Hi All.
This is a solution looking for a VFD noise problem.
I have installed several VFD's and always used a good two or three
stage noise filter on the input power line to stop line noise
to and from the VFD. Never had a problem with noise.
My last installation was different. A 15 HP VFD driving a 7.5 HP motor
as recommended by the VFD manufacturer because single phase 240 volt
input power. Something in the motor circuit was resonant at 3 MHZ.
Every time the VFD would pulse the motor (2 to 10 KHZ) the resonant
circuit would oscillate for 6 cycles. It was so intense a scope probe would
pick up 5 to 10 volts two feet from the Lathe. In spite of filter capacitors
on every wire to the electronics cabinet the ringing was coming through
the isolation circuits and recking the electronics.
The solution was a 5% KDR filter from TCI in Milwaukee Wi in the VFD
output. That stopped the ringing and the problem completely.
The only noise the scope sees on the Lathe now is a tiny random spike,
probably the static discharge of the four V belts driving the gearbox.
VFD manufacturers are cautious about 5% filters. The filter takes extra
power to drive and lowers the DC voltage in the VFD. My situation was
a 100% overrating of the VFD and 7 to 1 reduction in the gearbox was
never going to load the motor near 7.5 HP, so not a problem.
These filters also help extend motor life by reducing surge in the coils
and the resulting spurious currents in the motor metal, especially the
bearings.
Hope this helps
Don
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