Chuck Yung
EASA Senior Technical Support Specialist
While there are many similarities between 3-phase AC stators and DC armatures, there are some unique aspects to DC armature design; these can be extremely helpful to those who understand some little-known tips. My goal in writing this article is to share those tips.
First, the poles of the armature have to match the number of field poles in the frame. In other words, an armature with 4 main field poles in the frame is a 4-pole armature, regardless of the nameplate rpm. When it comes to DC machines, there is no correlation between rpm and poles. I have personally rewound 4-pole rolling mill armatures that ran at 99 rpm and other 4-pole armatures that ran 3900 rpm.
Lap or wave?
Before even taking data, count the number of armature slots and commutator bars. Most lap wound armatures have an even number of armature slots, while most wave windings have an unequal (odd) number of slots. Look at the coil top side as it exits the slot on both ends of the armature. If both ends of the top turn in the same direction, the armature is lap wound. If they turn away from each other, it is a wave winding. See Figure 1.
Divide the number of commutator bars by armature slots; the result should be an integer (i.e., a whole number). If not, the armature either has a dead coil or conjoined bars. The combination of unequal (odd) turns with a dead coil might explain an abrupt change in the bar-to-bar test results. It’s better to know that when evaluating the armature rather than to discover it when recording data on what was a good (i.e., not defective until you stripped it) armature.
Coil pitch
Secondly, the coil pitch of an armature should always be as close as possible to full pitch. See Figure 2. The full pitch design places the coil sides at the same position of adjacent poles at exactly the same time. Full pitch can be defined as follows:
For a full pitch coil, the number of teeth spanned = slots divided by poles
For example, with 40 slots and 4 poles: 40/4 = 10; a full pitch coil would span 10 teeth, for a pitch of 1-11.
With only 1 more slot, a 41-slot, 4-pole armature requires a coil pitch of 1-11:
41 / 4 = 10.25; full pitch of 1-11.25 is impossible; coil pitch must be rounded to 1-11.
With 43 slots: 43 / 4 = 10.75; full pitch of 1-11.75 is impossible, so a 1-12 pitch should be used.
But what about a 4-pole armature with 42 slots? 42 / 4 = 10.5; full pitch would be 1-11.5; so a 1-11 or 1-12 would be equally close to full pitch.
That moves us into an area where designer experience often seems lacking. Ideally, the armature slots divided by poles should not result in any “integer point 5” (e.g., 11.5). Consider our example with 42 armature slots and 4 poles. Although the 1-11 or 1-12 pitch would be equally close to full pitch, neither is ideal. When the slots/poles results in a fraction of exactly ½, the neutral position becomes less defined, because the coil sides are not exactly a pole-pitch apart (see Figure 2). Difficulty in setting brush neutral is one consequence of this; arcing at the brushes is another. To address the arcing, the designer returns to the drawing board and adds equalizers (more on those in a moment). A more experienced design engineer would have realized the need for a split-pitch coil (more on these later, too).
Circuits
Winders understand that (except in a few special cases) the number of poles must be a multiple of the number of circuits. We know that a 4-pole AC 3-phase winding can have 1, 2 or 4 circuits. Likewise, a 6-pole can only have 1, 2, 3 or 6 circuits. The reverse ratio holds for lap wound DC armatures. The number of circuits is determined by multiplying the poles times the plex. As Table 1 indicates, a 4-pole lap wound armature can have 4 (simplex), 8 (duplex) or 12 (triplex) circuits. For a given hp / size, lower armature voltage requires more circuits. That means a 24v marine-duty motor operating on a boat requires more circuits (plex) than the same armature designed to operate at 500v.
For wave windings, the lead throw must equal the number of commutator bars divided by the pole-pairs. It should be intuitive, but that makes it impossible for a 2-pole DC machine to have a wave-wound armature. The number of commutator bars offers a clue to the plex of the armature. That is because the lead throw (commutator pitch) of a wave winding must follow this formula:
Lead throw = Bars +/- plex / pole pairs
Example: a 4-pole machine has 123 commutator bars and 41 slots.
41 slots / 4 poles = 10.25; coil pitch = 1-11
123+1 = 124; 124 / 2 = 62; lead throw = 1-63
123-1 = 122; 122 / 2 = 61; lead throw = 1-62
Note that either a 1-62 or 1-63 lead throw will work for this armature; one is progressive, the other retrogressive. For more depth on this, read the Armature section of EASA’s “Fundamentals of DC Operation and Repair Tips” manual.
