Re: motors/torque

[Steve Edwards <sedwards-at-awger.net>]

Just to add to what Joe has already posted --

Frank Pittelli wrote:

> We know from the Tiger that provides plenty of power for turning and 
> good speed for moving approx 100 lbs.

Can you quantify "good speed?"  If you could provide some acceleration 
and velocity numbers (ex: 3 sec to reach 10fps on pavement, 4 sec to 
reach 8fps on grass) I can plug them into a spreadsheet and get some 
representative numbers.

> Clearly, since we'll only be able to carry half the battery amp-hours 
> at 24v then we can at 12v (while keeping weight the same), the motors 
> will have to draw less than 1/2 the number of amps for us to 
> break-even. Does the reduced amp draw come from the fact that we have 
> lots more torque than we need for turning?  The relationship between 
> torque and current draw is the piece of the equation that I'm missing.


Current draw vs torque is (for our purposes) constant. The 
differentiating factor is the RPM and resultant HP attained for a 
specific current.

The answer to the efficiency question will be answered when we can 
determine the current profile (amps vs torque vs rpm) and drivetrain 
efficiencies (power loss in the drivetrain, tractive force, and tractive 
effort).  But, just as a for-instance:

If the 12V installation were moving the model at the desired speed while 
running at 60% capacity (1326 rpm, 17 in.lbs, 40 amps) at a 9:1 
reduction (for a 5.17 inch drive sprocket, this would be on the order of 
3fps, if my drivetrain efficiency WAGs are close), you're looking at a 
steady-state torque requirement at the drive sprocket of about 155 
in.lbs (17.27 * gear reduction). Switching to 24V and an 18:1 reduction 
would move the torque curve quite a bit, but the torque requirement for 
steady state would be the same -- so you'd only need a motor input 
torque of 8.65 in.lbs, which is generated at 90% capacity while drawing 
19.8 amps, and you'd get that input torque -at- 4000rpm, resulting in a 
222rpm sprocket, and a 5fps vehicle speed.

At least, on paper.  :-)

    - Steve "It's just a theory" Edwards