Chuck Yung
EASA Senior Technical Support Specialist
In our industry, we have a lot of terms that are almost the same. And we even use the same exact terms for multiple unrelated things.
Let’s start with a commonly misused pair of terms: span versus pitch. Most of us write both as “1-x,” which helps prevent errors, but span refers to “number of teeth enclosed.” Span is reported as a single integer. Pitch is when we describe it as “slots 1 to x.” For a span of 10, the pitch is 1-11. One well-known manufacturer even states this on their data sheets: “Span is teeth enclosed,” so a span of 7-9-11 refers to a concentric pitch of 1-8,10,12. If a winder does not understand this and installs a winding using the wrong pitch, they will not have a satisfactory outcome.
This makes a good segway to another term: “equalizers.” We find equalizers in both three-phase AC windings and DC machines. In a DC armature, the equalizers connect coils of equal voltage potential. That places them exactly at the midpoint of two field poles of the same polarity. The formula for equalizer pitch is simple:
Equalizer span (there’s that term again) = commutator bars divided by pole pairs. Equalizer pitch is one more tha n the span. For example:
162 bars divided by 2 pole pairs = a span of 81, so the equalizer pitch is 1-82.
For a 6-pole example:
162/3 = 54, so equalizer pitch = 1-55 – 109
In a three-phase AC winding, equalizers also connect points of equal voltage potential. (See Figure 1.) That means the mid-point of parallel paths within the same phase. When possible, the equalizers are connected to the exact middle jumper of parallel circuits. That is done separately for each phase.
Going back to DC machines, we find equalizing jumpers connecting the brush posts of the same polarity. (See Figure 2.) For a typical 4-pole DC machine with 4 brush posts, the equalizing jumper should connect both positive posts. Another equalizing jumper connects both negative posts.
If we number the posts sequentially for a six-pole DC machine with six brush posts, there should be equalizing jumpers connecting posts 1-3-5, and separately posts 2-4-6. It is not uncommon to find that a manufacturer has equalized the posts of only one polarity, but we as repairers should add equalizing jumpers to all posts of the same polarity. Failure to use equalizing jumpers at the brush posts of the same polarity will often result in different rates of brush wear. That is due to the brushes on one post, carrying less current than the brushes on the other post of the same polarity, wearing faster than the brushes carrying more current.
Speaking of polarity and DC machines, we use the term “interpole polarity” to mean that the interpoles should alternate polarity. If we have a 4-pole DC, the interpoles should be N-S-N-S. That check is made using a compass or Detect-a-Pole while applying DC to the interpole leads. We must be
careful with parallel circuits, and with designs where the armature is connected between the interpoles, as in Figure 3.
Interpole Polarity or Relative Interpole Polarity?
Here is where the confusion arises. We use the exact same term “interpole polarity” when referring to a test used to make sure the relative polarity of the armature and interpoles is correct. If the brushholder leads are accidentally swapped, that reverses the armature polarity relative to the polarity of the interpoles. When that happens, the motor will arc even without a load, and it will spark dramatically when load is applied. For the sake of clarity, use the term “relative polarity of in terpoles and armature.”
Here are several other synonyms our industry uses:
- Concentric winding = basket winding
- Random winding = mush winding
- Odd grouping = unequal grouping
- Partial discharge = PD or corona
- Interpole = commutating pole = intercoil
- Compensating winding = pole face bars
- VFD = variable frequency drive or ASD or adjustable speed drive
While the terms generator and alternator are used interchangeably by some, an AC device is an alternator, not a generator, while a generator provides DC output.
The bottom line is that we need to be careful about terminology and be aware that others do not always use the correct names for technical terms. A common miscommunication is when a caller has problems with a DC machine arcing under load. At some point in the conversation, we will ask if they checked interpole polarity. When a caller hears “interpole polarity,” they harken back to the use of DC and a compass to confirm that the interpoles are N-S-N-S. Reporting that “yes,” they did indeed confirm interpole polarity may cause us to move past the “relative polarity” and on to other possible causes of arcing. What we should ask is whether they checked the relative polarity of the interpoles and armature.
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