[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
From: "Steve Tyng" <STyng-at-ACPTrust.com>
Subject: RE: High-Low gears [TANKS]
Date: Thu, 3 Feb 2005 08:58:27 -0500
Reply-To: tanks-at-rctankcombat.com

Eric wrote:

> I thought about this one first, but it is working backwards 
> from what I 
> want. If I understand correctly: A specific gear ratio 
> running at 12v will 
> produce X rpm's and Y in/lbs of torque. If you switch the 
> same setup to 6v, 
> then it produces 1/2 X rpm's and 1/4 Y in/lbs of torque. Conversely, 
> switching to 24v will produce 2 X rpm's and 4 Y in/lbs of torque. So 
> lowering the voltage to gain a lower speed range actually 
> reduces it's 
> ability to handle rough terrain.

Any decent DC speed controller will use pulse width modulation (PWM) to
control a motors speed.  PWM controls a motors speed by pulsing the
current to the motor at a constant rate (modulation) and varies the
pulse width from 0% to 100%.  At full power the pulses meet and become
one long stream of constant current.  There is no reduction of voltage
to the motor, only a variable reduction of time when current is
available to the motor.  This all happens several thousand times a
second so looks very smooth at the motor end.  The big advantage PWM has
over a resistance type of speed control is that excess voltage is not
shaved off and wasted as heat thus conserving battery power.  As this
relates to the question at hand concerning a multiple ratio
transmission.  You should consider that an electric motor produces its
maximum amount of torque at 0 RPM (i.e. when stalled) and the least
amount of torque at it's max RPM.  So in the example sited (rough
terrain, skid steering, etc.) a PWM controlled electric drivetrain
optimized to give you the top speed performance desired will still have
the most available torque at close to stall conditions (skid steer in
tall grass as an example).  At least that's how I understand it and I'm
no expert.

Steve Tyng