Introduction - Phase Technologies DV Series User Manual

INTRODUCTION

Output filters are used to remove unwanted high frequency content from the output of a
Variable Frequency Drive (VFD). Large voltage spikes are produced when VFDs are used to
drive motors with long cable leads, which can damage motor or cable insulation systems or
motor bearing fluting due to electric discharge. This voltage spike is known as the Reflected
Wave Phenomenon. dV/dt filters extend motor and cable life by reducing the effects of the
Reflected Wave Phenomenon and removing high frequency voltage spikes. dV/dt filters
should be used when motor cables exceed 50 feet and sine wave filters should be used when
motor cables exceed 900 feet.
For additional protection, sine wave filters provide more robust removal of higher frequency
content. As well as providing protection against voltage spikes from reflected waves, a sine
wave filter additional removes lower order harmonic voltage and currents. These benefits are
realized in reduced motor losses (less heating) due to lower eddy currents and magnetic
hysteresis losses in motor steel laminations.
Figure 1 shows an unfiltered VFD output voltage measured line-to-line.
Figure 1 – Unfiltered VFD Output Voltage
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