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  1. #31
    Thanks guys, I think I'll pass on the capacitor route then. Worth mentioning though as it might help someone else.

    Out of interest what is the verdict on DC motor vs AC + vfd or is that a question for another thread
    ?

  2. #32
    It's not quite true you only need one cap? i had to use a few to get the balance right on all the phases. Takes a bit of messing but worked well in the end and saved me using a VFD for the job. Ross it is horses for courses, for me i prefer to use AC and VFD because it is easier. Brushless DC would be second choice but expensive (large servo would be even better). How deep are your pockets?
    If the nagging gets really bad......Get a bigger shed:naughty:

  3. #33
    Swarfing, once the correct value cap has been established, apart from the "start" cap you are still only putting "C" across one leg of the delta (or am I missing something?). To give the correct "C" value you may well decide to use a combination of caps to achieve that value, but a single item of the correct value can usually be found. If Ross already has an inverter then obviously that is the best way ahead for him. I only make this point because I would not like to be thought of as misleading anybody. G.

  4. #34
    G it was a long time ago when i did mine, you may be right so don't think you are misleading people. Mine would not balance across each phase so had to tweak with various caps to balance. If one phase runs out of balance then it would contribute to a short life of the motor. Bear in mind mine was 69 BP i applied this too. look at the second image up on the link i supplied earlier in this thread and you will see what i mean. That is the way i followed and it works a treat with only slight loss of torque.
    If the nagging gets really bad......Get a bigger shed:naughty:

  5. #35
    Yes, I think its a great way to do it for very little expense and whilst I am aware that it is "quasi" 3 phase, I have never noticed any power losses. Mind you I have never had the real thing, so I wouldn't would I? G.
    Last edited by GEOFFREY; 08-04-2013 at 02:51 PM.

  6. #36
    Quote Originally Posted by Swarfing View Post
    It's not quite true you only need one cap? i had to use a few to get the balance right on all the phases. Takes a bit of messing but worked well in the end and saved me using a VFD for the job. Ross it is horses for courses, for me i prefer to use AC and VFD because it is easier. Brushless DC would be second choice but expensive (large servo would be even better). How deep are your pockets?
    Cheers, I always thought that ac with vfd was more reliable but restrictive on high rpm also I had heard that torque at low speed can be an issue.

    I was more looking at cheap treadmill motors, mainly because I picked one one up on ebay for 1. Not had chance to break it down yet but the specs say 1.75hp continuous and 3hp peak. geared down to 3000 rpm for the mill as it has roller bearings then that would be about 3hp continuous. geared up 1:2 on another spindle it should still achieve 1hp at 12000rpm

  7. #37
    Sounds like your most of the way there then. Just need an appropriate controller and your away :-)
    If the nagging gets really bad......Get a bigger shed:naughty:

  8. #38
    Quote Originally Posted by Ross77 View Post
    Cheers, I always thought that ac with vfd was more reliable but restrictive on high rpm also I had heard that torque at low speed can be an issue.
    You can run a lot of standard induction motors at well over their rated frequency using a VFD and you will still get approximately rated power at the higher speed, however for that to happen clearly the torque drops proportionately. When you reduce the speed with a VFD, ideally the torque remains constant but in reality it drops a little. Even if the torque did remain constant, your power output will reduce with speed as power=torque*angular velocity.

    The cheap VFDs from China seem to be good:
    http://www.ebay.co.uk/itm/VARIABLE-F...item3cc89d5929

    I use one of those to power the 2.2kW motor on my lathe and it works well.

    Quote Originally Posted by Ross77 View Post
    the specs say 1.75hp continuous and 3hp peak. geared down to 3000 rpm for the mill as it has roller bearings then that would be about 3hp continuous. geared up 1:2 on another spindle it should still achieve 1hp at 12000rpm
    Ideal gearing doesn't change the power output - if the input is 1.75hp, then whatever gearing you put in between the output will still be 1.75hp continuous, not 3hp. Clearly in reality you loose a few percent due to the inefficiency of the gearing.
    Last edited by Jonathan; 09-04-2013 at 10:27 AM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. #39
    Looking at the size of the machine i think the DC motor will do you fine. Mine will be running with only 1HP motor and that is more than enough for my needs.
    If the nagging gets really bad......Get a bigger shed:naughty:

  10. #40
    Thanks Jonathan, is the torque drop off noticable? or is it dependant on the motor?

    With referance to the gearing I was talking about power at the tool cutting tip. obviously the motor will always be the same power. Or have I got that wrong as well?

    edit yes I have.....increase in velocity when gearing up and increase in torque when gearing down.
    Last edited by Ross77; 09-04-2013 at 06:45 PM.

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