copper

Can you convert your commutator from inserted to solid risers?

Possibly.

A solid riser commutator is one in which the riser, into which the coils are inserted, is made of the same piece of copper as the rest of the bar. In a solid riser v-ring commutator, a rectangular shaped bar of copper has the three sections machined away - at each end for the dovetail,
and along the brush track, leaving the riser.

If a riser is very long, a tremendous amount of copper would be wasted to leave the riser. Instead, an inserted riser design was developed.

Winders, however, often prefer to work with solid riser designs, so how can you tell if your commutator is a candidate for conversion?

Alloy 116 (silver bearing commutator copper) is typically readily available in sizes up to 4, or even 5, inches in width. If your commutator from the inside diameter of the copper to the outside diameter of the risers fits within this, conversion may be a viable option.

What if the bar needs to be wider than available copper sizes?
An alternative to a solid riser is a lap joint design. A separate riser is brazed to the copper bar, but shaped to match the trapezoid of the bar itself. Though typically more expensive than an inserted riser unit, a lap joint riser will provide all the benefits of a solid riser commutator, i.e. carbon contamination prevention behind the risers.

What should you consider in a conversion?
If the risers in question are quite long, you will be adding weight to the unit, potentially
affecting performance. In addition, if cooling and airflow are considerations for your
application, note that the solid risers or lap joint will cut off this air flow versus the inserted riser design.
Call your ICC sales representative to discuss the possibility of conversion of any
commutator you have in house.

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.

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


How can asking for a change in commutator riser style save you time on your next rewind?

By giving your winder the style that works best for personal preference... and by knowing where you can safely make changes, and where you can't. Inserted risers come in many different styles... from open, to closed, to straight blade, and with a large number of foldover configurations.

Different winders have different preferences, and generally speaking, risers can be designed to meet those preferences; as long as the overall material remains the same for current carrying capacity and stability in operation.

When working with foldover tabs, the biggest challenge can be in placing the coils. Though it may seem that the top coil should fall entirely between the two sides of the riser, in fact, the short side of the riser should come only halfway up the top coil. This original OEM design may cause problems in aftermarket rewinding, since you'll end up working with a very small amount of the long side of the risers which can be difficult to bend.

The solution? Have your commutator manufacturer make an adjustment to the design. Some of the conversions available (shown below), may fit better within your winders' preferred work scope and may save time in the rewind as a result. Also, if you choose to stay with a full foldover, you can always instruct your comm manufacturer to increase the length of the long side of the riser to make it easier to bend.

Should you change riser material thickness?
Not if it means going thinner. By going from 0.060" to 0.040", you reduce the current carrying capacity of the copper, and also reduce the strength of the material. However, the reverse (0.040" to 0.060" is typically possible, and it is also possible to convert from a double inserted riser of 0.040" material to a single inserted riser of 0.093".

Can I do anything to stop inserted risers from cracking in operation?
Riser cracking is typically due to one of two causes. Vibration can be addressed in some cases by adding a row of lashing to help minimize the effect. Hydrogen embrittlement is seen in copper which contains oxygen. Over time, it will react with the hydrogen in the air and cause the copper to become brittle and crack. Using Oxygen-free copper for inserted risers will solve this problem, and should be specified for virtually all inserted riser commutators.

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.