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.
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
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