Re: motors/torque

[Joe Sommer <anvilus9-at-adelphia.net>]

Frank,

At 09:20 PM 10/7/2004 -0400, Frank Pittelli <frank-at-rctankcombat.com> wrote:
>OK, let me see if I've got this straight.  Your Hetzer is producing
>         39.6 ft-lb and 460 rpms at 24v moving 80 lbs
>which is sufficient torque for turning and plenty of speed.  So let's use 
>that as the baseline.

1)  Remember - max torque does NOT occur at max speed

2)  The Hetzer was fast but under-powered with the old motors
that only produced 400 rpm and 26.7 ft-lbf torque per motor.  When
turning, it was probably wasting large current draw as the
motors approached stall.

3)  My conversion to 24 VDC included installing new motors
that have windings with higher resistance.  I did not simply
connect 24 VDC to my old 12 VDC motors.

4)  I have always carried two 18Ah 12 VDC batteries so
the Hetzer's weight did not increase.

5)  The Hetzer uses 4 inch DIA drive sprockets.  Larger
drive sprockets require proportionally more torque

>If the EV Warrior is operating at 12v according to the specs it produces:
>         3.6 ft-lb -at- 2210 rpm (no load)
>with a 9:1 gear ratio that yields:
>         32.4 ft-lb -at- 245 rpm (no load)
>We know from the Tiger that provides plenty of power for turning and good 
>speed for moving approx 100 lbs.

math is correct

>    So, if we increase the voltage to 24v, the EV Warrior should produce 
> (according to the specs):
>         7.3 ft-lb -at- 4480 rpm (no load)
>with an 18:1 gear ratio that yields:
>         131.4 ft-lb -at- 248 rpm (no load).

math is correct

>If that is correct (please check my calculations), that means that we will 
>have an excess of torque at 24v and (this is the question) we should be 
>able to decrease the gear ratio to gain more speed while still having 
>sufficient torque for turning.  Is that correct?

correct

>    Is it also correct to assume that the excess power will allow the 
> motor to work less in a turn, thereby conserving battery life?

first clause is correct if you substitute the word
"torque" for "power"

You will have excess torque, not excess power.
power = torque * rotational_speed

second clause is generally correct with a caveat - see below

>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.

If we momentarily ignore the excess torque issue, you
would draw about 2 times as much current because
you are simply switching motors from 12 VDC operation
to 24 VDC operation with the same motor windings.

>  Does the reduced amp draw come from the fact that we have lots more 
> torque than we need for turning?

The availability of excess torque may reduce your current
draw to about 1.6 to 1.8 times more current because the
motors are working more efficiently.

This ratio is probably 1.1 to 1.2 times more current for the
Hetzer because of items 2)  3)  and 4)  listed at the top of
this note.

How do your 26 Ah batteries perform at the end of a battle
compared to the beginning?  I know that John's battery often
feels weak at the end of battle.

I simply recommend sticking two 18Ah 12VDC batteries
in series into your Tiger with your EVWs and 9:1 ratio to
give it a try.  It should go like a bat out of hell and have
way too much torque but probably will suck the batteries
down in about 30 minutes.

>The relationship between torque and current draw is the piece of the 
>equation that I'm missing.

It is a simple relationship in theory for steady-state
operation of a DC motor at constant speed with constant
load.  I will see if I can find a suitable reference and provide a
synopsis.

However, the interactions between motor torque,
motor speed and current draw become very complicated
for mission trajectories with periods of idle followed by
sudden bursts at full loaded speed followed by brief
periods at near stall (turns).

Joe


**********************************************
Joe Sommer
2378 Nantucket Circle, State College, PA 16803
814.234.4773     anvilus9-at-adelphia.net
http://www.anvilus.com