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Basic motor question

  1. Sep 4, 2007 #1
    I am trying to find what are the V and I of a leg for a 208/230V 3ph 60hz motor at 40 A???

  2. jcsd
  3. Sep 5, 2007 #2
    What do you mean by leg? Do you mean just one phase?
  4. Sep 5, 2007 #3


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    Use a voltmeter to measure voltage from the "leg"/phase to ground. Use a clamp-on meter to measure the current in that leg. That's it.
  5. Sep 5, 2007 #4
    ok got it.

    I figure it out to be 120v 40A
  6. Sep 6, 2007 #5
    hmm i thought the idea of using 3 phase is to reduce 'stress' or current draw on each phase.
    For example if you want to draw 100A, you be better of using 3 phase so each phase draws less current (but all adds up the 100A) rather than using 120V 1phase and drawing 100A on that line and having to use a rather large gauge of wire
  7. Sep 6, 2007 #6


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    208/230 means you can run it at any voltage in that range, so it seems to me like you are asking something that you have to read yourself with a multimeter (or check the drawings or wiring to see how it is wired).

    Ie, if you have 208V, 3ph, then you get 120V per phase. If it is 240V, 3ph, you get 138V.

    You are right that amperage is amperage, though.
  8. Sep 8, 2007 #7
    Hi, first post and after viewing this thread I thougt that I would add what I have learned from working in the field and self taught. The initial thread question sounded like a question I would have asked 15 years ago. Sharper minds will most certainly correct any mistakes I make and three phase motors were new to me when I entered the industrial electrical field.

    1.) 208VAC is between any two legs (phases) in a three phase system from a wye/wye transformer providing system power and from any leg to ground you have 120VAC.
    2.) 230VAC is from a wye/delta transformer but it can get sticky describing voltage references to ground with a delta secondary and will not go into this at this time.
    3.) By 40A I assume you mean Full Load Amps from the motor data plate and this is the current the motor pulls when it is operating at designed horsepower with a service factor of 1.0.
    4.) When power is first applied to any three phase motor it pulls 5 to 7 times rated FLA (200-280amps for a FLA of 40A) for a couple of seconds until the rotor rises to motor data plate RPM. It does this coupled or uncoupled to the load and falls rapidly off as the motor comes up to speed and this is called "slip". This is for DOL (direct on line) motor starting and not from a VFD (variable frequency device). The motor starter heaters and fuses are designed to take into account the high current when starting.
    5.) In a properly working three phase motor the current will be equal on all three legs and total motor current is measured from one leg only. When the motor is up to data plate RPM there must and should be less than 10% deviation between the measured current on each of the three legs.
    6.) If a motor with a FLA of 40A and a service factor of 1.0 is pulling over 40A when in operation then it does not produce any more than designed horsepower and the excess energy is disapated as heat. Not a good thing that can burn up the motor but the motor protection devices are supposed to prevent this by opening the circuit.

    I'll give that for starters and feel free to ask any questions in depth but I am only describing things from a pratical viewpoint.
  9. Sep 9, 2007 #8


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    "...but I am only describing things from a pratical viewpoint."

    And a good viewpoint at that. Thanks for sharing your knowledge.
  10. Sep 13, 2007 #9
    As a quick answer, if you use 60HZ power supply, you will find 208/1.73 V and 40A in star connection motor in each leg .Also you will find 208 V and 40/1.73 A in triangle connection motor in each windings.


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  11. Sep 15, 2007 #10
    So far I think itstoast gave the best answer. I would like to add this to help clarify.

    All motors should have a nameplate with all the information you requested. There should be information for the voltages, FLA or RLA (Full Load Amps or Running Load Amps), and LRA (Locked Rotor Amps) for each voltage the motor can use.

    Measure voltage between any two legs (ie. ABv, BCv, CAv). Measure amps with an amp meter clamped around any one leg (Ai, Bi, Ci). If the motor normally runs at 240V, 40A then if you apply 208V instead, the motor will draw slightly over 46A. Think watts (or Volt-Amps).

    Measure voltage leg to leg (not leg to ground) for a three phase motor.
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