Comparison of the audible noise level emitted by inverter-motor combinations

The audible noise of medium-sized squirrel-cage induction motors is analyzed for various inverter-motor combinations. Two motors are analyzed, one standard 90-kW double-cage induction motor and one with heat pipe cooling, having a higher specific output. The latter is equipped with two rotors, one with 27 slots and one with 28. Both machines have four poles. The overall value of the audible noise is registered as a function of the supply frequency for three types of variable supply: a sinusoidal voltage generated by a DC-motor-synchronous-generator unit, a current source inverter with a six-step current wave shape, and a voltage source inverter with pulse-width modulation. Mechanical tests of the natural frequencies are performed to establish a full electromechanical analysis. As a general conclusion, it may be stated that only a total analysis of the inverter-motor system can lead to an appropriate analysis for audible noise.<>