Tom Bishop, P.E.
EASA Senior Technical Support Specialist
When the number of coils per group is the same throughout a three-phase lap winding, the grouping sequence is simply that number of coils repeated three (since it is three-phase) times the number of poles. For example, a 48-slot 4-pole winding has 12 groups of 4 coils.
The formula used to determine the average number of coils per group is:
Coils per group = slots divided by groups.
We don’t advocate using full slot coils with a lap winding; thus, the total number of coils is equal to the number of slots. The number of groups in an alternating pole winding is equal to the number of phases times the number of poles. In many cases, there are windings that have unequal coils per group, such as a 36-slot 8-pole winding, which has 24 groups with an average of 1.5 (36/24) coils per group.
The number of coils and groups for a winding with an unequal (odd) grouping can be calculated, as explained in the AC Motor Redesign manual (pp. 107-112). However, in most cases the group and coil combination, grouping sequence and permissible circuits can be found in tables in the EASA Technical Manual (pp. 2-206 through 2-209) or by using the online Grouping Calculator on the EASA website (https://easa.com/resources/online-calculators/grouping-calculator).
We will use examples to illustrate some of the pitfalls and subtle aspects regarding windings with unequal coil grouping. The first example is the 36-slot 8-pole winding mentioned earlier. Viewing the EASA Technical Manual coil grouping tables we notice that there are two sets of tables, one for adjacent pole (1-4 jumper) and the other for skip pole (1-7 jumper) connections. The 1-4 jumper table indicates there will be 12 groups of 1 and 12 groups of 2 coils, with a sequence of 121, 212 (that will repeat three more times). A subtle aspect of the table is that it also indicates the number of permissible circuits, which in this case is 1, 2 or 4. Any other number of circuits, such as 8 in this case, would result in an unbalanced and unacceptable winding. Note: For information about making the decision to use 1-4 or 1-7 jumpers, see the March 2011 Currents article “Skip-pole or adjacent-pole: Does jumper selection really matter?”
Continuing with the example, we will check the considerations if a winding with 1-7 jumpers is desired. As will always be the case, the 1-7 jumper table in the EASA Technical Manual indicates the same grouping combination as with 1-4 jumpers, i.e., 12 groups of 1 and 12 groups of 2 coils. However, the grouping sequence is different. It is 121, 212, 212, 121 (that will repeat one more time). Note that after the sequence that was used for 1-4 is completed one time (121, 212), the “mirror-image” of that sequence then follows (212, 121). If this pattern was not followed, the winding would not have the same number of coils in parallel circuits.
The table on the right in Figure 1 indicates that using the 1-4 jumper sequence for a 36-slot 8-pole 2-circuit wye or delta winding with 1-7 jumpers would result in each phase having 4 groups in parallel with 8 groups. The result would be tremendous circulating internal currents, high line currents, and the motor may not rotate. If it did rotate, it would have low torque and be electrically noisy. Note that the table on the left indicates a balanced winding with 6 groups in parallel with 6 groups in each phase using a 1-4 jumper arrangement.
Figure 2 illustrates the unequal heating pattern of a 216-slot 32-pole 2-wye winding with 24 groups of 3 and 72 groups of 2. A misconnection resulted in 6 groups of 2 being connected in parallel with 3 groups of 3 plus 3 groups of 2. That is, there were 12 coils (6x2) in parallel with 15 coils (3x3 + 3x2), resulting in circulating currents and high electrical noise as well as high line current and overheating of some coils.
The following method can be used to determine when the grouping sequence needs to be “mirror-image.” If the larger quantity of groups divided by the smaller number of groups is an odd integer (e.g., 1, 3, 5, etc.) the grouping sequence needs to be of the “mirror-image” type. For example, a 54-slot 8-pole winding has 18 groups of 2 and 6 groups of 3. Dividing 18 by 6 we obtain a ratio of 3, which is an odd integer. Inspecting the EASA Technical Manual table for 1-7 jumpers we observe that the grouping sequence becomes a mirror image after 4 sets of groups.
Grouping and grouping sequence cell from 1-7 jumper table for 54 slots and 8 poles.
18 groups of 2
6 groups of 3
322, 232, 223, 222, 222, 322, 232, 223
1, 2 cir
In addition to making certain that the correct grouping sequence for the number of slots, poles and jumper type (1-4 or 1-7) has been selected, also confirm that the number of circuits matches one or more of the values in the tables. In the example above, the 8-pole winding can have 1 or 2 circuits, thus a 4 or 8 circuit would not be balanced and would not be acceptable to use.
A closing note: If you are working with a winding with slots and poles that does not appear in the tables, contact EASA Technical Support to obtain a balanced and useable grouping sequence.
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