Re: System Voltage

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

erik kump wrote:
> 
> If I did just use a resistor to drop the voltage, can anyone give me an 
> idea of what my power loss might be? And whether that resistor would get 
> hot enough over time to be a heat risk to itself or other components? 
> Thanks,
> 

Using resistors to drop the voltage from 24 to 18 in one of our vehicles 
would waste a huge amount of energy as heat because of the currents that 
we draw.  That is, the resistor will have to dissipate roughly 6v x 30A 
= 180watts of energy.  That's a pretty serious amount of energy and it 
would all have to be converted to heat which will means a fairly large 
heat sink as well.  You essentially be throwing away at least 1/4 of the 
battery power as heat (probably closer to 1/3 because of efficiency 
considerations).

The only practical mechanism for reducing 24v to 18v at high current 
levels is a variable speed control, which essentially turns the voltage 
on and off over a given time period (about 1000 times a second) to 
average out the voltage seen by the motor at the desired level.  There 
aren't a lot of speed controls on the market that operate at 24v because 
most of them are built for the R/C car market, which rarely goes above 
10-12volts.  The ones that do regulate 24v are generally built for 
industrial or wheel-chair applications, which makes them more robust, 
but also more expensive.

Joe Sommer has developed a relatively inexpensive controller for the 
hobby, but I'm not sure if it can handle 24v on the input side.  He 
hasn't chimed in on this thread, so he may be on vacation.

You may want to consider running the motors at 24v but increasing your 
drive train ratio so that the motor spins at a higher RPM while keeping 
the axle at the desired 18v RPMs.  Such a setup will significantly 
increase your torque (ie. stopping, starting and turning power) while 
allowing the motor to run at a more efficient RPM.  But, I haven't used 
those specific motors, so you'll need to experiment to make sure they 
can handle the higher voltage and speed.

BTW: Balancing motors, batteries and drive train is perhaps the hardest 
thing in this hobby.  Change one dimension and everything changes.  Much 
of it can be designed out on paper, but it still takes benchtop tests 
and field trials to iron out the kinks.  Be prepared for change ...

        Frank "Dr. Trial and Error" Pittelli

        Frank P.