weird DMM ohms readings on transformer

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I came across a weird issue I've never seen before which is baffling.

My scope is down for the count so I can't look at the waveforms coming out  
of the meters which may explain this.

I was measuring the primary resistance of a 10kVA transformer. I tried a  
Fluke 87 V. The meter freaked out and was showing negative OL and all  
sorts of garbage. With autorange on, it was unable to sort of measure the  
approximately 315 ohms. Manual ranging worked, part of the time.

Ok, time for a new battery. Still no good.

weird, the meter must be blown out. Next I tried the known good Fluke 73  
series 2. Same problem. It can't autorange at all, but will sometimes read  
from 200 to 300 ohms if manually ranged, and if you let the meter sit for  
about 10 seconds.

Any other transformer or anything with resistance works can be read fine.

Now it's time for the HP 34401. No problem, the X1-X2 resistance in 315  
ohms, in autorange or manual mode.

So is the resistance range on a DMM really some sort of AC signal?  

I tried the same test with the H1-H2 terminals shorted out. Neither Fluke  
meter can autorange, but if set manually, they show about 600Ohms which  
then drops to about 310-320ish after a while.

The HP meter shows the same.

Without the scope, I'm guessing the ohms range is not really putting out a  
clean DC current, or there's some interference going on with the  
integrator in these meters and whatever the inductance is of this one  
transformer.

Has anybody come across this before? It's never crossed my mind to second  
guess ohm readings on a DMM as long as the DUT is off.




Re: weird DMM ohms readings on transformer
On Fri, 27 Dec 2013 09:37:35 +0000, Cydrome Leader wrote:

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Yup, seen it. Try shorting the secondary before measuring the primary DC
resistance. That should get rid of all but leakage inductance.

It's down to rate of current rise versus integrating time.

--  
"Design is the reverse of analysis"
                   (R.D. Middlebrook)

Re: weird DMM ohms readings on transformer
Fred Abse wrote:

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Why would a 10 KVA transformer have 300 Ohms resistance?  That is
WAY too high!  It should be well less than one Ohm, I'd think, unless
this is for UHV transmission lines (and they don't mess with tens of KVA
on those).

Jon

Re: weird DMM ohms readings on transformer
On Fri, 27 Dec 2013 14:22:09 -0600, Jon Elson wrote:

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I didn't say it should. You attributed someone else's quote to me.

--  
"Design is the reverse of analysis"
                   (R.D. Middlebrook)

Re: weird DMM ohms readings on transformer
Fred Abse wrote:

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Sorry, it was the OP (cydrome leader) who gave that very odd
reading.

Jon

Re: weird DMM ohms readings on transformer
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It's the 14.4kV winding in a distribution transformer so several hundred  
sounds ok, but doesn't quite jive with the %Z rating unless I'm doing  
something wrong with the math

What's a tech if the field supposed to do when they hit the "improbable"
combination of something their meter just doesn't work with? Heck, in all  
the fluke sales stuff it's a man in a raincoat probing some sort of motor  
control at an oil refinery or something rugged sounding.

I'm now wondering if there are magic capacitances that cause bogus results  
or intense confusion with these meters.

Cameras sometimes have published "grey zones" where metering or certain  
modes functions just don't work as expected. They should have these for  
test equipment as well.



Re: weird DMM ohms readings on transformer
Cydrome Leader wrote:


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OK, that would have a LOT of inductance, then.  I imagine many
digital meters may screw up with that big an inductor.  Probably
any D'Arsonval meter should do a much better job.

Jon

Re: weird DMM ohms readings on transformer
On Fri, 27 Dec 2013 22:46:29 +0000, Cydrome Leader wrote:

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Back in the day, the most frequent howler was attempting to set the
thyristor-controlled field current (5 to 10 amps), of a big motor
controller, using a DMM. People in the know would have a moving-coil, or
moving iron, ammeter just for that job. The man in the raincoat would blow
expensive fuses.

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My Fluke 87V gives really silly readings on capacitance, with capacitors
which have seriously deteriorated ESR. It applies a known charging
current, and measures time to charge. It's capacitance function isn't of
much use.

Knowing how your instruments work, and what they *actually* measure is
essential.

