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  • VFD drive

    Hi all, I have a new to me mill, it has a 3 hp 3 phase motor, im lookin at putting a VFD on it to convert my 1 phase to run it.....My question is do I need to get a VFD rated the same as the motor or a couple HP above my motors rated hp, its a step pulley mill.......thanks

  • #2
    3hp is probably at the edge of derating, I would check with the manufacturer.
    Opportunity knocks once, temptation leans on the doorbell.....

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    • #3
      I've got a 3 hp 3 phase. Baldor recommended a 5 hp VFD.

      Pete

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      • #4
        First check with the VFD manufacturer some do not suggest derating, also it most likely is a 4 pole motor, if so move to a mid range pulley and run up to 120Hz. This in itself should eliminate the need to derate.
        Max.

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        • #5
          Generally you don't need to derate, but it depends on the motor and mill loading. For example, the 2hp BP motors are quite capable of developing far more their rated 2hp, and even with careful parameter adjustment it's "on the edge" to use a 2hp Hitachi drive with rapid acceleration and deceleration (even with max external braking resistors). I find a 3hp VFD works better - less trips.

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          • #6
            Look at the motor's rated full load current and bump it up by the service factor(if there is one). Buy a VFD with a rated current equal to or greater than that number. VFD's are usually designed to supply 110% of rated current without tripping. So, you have a built in cushion.

            RWO

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            • #7
              If your service is single phase and your VFD is single phase rated you don't have to de-rate. Size the VFD from the motor ratings. 3 HP seems to be the defacto limit for single phase rated VFD's.

              If your service is single phase and your VFD is three phase rated. Then the next size larger is the preferred derate. 5 HP to drive a 3 KP motor.

              If the three phase motor drives a constant full load as in a compressor or pump then the VFD needs to be de-rated DOUBLE (that is a 5 HP motor requires a 10 HP VFD).

              Exception. Most manual machine tools are run below max motor HP most of the time. The motor draws only the current necessary to drive the mechanical load and supply its losses. If you seldom need full motor HP, you can reasonably gamble that a less expensive to-size three phase rated VFD will be suitable for a single phase service provided you determine a safe current limit and enter it in the VFD parameter settings. The VFD will then self protect. Chances are you will never notice the diminished power available unless you're a hard charging production worker.

              Story about available machine power. I was once running a powerful 5 ft 50 HP vertical boring machine to near capacity on a big winch drum casting on whick was a full two inches of stock. The chips were jingling off the two heads to the table at an impressive rate to pile on the floor gently smoking radiating heat like a coal stove. I had a know-nothing boss but his boss was a real machinist. My know nothing boss fluttered and clucked but I ignored him. His boss came by and dragged him off for a talking-to. They came back to look at the load meter which was running about 95%. End of the fluttering but the clucking continued that I was a machine abuser for some time after.
              Last edited by Forrest Addy; 05-12-2012, 06:30 PM.

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              • #8
                Thanks for the reply's....the motor data plate states 2.0/2.4 KW ( its got 2 speeds) and 8.3/9.7 Amps...the 2.4 kw comes to 3.2 HP, so it looks like I will need to bump up to a 5 HP VFD.....another question I have is about the single phase ratings or 3 phase rated VFD, I have single phase in my shop....which should i get ??? Why ??? what's the difference ??....thanks...........
                Last edited by metalmole; 05-15-2012, 10:59 AM.

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                • #9
                  Originally posted by metalmole
                  Thanks for the reply's....the motor data plate states 2.0/2.4 KW ( its got 2 speeds) and 8.3/9.7 Amps...the 2.4 kw comes to 3.2 HP, so it looks like I will need to bump up to a 5 HP VFD.....another question I have is about the single phase ratings or 3 phase rated VFD...which should i get ??? Why ??? what's the difference ??....thanks...........

                  some VFD's are single phase input = 3 phase out, some are 3phase in,=

                  3 phase out. 3 in 3 out usually means next size up vfd to run the motor. you would probably be fine with 3hp single phase in, provided your not trying to pull max power out of the motor all the time. this would work http://dealerselectric.com/item.asp?...ID=165&PID=737 or a fancy one=http://dealerselectric.com/item.asp?cID=28&scID=165&PID=1509

                  have fun ! brent

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                  • #10
                    Thanks Brent, $184 sounds like a great price...is there an advantage of getting one of the higher priced VFDs......

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