Chuck Yung
EASA Senior Technical Support Specialist

One of the recurring questions asked of EASA technical support specialists is:  “Should I use 1-4 or 1-7 jumpers?” This article is a tutorial on jumper selection to help the reader recognize when it does – or does not – matter. 

Let’s start with a short review for the experienced winders and good fundamentals (Table 1) for the newer winders. First, three-phase windings are symmetrical. The connection is log­ical if we apply some basic rules. The groups are positioned symmetrically, in sequence of A-B-C-A-B-C, with an equal number of coil groups required in each phase.

Phases inserted sequentially
Because there are 3 phases inserted sequen­tially, and the jumpers cannot “cross phases” (meaning we cannot con­nect the phases together except at the end of each circuit), it is possible to use 1-4 jumpers (adja­cent-pole, because the 1-4 jumper connects ad­jacent pole-phase groups in series), or 1-7 (called alternate- or skip-pole, because the 1-7 jumpers skip the adjacent pole, and connect to the next coil group of the same phase).

We can trace through each phase and draw arrows to track our progress from each lead to the other end of the phase. For a salient-pole connection, the arrows should alternate direction, indicating the alternating polarity of the sequential groups. This holds true for each phase, considered separately; it also holds true for the complete winding.

So long as a connection follows those rules, we can use any jumpers that connect coil groups of the same phase together and maintain the cor­rect polarity of all groups. If every third group is of the same phase, we can use 1-4, 1-7, 1-10, 1-13, etc. jumpers and the motor will operate correctly.

Most winders prefer the adjacent-pole (1-4 jumpers) connection of Figure 1 for form wound motors, because the jumpers are easy to lay out neatly and tie down to re­duce the chance of shorting between jumpers. Some winders prefer the skip-pole connection (1-7 jumpers) of Figure 2 for other reasons.

A personal preference
The decision is largely one of per­sonal preference. When a winding has unequal grouping (for example, a 48-slot 6-pole requires 12 groups of 3 coils and 6 groups of 2 coils) the choice of jumpers is dictated by the grouping sequence used. See the EASA Technical  Manual, Pages 3-182-185, for the most up-to-date grouping tables.

When a motor has more than one winding, the choice of jumpers is limited if either winding has more than 1 circuit.  See the EASA Technical Manual, Page 3-189, for more information.

Extra long jumpers
Other than those limit­ing factors, the adjacent- and skip-pole connections are equivalent. The other situa­tion where we recommend using “special jumpers” is when a winding with more than 4 poles has half as many circuits as poles. For instance, with a 3Y on a 6-pole motor, we recommend using 1-10 jumpers. This is to balance the magnetic forces acting on the airgap. It takes more current to force flux across the airgap than through steel, so a larger airgap requires more magnetizing current (Figure 3). 

When a rotor is not centered within the stator bore, the groups nearest the largest airgap draw more current than the groups near the smallest airgap. Since both the rotor body and stator bore are round, we can mitigate the effect of an imperfect airgap by con­necting the pole-phase groups that are directly opposite each other in series.

That’s the reason for the special extra-long jumpers in Table 2; they connect pairs of groups directly across the stator bore to better balance the magnetic forces and the current between parallel paths.

When a winding with more than 4 poles has half as many circuits as poles, special long jumpers connect groups that are directly across the stator.

Other than these special cases, a bored winder could use a variety of ways to connect a winding that would work electrically, but would be waste­ful of materials. In general, shorter jumpers take up less room and are less likely to short to other jumpers.

Categories: AC motors, Stators/windings, Winding connections, Design/theory/application

Tags: Winding connections Jumpers

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