Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Although aluminum magnet wire theoretically can be converted to copper magnet wire of about 5/8 of the original wire area, in some cases this is not advisable. In others, it may result in a change in the magnetic strength of a coil or winding. In this article we will address the most common aluminum-to-copper magnet wire conversions as well as how to deal with whether the wire area should be reduced.

The resistance per unit of length of aluminum AWG wire is equal within 2% to the resistance per unit length of a copper AWG wire that is 2 sizes smaller. For example, aluminum 14 AWG wire has a resistance of 4.084 ohms per 1000 ft., and copper 16 AWG wire has a resistance of 4.019 ohms per 1000 ft. – a difference of 1.6%. With metric aluminum wire, select a copper wire with about 5/8 of the area of the aluminum wire. For example, if the original aluminum wire was 1.0 mm, with an area of 0.785 sq. mm, the copper equivalent size would be 0.80 mm, with an area of 0.503 sq. mm. Note that the copper wire size is 4 metric gauges smaller than the aluminum wire size given in the “EASA Round Magnet Wire Data” tables found in Section 7 of the EASA Technical Manual. 

AC motor windings
When converting AC motor windings from aluminum to copper magnet wire, the turns and connection do not change; only the wire area and dimensions change. Although the wire size reduction described above would result in about the same winding copper losses, the copper coils would have an area that is about 63% of the original aluminum wire (see Figure 1) and would therefore be very loose in the slots. This could result in loose un-bonded (by varnish) wires that could chafe against each other and eventually become shorted. Also the loss of contact with other wires and slot insulation would reduce heat transfer and cause the winding to operate at a higher temperature. Especially for low-speed machines, slot reactance is affected by the position of the coil in the slot, so adding filler above the coil would have an adverse impact on performance.

The best choice is to use a copper wire of the same size (area) as the aluminum wire. That will maintain slot fit and fill and reduce losses, thereby increasing efficiency and reducing winding temperature. If a reduction in wire size is desired, it should be no more than one wire size, e.g., aluminum #14 AWG to copper #15 AWG. The reduced wire resistance does not increase starting, no-load or full-load current.

Shunt fields
When rewinding a DC motor or generator shunt field coil that originally had aluminum wire, select a copper wire to match the resistance per unit length (e.g., ohms per 1000 feet or ohms/km) as closely as possible. Do not change the turns! Doing so would affect the ampere-turns, which establish the magnetic strength of the coil.

As just mentioned, with DC coils, the field strength is determined by ampere-turns; that is, by the current through the coil and the number of turns in the coil. According to Ohm’s Law (I = E/R), the current (I) is determined by the voltage (E) applied to the coil and the resistance (R) of the coil. To maintain the same resistance per unit length of round copper wire, the first step in this conversion process is to decrease the aluminum wire size by 2 AWG sizes. Note: If the wire is metric, the reduction is the same as with AWG and would be reduced 4 sizes if using the “EASA Round Magnet Wire Data” tables. For example, if the aluminum wire is 0.9 mm, the copper equivalent would be 0.71 mm. 

The decrease of two AWG wire sizes (actually a numerical increase of 2 AWG wire size designations) represents about a 3/8 decrease in wire area, so the replacement copper coil would have a smaller cross-section than that of the original coil. It would also be more compact, so the total length of its conductor would be shorter than that of the original coil. To match the resistance of the original coil, the overall length of the replacement coil must be adjusted to arrive at the same wire length and mean length of turn (MLT).

To do this, wind half the total turn count onto the pole iron. Next, determine the size of the spacers to place between the first and second halves (in terms of turns) of the coil. The thickness of the spacer should be 3/8 times the coil extension. For example, if the coil extension is 1.5” (38 mm), use a 9/16” (14 mm) thick spacer. Insert the spacer and finish the coil by winding the other half of the total coil turns (see Figure 2).

Note that the coil width will be narrower than the original, resulting in a slight decrease in MLT with a subsequent decrease in coil resistance. In most cases, the width does not change the MLT significantly, so the benefits of extra room for coil insertion more than offset the slight change in coil resistance. That is a judgment call for the repairer to make. If the original coil is nearly square (i.e., width and length are almost equal), it makes sense to use spacers on all 4 sides of the coil (see Figure 3).

When coil temperature has proven to be a problem, corner spacers can be used to increase ventilation between the coil halves. This is useful when the finished coil will not be fully taped.

Series fields and interpoles
The two electromagnetic parameters that are most important for series fields and interpoles are current and turns. The current is determined by the armature, which is in series with the series fields (if applicable) and interpoles. When converting from aluminum to copper magnet wire, the turns remain the same to maintain the same ampere-turns coil strength; thus the only consideration is the wire area. As with an AC motor winding, to reduce losses, use the same copper wire size as the original aluminum wire for series fields and interpoles. If wire area must be reduced, do not exceed about 25%. That is, use a copper wire area at least 75% of the aluminum wire area.

Transformer windings
Conversion of transformer windings from aluminum to copper has been addressed in two prior Currents articles. See the articles titled “Replacing aluminum conductors with copper conductors in power and distribution transformers up to 10 MVA” in the August and September 2010 issues of Currents.

Categories: AC motors, Stators/windings, DC motors, Field coils, Design/theory/application, Transformers, Wire

Tags: Motor design Wire

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