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Best practice for large electric motors

  1. Apr 7, 2008 #1
    This may not be the place to ask this question, but I can't seem to get a good answer anywhere else...I have a manufacturing company. I am on a 3-phase demand meter. I'm trying to figure out if it is more cost effective to shut all the electric motors down during breaks and lunch and restart or just leave them running "idling" during these times? The motors are 2 - 100 HP, 4 - 75 HP, and 4 - 50 HP.

    The other question I have is about motor efficiency - I've had a salesman come by and talk to be about "phase aligners" (Don't really know what they are, but I'm guessing some sort of capacitor that "cleans up" the inbound electricity). Anyone know anything about these and if they are worth fooling with?
  2. jcsd
  3. Apr 7, 2008 #2


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    Staff: Mentor

    For the first question, it depends a little on what these motors are doing, but if they are fully loaded but doing something unnecessary, they are consuming a lot of energy that could be saved. About $60 per lunch hour.

    For the second, there are some products that advertise various power cleaning capabilities. I tested one once on a compressor and it didn't seem to do much. What you do really want to make sure of, though, is that your power factor isn't bad enough that the electric company charges you for it (it should be clear on your bill). If it is bad, you should have an engineer select and a contractor install a capacitor bank to correct it.

    Welcome to PF - there's no better place for such a question!
  4. Apr 8, 2008 #3
    Thanks for the response.

    The motors are not loaded during these times - I thought it would be better to shut them down, but the practice has always been to leave them running - someone told a previous supervisor that because of the demand meter, it was better just to leave them running than to restart them after breaks (15 minutes X 2) and lunch (30 minutes). Each motor is started independant of the other, so it's not like they are all restarting at once.

    You say that there should be something on my bill if the power factor is bad? Where should I look for that. I'm sure every power companies bill is different, but is there a certain nomenclature that would show up or stand out if I have a problem?

    Again thanks for the information.
  5. Apr 8, 2008 #4


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    Staff: Mentor

    For power factor, it should just say power factor, followed by a number between 0.5 and 1.0. The detail of the bill shouldn't be very long...
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