Chuck Yung
EASA Technical Support Specialist
Member Question: We recently received an 800 hp, 2-pole 460- volt motor for repair. It had a 4-Delta connection, and the windings show severe thermal stress. The customer confirmed that the motor was recently installed, drew high current, and failed quickly.
We don’t want to duplicate the connection if it had something to do with the failure. The groups were connected like this:
The sketch above indicates that someone tried to interleave the winding without fully understanding how it works. This is what they actually wanted to do:
The interleaved winding was developed as a workaround for these considerations in conventional winding design:
- Cannot have more circuits than poles
- Turns per coil are proportional to the rated voltage
- Turns per coil are inversely proportional to the hp/kW rating
A large 2- or 4-pole conventional design, rated less than 600 volts, may have so few turns per coil that it’s impractical.
The interleaved winding alternative lets the designer use twice the allowable circuits, thus doubling the turns per coil. To interleave a winding, each group is divided into subgroups, which are paralleled.
Dividing each group into paralleled subgroups sounds simple, but an incorrectly designed interleaved winding will have circulating currents. I2R losses and magnetizing current (that’s no-load amps) will both increase. That’s what happened to the 800 hp example (Figure 1).
For the interleaved winding to work properly, the electrical center of each pair of subgroups must coincide. Since voltage enters A and A' simultaneously, the offset between subgroups causes a corresponding displacement of the sine wave. If the midpoints of the subgroups are displaced (out of phase), the result will be circulating currents within the windings, which in turn cause an increase in I2R losses.
It’s a continuation of the symmetry required throughout electrical machines:
When considering an interleaved winding, we can calculate the electrical angle between slots and then between the subgroups to determine whether circulating currents would be a problem.
But there is an easier way to check our interleaved winding layout. First, let’s look at how the electrical displacement is calculated, then we’ll introduce the easy way.
The electrical angle between slots = 180 / number of slots per pole. For a 48-slot 2-pole motor, the angle between slots is 7 1/2° (180 / 24 = 7 1/2).
[For a 2-pole, the electrical angle = the mechanical angle (360 degrees/number of slots), but that’s only true for a 2-pole.]
Making It Easy
To calculate the displacement of the subgroups, start by numbering the coils of the group in sequence. For our example with an 8-coil group, numbering the coils from left to right:
By totaling these, we get a sum of 36. Dividing 36 by 8, the average value is 4.5—which is the midpoint of the group.
1+2+3+4+5+6+7+8 = 36
36 / 8 = 4.5
Our goal is for the midpoint of each subgroup and the complete group to coincide. This weighting system can be used for each possible interleaving scheme. It saves time and simplifies the calculations.
There are three ways to go about subdividing each group. Each is presented below, along with results of the weighting system and actual electrical displacement between the subgroups.
Method 1
First, here’s what happened with that 800 hp example. Someone divided each group at the midpoint and paralleled the two subgroups as shown in Figure 1.
1+2+3+4 = 10 5+6+7+8 = 26
10/4 = 2.5 26/4 = 6.5;
(6.5 – 2.5) x 7.5 = 30º
The 30° electrical displacement between the paralleled subgroups causes high circulating currents within the windings. That’s why the 800 hpwindings got so hot.
Method 2
This alternative divides the group into even and odd numbered coils. The even-numbered coilsare connected in series, and the odd numbered coils are connected in series as well; then the two circuits are paralleled.
1+3+5+7 = 16 2+4+6+8 = 20
16/4 = 4 20/4 = 5
(5-4) x 7.5 = 7.5º
The electrical angle between the two paralleled subgroups drops to 7.5, which could still cause excessive circulating currents. The displacement between the sub-groups is much less severe than with Method 1.
Method 3
The greater the angle between the subgroups, the greater the magnitude of circulating currents. To avoid circulating currents, connect the sub- groups so that their midpoints coincide as in Figure 5.
1+4+6+7 = 18 2+3+5+8 = 18
18/4 = 4.5 18/4 = 4.5
4.5 – 4.5 = 0º
The midpoint of the complete group coincides with that of each subgroup. There should be no circulating currents with this interleaved method.
To assess the displacement for the 3 options above, we sometimes compare the percent displacement.
Each coil group covers 180 electrical degrees,so the percent displacement is calculated by dividing the electrical angle between the subgroup midpoints by 180.
Method 1: The electrical displacement angle between the midpoints of each subgroup was 30 degrees, so the voltage passing through the paralleled halves of each group of the stator windings would be significantly out of phase:
30/180 = 0.167, rounded to 17%
Method 2: 7.5/180 = .0417, or 4.2%
Method 3: With option 3, the total of each group half is zero, so the electrical displacement angle between subgroups is zero. No circulating currents should result.
This figure shows the electrical angle from each coil in the group to the midpoint of the group. Numbering the coils is much simpler for our purpose.
It is hard to find much literature about circulating currents in electric motors, but some texts suggest 3% (Liwschitz-Garik)* to 5% (Veinott)* as an acceptable limit. Even if the performance is satisfactory with circulating currents, expect increased losses that can increase winding temperatures and reduce efficiency.
Summing up
When rewinding a large, low-voltage motor with few poles, the interleaved winding—properly done—can be a useful alternative. When increasing the circuits in any motor, remember: The voltage per coil changes in proportion to the number of circuits. If the circuits are doubled, so are the volts per coil.
Voltage stresses in form-coil windings are controlled by keeping the adjacent turns in sequence, but turn position in random windings is random. That increases the importance of insulation between adjacent coils.
For a successful interleaved winding, these conditions must be met:
- Even number of coils per group
- Total turns per subgroup must be equal
- Subgroups should share a common midpoint
- Symmetry – In the spacing of poles, phases and groups
It should be clear that a winding with odd turns, or odd grouping, is not a good candidate for interleaving.
In the case of a form-coil winding rated for low voltage (e.g., 230 or 460 volts), the interleaved winding may permit an otherwise impossible design. The key is to minimize the displacement angle between the paralleled subgroups to prevent circulating currents.
The calculated turns per coil can be impractical for a relatively large, low voltage (under 600 volts) machine. The fewer the poles, the fewer circuits are permissible using conventional winding methods. For 2-pole or 4-pole designs, the interleaved winding offers the careful designer a way to increase the turns per coil (and circuits) beyond conventional limits.
For a low-voltage form coil machine the interleaved winding layout is especially practical, since it does not increase the labor required for coil- winding or connecting the windings.
For a specific design, voltage stresses per coil are proportional to the number of circuits used, so when a random winding is interleaved, additional phase insulation between each coil is required.
* These books are out of print, but can still be located: Winding Alternating Current Machines by Michael Liwschitz-Garik and Theory and Design of Small Induction Motors by Cyril G. Veinott.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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