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Thread: 3 Phase Voltage Problem

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    3 Phase Voltage Problem

    Got a problem with a 3 phase voltage machine using 2 compressors for heating. When the isolator is in the open position - that is off :-) I get the normal phase to phase voltage of 400V. As soon as I switch the isolator on the machine's phase failure prevents the machine from turning on as the voltage is not the same across the phases. When I measure I get 500 on one then 270 on another. These are really weird voltages I'm getting. I have swapped the phase conductors around which did not help. Could the phase failure be faulty? Any help would be awesome!

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    Diamond Member AndyD's Avatar
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    If you're getting 500v across 2 of the phases and only 270 across another two then it sounds like you're missing a phase.

    Test from each phase to an earth at the supply side of the isolator with it in the off position you should get 220-230volts. Disconnect the load and repeat the test on the load side of the isolator with it switched on, you should get the same results.

    Be aware if you're using a digital tester you might get odd readings on the missing phase due to induced or capacitively coupled voltages.
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    Gold Member Sparks's Avatar
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    Few years ago I came acrross an old 2ph+N supply. Even the Boss man at the monkeypality was surprised. Very old supply which was never changed as the property stayed in the family all the years.

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    Hi Guys thanks for the replies...nice to know there are fellow sparkies out there who are prepared to share their knowledge and experience! It was a the problem was with the neutral not being connected to the correct neutral bar. The machine was connected to a really old Panel with some "shockingly" bad labelling but eventually found the correct bar and bobs your uncle.

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    Diamond Member AndyD's Avatar
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    Thanks for letting us know BlueArc, it's always great to get the ending to the story and I'm happy to hear it was something relatively simple and easily fixable.
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