Aluminum as a winding material

There is more to the economics argument than just the cost of the wire itself. Beyond relative commodity prices between aluminum and copper, the electrical properties of aluminum itself creates other economic issues. Since aluminum is not as efficient a conductor as copper, it takes more aluminum magnetic wire to equal the performance of copper. So any temporary per-pound cost advantage can be quickly eliminated.

Aluminum magnet wire is generally found in smaller motors, where the ratio of wire to iron is relatively low and physical size is not an especially important factor. It is also found in applications where the motor runs intermittently and in applications where motor efficiency is not a major factor. Examples of these types of applications include hobby compressors, garage door openers, stationary power tools, and garbage disposals.

Motor performance is another factor that tends to limit the use of aluminum magnetic wire. In applications where the highest possible efficiency and the physical size of the motor are the main criteria (in equipment that runs continuously, such as air handlers) aluminum tends to be harder to apply than copper. Since aluminum’s conductivity is lower than copper, aluminum magnetic wire must be made in larger diameters (gages) to achieve the same linear conductivity as an equivalent piece of copper wire. If you factor the larger gage wire, multiplied by the number of coil turns in the winding, you quickly see that if you want to accommodate sufficient quantity of aluminum wire to equal the conductivity of a copper version, you’ll need a larger diameter motor. In high-efficiency designs, such as room air conditioners, furnace fans, or central air conditioners, this increased size requirement tends to rule out aluminum in favor of copper.

In some circles, aluminum suffers from a bad reputation as an electrical conductor—which it does not deserve. When aluminum was originally approved by the National Electric Code (NEC) for house wiring in the 1960s, problems occurred in instances where the wire was attached to devices such as switches and receptacles. Improper connections tended to develop high resistance, which led to thermal failures. In some cases, these thermal failures created enough heat to cause smoke and occasionally fires. A small number of these fires were catastrophic—in some instances, burning down entire houses. The trade press (and others) denounced aluminum as an electrical conductor. To this day, many people remember the "bad press" and still view aluminum as an unsuitable material for electrical use.

None of these problems ever affected motor design, where connections are made with equipment suitable for the purpose. Manufacturers use oxide-piercing connectors and specialized, high-pressure connector crimpers to create a uniform, high-conductivity connection which virtually eliminates the possibility of oxide affecting the integrity of the connection. They have found that it is critical to motor reliability to have the right connection design, materials, and manufacturing processes.

Motor makers have conducted extensive studies on the impact of adverse environments on aluminum magnet wire. These studies have centered on some of the more challenging applications, such as domestic water pumps (where the motor windings are subjected to mineral salts) and pool pumps (where the windings must withstand chlorinated water). Aluminum is well-suited to adverse environments and is used, along with copper for magnet wire, in many motors.

One last note on aluminum magnetic wire. The polyester insulating varnish commonly used on a motor’s magnetic wire has a copper hue. This makes it almost impossible to distinguish the material used in the motor windings. Other than understanding the application and the limitations of aluminum, you typically have no way of knowing if you have a motor with aluminum or copper windings.

by Neil Simon, aka the Motor Doctor - regional sales manager for A. O. Smith Electrical Products Company.