Re: Servo motor selection

Jake,

Sorry about that, I misread the 50 in-oz per amp. That is the Kt
(torque constant) for the motor. There is a fixed relationship
between Kt and Kv (volts / 1,000 RPM) of Kv = 0.7402 Kt, meaning your
motor's Kv is 37 volts per 1,000 RPM (0.7402 times 50 in-oz per amp).

You mentioned 900 RPM so the motor is probably rated at 36VDC.

You also mentioned 600 in-oz of stall torque. That means a stall
current of 12A (600 / 50). The power curve is parabolic and peaks at
1/2 of stall torque; the speed at that point is 1/2 of no-load speed.
Power (Watts) is RPM times torque divided by 1351, or in this case
500 RPM times 300 in-oz / 1351 = 111 Watts. Figure on using no more
than 150 in-oz continuously at 750 RPM.

The good news is you not have to use reduction.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "jbordens" <jake@a...> wrote:
> Mariss,
>
> Thanks again for your great 'top 10' post. Really awesome stuff.
I
> have it printed out and sitting right next to me actually...
>
> I've gone through your calculations and they make perfect sense.
> Except for one thing, I'm not sure where you got 6000 RPM, but
> according to the Camtronics site, these motors are 900 RPM. Of
> course, this is significantly less, and things don't look good from
> there.
>
> Did I miss something? I do see that they say 50 oz-in per amp...
> but they don't mention how many amps the motors are rated for as
far
> as I can find. Looking at the power supply, they show 25amps. Is
> it a good assumption to think each motor is about 5 amps? That
> would mean 250 oz-in, which would mean about 155 watts.
>
> Also, the machine I'm getting is 10tpi. If I use 2:1 reduction
> instead of 4:1, that would pretty much make the equations come out
> the same. These motors can come with a 500 CPR encoder so I think
> I'm good there for .0001 accuracy.
>
> I'm probably off in left field somewhere, so please set me straight
> if I've managed to confuse the situation again... I'll get it
> sooner or later.
>
> Thanks again.
>
> Jake
>
> --- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> > Jake,
> >
> > Those are good motors. Observe the continuous torque rating of 50
> in-
> > oz. Think in terms of power, as in Watts. 50 in-oz at 6,000 RPM
is
> > 222W (torque times RPM divided by 1351 equals Watts). That also
> > equals 3/10 of a HP (222 / 746 = .297HP). 1HP also equals 550 ft-
> lbs
> > per second or 396,000 in-lbs per minute. You have 118,800 in-
> lbs/min
> > at your disposal. This means if you want 300 IPM, you have 396
lbs
> > of "push" available (118,800 / 300). That takes care of the power
> > part.
> >
> > Now for the reduction ratio. From the previous example, you want
> 300
> > IPM. Assuming you have a 5 turns per inch (5 TPI) screw, it would
> > have to turn at 1,500 RPM (IPM times TPI). The motor turns at
> 6,000
> > RPM so the reduction ratio is 4:1 (6,000 / 1,500).
> >
> > Now a reality check. A 4:1 reduction on your 50 in-oz motor
> applies
> > 200 in-oz on the leadscrew. Force (lbs) = in-oz times 6.28 times
> TPI
> > divided by 16. For you that is 200 times 6.28 times 5 divided by
> 16
> > equals 392.5 lbs; close enough to 396. Use non-backlash reduction
> > (toothed belt).
> >
> > Finally, the encoder. Say you want 0.0001" resolution (10,000
> counts
> > per inch). You have a 4:1 reduction and a 5 TPI screw, so the
> motor
> > has to turn 20 revs to move 1". The encoder has to have 500
counts
> > per rev (10,000 / 20). Since there are 4 counts per encoder line,
> use
> > a 125 line encoder (500 / 4).
> >
> > The motor has a 600 in-oz stall torque. This is 12 times the
> > continuous torque. That means for brief periods of time (1/2 sec)
> you
> > would have 4750 lbs of "push" available (12 times 396).
> >
> > Mariss
> >
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@y..., "jbordens" <jake@a...> wrote:
> > > First, an introduction...
> > >
> > > I'm Jake, and I'm new to the group. I'm currently waiting for
> my
> > new
> > > Grizzly G3012 mill to arrive. Its backordered, so I've got a
> few
> > > weeks till it arrives.
> > >
> > > I'd like to do a motion control setup for it. I'm pretty handy
> > with
> > > electronics, and I understand most of how this works. However,
> as
> > > for chosing motors I'm at a loss.
> > >
> > > I'd like to thank Mariss for the wonderful top 10 list. Its
> helped
> > > me greatly. To quote:
> > >
> > > "7. Bewildering choice of motors, encoders, servodrives"
> > >
> > > I'm offcially bewildered. I'm deathly afraid of taking all
this
> > time
> > > (and money) and having underpowered motors.
> > >
> > > The camtronics people sell peak-600oz/in motors 50oz/in per amp
> for
> > a
> > > price I can afford. My question is: is this a good way to go?
> > Will
> > > these be strong enough to handle a mill this size? Should I go
> > with
> > > a direct drive, or timing belts? Is there a good conversion
> factor
> > > from stepper to servo oz-in ratings?
> > >
> > > Any help is appreciated, and I do appologize for the post. I
> > haven't
> > > seen a "guide to chosing motors", other than a really confusing
> bit
> > > about coiled ropes and such that I found in this forum. Maybe
> I'll
> > > put something together with all my notes once I've gained some
> > > experience.
> > >
> > > Thanks for your time.