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
James your post is pretty confusing; not sure what you're asking? The differences between voltmeters may be explained by the higher input impedance of the DVM compared to the analog meter.
You might obtain more sensible results if you connect loads to the transformer secondaries for your measurements; try using low power (~25 to 40 watt) light bulbs.
Sorry Bob, I guess I should of given more of an introduction to the post. A lot of audiophiles are using isolation xfrms for powering their system. My intent of the post is to let the members know how to check the xfmrs for the correct, wired, proper polarity orientation.

Up until today I personally have not used any power conditioners or isolation xfmrs for my equipment. Always just plugged into the wall recepts. Just tonight I am now running the 2KVA xfrm wired 240V primary/ 60V-0V-60V balance secondary, 120V, with my Arcam Alpha 9 CDP connected to it. Sound, pretty good.

And that was the main point of my thread. For those who may be using an isolation xfmr, or xfrms, for their equipment. Imo it is important that they have all the AC polarities the same. Just as the Ac polarity of the audio equipment power supply transformer primary winding, have the same polarity orientation.

Just so I could give you an accurate response to your proposal of using a connected load, a 25 to 40 watt light bulb for the test I did so. I used a lamp with a three way bulb. 50 watts being the lowest.

Primary 246V
Secondary wired balanced 60V-0V-60V (120V nominal)
actual reading with load connected 124.9V

With the center tap lifted from ground:
X4 to ground measured 31.V
X1 to ground measured 145V

X4 being the lowest reading of the two readings will connect to the neutral silver color screw on the receptacle and X1 will connect the hot conductor screw, brass color. Center tap bonded to earth ground.

By the way Bob even with a load connected to the xfmr the cheap Sears Multimeter gave 0 readings for the test. I even turned the lamp to the highest wattage, 150W. The meter would only register the secondary line to line voltage as well as the line to center tap reading. Apparently the digital meter must be of a fairly high resistance, like the Fluke, to do the test. Your thoughts.
Jim
Jea48,
Thanks for the test procedure for polarity of an ISO. Yes, a lot of us are interested in using ISOs on our systems, and this should be useful. I would have assumed that a manufacturer would mark the polarity somehow on the case. I guess not?
It might be helpful if you explained the labling conventions (X and H) to the general audience, especially if these are commonly used across manufacturers.
Also, I am interested in whether you are leaving the secondary floating (no connection between common and ground on the output side) when connected to your CDP? I believe that is the recommended approach in order to maximize noise isolation.
Finally, can you report to us in more detail the differences in sound now that you can listen both with and without the ISO?
>>"I would have assumed that a manufacturer would mark the polarity somehow on the case. I guess not?"<<
[Zargon]
>>>>>>>>

It is important that the user follow the wiring configuration diagram when hooking up the xfmr. Usually the winding leads are marked in some fashion. Most commercially sold transformers sold for power applications are marked with letters and numbers.
Example,
H = high voltage, primary side of the transformer.
X = low voltage, secondary side of the transformer.
Numbers on leads,
if the xfrm only has one primary winding the the leads may be identified by H1 and H2. If the primary has two winding the leads will be identified by H1 and H2 first winding, and H3 and H4 the second winding.

For the secondary of an isolation transformer most power application transformers have two secondary windings. Winding one leads are identified X1 and X2. The second winding X3 and X4.

