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Mystery Motor, 7.5 or 10 hp?

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  • Mystery Motor, 7.5 or 10 hp?

    I have a unmarked motor that I using to make my RPC with.
    The motor was on a commercial RPC, so it has the shaft cut off and no label on it.

    Anyway it figure out if it is a 7.5 or 10 hp?
    Wire size?
    Amp draw?


  • #2
    Haven't a clue but HP is in part a thermal rating for the motor. How much current will the windings pass before they toast? From that, I'd solve the problem by connecting one leg to a DC welder with continuus current control and adjusting Amps upward until you get 30 degrees F over ambientafter 20 minutes. Note: only one part of the stator will warm up. Note the measured current by actual Ammeter - NOT what it says on the welder..

    1 HP in most 3 phase motors is about 3 Amps per HP. 7 1/2 HP will draw about 23 Amps. 10 HP about 30. There are LOTS of niggling factors that could influence the 3 Amps per HP rule of thumb figure and even more about the 30 degree F temp rise. YMMV

    Also I seem to recall that some idler motors purpose built for phase converters were not symmetrically wound. The line leg winding was a lttle different from the two windings connected to the generated leg. Check winding resistance (or better impedence). The Line leg will show one reading the other two may be a little different. How much and which way I wouldn't know. It may be just a plain old motor but it woldn't hurt to check.
    Last edited by Forrest Addy; 05-01-2012, 03:05 PM.