A Variable Frequency Drive (VFD) regulates the speed of a 3-phase AC electric motor by
controlling the frequency and voltage of the power it delivers to the motor. Today, these
devices (also known as Adjustable Speed Drives or Variable Speed Drives) are becoming
prevalent in a wide range of applications throughout industry, from motion control
applications to ventilation systems, from wastewater processing facilities to machining
areas, and many others.
VFDs offer many benefits; principle among them the ability to save a substantial amount of
energy during motor operation. In that sense, these devices represent both an attractive, “green”
engineering solution, and an economical choice. Other benefits worth mentioning include the
following: they can maintain torque at levels to match the needs of the load, improve process
control, reduce mechanical stress on 3-phase induction motors by providing a “soft start,” and
improve an electrical system’s power factor. What’s more, legacy systems that now use throttling
devices to regulate motor speed can be retrofitted with VFDs to make speed regulation much
more efficient and precise.
Special consideration must be given to the proper installation and operation of the overall
system that comprises the VFD, the motor it controls, and the cable that connects them.
See Figure 1 for a schematic depicting a generalized VFD system. The way in which VFD-based
systems are constructed and operated will have an impact on both the longevity and reliability
of all the components of the system, as well as nearby or adjacent systems.
This paper is primarily concerned with the motor-supply cable in the VFD/motor system. It looks
at some fundamental cable design considerations, and presents suggestions for installation.
However, to give the reader some context, it makes sense to first describe VFDs, their benefits
and potential problems, and their relationship to the motors they control.
Overview of VFD technology
VFDs are solid state devices for converting 3-phas