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  1. #1
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    Default VFD for my RF-30

    I previous posts I have mentioned that the smallest of the stepped motor pulley is broken and the difficulties I have with the pulley cover.
    So I'm now looking at using a VFD.
    With a fully working mill I would have speeds : 110, 180, 270, 400, 470, 830, 1110 and 1560. With that small broken pulley I can't use 110, 180 or 400.
    The motor that's on it runs single phase which is good. The interesting thing is it looks to be a 3 phase motor with 2 capacitors in it's connection box so that it runs on single phase.
    The motor is 2HP 1440 RPM

    I found another problem the other day when I went to mill with a 4mm cutter. With the pulleys set on the highest speed (1560), the motor putt putt's quite a few times before it spins up properly. It even makes the shed lights flicker. Looks like the load is more than the caps will sync with the motor (bad timing, if you know what I mean). So a VFD will kill 3 birds.

    That brings about some questions :
    What might be the slowest RPM I can run the motor and still attain full torque ?
    What would be the highest RPM I could run the motor without damaging it or the pulley belt train and spindle bearings ?

    It would be nice if I could get the spindle up to 2000RPM or more as well as being able to run slowly for a slitting saw.
    If the motor is capable of delivering many more revs safely than its standard 50Hz I would leave the pulleys set to say 830 and rarely have to lift the lid.

    Thoughts ?

  2. #2
    Join Date
    Oct 2007
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    Default

    It is fairly common to set up a single phase motor with 2 caps, a start one and a run one. The motor has two sets of windings, the normal main set which receives the full 230V when the motor is energised and an auxilliary winding that gets most of the 230V to start the motor, then via speed sensitive switching gets significantly less voltage once the motor is almost up to speed.

    As with most single phase motors, the auxilliary winding is arranged to carry a lot of current for starting the motor, but would be prone to burning out if that amount of current was flowing continuously through it. The auxilliary wining is fed through the start capacitor and a centrifugal switch to start the motor, and once the motor reaches about 80% of its rated speed, the centrifugal switch will open, disconnecting the start capacitor from the circuit.

    In the case of a cap start motor, the start winding remains out of circuit until the motor slows enough for the start switch contacts to close. This ensures that the start winding does not burn out due to overheating. In a cap start/cap run motor, the run cap (somewhat lower value than the start cap) is connected across the centrifugal switch and start cap, and allows a much lower current to flow through the auxilliary winding continuously. This current aids motor torque and operation, and is limited by the run capacitor to the extent that it cannot cause the auxilliary winding to overheat.

    I suspect that you have a single phase cap start/cap run motor rather than a 3 phase motor and caps to create artificial phases. The simplest way to tell is to listen for the centrigugal switch to close as the motor slows when turned off. If you hear the switch closing as the motor slows, it is definitely single phase, as 3 phase motors don't incorporate switching in the motor, only in the control systems which are normally in a separate box.

    From the symptoms you describe, (motor putt putts a few times starting with belts set for highest speed), your start capacitor has decreased in value and is limiting the starting torque. This would become more obvious as you arrange the belts for higher speed, as this in turn reduces the torque multiplication effect of setting up for lower speeds, to friction and inertia have a greater effect on the motor while starting. If the motor was also lacking in power while running at speed, I would suspect the run capacitor to be decreasing in value.

    If possible, check the spindle with the belts removed to verify that the spindle bearings are not at fault, check the intermediate pulley in case there are issues with it's bearings, and rotate the motor by hand to check it's bearings and drag. If all of these feel OK, I would be looking to replace both of the capacitors in the motor, similar microfarads and the same or higher voltage as the originals.

    This won't solve the issue of the missing speeds caused by the damaged pulley, but I suspect that a VFD connected to the motor would blow either the motor windings, the VFD or both.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  3. #3
    Join Date
    Jul 2010
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    Default

    Leaving the single phase motor question to the side. (though I have run a small single phase motor on two legs of a VSD just to see if it was doable. Not a cap start one though)
    Quote Originally Posted by phaser View Post
    What might be the slowest RPM I can run the motor and still attain full torque ?
    Torque is not power.
    Short answer that isn't a great deal of help. "it depends on the type of VSD and the motor"

    Long answer BobL did a lot of testing on one combo, though I dont recall any tests with a vector drive which should improve those numbers at least at lower rpm.
    https://metalworkforums.com/f65/t182106-vsd-power-tests


    Quote Originally Posted by phaser View Post
    What would be the highest RPM I could run the motor without damaging it or the pulley belt train and spindle bearings ?
    Again is depends on the motor, but a common line of thinking says that the rotor is the same in the 2 pole and 4 pole motors so the rotor will be happy enough at 2800rpm, How close to that you get again depends on the VSD and motor, power will be down when you get that high, but then you are hardly likely to be needing much power up there as it would assume pretty smaller cutters.

  4. #4
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  5. #5
    BobL is online now Member: Blue and white apron brigade
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    I doubt it but if it really is a 3P motor with added caps to run on SP it will not generate as much power as when it runs on 3P.
    If it is a 3P then it could generate as much a 30% more power.

    The first thing to do is see if it really is a 3P motor. Disconnect everything (including the mains power) and check if the 3 sets of coils have the same resistance, if they do then it could be a 3P motor. Then we don't know if its a 240Y or 240∆ motor. The way around that would be to try the motor (completely unloaded) for a few seconds in either mode using a VFD and carefully watch the current with an ammeter. then try it loaded and carefully watch the ammeter. IE fair bit of mucking about involved.

    Maybe consider getting a 3HP 3P motor but then here are the restrictions.
    Running the VFD in standard mode would give you 2HP or more from 33Hz up to ~120Hz that's about a ratio of 4:1
    A vector drive VFD would give you 2HP or more from about 16Hz up to about 120Hz which is a ratio of 7.5 to one which is still only half of what you need.

    If you put the pulley on 470 rpm then 16/50 X 470 will give you 2HP at 150 RPM
    However I also wouldn't want to run too long fully loaded at 16Hz without extra cooling as the motor will probably overheat.

    If you absolutely need 2HP at 110 rpm then its starting to sound like it would just be easier to make a new stepped pulley?

  6. #6
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    I see where you're coming from there.
    I didn't really think about the motor running so slow at low spindle speeds. Sounds like the best thing would be to get the pulleys back up to spec.
    I'll have to have a good look at the coils on that motor when I've got time and in the mean time replace caps if necessary.
    If it is 3 phase, putting a VFD on it together with the pulley ratios would be a nice upgrade.

  7. #7
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    Quote Originally Posted by BobL View Post
    Running the VFD in standard mode would give you 2HP or more from 33Hz up to ~120Hz that's about a ratio of 4:1
    A vector drive VFD would give you 2HP or more from about 16Hz up to about 120Hz which is a ratio of 7.5 to one which is still only half of what you need.
    I cant even read a graph
    https://metalworkforums.com/attachme...7&d=1404643086

  8. #8
    Join Date
    Sep 2009
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    Default VFD for my RF-30

    My RF30 runs on a 3PH 1.5KW motor that I added a VFD to because I donít have 3PH in my garage.

    I have it set on the middle belts (I think) and it will readily rip through anything from 300 RPM up to a max of about 1400 on the one belt setting.

    Itís a monster and never misses a beat (other than the table being pecked and loose on the gibs in the middle).

    3PH motors are relatively cheap if you keep looking (I know because I acquired 3 in the last 4 months for future projects), so I would strongly recommend going down that path if you can. However the VFD isnít that cheap.

    You can also set up a nice control box for forward/reverse and speed.






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