A variable speed motor operates by placing an electromagnet in a magnetic field made by a permanent magnet. Constructed from steel or iron with wire wrapped around it going in one direction, the electromagnet turns the drive shaft, which then spins freely along a single axis.

For electricity to be supplied to the wires from the outside where wires do not spin with the electromagnet, moving contacts must be made. This also ensures that outside variable speed motor wires do not need to wind around the driveshaft. Because stationary electrical contacts brush or rub against contacts that move, the moving contacts are commonly referred to as “brushes.”

The electromagnet can feel the magnetic field of a permanent magnet around it. As a result, it naturally wants to line up correctly. The goal at this point is to reverse the direction of the electrical current so the electromagnet is prompted to turn around and then line up the other way. Brushes are aligned, causing them to rub on one contact when the electromagnet points in one direction and then rub on the other contact when the electromagnet points in the opposite direction.

When the electromagnet points in one direction of the permanent magnetic field, it suddenly changes direction in order to line up with the other direction. Dissatisfied with its direction, the electromagnet always wants to spin. Therefore, a variable speed motor is designed to spin in either direction.

The speed of a variable speed motor is determined by several factors, such as how fast the current can change direction within the electromagnet, how strong the electromagnet is attracted to the opposite pole on the permanent magnet, and the amount of load on the driveshaft. Attraction to the permanent poles depends on the applied DC voltage, so the more voltage applied, the faster a variable speed motor runs.