In applications where multiple motors are each powered by a separate VFD, care must be
taken regarding the selection of the inverter to motor cables. Cable selection is even more
critical if the cables are to be run any distance together in a raceway. Single conductor
cables, while commonly used for some drive applications, can cause issues in such an installation.
In addition to safety issues (see Southwire application note number 2012, VFD
Cables – A Safe Bet), electromagnetic coupling can cause issues with drive performance.
Electromagnetic coupling, also known as inductive coupling, is a phenomenon that takes
place in electrical circuits where the electromagnetic field in one conductor results in an
electrical charge in another. It transfers electromagnetic properties from one cable to another
and occurs without physical contact taking place. For electromagnetic coupling to
take place, there must be a change in the electromagnetic field. Direct current (DC) circuits,
which have no variation in the electromagnetic field, are immune to this effect. The
amount of electromagnetic energy generated is proportional to the square of the frequency.
The high frequency, pulse width modulated (PWM) waveforms generated by today’s
drives have significant energy and can cause serious issues. The longer the distance cables
are run together, the more severe the issue can be.
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