Cable for variable-frequency drives

Jan. 26, 2006
Variable-frequency drives (VFD) provide economic and performance benefits that manufacturers and designers find hard to resist.

But to get the most out of VFDs, the power and signal cables must be able to perform in mechanically and electrically harsh environments.

VFDs typically convert 480-V, three-phase ac sources to dc, which is then switched at up to 20 kHz using IGBT-based electronics. High-speed switching, coupled with long cable runs, can create reflecting electromagnetic waves. When these waves meet, voltage doubles or triples. With a 480-V source and 700-V operating voltages, 1.4-kV spikes are possible. This can superheat air trapped inside the cables, raising temperatures, generating ozone, and creating coronal and chemical reactions, all of which degrade normal PVC insulation.

To solve this problem, engineers at Alpha Wire Co., Elizabeth, N.J. (alphawire.com), designed a VFD cable that uses cross-linked polyethylene (XLPE) insulation. Cross-linking strengthens the molecular chains against meltdowns during voltage spikes and corona discharges. The cables are made using pressure extrusion for a round, symmetric profile. The low-capacitance cable also permits longer cable runs without signal degradation.

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