Explain heat rise difference 60hz vs. DC 2

[ramjet]

I'm a ME designing electrical connectors and would like help understanding what I've observed in testing. The connectors under test are used to pass 3 phase current through pressure zones in oil wells and consist of large thick steel shells with 3 insulated conductors within. The surface temperature of the steel shell ends up being about 10 to 30 percent higher when using 140Amps 60hz vs. 140Amps DC. Is the increase due to skin effect? Eddy Currents? Hysteresis? If the shell was non-magnetic or non-ferrous would that make a difference? Remember, I'm an ME, not an EE, so be gentle with me

 

[logbook]

The clamp-on meter can give rise to errors. You need the RMS current. Some cheaper meters are "mean reading, RMS calibrated". They give the correct RMS reading on sinewaves, but give errors on non-sinusoidal waveforms. The correct sort of meter will say "true RMS".

 

[ramjet]

logbook - Thanks for thinking of this. The clamp-on meter is a Fluke 80i-400 which is under current calibration. This is not a cheap meter, so I believe is reading true RMS. Also, I've simplified my test setup to use only a variac and transformer, so the clean 60hz sine wave is preserved at the high currents.

MJR2 - While I certainly don't understand all that is going on, I do think this is a magnetic effect. I can feel the magnetic pull of the 2 steel halves around the shell which correspond directly to the increase of circuit resistance as evidenced by the 3 amp drop in current. I will try the same with aluminum blocks, but I'm pretty sure I won't see any resistance increase. Also, the cable with aluminum armor has the same heat rise AC or DC, the steel armor cable has a somewhat higher heat rise with AC (armor is very thin), and the part that has the thick steel shell has the big increase in heat rise.

 

[ramjet]

I did some more experiments today and as I expected, the presence of non-magnetic conductive metal has no effect. I tried aluminum blocks around the shell and the circuit resistance did not change. Also, as I mentioned before, grounding the shell has no effect. My opinion right now is that Magneticflux explained what is going on correctly and that the term "Eddy Currents" does not really apply to the heat being generated in the shell due to the alternating magnetic flux. Is "recirculating currents" the proper term?

Does anyone know an easy way to create a 3 phase test setup for low voltage, high current (160A), testing?

 

[MagneticFlux]

Ramjet,

First let me correct myself on a previous post in which i stated "eliminates any field interaction that would normally take place between conductors, which has a canceling effect on the field". The point i meant to make is the field interacts with the adjacent field(s) and would, by some amount, reduce the amount of flux acting on the "shell".

Here is a link that I think may be some of help to you...
AMPS LAW

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html#c1

ELECTRO-MAGNETICS

http://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#emcon

 

[MJR2]

Eddy currents when generated AND have no place to flow make heat. MagneticFlux is describing the process. It is a common problem in dealing with AC devices which generate external fields. Permanent magnet devices with alternating poles which rotate and change polarity at high speed relative to a point will heat that point if it is metallic. The heating is in direct relationship to the electrical conductivity of the object.

Do a Google search for things like 'eddy current magnetic separators' or 'inductive heating coils'. An aluminum can held close to an eddy current separator will burn your fingers in a few seconds.

My Grandmother, about 30 years ago, got a stove which had no visable burners, cool to the touch. It would only work with metallic pans. It used eddy current heating from a coil (I think. She wouldn't let me take it apart.) under the non-metallic surface to heat metallic pans.

Moving metallic objects in a alternating permanent magnetic field or having an alternating field near metallic objects generates eddy current heating.

 

[ramjet]

MJR2 - My dad has the same type of stove that your grandmother has - an induction stove. The thing you're missing though is that it only works with ferrous magnetic pans, not just metallic. Aluminum pans don't work.

Here's a link explaining induction stoves:

http://www.wisegeek.com/what-is-an-induction-cooktop.htm

Just to verify the effect in my case, I'm having our machine shop cut a shell out of 304 sst and I will perform the same heat rise test. I'll have results i a week or two, but i think the rise will be closer to the DC test than to the AC test because the shell is non-magnetic.