If the manufactured labeled the windings correctly, and the big boys do, then the correct polarity of the primary to secondary winding should be,
H2 > X2
H4 > X4
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>>"Also, I am interested in whether you are leaving the secondary floating (no connection between common and ground on the output side) when connected to your CDP? I believe that is the recommended approach in order to maximize noise isolation."<<
>>>>>>>>>>>>

No, per NEC the center point must be bonded to earth ground. When using an isolation transformer for power application you are creating "A separately derived AC system." In simple terms you are creating a new electrical service, basically like the one feeding your home. NEC requires all AC power systems shall be AC grounded systems.
So if the secondary is 60/120V or as many call it
60V-0V-60V the center tap of the two windings shall be bonded, connected, to earth ground. What this means is the center tap shall be connected to the grounding electrode system of the main electrical panel system of your home. You can run a separate ground wire all the way back to your main incoming metallic domestic water line and install an aproved water pipe ground clamp there if you want.
Why does NEC require the secondary to be grounded? Safety, in the event of a leak or ground fault from one of the two hot conductors the fault current will travel back to the source, the isolation transformer. If the fault current is large enough it will cause the secondary over current device to open. If there is not overcurrent protection on the secondary the transformer will overload and cause the overcurrent device of the circuit feeding the transformer to open, a breaker in your main panel.
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Phase relationship
http://www.ibiblio.org/obp/electricCircuits/AC/AC_9.html

>>"Finally, can you report to us in more detail the differences in sound now that you can listen both with and without the ISO?"<<

I will try to report back this week

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.
>>"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.
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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?
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.
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!
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.
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Are you going to install and wire this yourself?
Jea48,
Hi, I am an electrical engineer by training and have quite a bit of experience in AC wiring, although not certified. So, I feel confident doing this if I can fully understand the hookup.
The 4kva iso is fed from a 60amp breaker (single phase) in the main panel, and feeds a subpanel, all with #6awg wire. The main panel, iso, and subpanel sit within 2 feet of each other.
The subpanel has dedicated circuits, each having 3 - #10awg solid conductors, with the hot and common twisted and star grounds at the subpanel. The circuits average about 15 feet in length.
The "idle" load on the iso is about 500w, with almost 80% of that being the tube amp. The peak load, worst case, is about 2000w, or less than 50% of the iso rating. If iso loading is a concern, I could move the tube amp and subs outside the iso, but on the same phase.
The normal voltage is 121-122v, and is quite consistent over 24 hours.
I will send you an AC power distribution and conditioning architecture diagram separately via email (and to anyone else who would like to see it).
Below is my response to a member on another forum. I attemped to to speak more in layman terms.
======================

Jon, I have to admit I did a lousy job on my presentation of this thread. I should of kept it simple and to the point. I should of left out all the voltage measurement values. They only confused people, and the issue.
The test is a simple way to check the polarity, phase relationship, of the secondary output of an isolation transformer to the AC grounded system the transformer is fed from. I have used this test procedure for years in commercial and industrial applications. It has never failed.
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Example:

Test procedure.
Primary
I connected the primary of the single phase isolation xfmr to 123V from the electrical panel in my home, connected to (L1) and the neutral. The two primary windings are parallelled, (H1&H3) connected to the Hot. (H2&H4) connected to the neutral.

Secondary
I had to series the two secondary windings to achieve 120V. X2&X3 tied together insulated. X1 and X4 floating with a difference of potential between them of 124Vac.

I measured the AC voltage from each of the two floating secondary leads, X1 and X4, one at a time to the equipment ground or to the line neutral. The lowest AC voltage reading of the two will become the neutral and be bonded to earth ground. The lowest voltage reading was from X4 to equipment ground.

I then bonded X4 to the equipment ground. X4 is now the neutral, the grounded conductor. X1 the Hot conductor. X1 to ground measures 124Vac.
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Now if every thing is correct I should be able to measure from the new X4 neutral to the electrical panel neutral and the reading should be 0. The reading was 0.

Next measurement to take is from new Hot conductor, X1, to L1 of the electrical panel. If the secondary polarity of the isolation transformer is correct, in phase, with the electrical panel L1, the difference of potential should be 0 or very close to it. The difference of potential was 1.8Vac unloaded. Good.

