G320 and a 850oz-in motor

[edwardvm]

At Homeshopcnc.com, in the servo section (http://homeshopcnc.com/servo2.html), i found advertised the following motor:

skewed rotor design
continuous torque: 170 oz/in <<
peak torque: 850 oz/in <<
terminal voltage: 72 VDC
continuous current: 8.5A <<<<< here are the specs
peak current: 38A <<<<< of interest for now
inertia: 3.39 kg.cm^
maximum operating speed: 4200 rpm
torque constant (KT): .1757 N. m/am
voltage constant (KE): 15.71 volts/100 rpm
resistance: 85 ohm
inductance: 2.35 mH
weight: 8.8 lbs
7 inches

on this same page (which only shows one other servo), a slight discount for a Gecko G320 is offered with motor purchases.

The G320 has a maximum current of 20A, just over half of the possible servo peak current.

I am curious as to whether i misunderstand the implied suitability of the G320 for this motor, or if there is a valid design compromise to be made here.

The other motor shown:

skewed rotor design
continuous torque: 50 oz/in <<
peak torque: 350 oz/in <<
terminal voltage: 60 VDC
continuous current: 3.5A <<
peak current: 20A <<
inertia: 0.0043 oz/in second^
shaft diameter: 1/4"
maximum operating speed: 4700 rpm
torque constant (KT): 0.102 N. m/am
voltage constant (KE): 11.0 volts/K rpm
resistance: 2 ohm
inductance: 5.2 mH
weight: 4 lbs
5 inches


Looks more like a match to a G320, but i am wondering if the higher higher continuous torque of the larger motor (170 vs 50) might make it worth it to throw away the torque peak of the motor:

850ozin@38A > ~400ozin@20A

I probably would end up close to the peak torque of the smaller motor, but would have more useful constant torque otherwise.

Is this a valid way to look at this issue, and If it is even valid, is it reasonable?
/edit
NB: i did read the FAQ on this forum, and i know it says motor V <20 amps, but Keling claims to have tested a motor just as big as this one with a geckodrive, and homeshopcnc seems to imply it as well.

[diycnc]

i am pretty sure that the 38a is locked rotor current.

you will get more torque out of the first motor as its rated for 72v

72v at 20a is 1440 watts

60v at 20a is 1200 watts

[edwardvm]

yup, that's what i figured....but it still leaves my basic concern:

as long as i don't attempt to shove the whole 38A through the G320, then i'm ok to use a motor rated higher in current than the drives?

I don't see why not, but it's in the "DO NOT" section of the Gecko forum FAQ, unless i'm misreading that as well.

quote
SECTION I – GENERAL FAQ
Q.) The Gecko Dos and Don’ts:
A.) Here is a quick list of what you should never do to your gecko:
.....
f. NEVER use a motor rated higher than 20A with a servo.
/endquote

.....but all the other kids are doing it!

[diycnc]

my motors are rated 40a running on a gecko. the 40a is locked rotor and my motor would fry in seconds with 40a at 72v, so its silly to worry about it. as long as your continuous current is under 20a you should be ok. for this very reason the max torque numbers on these motors are silly and pointless. you cant run them at those numbers without frying them.


so with proper limit switches and proper programing you should be fine.

[edwardvm]

thanks for the confirmation d.