Editor's Note: This "encore" technical article first appeared in the September 2003 issue of Currents. It was written by former Technical Support Specialist Cyndi Nyberg Esau.

To make more efficient use of time and materials, winders may want to increase the number of parallel circuits when winding an AC stator (or wound rotor). However, there are limits to the number of parallel circuits that can be used in an AC stator (or wound rotor) design. In this article, some of the potential problems associated with increasing the number of parallel circuits will be analyzed.

If the original design of a mo­tor has few turns with large wires, or many wires in hand, it may be easier to rewind if the number of parallel circuits can be increased (see Figure 1). Doubling the circuits, for example, doubles the turns per coil and cuts in half the wire size or the number of wires in hand. Of course, doubling the circuits also doubles the volts per coil.

It seems like more and more, motor manufacturers are winding relatively large motors with concentric coils. Unfortunately, not many service centers have winding heads that are big enough to make these concentric coils for a rewind. Consequently, by rewinding these to half-slot lap wind­ings from full slot concentric wind­ings, the turns per coils must be cut in half to keep the same connection. Very often these designs also have a low number of turns, with unequal turns being common as well. Therefore, it is often desirable to increase the number of parallel circuits to make winding the coils less difficult.

Maximum circuits
The number of par­allel circuits cannot be more than the number of poles. One excep­tion is the interleaved winding, where it is possible to have a four-circuit connection on a two-pole motor. With the interleaved wind­ing, each group is di­vided into paralleled subgroups, resulting in double the allow­able circuits, as well as double the turns. For more information on interleaved wind­ings, refer to the Janu­ary 2003 CURRENTS, “Interleaved Windings Provide Useful Alter­native.”

Additionally, the number of poles must be equal to or a multiple of the number of circuits. For example, you cannot change a 2-delta connection on a 6-pole winding to a 4-delta connection.

Unequal grouping
Unequal grouping limits the num­ber of circuits. There must be the same number of coils in each leg of the parallel circuit for a winding to be balanced. The Coil Grouping tables in the EASA Technical Manual indicate both the grouping and the allowable circuits for the number of poles and slots in the stator.

With an even grouping (i.e., all groups have the same number of coils), it is always possible to have a balanced winding with the maximum number of parallel circuits. However, with an unequal grouping sequence, it is important to check if it is even pos­sible to increase the number of circuits without creating a circulating current. The following example will illustrate this point:

Example:    6 poles, 48 slots, 6 groups of 2 and 
                   12 groups of 3 coils
                   Grouping sequence: 
                   2 3 3, 3 2 3, 3 3 2
For the 6-pole, 48-slot winding, only 1 or 2 circuits are allowed. 
 
Here is what will happen if the con­nection is changed to 3 circuits:
With a total of 48 coils, there will be 16 coils in each phase. You cannot di­vide 16 by 3 and get a whole number. If connected for three circuits, two of the parallel circuits in a phase will have 
5 coils in series, and the other will have 6 coils in series (see Figure 2). When the 3 circuits are paralleled, it will create a horrendous circulating current. This translates to high current and heating.

Volts per coil
For form coils, the turn-to-turn stresses are used to determine the required insulation for each coil. For random wound coils, the volts per coil (v/c) is the main concern. This doesn’t necessarily mean the stresses between two adjacent coils in a group. Any place where two wires can come into contact can be a potential concern. This can mean between two coils in the same slot, or coils on the end turns. They can be from different groups or phases, raising the voltage potential between them. 

Volts per coil = 
3 x rated voltage x number of circuits
K x number of coils
where
K=1 for delta connection
     1.732 for wye connection

Example:    A motor rated 460v, 4-delta, 48 slots
                    V/C =  3 x 460 x 4  = 115
                                 1.0 x 48 
Historically, the rule was that the volts per coil of a random wound coil should not exceed 40, if possible (see EASA Technical Manual, “Voltage Stresses in Three Phase AC Motors”). However, this number is really too conservative. The windings materi­als used in today’s motors, including better wire and better insulation, bet­ter protect the winding from voltage stresses. We now know that winding failures are more likely to be caused by inter-turn failures than by steady-state volts per coil. A more realistic value for the maximum volts per coil in a ran­dom winding would be 80, although we see many designs from reputable manufacturers with over 100 v/c.

Rather than using a specific value for volts per coil to decide when more insulation should be a concern, it is more important to look at the applica­tion and environment in which the motor operates. Things like starting frequency, power supply, across-the-line starts, and the application are all considerations. If there seem to be a lot of turn-to-turn failures in the same area/application, then the volts per coil might be more of a concern.  

If a strand from the first turn and a strand from the last turn come in contact with one another and the volts per coil are too high, a blown coil could result. 
However, there are many designs where the volts per coil are high. Cau­tion is advised in selecting materials and in handling the wire. 

It is not always possible to design or redesign a winding without using multiple circuits.  Say, for example, it is necessary to redesign a motor from 2300v to 460v.  By reducing the rated voltage, the resulting turns per coil can be very low for a large horsepower motor. To offset the low number of turns, we normally need to increase the number of circuits. This, in turn, will cause the volts per coil to increase. The real danger here is that often this type of voltage change also involves a change from form wound to random wound coils, increasing the potential voltage stresses on the coils.

When a winding has as many cir­cuits as poles, some winders use phase insulation between each coil, or tape each coil, as extra protection for the potential voltage between turns. An alternative is to insert phase insulation halfway through each group.

Connection time
Typically, it is more time consuming to connect a winding with multiple cir­cuits. It may be easier to wind the coils, but you may spend the same amount of total time winding because of the connection (although some winders find it easier to make multiple circuit connections). More time is spent mak­ing the connections and the connection becomes more bulky.

Splitting groups
There are a lot of random-wound large, low-voltage 2-pole motors around. In many cases, even using a 2-delta connection (maximum number of turns per coil), there are still relative­ly few turns per coil, with many wires in multiple (of large wire to boot!)

To make the winding process easier in these cases, each group can be wound in two steps, and then paral­leled. Each “half coil” is wound with the turns per coil remaining the same, but with half the circular mils of wire of the original coil. If the coil head can accommodate the wire, simply wind the second half over top of the first half, then tie the group and treat it normally. If there is too much bulk for the coil head, treat it like two separate windings. If the volts per coil are above 80, consider using phase insulation between the coil halves. Then, each coil is wound as two halves, in the same slot. The two group ends on the start and the two on the finish of the group are each then brought out as one group end.

2-speed, 2-winding designs
The easiest way to avoid circulat­ing currents in a 2-speed, 2-winding motor is to use a 1 wye connection for both windings. In the case of a voltage change or a relatively large horsepower motor, it may not be possible to get the desired design characteristics by using a 1wye connection—the turns may be too low or calculate to a frac­tional turn.  

When changing one or both of the windings of a 2-speed, 2-winding motor, you not only have to deal with the issues previously covered, but you also need to consider circulating currents.
If one or both of the original wind­ings has multiple circuits, it isn’t always necessary to have a 1wye, but you will need to document the origi­nal jumpers used, and duplicate them.

To change a 2-speed, 2-winding motor from 460 to 230 volts (or any other 2:1 ratio), it may not be nec­essary to rewind the motor. If the original windings both have a 1wye connection, then reconnecting both windings with a 2 wye connection will cut the voltage in half. However, make sure that the correct jumpers are used in the connection to avoid circulating currents. Refer to the EASA Technical Manual Pole-Group Connections table showing the proper jumpers to use for the combination of poles and circuits for the windings. 

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

Tags: Winding connections

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