The Magnetic Noise of Polyphase Induction Motors [includes discussion]

The wide use of induction motors in the home, the office, and the factory requires that they both look and sound well. The pleasing, if not downright handsome, appearance of present-day motors attests to the skill of the appearance-design experts who have worked with motor engineers in recent years. There is now a growing demand that motors should have good "sonance design" also. They should give out a steady pleasing hum, just loud enough to show that they are performing their duties properly, but not loud enough to be noticed. This paper describes the magnetic forces that cause the magnetic noise of polyphase induction motors, and the corresponding modes of motion of the motor cores and frames. Approximate equations are derived for the decibel sound levels that motors of normal design may be expected to produce, and variations due to core and frame resonance are discussed. The two principal types of magnetic noise, due to the radial forces of the fundamental air gap field, and the tooth frequency pulsations of these forces, are considered separately.