Explain heat rise difference 60hz vs. DC
Explain heat rise difference 60hz vs. DC
(OP)
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 :)





RE: Explain heat rise difference 60hz vs. DC
If however you are running a Variable Frequency Drive motor using 140A/phase then it's a different story. You are not really driving 60Hz but more likely up to 16KHz pulses simulating a 60Hz waveform.
My suspicion is that you are not actually comparing 3 x 140A AC with 3 x 140A DC.
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
thread238-128438
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
There was a recent thread about the heating of steel conduit elbows in a parallel installation, each conduit containing a single phase. The heating effect was considerable.
William
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
If it is the issue and the connector body must be made of ferrous metal then you are stuck with the heating. Grounding will not help. The RMS value of the current gives the same resistive heating effect as DC, that's why they use RMS. The magnetic effects are a function of frequency.
I immagine that it's no trivial exercise to try a brass connector body, but that is worth a go if possibel, as it's non-magnetic. Also using a larger diameter shell with the cable spaced to the centre will reduce the magnetic coupling.
RE: Explain heat rise difference 60hz vs. DC
In oil filled transformers the following is usually done:
For low currents, do nothing.
For medium currents, cut the tank between the bushing and welded with non magnetic stainless steel.
For high currents, mounted the bushings on stainless steel.
Try doing your test with a cold rolled steel plate and then a non magnetic stainless steel plate. Just drill or punch a hole through the steel and put an insulated conductor through. A plate 1/2' thick should give good test results.
The stainless steel should have low temperature rise and the cold rolled steel should have a high temperature rise.
Use a magnet to be sure that the stainless steel is non magnetic.
Good Luck
Carl Pugh
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
Here goes....
a) Whenever current flows thru a conductor, a magnetic field is produced around that conductor.
b) Magnetic Induction is the reverse of that law; whenever a conductor cuts through magnetic lines of flux, a voltage is induced into the conductor.
AC Issue:
You have a conductor (the ferromagnetic shell) which has a magnetic field passing through it (the three phase conductors) AND its alternating 60 times per second. The alternating field would induce some level of current into the "shell" causing a temp rise. Also, there is friction, current flow is electron flow and each time a electron strikes another, heat is produced and the energy of the striking electron is divided. Next, there is the magnetizing and de-magnetizing of the ferrous "shell". The magnetic domains are in a state of disarray while de-magnetized, and are spun into order (lined up) when magnetized, again, friction. And this is repeated 60 times per second.
Another thing worth noting is that there is 200 amps on each conductor which would mean there is 600 Amps of current flow thru the “shell” vs only 200 Amps for the DC wired (+) (-) or 100 wired (+) (-) (+)
DC issue:
Same as above however the current is constant, equal and opposite in polarity, conducted by 2 conductors with a 3rd conductor being used as ??? My guess is unconnected as a grounded conductor would be redundant. Copper is a good thermal conductor so its probably acting as a heat-sink thus lowering the temp by some level. The magnetizing of the "shell" takes place only once and is steady.
RE: Explain heat rise difference 60hz vs. DC
To clarify the setup - the current is daisy-chained through the three conductors in the shell, for both AC and DC. In other words, current goes in on the left end, lead #1, while lead #2 and #3 are joined on the left end. On the right end, lead #1 and #2 are joined and the current returns to the supply on lead #3.
Since my last post, i've experimented some more and found that the AC heat rise remained the same whether the shell was grounded or not. I was hoping, and kind of expecting that grounding the shell would reduce the heating, but it did not.
I also tried holding 2 steel blocks about 4"x4"x2" each, with a 1.5" dia half cutout on each such that they can be held around the shell with only .001 clearance. The blocks pull together with a fairly strong magnetic force when the gap is closed to around .125 inch. The current level drops about 3 amps, indicating that the circuit resistance has gone up, simply by adding steel around the shell.
RE: Explain heat rise difference 60hz vs. DC
Such as brass or aluminium?
Or stainless steel?
RE: Explain heat rise difference 60hz vs. DC
Life is what happens while we're making other plans.
Wally
RE: Explain heat rise difference 60hz vs. DC
Its possible that the way you are testing the assembley is the reason for a good deal of the heating. You mentioned the cables are connected as follows:
(1)----------------(2)
|
(3)----------------(4)
|
(5)----------------(6)
Power supply at point 1 and return path on 6, is this correct?
If so, the looping of the conductors will increase the amount of flux induced in the shell. The shell would be act as a "core" like that of a transformer with AC applied.
Also, your using one-half of a single phase signal for the AC test ( A - N) ? This eliminates any field interaction that would normally take place between conductors, which has a canceling effect on the field.
I believe if you were to test the assembly connected in circuit as if it were actually installed for operation, you would see a decrease in the temp rise. My thinking is that you have a Pin and Sleeve type connector for a 3 phase, 3 wire circuit. With all 3 phase signals present, field interaction will reduce the flux available for induction, thus reducing the amount of induced current into the shell.
This is why electricians never group feeder cables, of like phases, in the same metallic raceway. Its always A-B-C-(N) or A-B-(N). There are some exceptions, but that's for another discussion.
Again, if I am in error, please correct...
RE: Explain heat rise difference 60hz vs. DC
BigWally - will a thin sheet of brass or aluminum under the shell really break the magnetic effect? i can try experiment and i'll let you know.
RE: Explain heat rise difference 60hz vs. DC
Magnetic fields will pass through brass or aluminum.
Eddy current heating only takes place with AC not DC fields as generated by the conductors.
Interesting idea the phase cancelling concept.
RE: Explain heat rise difference 60hz vs. DC
RE: Explain heat rise difference 60hz vs. DC
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.
RE: Explain heat rise difference 60hz vs. DC
Does anyone know an easy way to create a 3 phase test setup for low voltage, high current (160A), testing?
RE: Explain heat rise difference 60hz vs. DC
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:/
ELECTRO-MAGNETICS
http://hy
RE: Explain heat rise difference 60hz vs. DC
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.
RE: Explain heat rise difference 60hz vs. DC
Here's a link explaining induction stoves:
http:/
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.