Mica

When should you buy a complete new commutator?

Most of the time, don't.
A "new" commutator refers to a commutator designed and built from scratch, including all copper, mica and steel components.

The key is steel:
The only time you need a new commutator is when you do not have an existing steel part in good condition to fit the style and size of commutator you need.
• Conversion
• Replacement of damaged parts
• Spare
• Design improvement
Conversion: When an existing unit is molded (i.e. nonrefillable), the commutator needs to be replaced. Either a new molded unit, if available, can be purchased, or a new v-ring or glassband refillable commutator can be manufactured. None of the steel components of molded commutator should be re-used (see Motor Fax Issue 11) due to design limitations and space requirements for maximum stability. Conversions between refillable types also require new steel parts.

Replacement of Damaged Parts: Whether damaged from arcing in operation, galded or warped from removal from the shaft, or cracked due to porous castings, damaged steel parts must be replaced to ensure stability of the commutator in operation.

Spare: With the aim of reducing down time for the future, manufacturing a spare new unit is another circumstance in which new steel is a requirement. Either purchased from the OEM or designed by an aftermarket manufacturer, key dimensions can be obtained from prints or from the existing unit, ideally upon refill. Alternatively, if shaft profile information is available, along with any flange mount details if required, a new unit can usually be designed. As long as accurate and detailed information about the external dimensions of the commutator and its application environment is available, a new commutator can typically be designed to fit.

Design Improvement: In some cases, an existing steel design may cause problems in a specific application. For example, a v-ring commutator with a floating front cap and no spool may prove unstable or particularly susceptible to contamination in the wrong environment.
New steel can be designed to incorporate a front bore fit and spool to address the operational issues. In short, buying a new commutator when a refill is an option is rarely the most cost effective or timely choice. With no sacrifice in quality by re-using good quality steel parts, refilling a commutator typically represents the best option available in motor repair.

Should you or your customer have any questions regarding this issue, contact an ICC representative and we will be happy to assist you in making a decision on your specific
situation.

When should you dip a commutator in varnish?

Never.
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.

When is reinsulating a comm an option?

Usually if it has inserted risers, is a v-ring style commutator, and uses over 1000 pounds of copper. 

There are, of course exceptions to this. If the commutator doesn't use quite that much copper, but it has a very thick or very wide copper bar, reinsulating may still be a great cost saving alternative.

What is replaced in a reinsulated commutator?
Typically, reinsulation refers to the replacement of segment mica between the bars, and replacement of the mica v-rings and mica tube. It should also include new risers, and any 
peripherals like lashing or wrapped caps.

Why would you want to reinsulate instead of refill?
Cost. Reinsulating a commutator typically doesn't save any labor hours, since each bar has to be cleaned thoroughly before re-use. However, if the comm uses a significant amount of copper, the material savings can quickly amount to several thousand dollars. On a smaller comm, however the material savings would be quickly outweighed by the disadvantages of reinsulating.

When shouldn't you reinsulate a commutator?
When there is limited brush life left, and when the copper is damaged in some way.
If the copper has been overheated and annealed, it will no longer be re-usable. If the dovetails have been bent or cracked, a refill will be necessary. Finally, if there is insufficient flat on the interior of the comm to allow for removal of copper for reassembly, reinsulation would not be
recommended.

How can you tell if reinsulating is possible?
For thorough inspection and final determination, the commutator will have to be entirely disassembled. While an external inspection will show the condition of the bars and the amount of brush life left, inspecting the dovetails and copper hardness requires individual bar inspection.

Can you reinsulate a solid riser comm?
Not typically, and not cost effectively. Solid riser comms are slotted, and the slots would need to be plugged to withstand the new compression for reassembly and subsequent machining. In addition, in reinsulating, in order to maintain brush diameter, mica thickness is increased. On a solid riser comm, this would also increase the riser diameter, and would change the slot depth and position.

What about glassbound commutators?
Glassbound comms are a totally different design. Since the copper shrinks to a mica wrapped hub, any additional removal of material would adversely affect the fit. In addition, once grooved for banding, the copper material remaining under the band is very thin, and unlikely to withstand compression.

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