tjs wrote:
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 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.
If the MOV voltage is well below its conduction threshold, then it
looks to the outside world, much like a small capacitor. So you can
expect that a signal with significant high frequency content will pass
more current through it than one with only line frequency content.
I don't recognize the part number of your MOV, but this data sheet for
some small MOVs lists the capacitance for a 132 volt AC unit is only
21 pF.
http://rocky.digikey.com/WebLib/Litt...r%20Series.pdf
This bigger disc type lists the capacitance for the 130 VAC unit as
100 pF.
http://rocky.digikey.com/WebLib/Litt...r%20Series.pdf
If you can find a data sheet for your unit, the capacitance should be
listed.