Experimental investigation of a naval propulsion drive model with PWM-based attenuation of the acoustic and electromagnetic noise

An extensive experimental investigation of a 40-hp ac drive has been conducted with respect to mitigation of the acoustic and electromagnetic noise, and vibration, by means of randomized pulse width modulation employed in the drive´s inverter. The drive was a laboratory model of an electric propulsion system for naval vessels, particularly electric submarines, in which the noise mitigation is crucial for survivability. Three PWM methods were used. The deterministic PWM is characterized by a constant switching frequency equal to the sampling frequency of the digital modulator. In the known random PWM, RPWM I, technique, the switching and sampling periods vary simultaneously in a random manner. Finally, in the novel, RPWM II, method, the sampling frequency is constant, while the switching frequency randomly varies about an average value equal to the sampling frequency. Spectral properties of the three PWM techniques are described in the paper. The experimental results have confirmed the noise and vibration mitigating properties of random pulse width modulation. The technically convenient RPWM II technique is only marginally less effective than the classic RPWM I strategy. Conclusions drawn from the described study are also valid for industrial ac drives.