MOV protection of 120vac circuits with high freq content
 

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

"tjs" wrote:

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

Well this isn't rec radio either!
A mov does not model as a "capacitor" it models as a (verY) non linear
resistor.

What is the continuous rating of the MOV.

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


jk

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.

Sorry , I crossposted to rec.radio.amateur.homebrew and sci.eng.electrical
sys-protection

I'm having trouble finding data on the MOVs. The markings MDC Z181 04UL seem
to point to a 175 vac 225vdc 17-19 joule device with 130 pf capacitance,
1200 amps Ipeak, 25mW transient dissipation. I searched Digikey and NTE. It
is about 10mm diameter disk type, maybe 8.5mm per some datasheets.

Tim



"tjs" wrote in message ...
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

This is close the exact component I have.
http://www.maida.com/content/product...6121ZOV181RA04
It shows 320pF. Its marked 04UL is supposed to mean 40 joules, although the
data sheet says 60J.
Tim

I want to conclude that the MOV can short pulling over 25ma thru the 1/8th
watt resistor, heating it up, due to 10khz components on the voltage, and
not an actual high voltage. Would this be a true statement?

Thanks

"tjs" wrote in message ...
Sorry , I crossposted to rec.radio.amateur.homebrew and sci.eng.electrical
sys-protection

I'm having trouble finding data on the MOVs. The markings MDC Z181 04UL
seem
to point to a 175 vac 225vdc 17-19 joule device with 130 pf capacitance,
1200 amps Ipeak, 25mW transient dissipation. I searched Digikey and NTE.
It
is about 10mm diameter disk type, maybe 8.5mm per some datasheets.

Tim



"tjs" wrote in message ...
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

tjs wrote:
This is close the exact component I have.
http://www.maida.com/content/product...6121ZOV181RA04
It shows 320pF. Its marked 04UL is supposed to mean 40 joules, although the
data sheet says 60J.
Tim

I want to conclude that the MOV can short pulling over 25ma thru the 1/8th
watt resistor, heating it up, due to 10khz components on the voltage, and
not an actual high voltage. Would this be a true statement?

Without having the exact waveform, it is impossible to say exactly how
much current the MOV capacitance allows through. I suggest you try an
experiment of removing one of the MOVs and see how much cooler the
resistor runs.

Yes I think I will have to test the MOVs to really know. Or as you suggested
take a MOV out and measure resistor temperature.

"John Popelish" wrote in message
...
tjs wrote:
This is close the exact component I have.

http://www.maida.com/content/product...6121ZOV181RA04
It shows 320pF. Its marked 04UL is supposed to mean 40 joules, although
the
data sheet says 60J.
Tim

I want to conclude that the MOV can short pulling over 25ma thru the
1/8th
watt resistor, heating it up, due to 10khz components on the voltage,
and
not an actual high voltage. Would this be a true statement?

Without having the exact waveform, it is impossible to say exactly how
much current the MOV capacitance allows through. I suggest you try an
experiment of removing one of the MOVs and see how much cooler the
resistor runs.

tjs wrote:
Yes I think I will have to test the MOVs to really know. Or as you suggested
take a MOV out and measure resistor temperature.

I was suggesting that you might prove that the MOV current is the
cause of the resistor overheating, by removing an MOV and applying the
input signal. I can't guarantee the input will work, correctly, however.