Chuck Yung
EASA Senior Technical Support Specialist
One of the least understood parts of a DC motor is the commutator. With a little understanding and some helpful tips, commutator life can be maximized.
Commutators are made of copper bars* separated by insulation from each other and from the steel hub. Viewed from the end, each bar is wedge-shaped, tapered radially with the thickest portion towards the outside. The insulation material most often used is segment mica because it remains stable at the temperature and pressure required during assembly and operation. By alternating copper bars with mica segments, each bar is isolated electrically from the other bars. The resulting cylinder of bars and mica is mounted on an insulated steel hub.
Each Design Unique
How the commutator is mounted to the steel hub defines the different types of commutators, which include v-ring, glass-banded, molded and steel shrink ring. While externally similar, each design is unique and must be handled differently. To understand those differences, and why they are important, it helps to compare them. (See types of commutators below.)
To hold the commutator together, designers use either a clamping hub arrangement on the interior or banding on the circumference. Each design has merit, and each has advantages for different applications. V-ring commutators are available in the greatest range of sizes, from 2 inches to 20 feet (50 mm-6 meters) in diameter, and can be used in almost all applications. Glass-banded commutators are lighter in weight and require less maintenance. Molded commutators are typically smaller and less expensive, but cannot be repaired. Steel shrink ring commutators are designed for extreme duty applications and can withstand instantaneous acceleration and high speeds.
In manufacturing, all commutator segment packs (copper and mica) are compressed under considerable pressure (a 12-inch or 300 mm diameter commutator may be closed at 40 tons of pressure), to hold the bars together at high speeds. Tension in the commutator is the key to satisfactory performance and life. One expert described the commutator as a spring; it’s a good analogy.
A v-ring commutator uses the traditional clamping method, relying on a v-notch (also known as a dovetail) cut in the front and back of the commutator bars. A corresponding “V” shaped ring, insulated with mica, holds the commutator together. Tension in a v-ring commutator relies on the pressure exerted downward against the interior angle of the dovetail. This force holds the bars in place during operation, maintaining the compression established by the manufacturer. In commutator design, it is important that the designer allows enough bar height to accommodate the v-notch without weakening the bars. A shallow v-notch designed incorrectly will allow bars to lift in operation, causing commutator failure.
A glass-banded commutator differs from a v-ring commutator in that the segment pack compression is retained on the exterior of the commutator by cured glass banding applied in grooves on the brush track. Steel shrink ring commutators use forged steel rings on the exterior of the commutator brush surface, while molded commutators are “glued” together with resin compounds.
The bottom line is that each style of commutator has different bar configurations, designed specifically for the intended clamping method. Any attempt to change the commutator design based only on the outward appearance of the commutator is destined for failure.
Checking For Tightness
Regardless of design, the problems can start when a commutator is not properly maintained. The v-ring commutator clamping device should be checked for tightness when a motor is cycled through the service center for routine repairs. Commutators of this type are clamped by fasteners, either through-bolts or a large nut (like a bearing lock nut). The bolts are normally grade 8, and the bolt-heads are often tack-welded to prevent movement. That leads to the mistaken assumption that you should not tamper with the commutator bolts.
As a commutator seasons in operation, bars can loosen and lift, chipping brushes and causing arcing. The solution is to check the tightness of the fasteners each time a DC machine is serviced. Bolts should be torqued to approximately 50% of the published ‘bolted joint’ values for the bolt size and grade, while the armature is still hot from the oven. If tightening is required at the prescribed values, the tightness should again be checked when the commutator cools to ambient temperature.
These values are typical for 4-8 bolt units of one manufacturer. In general, longer bolts require higher torque, but decrease torque for higher bolt counts, fine threads, smaller bolt circle diameters, thin copper segments or for thinner or cast steel sections.
A banded commutator, lacking bolts or a nut, cannot be checked for tightness, in the same way as a v-ring commutator can, causing the banded commutator to be unfairly maligned. Instead, if the segment pack has been repositioned or if the segment pack does not “ring” when a bar is tapped, the commutator has loosened in operation. Though there are no bolts to tighten, the banding can be replaced in certain situations. To replace the banding, the brush surface must first be banded—under tension—to retain the compression. Only after the temporary banding is cured should the old banding be removed, the banding grooves cleaned, and new banding applied under appropriate tension [at least 600 pounds (272 kg) for a 1 inch (25.4 mm) wide band].
Replacing V-Rings
V-ring insulation may deteriorate over time, resulting in low resistance to ground. Commutator life can often be extended by replacing the v-ring when appropriate, but the tension in the commutator must not be released. The commutator should also be thermally seasoned as part of v-ring replacement. In manufacturing, commutators are assembled or “closed” using a combination of torque and tonnage. If feasible, the use of both these elements after v-ring replacement is preferred. In all cases, it is essential to ensure that the cap is installed square, and is not cocked to either side. Bolts should always be cross-tightened incrementally.
If the commutator fasteners are over-tightened, the commutator bars may become distorted. Since steel is harder than copper, applying too much pressure to the interior angle will bend the copper dovetail. Check for cracks at the bottom of the dovetail before replacing the v-ring. When replacing the mica v-ring, use a straightedge to inspect the underside of the bars. The bar bottoms should be straight; concaved bars indicate that the bars were bent—probably by over-tightening the fasteners. If the dovetail is bent, the clamping hub will not seat correctly, and bars will continue to lift in operation. If the commutator brush-surface diameter is undersized (below the minimum commutator diameter) the bar height may not provide enough strength to withstand the centrifugal forces. Because each bar is supported only at the ends, centrifugal force could bow the bars.
The gap at the end of the bars is vulnerable to contamination, so it requires protective measures. A string band, placed over the mica, keeps the mica from flaking and keeps carbon and other contamination out of the commutator. Traditional protection—a string band treated with varnish— eventually deteriorates. Better options include resin-rich glass banding, Viton®, various specialty epoxy products and Teflon® bands (which must be custom made to size). A silicone product is also available, but only electrical grade silicone should be used. Never apply normal silicone products to a DC machine.
The molded commutator differs in several ways. First, it does not incorporate a clamping device; there is no v-ring. For this reason, the molded commutator cannot be re-torqued. The bar profile is lower, since there is no v-ring. While that results in a commutator weight sometimes closer to that of the glass-banded commutator, it also means the commutator bar probably cannot be successfully modified to include a v-ring. Even though some molded commutators use a steel core with molding resin, there is normally not enough room between the core and the copper to machine a v-groove to accept a v-ring and have enough copper remaining to resist bending under centrifugal force. Molded commutators therefore must be redesigned when replaced. When grounded or shorted, the molded commutator cannot be repaired.
Never Dip A Commutator
A commutator should never be immersed in varnish. To do so will result in varnish buildup in the area of the v-ring, eliminating the gap needed for expansion during thermal cycles. That also interferes with the clamping method, preventing the commutator from being properly tightened.
If properly maintained, the commutator of a DC machine provides long life and reliable performance. Regular cleaning and maintenance are critical to getting the most from a commutator. An important aspect of commutator maintenance is maintaining the spring tension established by the commutator manufacturer or remanufacturer.
* Commutator bars are usually an alloy of copper and silver. Alloys can be heat-treated, so the commutator should not be heated above 500°F (260°C). To check an armature for heat-related damage, use a spring-loaded center punch and compare the indentation size on several bars.
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