having banded the unit and taking it apart, you discover that the varnish deposits are minimal and contained mainly to the dovetail area, you may be able to simply clean the dovetails and replace the v-rings. Sanding or taking a very light skim cut should do this effectively.

For tips on v-ring replacement, see Motor Fax "Replacing V-Rings", or call us for information.

How much damage can flux contamination cause?

Enough to kill a commutator.

When soldering coils into the risers of a commutator, the worst potential consequence is flux contamination, especially in solid riser comms. The same flux that cleans the copper to permit the solder to adhere uniformly to the risers creates a potential for contamination that can destroy a commutator. Flux contamination can occur during the soldering process when flux and/or excess solder seeps away from the riser slots and finds its way to the underside of the commutator. Because the flux and solder are conductive, they render the mica insulation useless. When the insulation can no longer prevent electricity from jumping from bar to bar, the commutator shorts out. Once the mica insulation is contaminated, almost nothing can be done to restore its insulating properties. When the commutator shorts out due to flux contamination, there are usually only two options: Reinsulate (replacing all mica insulation throughout the commutator) or rebuild.

When inspecting a commutator for flux contamination, look for discoloration of the copper bars. The discoloration may take several forms. Streaks of solder and flux are silver in color; carbon residue where arcing has occurred is black; contaminated copper bars can take on a darker, mottled and greenish coppery hue, or the contaminated area may just appear lighter than others. Compare the rear dovetail to the front, since the front will be free of contamination. As always, when taking a comm apart, be sure to band it tightly and bake it before attempting to remove the v-rings.

There are several ways to reduce the risk of flux contamination during the soldering process:

  • While soldering, angle the comm so that flux and solder run away from, and not toward, the bottom of the comm.
  • Use flux and solder sparingly.
  • Flux and solder small sections of the comm at a time.
  • Use a rosin-core solder to reduce the need for flux, or a flux- based solder so the solder contains the flux.

If the commutator is contaminated, some solvents will clear minor contamination, but call your ICC representative at any time for advice on how to proceed.

If you have any technical questions don't hesitate to call, we can talk you though the troubleshooting process. 865-983-7444