Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electrical Consumption of Machine

  1. Dec 3, 2014 #1
    I need to know how much electricity a machine uses in kW please.

    On the machine information plate it states: 460 volts, 60 Hz, 161 amps, and it is a 3 phase.

    I took the readings for the volts and amps:
    Phase 1- 271 volts & 37.68 Amps
    Phase 2- 271 volts & 37.62 Amps
    Phase 3- 273 volts & 38 Amps

    I cannot figure out the Power Factor to apply to the formula I am using:
    kW= (Volts x Amps x Power Factor x Sqrt 3) / 1,000

    I know Power Factor = Real Power/ Apparent Power, however I cannot figure out the real power. I believe the apparent power is (460 x 161 x Sqrt 3) / 1,000 = 128.27 kVA?

    Could I just average the voltage readings and then average the amperage readings for the 3 phases and then do (Avg volt x Avg amp x Sqrt 3) / 1,000 to get the true/real power without using the Power Factor?

    Are my assumptions correct? Can someone please help me calculate the electrical consumption please! Thank you
     
  2. jcsd
  3. Dec 3, 2014 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Welcome to PF!

    You're pretty close: you don't have a measurement of power factor, so the best you can do is estimate. If you describe the application, we can help with that. If this is a 1/4 loaded motor, I'd use 60%. Otherwise, your calculation is fine.
     
  4. Dec 3, 2014 #3
    The machine in question is an industrial parts washer. How would I find out if it is a 1/4 loaded motor? When you say my calculation is fine, are you referring to using the nameplate amps x volts or the avg amps x avg volts readings that I got with the device?
     
  5. Dec 3, 2014 #4
    I'm afraid not. You need to know or measure power factor.
     
  6. Dec 3, 2014 #5
    How would I measure the power factor with the given information? Or do I need additional information? I have the voltage and amps for each phase and the kVA.
     
  7. Dec 3, 2014 #6

    russ_watters

    User Avatar

    Staff: Mentor

    The readings you took are the actual load and they were about 1/4 of the nameplate. So that's the second calculation.

    Also, when you say a parts washer, that makes the load pumps and a conveyor? Or is there electric heat as wel? And do any of the motors have VFDs?
     
  8. Dec 3, 2014 #7

    russ_watters

    User Avatar

    Staff: Mentor

    Measuring power factor is done with a power meter.
     
  9. Dec 3, 2014 #8
    You can measure/determine it with cos(φ)-meter or clamp on-power meter or any kind of wattmeter . To calculate/estimate it you should now what motor exactly you have. So, nothing about cos φn on the nameplate?
     
  10. Dec 3, 2014 #9

    russ_watters

    User Avatar

    Staff: Mentor

    For most purposes you can estimate it close enough for a useful answer, though it does depend on the required precision.
     
  11. Dec 3, 2014 #10
    I do not know if the motor has VFDs, and the parts washer is basically a big conveyor belt system. I believe there is electric heat as well, not sure though.I cannot attach the manual because it is larger than 3 MB. I have 2 tools at my disposal, a Fluke 115 True RMS Multimeter & a Fluke 325 True RMS Clamp Meter. If you could please guide me through the steps needed to calculate the most accurate kW (true power) with the resources at my disposal, I will be eternally grateful. Thank you!
     
  12. Dec 3, 2014 #11
    cos φ estimations get more shaky more you're off the nominal power of machines.
     
  13. Dec 3, 2014 #12

    russ_watters

    User Avatar

    Staff: Mentor

    With your equipment you can't measure or calculate power factor. You need a power meter and unfortunately, they aren't cheap:
    http://www.fluke.com/fluke/m3en/solutions/pq/

    To estimate the power factor, you need to know how much of the load is resistive (from the heaters) and how much is inductive (from the motors). You'll need amperage readings for each motor feed and the nameplate amperage of each motor.

    If the pumps or motors change speed, they are probably on VFDs, but if they were, you'd probably also get true power readings from the VFDs, so I'm guessing they are not.
     
  14. Dec 3, 2014 #13

    russ_watters

    User Avatar

    Staff: Mentor

    Yes, but if he has a name brand motor with performance curves, he can get very close. If not....
    [edit] oops....looks like it was worse than I remembered. Below 50% load, it starts getting pretty bad:

    motor%20powerf_doe-med.jpg
     
  15. Dec 3, 2014 #14
    What is the cheapest Wattmeter I can buy that can be used on a 460 volt, 161 amp machine?
     
  16. Dec 3, 2014 #15

    russ_watters

    User Avatar

    Staff: Mentor

  17. Dec 4, 2014 #16
  18. Dec 4, 2014 #17
    Performance graph (p6).
    Line current drawn at 460 V (266V phase to neutral) → Power factor (cos φ).
    This should give you a good estimate of a true electrical power of the motor since Pel= 460⋅I⋅√3 ⋅cos φ
     
  19. Dec 4, 2014 #18

    NascentOxygen

    User Avatar

    Staff: Mentor

    If you were able to switch off all but low-power machinery (and restrict the lighting) in the shop, and just run that motor alone at the intended loading, you might be able to see how much your workshop's power meter advances in 15 minutes of running, then x4 for the figure of actual power drawn. Depends on the meter―some of these electronic ones I think only the utility company can read.

    It might be more practiceable to switch off everything else on just one of the phases, at least, to devote one phase's watthour meter to this motor exclusively, for 15 mins to do the test? (Monitor the 3 currents to confirm loading is balanced.)
     
    Last edited: Dec 4, 2014
  20. Dec 4, 2014 #19

    russ_watters

    User Avatar

    Staff: Mentor

    Is that the only motor in the washer? Doesn't it have a water pump and a conveyor?

    If it is the only motor, that motor is so small compared to your nameplate electrical capacity that you could probably ignore it and assume all of your electrical usage is the electric heat at a 100% power factor. I'd probably still measure its amperage directly though.
     
  21. Dec 5, 2014 #20
    Well there are 2 of those motors actually as well as a few heating elements (pads). I'm figuring that the calculations are going to be fairly complex, so I am trying to find someone who will lend me an industrial wattmeter.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Electrical Consumption of Machine
  1. Electrical Machines (Replies: 3)

Loading...