Is this really of any importance?? Probably not if all of the audio connected ic equipment is fed from the output of the xfmr. What if the transformer is only to feed the preamp, and the Amp is plugged directly into the wall recept. Notice I used L1, line one, for my test. The xfmr is fed from L1 and I would assume the power Amp would be connected to L1 from the electrical panel.
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What would the result if I had Bonded X1 to ground instead of X4 of the secondary of the transformer?
I did so,
X1 bonded to ground now the neutral, the grounded conductor.
X4 is now the Hot conductor. X4 measured to ground 124Vac, should be...

Measurement from X1, new neutral, to the line neutral, electrical panel, 0 Vac. Ok.
Measurement from X4, Hot, to electrical panel neutral 124Vac..ok
Measurement from X4, Hot, to L1 of the electrical panel, 248Vac....

Real power?? You bet! I connected a table lamp with the three way bulb set on 150 Watts across the Hot X4 and the panel neutral, it lite full brilliance. I wished I had a couple pigtail lamp sockets on hand, but no such luck. I took the plugs of two lamps and series two of the blades for the next test. With the two 150 watt bulbs in series and a couple of make shift pigtails I connected one end of the series two lamps to the hot X4 of the xfmr secondary, and the other to L1 of the electrical panel. They lit full brilliance.
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I tried to use the most direct way of explaining the test procedures.
I hope it came through that way. My attempt was to explain in layman terms best I could.
The actual test was preformed with two three wire cord and plug connected to the same electrical duplex outlet. The outlet is fed from L1 but the results would of been the same for L2, of the electrical panel. Primary of xfmr was connected from one cord and plug. Test measurements for the electrical panel reference taken from the three leads from the other cord and plug. Polarity, continuity, of both cords and plugs are the same.
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If we have learned from experience that audio equipment that is connected together by ICs should be connected to the same line. Then should not all isolatation transformers have their output polarity, phase relationships, in sync the same? Or at least as close as possible?

I would appreciate your comments Jon.
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>>"It can take just a bit of a load to read near correct voltages, and this is related to how much capacitive coupling there is between the primary and secondary, and how much capacitive imbalance there is between the various windings/taps. Very little capacitive coupling (which usually indicates a higher quality transformer, or one designed specificaly for a low amount of capacitive coupling) require very little loading to get nearly correct voltage readings."<<
>>>>>>>>>>>

Jon as I had stated earlier in a post I did connect the three way lamp set at 150W for a load on the xfmr. The only other heavier load I could think of was a 1200W cloths Iron. One hell of a transformer all I can say. Unloaded the output was 125V. With the Iron connected the voltage dropped to 100V With the secondary windings in series my 2kva xfmr was a 1kva. I quickly took my test readings from the two floating 120V secondary leads though the readings were a little lower when measured to the equipment ground one was high and the other was low. The readings matched the polarity orientation of the xfmr leads.
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Jim
I also posted this thread on AA. Here is the link for the thread and responses I received.
http://www.audioasylum.com/audio/tweaks/messages/131762.html
Jim
I would like to thank Jea48 in public for assisting me off line with a fairly complex, AC system with dedicated lines and multiple isos. His years of electical experience and passion for this hobby, combined with the willingness to share makes him a great AGONNER!
This might help for people getting strange voltage readings.I just bought a 500 watt power inverter for the TV in the camper. The manual states that a true RMS meter must be used to measure the output voltage. If a non RMS meter is used the voltage reading will be 30 to 40 volts lower.
This might help for people getting strange voltage readings.I just bought a 500 watt power inverter for the TV in the camper. The manual states that a true RMS meter must be used to measure the output voltage. If a non RMS meter is used the voltage reading will be 30 to 40 volts lower.
Nerspellsner (System | Threads | Answers)

I had the shop buy me a new "Fluke 87 5 True RMS Multimeter" for somewhat the same reason. The old Fluke would not read the output voltage of a 480V 3ph frequency drive. The new meter will.
I did run a test on checking the proper AC polarity orientation of the primary to secondary polarity of a transfomer and got the same measurements as the old Fluke.