On 4/24/2013 4:27 AM, Mike King - KM0T wrote:
A local college is putting in a well
pump, 60HP 480V 3 Phase. (they are on a very high hill, its super deep) It
is controlled by a VFD.
Variable frequency drives are addressed in my RFI tutorial. They are
essentially square wave generators, the shape and frequency of which are
varied to control speed. The wiring carrying this current is an antenna,
and it also produces a strong magnetic field. The long the wires, and
the more widely separated they are, the greater the magnetic field and
the EM field.
One of the major design issues with most large VFD installations is that
the controller is at considerable distance (across a large room, for
example) from the motor being controlled, so the antennas are long. A
contributing factor is that the wiring will usually have bypass
capacitors between the phase conductors and ground, and there is stray
capacitance between the phase conductors and ground at the motor end.
The result is that much of the return current flows in an un-controlled
ground path rather than on the neutral, which produces both radiation
(EM field) and a strong magnetic field. The magnetic field dominates at
lower frequencies.
The single most critical thing that must be done to contain the trash
produced by a VFD is to locate both the 3-phase transformer and the
motor controller AT THE MOTOR, so that the wiring between these three
elements has "zero length."
73, Jim K9YC
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