steel

Why is oven baking time such a hot issue?

Time is money...is the short answer.

“In your kitchen, turning up oven temperature will not bake a cake faster, it will just burn the exterior leaving the inside raw. So too, is the effect in commutator construction.”
— Excerpt from Connections Commutator Repair Guide
DROOPY CAKE.jpg

Do you expect to season your commutator while it is operating? Probably not.

That’s our job. Thorough baking cycles are imperative to stability in operation. Mica v-rings aren’t completely cured until a comm is fully at temperature (350° F) for a minimum of 4 hours.  But, there’s more. Copper and steel expand and contract at different rates and by different amounts.

Full thermal cycling ensures that none of this has to happen when your production is on the line.

We are ICC, and improving commutators is our only job. 
Our industry depends on it.

CONNECTIONS-BANNER.jpg

If you would like a copy of our technical commutator repair guide, that contains 31 chapters on everything from troubleshooting and diagnosing field issues to frequently asked questions. Please send a request to Info@iccinternational.com.

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.

How do you avoid loosening a comm when you take it apart? And why is it important?

It's important because a loose commutator will cause endless amounts of problems in operation. High bars, chipped brushes, and poor commutation make your job of checking for commutator tightness an important part of routine maintenance. 

How is tightness built into a commutator?
In manufacturing, the copper and mica segments are assembled in a circular form. The resulting "segment pack" is checked for skew and angle. A steel ring which has been machined to the rough OD of the commutator, is then compressed over the segment pack in a press. The tonnage used will vary depending on the size of the comm, but can easily range from 10 tons to 90 tons. The resulting compression provides the commutator tightness which all following procedures are designed to maintain. In some instances, the commutator may be banded to obtain additional stability.

What keeps a v-ring commutator tight?
The dovetail angles that are cut into the segment pack are the foundation for keeping the commutator tight. With the compression ring still in place, the segment pack is assembled to the steel caps and hub, and insulated with mica v-rings. The caps seat against the internal angle (typically 30°) and retain the compression after a series of thermal closings under torque and tonnage. When the compression ring is removed, this assembly keeps the commutator tight. In effect, we have created a spring, which is held tight by the pressure exerted internally.

How do you take a comm apart without releasing the tightness?
Since the steel caps are keeping the commutator tight, removing the outboard cap will immediately release this pressure, loosening the comm. It is almost impossible to get the compression back in the segment pack once it has been released. Before pulling a commutator cap, always band the brush track, preferably with a resiglass tape, applied hot and cured (see banding material specifications for temperatures). You should be applying the tape at 300-400 psi, covering approximately 3/4 of the brush surface, and building it up to about 1/4" per side. To finish, wrap 8-10 wraps over your tucking loop to ensure that the banding stays in place. Alternatively (though not preferred), a steel ring machined to between 0.010" and 0.020" smaller than the brush diameter (depending on the size of the unit) can be heated and applied over a nomex sleeve covering the brush track. The ring should be first measured cold before installation, and then measured again once cooled after installation to ensure that sufficient fit has been obtained. With banding or a ring in place, the cap can now be safely removed, and the v-rings replaced or other maintenance performed.

Checking for tightness.
During routine maintenance, bolt and nut v-ring commutators can be checked for tightness by applying a specific torque to the bolts, or to the nut with a spanner wrench. Typically, torque values are approximately 50% of the maximum rating for the bolt grade and size. If you have questions regarding a specific unit, call us for a recommendation