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weird DMM ohms readings on transformer

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Cydrome Leader
Guest

Fri Dec 27, 2013 11:37 am   



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.

Fred Abse
Guest

Fri Dec 27, 2013 10:46 pm   



On Fri, 27 Dec 2013 09:37:35 +0000, Cydrome Leader wrote:

Quote:
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.


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)

Cydrome Leader
Guest

Sat Dec 28, 2013 12:46 am   



Jon Elson <jmelson_at_wustl.edu> wrote:
Quote:
Fred Abse wrote:

On Fri, 27 Dec 2013 09:37:35 +0000, Cydrome Leader wrote:

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.

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).


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.

Cydrome Leader
Guest

Sat Dec 28, 2013 12:55 am   



Fred Abse <excretatauris_at_invalid.invalid> wrote:
Quote:
On Fri, 27 Dec 2013 09:37:35 +0000, Cydrome Leader wrote:

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.

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.


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?

Jon Elson
Guest

Sat Dec 28, 2013 3:22 am   



Fred Abse wrote:

Quote:
On Fri, 27 Dec 2013 09:37:35 +0000, Cydrome Leader wrote:

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.


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

Fred Abse
Guest

Sat Dec 28, 2013 3:26 am   



On Fri, 27 Dec 2013 14:22:09 -0600, Jon Elson wrote:

Quote:
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).


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

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

Jon Elson
Guest

Sat Dec 28, 2013 5:49 am   



Fred Abse wrote:

Quote:
On Fri, 27 Dec 2013 14:22:09 -0600, Jon Elson wrote:

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).

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

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

Jon

Jon Elson
Guest

Sat Dec 28, 2013 5:59 am   



Cydrome Leader wrote:


Quote:
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

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

Fred Abse
Guest

Sun Dec 29, 2013 12:13 am   



On Fri, 27 Dec 2013 22:46:29 +0000, Cydrome Leader wrote:

Quote:
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.


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.

Quote:

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


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)

Jon Elson
Guest

Sun Dec 29, 2013 8:30 am   



Cydrome Leader wrote:


Quote:

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

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

Baron
Guest

Sun Dec 29, 2013 6:21 pm   



Jon Elson wrote:

Quote:
Cydrome Leader wrote:



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


Come back AVO8, all is forgiven !

Fred Abse
Guest

Sun Dec 29, 2013 10:08 pm   



On Sun, 29 Dec 2013 11:21:06 +0000, Baron wrote:

> Come back AVO8, all is forgiven !

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

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

Baron
Guest

Mon Dec 30, 2013 5:06 am   



Fred Abse scribbled thus:

Quote:
On Sun, 29 Dec 2013 11:21:06 +0000, Baron wrote:

Come back AVO8, all is forgiven !

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


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.

Cydrome Leader
Guest

Mon Dec 30, 2013 9:36 pm   



Fred Abse <excretatauris_at_invalid.invalid> wrote:
Quote:
On Fri, 27 Dec 2013 22:46:29 +0000, Cydrome Leader wrote:

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.

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.


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.

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

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.


It really is.

Quote:
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.


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.

Cydrome Leader
Guest

Mon Dec 30, 2013 9:40 pm   



Jon Elson <elson_at_pico-systems.com> wrote:
Quote:
Cydrome Leader wrote:



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


That's pretty impressive. Did it give equally amazing AC readings too?

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