Chuck Yung
EASA Senior Technical Support Specialist
While manufacturers use concentric windings due to their ability to wind the coils directly into a core, many repairers convert them to lap windings to take advantage of the superior MMF (magneto-motive force) curve.
Although the former Tech Note 12 (see page 2-187 of the EASA Technical Manual), and the AC Motor Verification and Redesign Program, Version 4 allow us to convert a concentric winding to a comparable lap winding, there are still some winders using “shortcuts” they have learned over the years.
One such shortcut is the “full slot lap winding” when a winder decides that six groups of three pitched 1-16 is equivalent to six groups of three pitched 1-14,16,18. They reason that, when drawn out, the coils occupy the same slots, so they must be equivalent. They’re not, but let’s look at why that is a bad idea.
With a 36 slot 2-pole, the chord factors are:
1-14 0.906
1-16 0.966
1-18 0.996
Using a 30 turns/coil example, the effective turns would be (30 x .906) + (30 x .966) + (30 x .996) = 86 effective turns for the concentric group, compared to 87 (90 x .966) for the “full slot lap winding” group. Magnetic flux density is inversely proportional to the turns, and torque (HP or KW) is proportional to the airgap density squared. So, if we only consider the chord factor the full slot lap develops less than 98% of the concentric winding torque.
That doesn’t sound too bad, but let’s look further. The other variable we must consider is the distribution factor. A concentric winding is also called a “concentrated” winding. That’s because the coils of a concentric group share the same centerline. The distribution factor is therefore 1.0. A lap winding is sometimes called a “distributed” winding because the coils are distributed across the pole phase group. If we convert that concentric winding to a conventional lap winding, using six groups of six coils, the distribution factor for a standard lap winding is 0.956.
Winder #1 says, “I’m going to lay three coils, skip three slots, so the coils occupy the same slots as the concentric group” when inserting the coils, the distribution factor is 0.831. As Figure 1 illustrates, the MMF curve is less than ideal.
Winder #2 says, “I’m going to insert a coil, skip a slot, insert a coil, skip a slot, so that my coil group covers almost the same angle as a lap winding group would” for his six groups of three coils, the distribution factor becomes 0.990.
That difference doesn’t sound too bad, but now what do we have? As Figure 2 illustrates, the MMF curve is better than Figure 1 but still not optimal.
A properly converted lap winding will produce the same torque as the original concentric winding.
Winder #3 - “If we convert that concentric winding to a conventional lap winding, using six groups of six coils,” the distribution factor for a standard lap winding is 0.956. Assuming the redesign was done correctly, the power matches that of the original concentric winding. That makes the .956 value a baseline for comparing the three winders’ results. In Figure 3, we see the MMF curve is more sinusoidal than the other options.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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