Motor De-Rating for Slow Speed VFD
Close
Login to Your Account
Results 1 to 14 of 14
  1. #1
    Join Date
    Jun 2020
    Country
    UNITED STATES
    State/Province
    California
    Posts
    41
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4

    Default Motor De-Rating for Slow Speed VFD

    Hi All,

    I have an old Delta-Milwaukee drill press I rebuilt still has the original 3 phase motor, 3/4hp 1750 RPM, the drill press run well and has a cool turret attachment on it. But it goes a little fast for my application, even in the lowest drive ratio.

    In the lowest ratio its 4.5:1 so 388 RPM'ish That's a little fast for a larger bit in the materials I use so I'm going to swap the motor.

    I found 3 phase 1200 rpm motors 'reasonably' priced and that get's me down to 260'ish, I was considering over-sizing the motor a little and use a VFD to get me down lower when necessary, but I don't know the VFD de-rating rules so I can properly oversize.

    Any Info would be nice.

    Attached is it and it's glorified drill press friend, that has really good torque, don't need that much, but a little more than what I currently have.

    Thanks


    drill-presses.jpg

  2. #2
    Join Date
    Jul 2007
    Location
    Missoula Mt
    Posts
    1,175
    Post Thanks / Like
    Likes (Given)
    52
    Likes (Received)
    524

    Default

    I go on the sqr rule... 1/2 the speed is 1/4 the hp, it may not be textbook right but it works for me...Phil

  3. #3
    Join Date
    Sep 2010
    Location
    Oklahoma City, OK
    Posts
    5,377
    Post Thanks / Like
    Likes (Given)
    1024
    Likes (Received)
    2333

    Default

    Quick and dirty answer is that 3ph motors are constant torque below rated speed (how low depends on the quality of your VFD) and constant HP above rated speed.

    I'd not hesitate to use that motor at 20hz, 1/3 speed, for short durations. Keep an eye on the motor temp, but it's also a small drill. I doubt you'll be tapping 1/2-13 in 304 over and over.

    Sent via CNC 88HS

  4. #4
    Join Date
    Aug 2008
    Location
    Sussex, England
    Posts
    3,482
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    796

    Default

    Quick'n dirty rule of thumb concerning VFD use on industrial rated machine tools is that you will pretty much not notice torque variation effects between about ± 1/3 speed variation from data plate unless you are overloading past the original design limits. Objectively anything made for serious industrial use will be overpowered.

    With modern vector drive VFD boxes odds are you can get down close to half speed before hitting trouble if you sort out the torque boost settings. Which may not be easy if you don't understand whats going on and the manual is a simple do this to do that affair.

    Simple explanation of whats going on here HowTo: How to get Maximum Torque at Low Speed from a Simple VxF Inverter should give you the idea but obviously what you actually need to do depends on the inverter.

    Clive

  5. #5
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    20,694
    Post Thanks / Like

    Default

    I think your gonna start hitting the wall on how much torque the standard V belt system can handle.

  6. Likes Limy Sami liked this post
  7. #6
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,946
    Post Thanks / Like
    Likes (Given)
    2069
    Likes (Received)
    3878

    Default

    The torque stays about the same over the range so there is no problem with belts.

    Square law applies to things like heaters where when you halve the voltage you also halve the current. It doesn't quite hold there, either, because most heaters' resistance is lower at the lower temperature so it draws more current than you might expect.

    The motor draws about the same current at any speed so the hp is a simple speed X torque.

    Bill

  8. #7
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    20,694
    Post Thanks / Like

    Default

    Quote Originally Posted by 9100 View Post
    The torque stays about the same over the range so there is no problem with belts.
    OP stated applying bigger motor, for more torque.

  9. #8
    Join Date
    Jun 2020
    Country
    UNITED STATES
    State/Province
    California
    Posts
    41
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4

    Default

    Thank you all,

    It's one of those do it right and do it once type thing. I think I'll pop for the 2 hp and see where that goes, and then VFD as necessary, keep an eye on the motor for heat and mount an additional fan as necessary.

    Belt slippage is rarely an issue with anything I do, I keep a can of that belt spray around the belt machines just in case, but for the most part I do ok.

    Thanks Phil, that's what I was looking for, that little 'this is the rule...' answer.

    Thanks all
    Aaron

  10. #9
    Join Date
    Dec 2003
    Country
    UNITED STATES
    State/Province
    Washington
    Posts
    5,077
    Post Thanks / Like
    Likes (Given)
    2655
    Likes (Received)
    4875

    Default

    If it is a TENV motor, you can run all day long at full torque at stall and not worry about over heating. Given the duty cycle of your need and the turn down of your belt ratio I wouldn’t worry about it.

    Also, if the motor has a thermal click switch embedded in the windings, wire that into the drive’s overload contact input and drill to your hearts content at as low a speed as you want. The overload will let you know when the motor is getting hot and you can adjust accordingly.

  11. Likes LexD liked this post
  12. #10
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    5,980
    Post Thanks / Like
    Likes (Given)
    114
    Likes (Received)
    1170

    Default

    If you configure the correct numbers for the VFD you will know how far you can go. The VFD will tell you where your boundary condition is.

    BTW, have never seen a chopped down table for a mill-drill. Very compact.

  13. #11
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,946
    Post Thanks / Like
    Likes (Given)
    2069
    Likes (Received)
    3878

    Default

    Quote Originally Posted by Aaron-M View Post

    Thanks Phil, that's what I was looking for, that little 'this is the rule...' answer.

    Thanks all
    Aaron
    That isn't the rule for motors.

    Bill

  14. Likes JST, digger doug liked this post
  15. #12
    Join Date
    Jun 2020
    Country
    UNITED STATES
    State/Province
    California
    Posts
    41
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4

    Default

    Rons,

    Once you realize it's just a glorified drill press, with a big ass 10" 4 jaw mounted as a vice cutting the table was a no brainier. I think it has move plus minus 1" center in the last year. Getting ready to add a cross hair laser setup to it, to add to the functionality.

    Thanks

  16. #13
    Join Date
    Dec 2000
    Location
    Bremerton WA USA
    Posts
    10,718
    Post Thanks / Like
    Likes (Given)
    41
    Likes (Received)
    4424

    Default

    My solution to the same problem was to install a 3 HP, 3 ph motor. I run it with a 20 year old MotoTronic FVD with few bells and whistles. Years ago when I was building architectural iron work, I drilled and tapped 1/2 - 13 holes by the hundreds in 5/8" plate flanges using plain vanilla gun taps out of the drill chuck. Same job with 1 1/2" annular cutters. Hole sawed bird blocks, etc all sorts of high torque fhole making.

    When I was in the talking stage of my drill press conversion, the doubters predicted all sorts of trouble overpowering an othewise sturdy but consumer grade drill press. Never has a speck of trouble and I made lots of big holes. Probably the most cost effective project repowering my personal equipment.

    Just saying.

  17. #14
    Join Date
    Jan 2015
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    211
    Post Thanks / Like
    Likes (Given)
    71
    Likes (Received)
    85

    Default

    Quote Originally Posted by Aaron-M View Post
    Hi All,

    but I don't know the VFD de-rating rules so I can properly oversize.

    drill-presses.jpg
    If you're wanting the rule for de rating the VFD for split phase input I don't think that has been addressed yet. I don't remember, but you can surely find the answer in the Transformers, Phase Converters and VFD sub forum.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •