Re: Does speed controller affect battery runtime ?

Wow - thanks for the detailed explanation. This ones goes in my ever
expanding notes folder !
Thanks !

"mlw" <mlw@xxxxxxxxxxxxxx> wrote in message
news:X-GdnWbZmdzosLbZnZ2dnUVZ_s2dnZ2d@xxxxxxxxxxxxxx
pogo wrote:

I just completed a "running test" of my latest robot frame ( just
letting
it run in a circle to see how long the battery lasts, how hot motors
get,
etc ), with the motors connected straight to the battery.

Would using a speed controller, in general, sacrifice any battery time
as
a trade-off in controlling the speed ? For example, if I rigged up the
speed controller to run full speed (like I have it now) should I expect
about the same amount of run time or less ? What about half speed? When
I
ask this, I am talking about using controllers such as the Devantech
MD03,
MD22, or the L298 Dual H-Bridge kit from Solarbotics.

I am running 2 12vdc vending machine type gearmotors + small caster on
an
8 lb frame, lawnmower wheels, level hardwood floor, from one 12vdc 5Ah
SLA
battery. I was quite surprised that it ran for over 25 minutes with the
motors barely generating any heat! Seems like it could easily go another
25 minutes! So when I add in the controllers should I expect any
performance changes. I plan on using a separate battery for the
logic/motherboard, etc.

As for battery life, only one thing affects how long the batteries will
drain, and that's the amount of current the load draws.

Now, the interesting part about a good PWM motor controller is that
theoretically possible to get better battery life using the controller
than
not. Sounds unrealistic? Wait, and listen up its fun.

What drives the motor is the magnetic field generated by voltage across
the
coil windings. The current into a coil increases over time until the coil
reaches saturation, then the current limitation, or maximum current, is
the
internal resistance of the motor. Once you hit this point, you are burning
power in the form of heat not EMF.

Now, current into a motor produces torque. Torque produces acceleration.
Acceleration is limited by friction or load. When torque and load are in
equilibrium, you have a steady speed. A rotating motor produces "back EMF"
in the same polarity as the voltage applied. So, if you apply 12 volts to
a
motor that is free spinning, the motor may be producing 11 volts back EMF,
giving you 1 volt differential. While a motor under load may spin at a
lower RPM and produce less back EMF, say 8 volts, or at stall, 0 volts.
That's why when a motor is free wheeling, it draws much less current than
at stall.

Now comes the theory about how your controller can save power. The trick
is
to create the equivalent of a PWM current source. Remember, the current at
"stall" is when the motor is not moving and the coil is saturated. For the
sake of discussion, lets say it is 12A at 12 volts, that's 1 ohm effective
internal resistance. At stall, your motor will be heating up to the tune
of
144 watts. Measure the stall torque. Now, with a dynamometer with the
motor
not stalled (spinning), see what the current is just before the motor
stalls. It should be less than stall. What would even be better, is to
measure the current against the torque curve. Find a current that produces
the most torque per amp. Set your PWM current source's maximum effective
current to this current because any more is just wasting power.

So, you can save quite a bit of power with almost no noticeable loss of
power.








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