An EMC evaluation of the use of unshielded motor cables in AC adjustable speed drive applications

The most common solution for modern adjustable speed drives (ASD) is the use of induction motors (IM) fed by voltage-source inverters (VSI). The inverter generates a pulsewidth modulated (PWM) voltage, with dv/dt values of about 6 kV/us or even more. In three-leg inverters for three-phase applications the occurrence of common-mode voltage is inherent due to asymmetrical output pulses. As a result, for electromagnetic compatibility (EMC) reasons, in most applications shielded cables are used between the inverter and the motor, implying high installation costs. The present paper discusses the use of cheaper, unshielded cables from an EMC point-of-view. A new method for measuring electromagnetic interference (EMI) from unshielded cables is
\nproposed and measurement results are presented. The level of
\nEMI is evaluated in different situations: without an output filter,
\nwith a classical LC output filter and with an advanced output filter
\nwith dc link feedback. It is concluded that, from an EMC point of
\nview, unshielded cables can give very good performance provided
\nthat a common-mode (CM) output filter is used.