Mike Howell, PE
EASA Technical Support Specialist
This article should be helpful to anyone working with a winding that meets the following criteria:
- Three-phase, two-layer lap winding
- Unequal-turn (odd turn) winding
- Integer-group winding (same number of coils in every group)
The purpose of this article is to provide some tips for working with unequal-turn (odd-turn), two-layer lap windings, with emphasis on how to sequence the coils within the group for optimum slot fill. For these windings, the total number of turns in at least some of the slots is an odd integer greater than 1, i.e., 3,5,7,9…n. This means that the top and bottom coil sides in those slots have a different number of turns.
We’ve further limited the scope of this article to integer-slot windings, which means all groups have the same number of coils. That is, the number of slots per pole and phase must be an integer greater than 1, e.g., 2,3,4,5…n. So, the number of coils will equal the number of slots, the number of coil groups will equal the number of poles multiplied by the number of phases, and the number of coils per group or slots per pole and phase (SPP) will be an inte ger calculated by:
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 poles
M is the number of phases
It’s also worthwhile here to define the terms “teeth spanned” and “coil pitch,” as they will be used in this article since terms and definitions can vary. 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 were inserted into slot 1. The number of teeth spanned is then X – 1. This represents the number of stator teeth that can be counted between the two coil sides. These terms are demonstrated graphically in Figure 1, where there is a concentric winding on the left and a lap winding on the right. Note that both windings shown occupy the same slots.
We will limit this article to windings where half of the coils at most have one turn per coil less than the remaining coils, and we want to place them in the stator to make the best use of slot space. An example of this is a 72 slot, 4 pole winding with 12 groups of 6 coils and a 1 to 16 pitch (15 teeth spanned). If each group has 33 total turns, the group requires 3 coils with 5 turns and 3 coils with 6 turns. One turn sequence to install each group of 6 coils, resulting in 11 turns per slot is 5,6,5,6,5,6. If the same winding had a 1 to 15 pitch (14 teeth spanned), things would be 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. To get 11 turns per slot, an unfriendly sequence of 6,5,5,6,6,5 / 5,6,6,5,5,6 could be used.
This is the type of scenario winders encounter, and the solution isn’t always trivial. Sometimes, we can increase the number of parallel paths per phase to eliminate the unequal turn problem. Other times, we can utilize a different 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 unequal turns and a pitch requiring a special sequence for optimum slot fill. Examples of this are generator windings where the pitch should not be changed and relatively high power/low voltage windings that already have the maximum number of parallel circuits per phase and a low turn count.
A
“Turn Sequence” calculator has been added to the EASA website (
easa.com/calculators) to help determine a sequence for distributing the total turns within a group of coils to achieve reasonable slot fill. Using the calculator should be straightforward for most, but the “Turns per group” input may seem unusual – simply add up the turns of each coil in the group. Below in Figure 2 are a couple of examples of using the calculator, and for demonstration purposes, coils with 5 or 6 turns are used. Note that this calculator is not a tool for modifying winding designs – only a tool for how the coils may be installed once a design is established. For example, the two instances shown in Figure 2 would not be equivalent windings for the same machine.
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 have a different section 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 approach 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 fewer turns are close to the same height as the coils with more turns. For example, a 4000 V stator coil design with 9-turn and 10-turn coils might be manufactured with a wire thickness of 0.091 inches (2.3 mm) for the 10-turn coil and 0.102 inches (2.6 mm) for the 9-turn coil. Assuming a heavy film strand insulation, the two coils would be within 1% of each other in nominal height. (See Figure 3.) One common question that arises in this scenario is related 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.
Unequal-turn windings can be a challenge, but any winder can be successful, provided proper evaluation and planning is done up-front. Don’t hesitate to contact EASA technical support for assistance with these. As mentioned, there may be a reasonable redesign that could eliminate the unequal-turn configuration or a pitch modification to simplify the turn sequence required.
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