Not a REC.Radio issue but still appropriate here...


Question: will 1-10khz harmonics riding along on 60 hz power cause
protective MOVs to charge and short out thusly drawing damaging currents.

The scenario: I have an Allen Bradley PLC digital input card and it contains
1 MOV across ac input channel terminals (I think they are MDC Z181 spec).
They are there to protect against high voltages at the terminals which I
contend never occurs, never seen it happen. What does happens is that a 232
ohm 1/8th watt (current limiting?) resistor just ahead of the MOV will
slowly heat up and burn to open circuit in about 30 seconds after
connection. Using 0.125W (P=I^2 x R) 232 leads me to exceeding a current
of ~25ma drawn to burn the resistor.

The IO being monitored is motor run status from a VFD driven motor (variable
frequency drive, PWM type, naturally using ~50-100khz synthesis methods).
The control power is riddled with harmonics from 500hz to 10khz, and I
estimate 10vp-p maybe less (as seen on the oscilloscope). As soon as I
close the electrical connector to the IO card the resistors start heating
up, smoke, then fail.

I know I need to isolate the control power and rid the plc of the harmonics.
I just want to confirm the high frequency components can cause MOVs to short
as if there was a high voltage event when there isnt one. I beleive the
capacitor model of a MOV means it should charge up at higher frequency, and
maybe this is why it takes 30 seconds for smoke to appear.

Regards
Tim KF8XW