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  • 3 phase question

    What would be the practical/useable rpm range of a 3 phase 1750 rpm motor with a VFD?
    Len

  • #2
    We've got 20 Hp and 50 Hp motors we consistenly run from 10% to 110%. With extended use (40-50 hours) in the 20 to 30% range. Frequency drives are the bomb.

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    • #3
      "Frequency drives are the bomb."
      I assume that is good.

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      • #4
        Originally posted by cruzinonline
        We've got 20 Hp and 50 Hp motors we consistenly run from 10% to 110%. With extended use (40-50 hours) in the 20 to 30% range. Frequency drives are the bomb.
        Well how about a 2-3 Hp motor in the home shop? Same %?
        Len

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        • #5
          As I under stand it a VFD can wind a motor up to 200% for brief times,I dont think I would,Maybe 150% tops.

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          • #6
            It depends on the physical size of the motor. The bigger they are the less overspeed it can tolerate and the smaller they are the less slow they can run. It's all a matter of momentum and centrifugal force as well as the mass which increases as the cube of the size.
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            • #7
              I got VFD's on all of my machinery. I run the motors from zip or barely creep to 150% of nameplate rated speed. I've also run motors at 5 Hz for hours under near idle load without a problem. '

              The conventional wisdom is AC motors run too slow will overheat because the cooling is ineffective. It's not that simple. Motors draw Amps according to the mechanically driven load. It's true if the motor is under significant load; the motor amps results in some heating but that will also depend to an extent on the VFD'sparameter set-up. If you run the motor slow for jogging or to take very light cuts motor heat will build up according to the duty cycle.

              My reccommendation for motors to be run at low frequency for extended times is to mount a snap action thermostatic switch to the motor stator and run the N/C leads the fault terminals or the series control string on the VFD. Select the temp switch to suit the motor's thermal ratings and designate it as the "over temp switch" (O/T). Also install a temp switch on the motor stator set 20 degrees cooler than the O/T switch and use to to switch a biscuit fan whose draft is directed to cool the motor. Generally in the case of a TEFC motor a couple of tie wraps will hold the fan onto the fan end bell.

              I have no auxiliary fans on my motors and they've never overheated in service unless I was being cruel.
              Last edited by Forrest Addy; 06-26-2006, 08:49 PM.

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              • #8
                VFD's

                A lot of good advice here but one detail has not been mentioned.
                When running an ac motor above rated speed on a VFD it is in the constant horsepower range same as a dc motor being run overspeed by field weakening.
                Here is an example to illustrate:
                Say we have a 10 hp motor rated 1750 rpm on 60 hz. This will have a rated torque of 30 lbft. If we run it at 30 hz we can still get 30 lbft torque and this translates to 5 hp and 875 rpm. However if we run it at say 120 hz we will get 3500 rpm but we are still limited to 10 hp therefor the torque must be reduced to half the rated value or 15 lbft.
                I would expect a full feature VFD to look after this.

                Ken

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                • #9
                  Originally posted by IOWOLF
                  "Frequency drives are the bomb."
                  I assume that is good.
                  Yeah I never figured that one out either....kinda like "that's the cat's ass".....

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                  • #10
                    Or..........."I dont give a $hit" Why would you want to keep it?

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                    • #11
                      I run the motor on my milling machine from 10% to 100%. 10% starts to get a little wimpy, but it's quite usable.
                      ----------
                      Try to make a living, not a killing. -- Utah Phillips
                      Don't believe everything you know. -- Bumper sticker
                      Everybody is ignorant, only on different subjects. -- Will Rogers
                      There are lots of people who mistake their imagination for their memory. - Josh Billings
                      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                      Don't own anything you have to feed or paint. - Hood River Blackie

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                      • #12
                        Originally posted by D. Thomas
                        Yeah I never figured that one out either....kinda like "that's the cat's ass".....
                        It's from ancient Persian history. The idea is that, well, what use does a feline have for a donkey? So if it does own one, you can probably borrow it, and the cat won't mind. Of course!

                        "It's the bee's knees."
                        The curse of having precise measuring tools is being able to actually see how imperfect everything is.

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                        • #13
                          kinda like "that's the cat's ass"
                          You must not be a cat owner. Ever notice how proud cats seem to be of their ass? They walk around with tail high so it is highly visible.

                          As for sayings, why does **** mean anything good? Like, "that's some good ****!" For that matter why is "hot ****" especially good? If I have to deal with **** I want it as old and cold as possible. A notable exception to that is my septic tank which definitely does not improve with age.
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                          • #14
                            This post is to work around the date stamp problem and will throw the post off the top of the list.
                            Free software for calculating bolt circles and similar: Click Here

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