Abstract |
The electromagnetic interference and the radiated acoustic noise from a microprocessor-controlled
induction machine are investigated, with particular reference to comparing the influence of PWM
strategies, viz. space vector and random PWM, with direct torque control. In contrast to space vector
PWM which results in concentrated harmonic spectrum and tonal acoustic noise emission, random
PWM and direct torque control strategy result in similar wideband harmonic spectra and indeed less
irritating atonal acoustic noise. However, they produce almost identical conducted and radiated EMI
spectra, on which it was found that there exist dominant discrete peaks due to the energy concentrated
into discrete bands by the action of periodic signals on the external and internal buses of the
microprocessor system. A technique, referred to as software waiting loop length control (SWLLC), is
proposed to reduce the radiated EMI from digital PWM drive system. By randomly changing the
length of the software waiting loops, discrete EMI components can be spread across a larger
frequency range. Using this technique, significant reduction of radiated interference levels has been
achieved in a typical PWM drive system. |