Electromagnetic wave emissions generated from PWM inverter and motor drive systems are rapidly increasing as switching characteristics of power switching devices used in PWM inverters are increased, and as operating frequencies of microcomputers in the motor drive controllers are also increased This paper discusses emission characteristics generated due to common mode noises which are produced by circuits having with stray capacitances between the PWM inverter frame and the motor frame. It is difficult to cope with reduction of common mode noises emissions in comparison with other differential mode noises. The emissions are analyzed by computer simulations using the method of moments, especially, for the case of emissions connected to the PWM inverter frame and motor frame by a conducting plate. The effects of cable length between the PWM inverter and motor on emissions is also examined In low frequency regions where the structure length of the drive system compromised of the PWM inverter and the motor is less than one-fourth of the electromagnetic wavelength, emissions are remarkably reduced by using a conducting plate between the PWM inverter and the motor. In high frequency regions there is hardly any reduction in emissions. Finally, the resonance phenomenon is shown to occur when cable length between the PWM inverter and the motor is equal to one half of the electromagnetic wavelength.
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