Does speed controller affect battery runtime ?

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.
Thanks !
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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
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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
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pogo wrote:

Yes. Quick Google find (first hit, in fact):
http://www.merkle-korff.com/technical_advice.asp?idP
-- Gordon
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What
?
Thanks !
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pogo wrote:

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

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