1. #1
    Could someone put me right on servos.

    I have a mill which I retrofitted some years ago which has a standard 3 phase spindle motor of 3hp. I have a 7hp motor and a VFD on the shelf which I intend to replace it with at some point in time. Lately though, with the price of servos and drivers becoming more affordable, my thoughts have been going in the direction of using a servo instead that would provide a reasonable power. I don't know enough about servo power outputs to take this any further and would like some help.

    On the face of it a 3KW servo is the same as a 3KW AC induction motor, but I know it cannot be as simple as this, or is it? Ignoring gear reductions and servo drivers for the moment, how does the power output rating between the two units compare. Thanks.

  2. #2
    Hi and welcome to the forum.

    Have a look at this I know that it is not directly related but there is a chart that shows a few differences.

    I am no expert on motors. I think there are a few on the forum though.
    Last edited by Clive S; 23-08-2017 at 01:37 PM.
    The more you know, The better you know, How little you know

  3. The Following User Says Thank You to Clive S For This Useful Post:

  4. #3
    Thanks Clive, that's very interesting and encouraging. To add a little more to the picture, I run a workshop with all manual machines but for the one CNC mill I mentioned earlier. I don't have three phase to the building so I run of a rotary converter, which does the job pretty well. My CNC mill is all powered by servos and 240v input AC drivers, with the exception of the main spindle motor which is three phase. I mention all this because looking on the Chinese sites, it seems that the biggest driver I can get with a 240v input is around 3.5 kW. The spindle runs via a variable drive belt system which for the most part does its job well, but I would like to remove it completely and replace it with a toothed belt and suitable gear ratio. Looking at the info you gave me it would seem that torque from a servo would be in my favour. Money aside, this seems too simple, are there any other issues. Again, thanks.

  5. #4
    are there any other issues.
    I have installed a few rotary converters but I don't feel qualified to answer.
    I think that it has been discussed on the forums some time ago. So I will pass and leave to brighter people.
    The more you know, The better you know, How little you know

  6. Power is power regardless how you look at it. KW = torque (Nm)* Speed /9550

    The difference is how the motor is controlled and how the torque is delivered at what speed.
    Last edited by Gary; 23-08-2017 at 09:54 PM.

  7. #6
    Lemme get my oar in...
    On a very good rigid 12x24 lathe, chicom, chester crafstman, bought new from chester around 2007 perhaps..
    it is likely the most "cnc" shop built lathe in the world.
    At vast cost, changes, tests, hw, hours, etc etc.

    The original spindle was 1.5 kW AC 220V.
    And really worked well, had excellent torque, could do blue/black chips on 200 mm steel discs for hours, at 1-1.2 mm DOC.
    Note its 200 mm D, at 1 mm doc. About 200 rpm afair.

    I put on a servo.
    AC brushless, 220V (50 hz, spain), 3000 rpm max, 10.000 counts, 10 Nm cont, 30 Nm peak for 3 secs.
    Very good mount.
    Ie very rigid, 30x40 mm frames in tool steel.
    Motor mount plate 250x200x20 mm tool steel. Etc. About 50 kg in mass.
    IE very rigid.

    8 mm profile, HTD belts, 24:72 so 1:3 drive.
    30 mm wide. This is important.
    Taperlock pulleys.
    Gives me 0-1000 rpm.

    The setup gives me 90 Nm torque, 0 rpm to 1000 rpm.
    And 0.01 degree indexing, c axis, +/-. 30.000 /rev.

    For comparison, a 10 kW HAAS ST10, 50.000€, gets 102 Nm max at 1200 rpm.
    And quite a lot less, at lower revs.

    The motor is silent, belt has some noise.
    Lifetime should be about 10k hours - industrial.

    I was always of the opinion that I had more than enough torque.
    Now I have much more, and I find it wonderful.
    I was wrong.
    Much more torque is extremely useful. Extremely.
    I don´t exactly know why ...
    and I am pretty sure that this works so well because my lathe is very short/heavy for its size, and thus very rigid, for its size.

    Variable-speed 0-1000 rpm, perfect speeds, perfect surface speed, is great.
    Zero belt changes ever (could add pulleys, at need - if I made small sub 10 mm steel parts- I dont, so far).
    Good 30 mm wide belts and pulleys cost money.
    About 300€ to me (some spares).

    Making a big heavy frame is not difficult, but is hard work, heavy, expensive parts, lots of hours.
    Servo set all in 1500€ at 22% VAT in Spain - I am a wholesaler/importer.
    A 3kw+ servo maybe 2500€.

  8. #7
    Thanks Gary, I had hoped as much.

    Hanermo2: Thanks for taking the time to put together some of your experiences. It looks like I am on the right track for this setup to be a better proposition than what I have or had planned for the mill.

    I'm grateful for all the replies, cheers guys.

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