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Isolation xfmr correct polarity orientation test

I pulled out an old single phase 2KVA xfmr I have. I thought I would do some bench testing for the correct primary to secondary polarity orientation of the windings. I was especially curious to see if the primary was connected to 240 volt if the secondary would read equal AC voltage from each ungrounded lead to ground as I thought.

The Xfmr, transformer.
2KVA
Dual primary windings.
Dual secondary windings.
Data plate on xfmr:
Primary 240V/480V
Secondary 120V/240V

I first connected the primary to 120V. (Parallelled the primary windings). (Hot H1&H3), (Neutral H2&H4) Line voltage 123V.
I configured the secondary windings for a balanced output. X2&X3 tied together but left floating from earth ground for the test. I energized the xfmr and measured the secondary output voltage across X1 and X4, 124.7 VAC. I need to stop here and mention the meter I first used for the test. My Fluke 23 series Multimeter was at work so I used a cheapo Sears Digital Multimter #982015 I had at home. So for the polarity test. I first touched one meter probe from earth ground to X4. This should be the correct polarity as H2&H4 that is connected to the neutral from the source 120V. This reading should be the lower of the readings if correct. The reading was, 0. I them measured from X1 to earth ground, again 0. What did I do wrong. Check earth ground being used, was ok. Double check again, Still 0. This can't be I have done this test several times in the past. Darned if I did not try again, still 0 from each secondary lead when measured to earth ground. Note here you do know when I refer to earth ground it is the equipment ground. But really the measurement is being made back to the source grounded conductor the neutral. Sorry, back to the test. What was going on. I was puzzled to say the least. What was different from any other time I had checked for the proper polarity orientation of commercial power xfmrs. The cheap multimeter.

To make a long sory short I went and got my Fluke meter. Energized the xfmr measured the voltage from X1 to X4, 125.3 Vac. I then measured from earth ground to X4, 50.5V. I then measured from X1 to earth ground, 163.2V. I should mention here as all of you may know this is not a usable AC voltage. Will it bit? Well I for one did not try it....

The 240V primary test.
For this test I connected the two primary windings in series, H2&H3 connected together. 240Vac source connected to H1 and H4 repectively. My power runs about 246VAC as measured at the time of the test.

Secondary wired the same as the first test.
Output secondary voltage, 125.2Vac.
X4 to earth ground reading, 36.6V.
X1 to earth ground reading, 149.8V.

I was surprised these two reading were not closer. I do not have an explanation why.

In either of the two tests when I bonded the center tap of the secondary to ground, X2&X3 the voltage from X1 and X4 repectively to ground was 60V nominal.

Final test.
Primary wired for 240V. windings parallelled.
Secondary 120Vac with secondary windings parallelled. Full 2KVA rating of the xfmr.
(X1&X3), (X2&X4) X1 will be the hot ungrounded conductor. X4 the grounded conductor,neutral.
X2&X4 is left floating for the test.
Primary energized 247Vac.
X4 to earth ground measured 21Vac
X1 to earth ground measured 135Vac
X4 has the lowest AC voltage reading to ground. This will be the secondary lead that will bond to earth ground.
Jim

jea48
herman
1,974 posts
 
I agree that the post is confusing but something like this begs for drawings to help us visualize the set up. Since Audiogon won't let us post pictures then it is probably as good as it gets.

If the secondary is floating i.e. nothing on the secondary is conected to earth ground, and you measure from ground to a secondary terminal, then this reading will not make any sense.

Your meter measures the difference in potential between it's leads. You can't get a meaningful measurement between 2 points that don't share a common reference point. Since the floating secondary is isolated from the ground they do not have a common reference and any measurement is invalid.

You proved this by measuring with the secondary center tap grounded and you got what you expected, but got nonsense when it was floating. The difference in the meters can probably be explained by their impedance as suggested above.
jea48 OP
2,944 posts
 
>>"Your meter measures the difference in potential between it's leads. You can't get a meaningful measurement between 2 points that don't share a common reference point. Since the floating secondary is isolated from the ground they do not have a common reference and any measurement is invalid.<<"
>>>>>>
Thats funny Herman, I and many other electricians have used this test to find the correct phasing orientation of secondary windings of transformers for years. I have used the procedure where winding lead identication are missing. The test is reliable. Don't knock it till you try it.
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>>"Since the floating secondary is isolated from the ground they do not have a common reference and any measurement is invalid.<<"
>>>>>>>>

AHhh, but how about the grounded AC system that is feeding the primary of the isolated transformer? And that difference of potential that exists and can be measured when the floating secondary leads, is due to capacitive coupling between the primary and secondary windings. The fact is the test will match the actual polarity, phase, identifications that are known for the transformer every single time.
==
Herman have you ever checked for the proper AC polarity orientation of a preamp or power amp power supply transformer? You are preforming the same test. The only difference is the secondary of the transformer is fixed so if it is reversed you change the primary. Correct?
herman
1,974 posts
 
I guess I got lost in your post. I thought you wanted to know why you didn't measure 60 - 0 - 60 referenced to ground when it was floating.
zargon's avatar
zargon
453 posts
 
I have a 4.2kva Topaz iso to connect into my system. Here is the information off the plate. I have inserted periods to separate the 2 colums.

INPUT SIDE................OUTPUT SIDE
126v H4 dot................120v X2 dot
120v H3...................
114v H2...................
000v H1......................X1
S1.............................S2
CHASSIS...................CHASSIS

The input side has a chassis lug, an S1 lug (this is the shield covering the primary winding), an H1 lug (for common), and H2-H4 lugs (hot lugs which allow one to adjust for a slightly high or low input voltage to get 120v on the output). H4 has a polarity dot.

The output side has a CHASSIS lug, an S2 lug (for the shield covering the secondary winding), an X1 lug (for common), and an X2 lug (hot). X2 has a polarity dot.

The question is: "How should this be wired?"

On the INPUT side, I would assume that the CHASSIS is grounded, and that common connects to H1 (which is already grounded at the sub pannel), and that hot goes to H3 (in my case).

On the OUTPUT side, I would assume the ground wire from the outlets goes to CHASSIS, X1 goes to the outlet common, and X2 goes to the outlet hot.

Where do the shields get connected?

I have been advised that the secondary common, X1, is left floating or it will compromise the iso's ability to isolate and filter out the garbage.

Any assistance here would be appreciated!
jea48 OP
2,944 posts
 
Could I ask you a few questions?

Sub panel?
Is this for the output of the iso unit? Or the input?

4.2kva at 120V input.
Xfmr is good for 35 amps at 120V.


Wire size to feed the xfmr. 125% of 35A = 43.75A... #6awg min. What is the total distance from the feed panel to the xfmr, if it is not being fed from the subpanel?

Does the unit have it's own primary safety disconnecting means? Is the unit insite of the panel it will be fed from?

What is the nominal Ac 120 voltage measured at a kitchen receptacle? 115V, 118V, 120V, 125V, ?

If the xfmr is going to be fed from the subpanel.
How big is the wire size feeding the sub panel?
How far is the sub panel from the main electrical panel?

Aprox how much load, amps, watts, va, will you be connecting to the xfmr?

If the unit is to be fed from the sub panel, does the xfmr unit have it's own individual overcurrent devices, breakers or fuses, on the output to protect it from the loads to be connected to it?

>>"I have been advised that the secondary common, X1, is left floating or it will compromise the iso's ability to isolate and filter out the garbage."<<
>>>>

Not by any licensed electrician.
===
Are you going to install and wire this yourself?
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