Re: Mechanical Speed Control [TANKS]

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

Martin Helsing wrote:
> Has anyone considered using variable resistors controlled by servos as
> an alternative to ESCs?
> 
> I know there is a lot of heat built up but I would think the one main
> disadvantage (current draw) can be overcome by bigger/more batteries

At the current loads required for our purposes, "a lot of heat" is 
better stated as "a pretty serious amount of heat".  Aside from the 
wasted battery energy (which is significant), the problems caused by the 
heat itself will have to be overcome.  Furthermore, as you reduce the 
voltage seen by the motors, they start to lose torque (our friend). 
People used to use resistor motor controls in warships in the early 
days, but even there the disadvantages make them impractical.

An ESC constantly hits the motor with full voltage (thereby maintaining 
torque) and limits speed by turning off the voltage completely, thereby 
conserving energy and reducing heat generation.  The heat generated by 
an ESC is due to the FETs switching on and off, but it is no where near 
as much heat as a resistor would generate.

        Frank P.