Re: EV transmission [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Joe Sommer wrote:
> Switching an existing motor/reducer from 12V to 24V will double speed
>  and double torque. However it will also double current so be certain
> to use heavy wiring.
> 
> 1)  Keep your existing 9:1 reduction ratio for initial testing at 24V.  
> It will drain batteries twice as fast.

Agreed.  If you keep all other variables the same and double the 
voltage, then you'll also double the current and therefore cut your 
endurance in half.  In other words, if you want to double your speed by 
doubling the voltage, you'll also need to double your battery weight.

> 
> 2) You may actually be able to switch to a 7:1 or even 6:1 reduction 
> ratio at 24V due to increased torque capability and improved
> efficiency.

Let's all be careful about optimizing for "efficiency", "speed" or 
"current draw".

With a 9:1 ratio -at- 12v, the EV Warriors drive the Tiger drive wheel at 
about 200 RPM, achieving a speed of about 3 MPH, drawing about 20 amps 
(according to Steve's spreadsheet model).

If you increase the voltage to 24v with a 9:1 ratio, then that will 
basically increase the drive wheel speed to 400 RPMs, achieving a speed 
of about 6 MPH, drawing about 40 amps.  Go twice as fast and pay twice 
the energy bill.

So, if you decrease the ratio to 6:1, then at 24v the drive wheel will 
be going 600 RPMs, with a top end speed of 9 MPH, with a massive 60 amp 
draw.  Go three times faster and pay 3 times the energy bill.

Although I agree that that is in the "efficiency" zone of the motor, you 
still have to pay the current price, which I think is a little too high 
a price to pay.

Also, the simplified models don't take into account the extra weight 
required to pay the energy bills.  If you stick to the same battery type 
and double the voltage, then you'll need to double the weight to acheive 
the same run-time.  Of course, that extra weight increases the rolling 
friction of the vehicle, which decreases your top-end speed.  So, 
instead of doubling your speed, you may only get 1.8 times the speed (I 
think we can modify Steve's spreadsheet to include this variable).  (You 
can, of course, double your battery capacity without doubling your 
weight by purchasing different battery types, like NiCads, but get ready 
to pay a hefty price for the capacities that we use in a battle.)

In short, it's not just the motor variables we're trying to optimize, 
it's all of the vehicle variables.

> 
> In theory, you could try a 4.5:1 ratio.  Unfortunately frictional drag 
> on the hull and track also increases with speed.

Agreed.  Basically, the EV Warriors top out at approx 10 MPH when used 
in the drive train of our tanks.  If you try to go over that speed, 
you'll push the motor over it's efficiency hump and the drivetrain 
losses and energy consumption will eat you alive.

BTW: At 10 MPH, any tank rolling across the battlefield better have a 
real good suspension, because the kinetic energy of a 100+ vehicle 
traveling at that speed is very high.

        Frank P.