Cyndi Nyberg
Former EASA Technical Support Specialist
It is very important to take accurate data when you rewind a form-wound motor or generator, especially if the coils will be made by an outside coil manufacturer. There are a couple of variations to the standard coil design that is not common, but that you may come across from time to time.
Brick-type design
A brick-type winding uses two different sizes of wire arranged as shown in Figure 1. Rather than use one large rectangular wire for each turn, this example uses four smaller wires. When more than one conductor is used, each individual conductor must be insulated. However, it is not necessary to insulate every wire to achieve the required separation.
There are two different wire sizes used in the coils of Figure 1. The large wires are insulated (glass, film, mica, glass-over-film, etc.) and the smaller ones are bare copper, with the same total height as the insulated wire. The reason for this configuration is to reduce eddy current losses within each turn of the coil.
This type of winding can increase efficiency and cooling, based on the design requirements. Each strand of wire is separated by the insulation, and the bare copper wire maximizes the copper content in the slots.
Terrace design
A second type of coil is called “terrace wound,” also known as zig-zag or crossover. This type of coil is normally used when there is a high number of turns per coil. Instead of using a wire that is the width of the slot and very thin, a thicker wire that is half the width is used and is staggered in the slot.
Figure 2 shows a comparison of the standard coil and the terrace wound coil.
When you’re taking data on terrace wound coils, it’s easy to miss the number of turns and the wires in hand. Normally, when you see a coil with two vertical columns, you assume that
there are two wires in hand, with the number of turns equal to the number of conductors in the “stack.”
The way to tell that the coil is terrace wound and has only one wire in hand is to look at the coil lead and confirm that there is only one wire in hand. If you routinely calculate the flux densities for your designs, the incorrect data will have double the densities, and double the circular mils per amp that you would normally expect.
One reason that the terrace wound coils are used is because the thicker wires are easier to work with than a very thin wire. The drawback is that the volts per turn (V/T) are higher between some of the turns in the terrace wound coil. For example, in Figure 2, between turns 1 and 4, the V/T is equal to the sum of the V/T between 1&2, 2&3, and 3&4.
Unfortunately, most of these designs use a 1-circuit connection. If the connection had 2-circuits originally, a new coil with the incorrect data could be changed to a one circuit connection. But if the connection was only 1-circuit, new coils would be needed.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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