Re: Declining motor torque with lower voltage.

Using your example

> of Z table weight I think your numbers are off. I think an empty Z
> weighs a couple of hundred pounds, and fully loaded with vises part
> ect, specs out at 750 lbs. So unless you added 300 to 400 lbs weight
> to your table to test it, you do not have real numbers yet.

Shipping weights from Bridgeport:

Knee 257#
Saddle 142#
48" table 365#

Total= 764#

The number for the table sounds a bit high. My 42" table I can
pick up and carry. I guess it is about 200#. Their weight maybe
includes leadscrews and all.


Scott

>
>
> Wally
>
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
> wrote:
> >
> >
> > cnc_4_me <cnc4me@g...> wrote:
> > Since you are running the motor aprox 38% below rated voltage you
> > cannot push the amps thru the motor to get full torque.
> >
> > I left out the current ratting before but here it is 18.5 amp.
> > Armature resistance would be 130vdc / 18.5A = 7 ohm. At 80vdc and 7
> > ohm you can only make 11.4 amps. The torque per amp is 373 0z-in /
> > 18.5 = 20 oz-in. At 11.4 amps max current you get 11.4 x 20 oz-in =
> > 230 oz-in.
> >
> > If we use your speed reduction number above 61.5% and multiply it
> > times the torque of 373 oz-in we get 229.4 oz-in. Seems to agree
> > with ohms law calcs.
> >
> > Wally
> >
> > /// Isn't it possible that delivering the volts and current by
> another means than a continual supply as is the case with a pulse
> stream, that the current delivered in tiny spikes rather than a
> continuous flow would counter the resistance of the windings? We're
> not running these motors at top speed either, and very rarely do.
> Would that also effect the resistance? All calculations aside, when I
> first started looking at B'port retrofit packages such as Ajax, they
> all told me a 28 lb-in motor was too small to drive the knee and that
> a 40 lb-in servo would be imperative. And nothing was guaranteed
> especially the Z motor. I'm running a 100 volt 13.7 continual amps
> servo rated at 40 lb -in torque rating with a 64 VDC supply. Using
> the above calcualtion, I'm only getting 64% of the stated torque.
> Which is 25.6 lb-in. It drives the Z at 60 ipm just as Ajax claimed a
> 40 lb-in motor would at full voltage. Calculations and theories
> aside, it works. Remember,the knee table and vise weigh
> > approximately 800 lbs. then add a few hundred pounds force of a
> drillbit plunging steel. The little Gecko320 tolrates this, much to
> my amazement. ///Ron
> >
> >
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