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Single phase generator problem

  1. Jan 4, 2017 #1
    A friend has a ?40-50 kVA, 240 V, single phase generator, of 1970s vintage, coupled directly to an engine. It was working fine but now produces no power at all. All the brushes and spring holders are fine, as are the fuses. I can't find a decent description/diagram of the internals, other than very basic animations of the principles of AC generation. Does anyone have any tips on what to check, or where to look for information?

    Within the generator body is a large capacitor, a 40A bridge rectifier (which is good) and four heavy-wound coils (?tranformers). It's hard to see what goes where as the wires are hard to get to and covered in dust. Also, there's a button marked 'excite' which seems to make no difference, although I've metered it and it works as a SPST switch shorting two wires together. What are all these bits for? I did clean the excite switch once and got the generator working again, but only briefly, and I suspect it was just a coincidence.
  2. jcsd
  3. Jan 4, 2017 #2


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    Staff: Mentor

    You might attract the attention of readers who have dabbled with these if you attach a photo or two of the generator's internals. :smile:
  4. Jan 4, 2017 #3


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    Is this a self exciting generator? In other words, does it rely on the residual magnetism to build up enough of an output to provide current to the field? Is there provisions for a battery? I would also assume there is some type of voltage regulation on a unit of this size.
  5. Jan 4, 2017 #4

    jim hardy

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    Have you checked the capacitor?
    Use an ohm-meter set to high scale at least RX1K . You should see an initial low reading that quickly increases to infinite, and reversing the leads should give a reading that's initially negative then increases to infinite.

    That's because the ohm meter charges the capacitor to a volt or two.

    The capacitor should hold that charge for several minutes - check this way... When reading reaches infinite remove one meter lead, wait some time and reconnect it. Reading should remain infinite, not dip.
    That's a "Poor Man's Capacitor Check" .

    Really it sounds like you have a broken wire that's intermittent. Examine them carefully anyplace they might flex in the wind, and right where they attach to something rigid. Mechanical things subject to vibration tend to fail where there's a change of cross section.
  6. Jan 4, 2017 #5
    I'm guessing it's self-excited, as there's no provision for a battery. The voltage output appears fairly well coupled to engine speed, i.e. Not regulated, but I'm not sure. It's an old generator.
  7. Jan 4, 2017 #6
    I didn't check the cap, but my meter does measure capacitance. I'll do that as well as your suggestion. The rectifier appears to take in 140V AC and put out about 100V DC. Would it be reasonable to assume the rectifier and cap are to provide DC for the field coils?
  8. Jan 4, 2017 #7
    Oh, and thanks for your replies.
  9. Jan 4, 2017 #8

    jim hardy

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    Yes. Reasonable but not guaranteed. Rectifier could be overvoltage protection for the field.

    At standstill, try re-magnetizing the field with perhaps a lantern battery or car battery charger between the brushes.... just touch it. If you get a spark on disconnect that's good, you have inductance.

    We had a 150 kva generator that we had to occasionally re-magnetize that way . We had access to field terminals at a cabinet so did not have to get near moving parts. A 9v transistor radio battery did the job.

    <<Use common sense precautions around rotating machinery - no long sleeve shirt, no necktie, orient hands so contact will push fingers free not pull them in.>>>
  10. Jan 4, 2017 #9
    I can access the rectifier quite safely - could I connect a car battery + and - to the recifier + and - and remagnetise that way?
  11. Jan 4, 2017 #10

    jim hardy

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    That should work. Of course observe + to + . Just touch the wires for if it's running it should come up to voltage quickly.
  12. Jan 4, 2017 #11

    jim hardy

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    Try it first at standstill, of course.

    Do you have pictures ? Just get them on your computer and use "upload" button.
    I use Paint to resize them about 600 pixels wide and save as jpg .

    Curious what that "excite" button is for.
  13. Jan 4, 2017 #12
    I'll take some next time I'm at my mate's place. The excite button has two wires connected to it with a resistance of 9.5 ohms between them. It shorts these together when pressed.
  14. May 26, 2017 #13
    The generator is sorted - we stripped it down to reveal a hidden area with various boxes and terminals which I assume was some sort of automatic voltage regulation kit. Amongst the components was a double capacitor unit with two taps and a common ground which was shorted. I replaced this (decoupler?), and we cleaned up all the terminals, and the genny has run without missing a beat since.

    I guess the cap had nothing to do with the fault, as the rig would work sometimes and sometimes not - often the signature of a poor connection as suggested.

    Anyway, we celebrated with a nip or two of Balvenie single malt from the local distillery and enjoyed the feeling of, "refer servicing to authorised personnel only - my arse! (Or ass if you prefer)".

    Thanks for all your advice.
  15. May 26, 2017 #14

    jim hardy

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    I enjoy that feeling from two perspectives.
    1. Beating the system. Or if you prefer, "Civil Disobedience"* style protest of today's 'Throwaway Paradigm' ", which keeps consumers in perpetual debt.

    2. You breathed new life into an old machine of which once upon a time somebody was very proud. See @dlgoff's signature...

    *"Civil Disobedience" may not be an everyday phrase in UK - Henry Thoreau was an eccentric American writer of 19th century whose essays still inspire modern day protesters. I think he was quietly removed from US high school curricula in 1970's.

    old jim
    Last edited: May 26, 2017
  16. May 26, 2017 #15


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    I know that's right. :approve:
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