Lathe Motor issue

  1. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Guys, I have a feeling my 3 Phase Brook Crompton motor is slowly dying on my Harrison M250.

    The lathes started bogging down even with lighter cuts, it slows down when on automatic feed.

    I'm running it through a VFD on single phase, checked the lathe mechanically everything seems in good order. its been great up until this week but now there is a definite lack of grunt.

    Before I start pulling the motor out, is there any clever way to diagnose motor issues across the input leads?
     
  2. Pete.

    Pete. Member

    Messages:
    8,672
    Location:
    Kent, UK
    Generally motors either work or they don't so I'd be looking for a mechanical or electrical problem. You're sure that there are no pulleys coming loose or belt slippage?

    What VFD is it? Some (many) of them will display the current per phase being used.
     
    WorkshopChris and daleyd like this.
  3. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    thanks Pete

    it is a strange thing, its a mitshibushi VFD ill have a look at the Phases I documented my setup in my restoration thread here

    #371

    it seems almost like its low on power/volts. I will double check the belt.
     
  4. piman Member

    Messages:
    1,709
    Location:
    Oswestry Shropshire
    Hello RDR,

    check the winding rsisitances, they should all be similar, it's possible one has gone open circuit?
    If they are all reasonably balanced I suspect the VFD is limiting current so check your base settings?

    Alec
     
  5. Stueeee

    Stueeee Old git

    Messages:
    392
    Location:
    N W Kent
    Have you checked that you have 240 (if delta) or 415V (if star wired) phase to phase on all three phases? Three phase motors will run on two phases but with drastically reduced torque.
     
    BarrieJ likes this.
  6. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Thanks all I’m going to have a look tomorrow.

    None of the wiring has been changed or VFD settings moved since I installed it last year. It’s had plenty of grunt up until this week.

    belt seems fine and I disengaged the change gears to rule out the screw cutting gearbox. It was still slowing down on anything more than a light cut.
     
  7. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Quick update as I got sidetracked servicing the saddle and cross slide

    I haven’t started electrical testing but verified the VFD parameters haven’t changed from my initial settings.

    with the change wheels disconnected if i try it at highest speed (1500rpm) it really struggled to get there it spun up but at about 1000rpm started hunting (phasing?) basically the rpm pulsing but not getting faster.

    The motor used to spin the lathe upto this speed like it was a v8.

    I’m going open up the control
    Box measure voltage across all three VFD outputs when running if they are all the same I’ll disconnect and measure resistance between windings.
     
  8. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Guys

    Finally grabbed some time to start investigating further. Resistance across the phases is not consistent

    7.5
    7.4
    6.1

    Is that enough of an inconsistency to indicate motor trouble?

    I’ll pull the motor out of the lathe tomorrow to have a better look.
     
  9. daleyd

    daleyd Member

    Messages:
    6,156
    Location:
    Wrexham, North Wales
    I’d say it’s certainly more than I’d be expecting - if it’s more than 0.1 -0.2 out then in my experience they are mammaries skywards, or at least heading that way. Sounds like the motor may be the issue.
     
    WorkshopChris and R-D-R like this.
  10. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    thanks I'm going to get it out on the bench tomorrow so i can test it properly but a quick google confirms what you say it should be very low compared to what i am getting. I tried running it again still no power, put the lathe in a high gear and it can't get it upto speed. At the highest gear(1500 rpm) it couldn't get past 700 rpm according to the spindle taco before stalling and the Inverters anti stall was giving everything it got before it finally stalled and errored with the Electronic motor thermal cut out kicking in. Reading the manual for that error amongst other things it mainly points to check the motor if it's not a new installation.
     
    daleyd likes this.
  11. m_c Member

    Messages:
    170
    Location:
    East Lothian
    If it's an old motor, the insulation isn't likely to be rated for inverter use, and can breakdown leading to things starting to short out.

    I'd be looking to either try the motor directly on three phase, or with a different VFD, to see if it makes any difference to power.
     
  12. daleyd

    daleyd Member

    Messages:
    6,156
    Location:
    Wrexham, North Wales
    Do you have (or can borrow) another motor to try, even if not the same size? It would at least prove the inverter is ok, though from the sounds of it I’d be 99% sure you’ve already found the problem.
     
    WorkshopChris likes this.
  13. WorkshopChris

    WorkshopChris Forum Supporter

    Messages:
    4,649
    Location:
    South East Essex
    A valid point but if the windings are now at a different resistance they won't get any better again.
     
    daleyd likes this.
  14. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    thanks it is the original motor, 1982 vintage if i recall. I do have a single phase i might be able to test the lathe with, once the motor is off ill give the lather a good inspection make sure everything's free and not binding anywhere.

    Worse case if i'm no further forward I know a motor firm in rotherham who test, rewind and sell new but from advice received so far and my own research i think I'll be able to confirm its the motor.
     
  15. daleyd

    daleyd Member

    Messages:
    6,156
    Location:
    Wrexham, North Wales
    Why size motor is it out of interest?
     
  16. RichardM Member

    If you want to borrow a 3 phase motor I have one from my old BUD, it will be less HP than yours.
    Check your ramp up time on the VFD, I set mine too small and it would trip on max speed until I backed it off a bit.
     
    R-D-R likes this.
  17. Pete.

    Pete. Member

    Messages:
    8,672
    Location:
    Kent, UK
    20% down on one phase. You've cooked a winding.
     
    Hopefuldave, DAPPH and WorkshopChris like this.
  18. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Thanks all

    I’ll check the VFD settings but am pretty resigned to the fact it’s the motor. Was always pushing it using the VFD and have been using it quite a bit this past month with some repetitive work I was heavy cutting on the roughing out to get it done.


    The VFD was set to the motor characteristics from its plate and then with the auto tuning feature where it spins it up and down doing some fancy trickery. The frequency was normal and I relied on the gearing for speed changes.
     
  19. R-D-R Forum Supporter

    Messages:
    1,017
    Location:
    Derbyshire - England
    Ok so pulled it out of the lathe and disconnected from the inverter I think that was throwing results but I still think I have a problem each phase is reading between 10.3 and 10.5 ohms that seems high?

    A74EB416-DBB3-4F48-B2B5-2C20F5E20835.jpeg
     
  20. daleyd

    daleyd Member

    Messages:
    6,156
    Location:
    Wrexham, North Wales
    Should there be any link bars fitted?
     
Advertisements