Mike Howell
EASA Technical Support Specialist

Manufacturers almost always utilize machine-inserted concentric windings for random-wound, three-phase stators when their processes can facilitate it due to lower manufacturing costs. Many service centers can produce concentric windings too, but the most common practice is to utilize the two-layer lap winding. For form-wound stators, the two-layer lap winding is almost always used by manufacturers and service centers alike.

The purpose of this article is to provide some tips for working with odd-turn (unequal-turn) windings, or two-layer windings where the total number of turns per slot is an odd number (e.g., 3,5,7,9…n).

In such cases, the top and bottom coil sides must have a different number of turns. We will further limit the scope of this discussion to windings where the number of slots per wound pole and phase is an even integer (e.g., 2,4,6,8…n). For the lap windings we are discussing, the number of coils will equal the number of slots, and the number of coil groups will equal the number of wound poles multiplied by the number of phases. So, the number of coils per group, or slots per pole and phase (SPP), is:

SPP = Q / (M·P)

where

• SPP is the number of slots per pole and phase
• Q is the number of stator slots
• P is the number of (wound) poles
• M is the number of phases

SPP calculations

Table 1 includes some examples of SPP calculations that would apply to this article and others that do not. It is also worthwhile to define coil pitch since different terms and definitions are used. We will express coil pitch here in the format of 1 to X, where X represents the slot number that the top coil side would fall in if the bottom coil side was inserted into slot 1. We will further say that if X is an odd number, we will call that an odd pitch. If X is an even number, we will call that an even pitch. Examples of odd pitch would be 1 to 11 or 1 to 15 and examples of even pitch would be 1 to 10 or 1 to 12. With the scope we have defined, we can now discuss some of the issues winders encounter with odd-turn windings.

We have limited our discussion to windings where half of the coils have one turn per coil less than the other half. We want to place them in the stator to make the best use of slot space and to ensure that we have a winding with the same number of total turns in each phase and each parallel path.

We will refer to the coil with less turns as 0 and the coil with more turns as 1. When we have an even pitch, the coils can simply be alternated 010101… An example of this is a 72-slot, 4-pole winding with 12 groups of 6 coils and a 1-16 pitch. If the winding had odd turns of 5 and 6, the turn sequence to install each group of six coils would be 010101 or 5,6,5,6,5,6. If the same winding has a pitch of 1 to 15, things get more complicated. If a turn sequence of 5,6,5,6,5,6 is used, some slots will have 10 turns and some slots will have 12 turns. This is the type of scenario winders encounter sometimes, and the solution isn’t always trivial. Sometimes, we can increase the number of parallel paths per phase to eliminate the odd turn problem altogether. Other times we can utilize an even pitch instead without significant performance changes. However, with some concentric-to-lap conversions and some original equipment manufacturer (OEM) lap windings, we get stuck with needing to use odd-turn coils and odd pitch. 

Table 2 includes some common odd-turn, odd-pitch windings and a permissible turn sequence for each such that every slot will have the same total turn count. Also listed are a few combinations where we don’t know of a good solution (e.g., 36 slots, 2 poles and 1 to 13 pitch). Also, note that the table restricts normal connection options in some cases.

Let us look at a couple of examples using Table 2. First, consider a 6-pole machine with 72 slots, 1 to 11 pitch and odd turns of 6 and 7 turns per coil. Using the table, the turn sequence 0011 should be used so the coil groups should be wound with 6,6,7,7 turns per coil. In this case, the winding could be connected with as many as 6 circuits and each slot will have 6 + 7 = 13 turns.

A different approach

A more complicated case is to revisit the 72-slot, 4-pole winding with a 1 to 15 pitch. Again, this is a combination where it may be desirable to explore use of a different pitch, e.g., 1 to 16 or a circuit change that would eliminate the need for odd turns. But, if odd turns and a 1 to 15 pitch are required, the turn sequence would be 001100 110011. This winding has 6 coils per group so this turn sequence represents 2 groups of coils.

For example, let us say the winding has 3 and 4 turn coils. Of the 12 total groups, 6 would have turn sequence #1 = 3,3,4,4,3,3 and 6 would have turn sequence #2 = 4,4,3,3,4,4 and the turn sequences would alternate #1,#2,#1,#2… This would result in each slot having 4+3 = 7 turns. An additional limitation with this winding is shown in the table.

Since half the coil groups have 20 turns per group and the other half have 22 turns per group, the number of circuits permissible is limited to two. This keeps the same number of turns in each parallel path. Also, the jumpers used for this turn sequence must be 1 to 4 or adjacent pole. This will result in each circuit having a total of 20 + 22 = 42 turns. If 1 to 7 or skip pole jumpers were used, one circuit would have 20 + 20 = 40 turns and the other 22 + 22 = 44 turns resulting in circulating currents.

An additional challenge

Form-coil windings present an additional challenge. When formed coils are manufactured with a different number of turns but the same wire sizes, the coils will obviously be a different height. Depending on the space available for vertical spacers, this can cause significant problems with crossover in the coil end turns. Manufacturers have typically dealt with this in one of two ways. First, the coils are manufactured with the same wire sizes and the bottom and center spacers are modified to facilitate proper clearance between coils, but this requires significant room for spacers. The second and more common approach is to use different wire sizes in the coils such that the coils with less turns are close to the same height as the coils with more turns. For example, a 4000V stator coil design with 9-turn and 10-turn coils might be manufactured with a wire thickness of 0.091 inches (2.31 mm) for the 10-turn coil and 0.102 inches (2.59 mm) for the 9-turn coil. Assuming a heavy film strand insulation, the two coils would be within 0.4% of each other in nominal height (see Figure 1). One common question that arises in this scenario is in regards to current density. Yes, the coil with the smaller wire size will have a higher current density and therefore higher temperature rise than the other coil. Manufacturers account for this in the design stage.

Odd-turn windings can be a challenge, but any winder can be successful providing proper evaluation and planning is done up-front. Contact EASA technical support for assistance with these. As mentioned, there may be a reasonable redesign that could eliminate the odd-turn configuration or pitch modification to simplify the turn sequence required.

Categories: Random wound, Winding connections

Tags: Winding connections Motor rewind/repair Unequal turns

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