Since most of the armatures we rewind are for industrial use, 240v or higher, most of them are simplex. We are accustomed to simplex connections, so it is easy to overlook a duplex, triplex, etc. I helped a member several years ago who had an armature with a 100-plex winding. I was certain they had made an error in tracing the leads, but the application turned out to be unique. That DC motor drove a telescope, and was designed to turn one revolution every 24 hours. Some of the things we are privileged to work on are fascinating!
Equalizers
Because of all those circuits and slight inequities in the physical airgap between different poles and the armature, there is often a current unbalance between the parallel paths. Current tries to flow to balance the paths, resulting in circulating currents that cause heating and arcing. To minimize the effect, a lap wound armature has equalizers directly connecting points of equal voltage potential. (Equalizers are unnecessary in wave wound armatures because the two coils are connected in series and therefore carry the same current.)
Visually, each equalizer must connect two coils exactly 2 pole-pairs apart. That gives us this formula:
Equalizer pitch = Bars / pole pairs;
Example #1:
A 4-pole armature with 84 commutator bars:
84 bars / (4 poles/2) = 42; equalizer pitch must be 1-43.
Example #2:
A 6-pole armature with 324 bars:
324 / (6 poles/3) = 108; equalizer pitch must be 1-109. Note that for a 6-pole armature, each equalizer should be comprised of a triangle: 1-109-217.
The same holds true for armatures with 8 or more poles. See Figure 3. Using the wrong equalizer pitch connects two points that are not at equal voltage potential, and will cause arcing and overheating for the brief time until the armature fails.
An armature with 40 slots and 120 bars has 3 coil sections per slot. With 20 equalizers, a coil section in every slot is equalized. That is normally sufficient to yield good performance. But in a case where the slots divided by poles = X.5, the designer usually must increase the number of equalizers. When taking data, finding 1 equalizer end per commutator bar should alert us that the manufacturer had to increase the normal complement of equalizers to manage circulating currents within the armature.
When evaluating an armature design, there are clues that, if recognized in time, indicate that the design can be improved. First, divide the armature slots by the poles to determine if the result is an integer or fraction. If the result is “X.5,” flag it for further review. Next, if the armature is lap wound, consider the number of equalizers. Most armature designs are satisfactory if the number of equalizers equals half the number of armature slots.
Split pitch guidelines
When we know to look for those conditions, and understand what they indicate, it’s possible to improve on the original design. For the example described earlier, when “slots / poles = X.5,” a better solution than adding equalizers is to use a split pitch coil (see Figure 4). In our 46-slot example, 46 / 4 = 11.5, so the split pitch design uses 2 coil sections at 1-12 pitch and the third at 1-13.
The top coil sides might all share a slot, and therefore the appearance of the armature looks normal, but the bottom sides are divided into two sections at 1-12 and the other at a 1-13 pitch. Because all the coils are made in this manner, it’s easy to miss when taking data. However, if the armature has more equalizers than usual, that is a strong indication that the original design can be improved. In a perfect world, the designers would never choose a lamination punching that results in slots / poles = X.5; but many motor designers are not cognizant of this issue. Since the cost to manufacture a die to punch laminations can be very high, the design engineer may not have had a choice.
The coil manufacturer can split the top or the bottom coil side; it is a matter of preference. Be proactive when preparing to record armature winding data. Count the slots and bars first; then divide slots by poles to determine whether or not a split pitch coil is likely. For a 2- or 3-section coil, the coil should be split such that one coil section uses the longer coil pitch. A 4-section coil should be split 3 and 1, while a 5-section coil may use either 3 (or 4) sections at the shorter pitch and 2 (or 1) sections at the longer pitch. In most split-pitch designs, there is only one “orphan” coil section. See Figure 4.
When the number of commutator bars divided by the number of poles results in an integer with a fraction of ½, the performance will improve if the armature is changed to a split-pitch coil. Additional clues are complaints of chronic arcing, difficulty in setting brush neutral, reduced brush life, and poor commutation. Any of these symptoms could indicate a variety of issues, which underscore the importance of dividing the armature slots by poles to determine whether or not a split-pitch coil is desirable.
When changing an armature to a split-pitch design, the number of equalizers can and should be reduced to half the number of armature slots. The equalizer conductor size should be one-third the size of the armature conductor, but of the same thickness so the riser can be slotted normally.
When we know what to look for, it is often possible to improve on an armature design to improve both reliability and performance. The key is to recognize the clues before ordering new coils and to confer with EASA’s technical support department before making substantive changes. And don’t forget to tell your customer what you have done to improve his motor. Take credit where credit is due!
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