3.1 Magnetic Noise

A property of ferromagnetic materials is magnetostriction, which causes magnetic materials to expand or contract in response to a magnetic field. The molecular dipoles and magnetic field boundaries shift in response to the magnetic field, changing length slightly in the direction of the applied field, as shown in Figure 4. In a stepper motor, magnetostriction deforms the iron and pulls the rotor and stator teeth toward each other in the air gap, causing audible noise.

Figure 4. Molecular Dipoles Changing With Applied Magnetic Field

For stepper motors, the audible sound of magnetostriction manifests itself as an intense low pitch hum. Stepper motors operating at low speeds show the worst effects of magnetostriction. Thus, magnetostriction has the worst noise effects for motors used in laser printers and copiers due to generally low motor operating speeds.

Noise resulting from magnetostriction can not be completely eliminated, but it is known that certain types of metals are more prone to magnetostriction than others. There are special materials that compromise between magnetostriction and core losses to achieve the best performance for a given application.