In your case, with that particular transformer, I'd use a DC PSU, a
voltmeter, and an ammeter. I'd short the secondary, too, to reduce the
nasty inductive spike when disconnecting the setup.

--  
"Design is the reverse of analysis"
                   (R.D. Middlebrook)

Re: weird DMM ohms readings on transformer
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I need to rescue a Simpson meter. I'm no old timer, but I'm quite  
skeptical of these digital meters. There's apparently so many weird things  
that they don't cope with. I've "discovered" problems with each and every  
one I've used. Analog meters just seem to lack the surprises.

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It really is.  

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I don't doubt any analog meter ever made would have no problems either.

It's just sort of sad that what should be a simple test can still require  
a lab style setup to get a reading.

I should try some power supplies in constant current mode on this thing to  
see if any get confused due to their switching power supply and whatever  
microcontrollers runs the front panel and some magic resonant frequency  
that causes self destruction.

Once I ran into a bizarre resonant condition with an induction motor and a  
variac. At at certain voltage (or maybe motor speed) the variac saturated.  
This was light dimming, horrible buzzing and a blown fuse, but only at a  
certain dial position.




Re: weird DMM ohms readings on transformer
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Shorting the secondary before taking a reading seems to belp, but it still  
takes over a minute for the reading to settle down. The lower ohms ranges  
settle much faster than the higher ones.

I need to replace the fuses in the analog meter and see how fast it  
settles. I'm going to guess it runs at a higher voltage and will build up  
the magnetic field faster than whatever these digital meters are doing.

What other goofy effects have you seen with these other instruments?

Re: weird DMM ohms readings on transformer
Cydrome Leader wrote:


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I have a couple Protek meters that I really like.  But, the autoranging
can sometimes screw up quite massively.  One time, I was checking the
+5 V on a board with some logic.  The meter gave an astounding reading
of 125 V or thereabouts!  I knew this had to be wildly incorrect as
the circuit was mostly working, rather than exploding in flames.
I locked the range and it gave a somewhat more reasonable reading, but
still quite off.  I used another meter and got 5 V on the nose.
Well, the fact the Protek went crazy on this circuit told me there
might be some high frequency noise on the +5, so I added some caps.

That't the most ridiculous reading I've ever gotten from a DVM.

Jon

Re: weird DMM ohms readings on transformer
Jon Elson wrote:

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Come back AVO8, all is forgiven !


Re: weird DMM ohms readings on transformer
On Sun, 29 Dec 2013 11:21:06 +0000, Baron wrote:

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I still have one, and it's still within calibration tolerance.

--  
"Design is the reverse of analysis"
                   (R.D. Middlebrook)

Re: weird DMM ohms readings on transformer
Fred Abse scribbled thus:

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Yup. There's a reason that they are/were industry standards.
If I remember correctly the model 7 was the low Z one, 200 OPV.

--  
Best Regards:
                     Baron.

Re: weird DMM ohms readings on transformer
On Sun, 29 Dec 2013 22:06:01 +0000, Baron wrote:

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Never had a 7. ISTR they were 1000 OPV. Certainly too low for electronics.

I was fortunate enough to get an 8 MKIII, on a trip to England. Bakelite
case, stud-and-leaf switches, and tropicalized resistors. Later models had
thick film resistors and printed circuit switches.

--  
"Design is the reverse of analysis"
                   (R.D. Middlebrook)

Re: weird DMM ohms readings on transformer
Fred Abse scribbled thus:

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Hi Fred, Happy new year.
I used to have a model 7 until it got borrowed.  Currently I have two
Model 8, a MKII and MKIII.  I did have an ex mill one in a leather
case, that one got stolen when I was burgled.  The swine also took my
scope and spectrum analyser along with a lot of other gear.

Never did get any joy from the police, but that's another story.

--  
Best Regards:
             Baron.

Re: weird DMM ohms readings on transformer
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That's pretty impressive. Did it give equally amazing AC readings too?

Re: weird DMM ohms readings on transformer
Cydrome Leader wrote:

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No, I think the AC mode has a low frequency cutoff, and this was likely
very high frequency noise.

Jon

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