High Current Draw from RPC

[jim rozen]

... I would not be concerned at this point. Run a part, and see what happens.
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Well there's one thing I agree with. If something gets 16 KW hot however maybe stop cutting. =)

 

[Cleetis]

At low load, but still SOME load, a rectifier takes current in narrow pulses. A "true rms" meter can give you a high reading because pulses have a disproportionately high rms value.

There may be other things going on as well, but a lowish source impedance, and a high current rectifier can really give you a high but very short pulse current, and a higher than expected "rms" current. Added to the normal draw from things in the control, it might look crazy-high.

In THIS CASE, if you had an "amp clamp" 'scope probe and a 'scope, you would probably see that.

Since the unit is evidently not drawing excess current, since it is not getting hot, and no breakers or thermals are opening, no faults showing, I would not be concerned at this point. Run a part, and see what happens.

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BTW, "amp clamp" is not a meter type, and there is nothing about an "amp clamp" that is a problem. The same type (true rms) series ammeter would give the same answer as the clamp-on.

It is, however, necessary to know what you are measuring, and how it is being measured, plus to have some idea what you ought to expect, to know what to think about the "number" you get as a result.

In this case, an old clunky "Amprobe" averaging-type meter might well give a better answer, even though it is still an "amp clamp meter" (see....? There are many types). The "average" of that sort of pulses is going to be much lower than the "rms" value.

I think "true rms" meters should be able to read "average" by changing the setting..... very few if any do that, though, the "true rms" is generally regarded as the "better" reading.... That is not always true, and the comparison between the two is often very telling.

Great info. I’ll see if the scope at my office is capable of a better measurement and see if it can go missing for just a few days!

 

[Just a Sparky]

That may cost as much as a decent meter..... depending on the fuses.

3 25A, 300V RK5s are $15. Slapping them into the disconnect and seeing what happens is cheap and easy. Peace of mind if nothing else. If the machine runs fine with the fuses then put the tools away and run it. If they blow then there's more going on than phantom current readings.

He did mention the #8 conductors are getting warm.

Great info. I’ll see if the scope at my office is capable of a better measurement and see if it can go missing for just a few days!

Don't forget to bring a current probe or shunt resistor. Voltage probes won't tell you much on their own. Detach or insulate the probe ground (alligator clip) if going the shunt route. Connecting it accidentally to the ungrounded conductors of a solidly grounded electrical system will make a big, expensive boom. You'll need a two channel scope with basic math capabilities to take a differential voltage reading across the shunt. Multiply the differential voltage reading by the shunt ratio to get amperes.

 

[JST]

Great info. I’ll see if the scope at my office is capable of a better measurement and see if it can go missing for just a few days!

Don't mess with shunts and differential measurements..... use a clamp-on probe.

The other ways of doing the job have nasty downsides UNLESS you have a scope that has high voltage isolated inputs.