Re: Declining motor torque with lower voltage.

> /// 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


Well I am still trying to digest post number 78682 from Stephen Wille
Padnos. He talks about back emf, sort of like getting free
voltage….But until I do I will stay conventional. 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. And no,
you can not deliver more current with tiny spikes.

What are the specs of your Z axis. Do you have an acme or ballscrew
and what TPI.


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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