Does speed controller affect battery runtime ?

The speed controller does consume some amount of power plus the
efficiency of the h-bridge comes into play.  But if you're running it
at 50% max speed, even with an inefficient h-bridge, you should see
longer run times.

Did you measure the current draw on the motors yet? See what the
current is while the motors are free-wheeling and when you have them
under load. The most current draw occurs when the motors stall. This
may be listed in spec sheet for the motor if you have it. Be sure your
speed controller can handle the current draw you expect the motors to
have.

Are you going to be using a PC or a microcontroller with this bot?

Eddy Wright
http://www.wrighthobbies.net

I tried and I get a reading of less than 1 ohm for resistance across the
motor which surely isn't correct ? When I put the meter in amp mode & in
series with motor + battery I get nothing - no reading and no motor turning.
It's an old Sears digital meter. Maybe I need to change the battery. I'll
check it on some resistors I have and post more on it later.

I noticed on the specs for the motor it says "Overhung Load 15 Pounds". What
does that mean ? Max weight I can put on the motor shaft without damage ?

Specs:
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?xi=xi&ItemId 
11577011

Yes. Quick Google find (first hit, in fact):

http://www.merkle-korff.com/technical_advice.asp?idP

-- Gordon

?

Thanks !

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.



 

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