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  1. #1
    Rick Rowlands's Avatar
    Rick Rowlands is offline Titanium
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    Can a slip ring 3 phase motor be used to generate 3 phase power? I ask this because I have a 10KW single phase generator head that looks remarkably like a slip ring motor, except it has two slip rings instead of three.

  2. #2
    JST's Avatar
    JST
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    A wound rotor (synchronous) 3 phase motor can be a stand-alone generator.

    For that matter, a regular induction motor can be a stand-alone generator.

    Not knowing exactly what you have, I'd have to say "very likely it can" be a generator.

  3. #3
    jim rozen is offline Diamond
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    Your single phase unit probably has the
    field winding on the rotor. The slip rings
    are probably passing DC to the field, which
    rotates.

    That's a more modern design than having the
    AC power output via slip rings.

    Jim

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    Jraef's Avatar
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    Slip rings are not alone enough to identify what you have. Slip rings in AC motors can mean either a Wound Rotor motor or a Synchronous motor. There is essentially no difference between a synchronous motor and a synchronous generator other than the direction of power flow and work performed.

    A Wound Rotor motor however can only be used as an induction generator in the same way as any standard induction motor can. The slip rings are irrelevant in that case. Induction generators need to be connected to the grid in order to function, so they are not used as what is refered to as "island" generators. That means they are generally unsuitable for off-grid living because without being connected to the grid, there is no source for exciting the magnetic fields necessary to facilitate generation. A synch. Generator/motor can have an external DC supply, i.e. battery, for the field excitation power through the slip rings and make power at any time the rotor turns faster than synch. speed.

  5. #5
    JST's Avatar
    JST
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    Induction generators need to be connected to the grid in order to function, so they are not used as what is refered to as "island" generators. That means they are generally unsuitable for off-grid living because without being connected to the grid, there is no source for exciting the magnetic fields necessary to facilitate generation.
    Not actually true.

    They are being used in many places due to the low expense of the motor vs the generator.

    For starting, it is only necessary to have some residual magnetism in the structure. A reactive load, typically capacitors, helps to maintain generation at low loading.

    I know for a fact that this is true, as I have set up and demonstrated single and multi-phase versions with no connection to the grid voltages. There are also government reports on the use of motors in this way, replete with techie stuff on optimal loading, etc.

  6. #6
    jim rozen is offline Diamond
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    There was at one time an *excellent* practical
    discussion of this effect, a web site put up
    by a ham radio operator, who discovered that
    an excellent field day generator could be
    made from a lawnmower engine and a large
    single phase motor. Basically you put a
    capacitor across the motor, and it does have
    to have some residual field like you say.

    Other than that, the only caveat is never start
    or stop the motor with electrical loads connected.

    Seemed like an inexpensive way to get on the
    air.

    Jim

  7. #7
    Jraef's Avatar
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    Yes it is possible, but most average users will find it difficult to implement unless they understand residual magnetism and capacitor excitation, that's why I didn't mention it. There are a couple of articles on implementing that scheme on the web, but they too will admit it doesn't work in all cases because it's dependent upon the amount of residual magnetism in the rotor, which varies from motor mfr. to mfr. I just thought this explanation would detract from the discussion of his slip ring motor. Maybe I was wrong.

  8. #8
    Rick Rowlands's Avatar
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    How can I tell the difference between a wound rotor motor and a synchronous? The motor I have is 5 hp, 1140 RPM. It has the standard 9 wires for the field and three wires from the slip rings. In addition to the HV LV wiring diagram is a Y diagram.

    I need enough 240V 3 phase to run a 2 hp hoist motor and I can use my Lister 6/1 to drive the generator. If the slip rings are irrelevant and it will operate with residual magnetism thats fine. What else might I have to do to it?

    Jraef my questions may indicate that I don't understand electricity but I certainly do. Whenever I start exploring another avenue of electricity or engineering I always ask a bunch of questions. If you have a couple of websites that might shed some light on AC generation please share them. Thanks.

  9. #9
    Jraef's Avatar
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    If it's a 3 phase motor and you have 3 slip rings, it's a Wound Rotor motor. A Synchronous would only have 2 slip rings because you would apply DC to it.

    As to sites, Google "self-excited induction generator" and start perusing the 187,000 odd sites it finds. You can narrow it a bit by adding "wound rotor".

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