Let's Make Robots!

Power Screwdriver Motor

So I bought a few power screwdrivers for the main drive motors for my new robot project. I have started creating a rocker bogie suspension system out of PVC pipe. This suspension will require 6 drive motors. The power screwdrivers run on 4 AA batteries. So the motors are 6 volt. By browsing the interwebs I figured out that one AA battery supplies about 2.4A. So I figured that each motor would require 9.6A. But since I have 6 motors I would need 57.6A. This would require a huge SLA which would weigh about 20lbs and cost around $100. This is simply unrealistic for my project (the weight, I would be able to pay for it somehow). So my question is do these numbers for the current draw of the motors seem accurate? If so, are there any alternatives to a 20lbs SLA? Sorry for the long question and thanks for your help in advance.

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The AA battery supplies 2.4 Ah (amp/hours) that is, it can supply roughly 2.4 amps for 1 hour, 1.2 amps for 2 hours etc. You can't simply take this number and multiply by 4 to get your motor draw --this won't work at all. Instead, you can simply measure the draw of your motors with a meter while actually running them or, measure the resistance of the coils and then use ohm's law to figure the current. To cut to the chase here, your math is not quite right. I personally like real-world tests, so I would actually test your motor's current draw, then multiply by 6, add a little fudge room (because the motors will draw more when under load) then figure out what kind of battery you would need. Also, you should do a test of the current draw when the motor is stalled to be sure your motor driver can handle the load when all 6 motors are locked up.

Ok thanks. I knew that something was probably wrong because that seemed like a lot of current. So to test the current draw I would just put a meter set to current in between the battery and the motor? Thanks so much for your help.

Your meter needs to go in series between your motor and batteries. There is probably a 10A setting on your meter and the leads may have to be plugged into different holes for this test. 

Ok. Thank you so much for your help.

So when I performed the current test my meter started at about 340mA but it then it started into a steady drop and I stopped testing at about 320mA. Should I just use the 340mA in my calculations?

That is actually a lot lower that I would have guessed. But great! That is going to save you some money when you get to buying motor drivers. Sure, you can calculate from there, maybe bump it up to 500mA for your math. Even better, I would perform the same test but this time, grab the shaft of the motor with your hand and physically force it to slow down. Create a "load" for it. See if your numbers bump up a bunch or not --this way you can start getting a better picture of what they will draw when there is a couple kilos of robot on top of them. Also, if you feel brave, go ahead and force the motor to stop completely (just for a second or two, just long enough to get a reading) --this is what the motors will draw during a "stall" and if you want to be 100% safe, your motor driver should be able to handle this reading x6. All in all, I woud say you are probably in the range of maybe 3 or 4 amps total draw, which (if you were just running your motors and not extra servos, etc) you could probably get a good 1/2 hour out of a AA pack. 

Ok thanks

Yeah I plan on running each motor simultaneously but I want to have control of each motor individually. I want to have 6 different H-bridges. Or possibly 3 dual motor drivers. I think I am going to steer it differentially for now. I plan on doing the stall test so that I can be more accurate when picking a battery. I will definitely do the voltage test as well. Thanks for your help!

I am pretty sure those draw use more than 340ma - that's just the freewheeling current. To get a sense of the maximum current you must power the motors with the amp meter connected, then try stopping the motor with your hand (or use somethng else if the motors are too strong). I think the current draw will easily jump to 3-4amps. I have also used drill motors and a 5amp controller worked just fine. Don't let it work at full load for long periods of time though...