commutator

Checking Skew

An integral part of the comm building process is checking the skew. After all the bars and mica are stood up into a segment pack we check the skew to ensure the bars are straight and not leaning to either side. It's a tedious process and takes a trained eye and patients to look at each and every bar individually.  It's a step that can not be skipped because if a bar is out of skew it will cause major problems throughout the process and the comm would then need to be rebuild from the beginning. 

Extension Banding and why it's CRITICAL

Check out this image of bad banding versus good banding.  See all the gaps and holes? 

Those tiny little slits between the banding and the steel put this commutator (on the left) in a vulnerable position. The fine particles of carbon that come off the brushes find their way into any little gap or hole.  Enough contamination will lead to a short or even worse, a failure.  To get the most life out of your commutators check the banding frequently and if needed replace the banding.  

ICC is more than happy to help. Call or email anytime if you have questions. We can walk you through what to look for and even guide you through the process, we'll even tell you exactly what products we recommend using.

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.

Why is oven baking time such a hot issue?

Time is money
     ...is the short answer.

And it's why the desire to shorten deliveries to customers, even on straight time work, is so strong. Since bake cycle duration can easily be one of the biggest chunks of time in scheduling a project, it seems to be the perfect candidate for cutting.

But there are technical reasons to keep temperatures relatively low and cycle times in place.

Baking is used both to cure materials, and to create an environment which replicates that
that found in operation. Controlling both the temperature and duration of the bake cycle is important to avoid overheating. During bake-off, for example, overheating can result in a reduction in motor efficiency.

Although today's insulation products can withstand higher temperatures, the resin compound requirements for curing must still be met. Further, the modulus of any given material will only allow it to accept a specific amount of thermal soaking. Raising the temperature unfor-tunately cannot speed this process. Fortunately, with the manufacturing software, process improvements and expedited shipping options available today, deliveries can still be improved, putting more of your time (and money) to the bottom line.

“Bake cycle duration is calculated to obtain the greatest differential expansion of the copper segment pack to the steel assembly. This results in the highest molding pressure on the mica, forcing it to its most stable operating situation. Shortening thermal cycles adversely affects commutator stability under rotational stress.”
— From the professional notes of R.F. Sharrow, PE GE Commutator Design Engineer, 36 years


Having trouble with banded commutators?

Glassband commutators were first designed by General Electric in 1960 for their redesigned line of 580 and 8000 frame machines, and later for their MD800 Armored Motors. Although performance of these commutators is good, end users and motor repair shops sometimes give them less than favorable reviews because of the difficulty in field repair of the units. With v-ring commutators, bolts can be tightened and vrings replaced, but glassbound commutators are designed to be virtually maintenance-free. In addition, the glassband comms are significantly lighter in weight than their v-ring equivalents, and under normal duty requirements, routinely out-last v-ring units by 50%.

However, how many motors do you see operating under anything but "normal duty requirements?" The problem arises when end-users in high contamination environments end up having to replace bands on a far too regular basis.

The only restraining force on glassband comms are the res-iglass bands. This material is made from high tensile glass yarns laid parallel and bonded with fully catalyzed thermosetting resins. The glassband commutator is set with an interference fit of approximately 0.030" to a mica wrapped and cured steel hub. The retained interference fit, measured by the growth of the segment pack, should be, at minimum, 0.015". The inside of the segment pack is bored smooth, specific to the cured mica-wrapped hub. Glassband grooves are cut after the segment pack is assembled to the core. Finally, the commutator is banded to an OEM specified number of wraps at 500 lbs PSI. It is then baked for several hours at approximately 300 degrees. The bands are applied multi-stage to ensure maximum strength.

Most repair shops will apply a coating of Viton® to the bands prior to the commutator going into operation; this step should not be omitted. If the bands have not been coated, carbon can creep behind the band, causing burning from the inside out. Though this might not be seen in a visual inspection, it can easily result in the failure of the unit.

What you should consider before converting from a glassband to V-ring comm:

Converting a glassband comm to a v-ring is an alternative to dealing with ongoing problems. But there are several things to consider when presenting this as an option to your customer:

Weight
A v-ring style commutator will be significantly heavier than the glassband it replaces. The v-ring design requires not only the addition of a hefty steel part including a spool and two caps, but also a substantially wider (and therefore heavier) copper bar.

Performance
As a result, performance may be affected and the weight gain should be measured against the motor as a whole and the application for which it has been designed.

Maintenance
V-ring comms are more familiar, but they do require ongoing maintenance such as checking for  tightness. However, if your glassband is being rebanded once a year, v-ring tightening is going to be a welcome change.

Cost
Converting to a v-ring will mean a substantial cost outlay (typically 3x) compared to the cost of a refill. However, when weighing the cost of contant band replacement, the capital expenditure associated with the conversion will often make sense.

Experience
Conversion to a v-ring is a major redesign and requires both engineering and production experience from your commutator manufacturer. Ensure that you are being asked to approve conversion drawings, and ask for references.

Your commutator supplier should be able to help you explain this alternative by providing you with support materials and technical references. If this is a topic which applies to one of your customers, let us know, we'll be glad to help.

Seamless Extension Banding Can Save a Commutator

When you band commutator extensions with skill and a few tricks of the trade, all seams are sealed and carbon won't infiltrate.  Preventing carbon from entering behind the copper bars, or between the mica v-ring and the steel can prevent problems with arcing bar-to-bar, as well as bar-to-ground.  The pressure used, technique applied, and finishing touches make a big difference.  A feeler gauge should not be able to enter a seam. If it can, your comm is at risk.  Contact us if we can help walk you through it the next time you have to replace a band.