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Topic: Motor Power (Read 834 times) previous topic - next topic

vek11

Hey, I recently bought an Adafruit Motor Shield 1.1. I have a 6-12V DC motor and I want to use it as a power supply for the motor. So would using 6 AA batteries(9V total) be enough to power the 6-12V motor?

zoomkat


Hey, I recently bought an Adafruit Motor Shield 1.1. I have a 6-12V DC motor and I want to use it as a power supply for the motor. So would using 6 AA batteries(9V total) be enough to power the 6-12V motor?


Hmmm..., gosh..., hmmm...

Not knowing the size of the motor, who would know?
Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

Krodal

I don't know if this is cross-posting, but it seems that you have a single problem. So it would be better if you started a single topic, instead of four.
http://arduino.cc/forum/index.php/topic,119001
http://arduino.cc/forum/index.php/topic,119005
http://arduino.cc/forum/index.php/topic,119006
http://arduino.cc/forum/index.php/topic,119007


CAP_Cadet

Hey

9V falls between 6-12V, so I guess you're fine!! Check the amps on your motor. You should be fine, but just check.
________________
Joseph Anand-Maker
joseph@anandfamily.org

vek11

I'm pretty sure my motors have good amps, check out the link;
http://www.abra-electronics.com/products/29DM12V-Single-Shaft-6V%252d12V-DC-Motor.html

cr0sh


I'm pretty sure my motors have good amps, check out the link;
http://www.abra-electronics.com/products/29DM12V-Single-Shaft-6V%252d12V-DC-Motor.html


From the above link - the following specs for the motor:

29DM12V Specifications:
Single Shaft 6V-12V
DC Motor: Operate range 6V-12V, Nominal 9V constant,
No load: speed 7600 RPM, current 40mA.

At maximum efficiency:
speed 5928 RPM, current 143mA, torque 12.1 g-cm, output 0.74W


Learning to read specifications is imperative to creating successful electronic projects. You've told us that you plan to 6 AA cells in series to get you 9 volts. However, you haven't told us anything about the cells (they have specs, too!). So - which AA cells do you plan to use?

The fact that you are saying you want to get 9 volts from 6 AA cells highly suggests that you are going to be using alkaline or carbon-zinc batteries for the motor power, as each cell from those chemistry types nominally outputs 1.5 volts each. If you are planning on using rechargeable AA cells, though (like NiCd or NiMH, for instance) - know that such cells have a different voltage output; for the two mentioned, it is 1.2 volts per cell (thus, 6 cells will only give you 7.2 volts - still enough to power the motor, but not the 9 volts you were expecting - also note that when you purchase R/C batteries, they are usually sold "by the cell" - so a "5 cell" battery is a 6 volt pack, a "6 cell" battery is a 7.2 volt pack, etc).

Ok - so, assuming you are going to use alkaline cells, but not knowing which manufacturer you are going to select, we can only guess as to how much current the battery will deliver (expressed in units called "amp-hours" - ie, "Ah"). From this wikipedia article:

http://en.wikipedia.org/wiki/List_of_battery_sizes (you might want to bookmark this for later)

...we can see that an "average" AA alkaline cell can deliver approximately 2700 mAh (millamp hours). Note that carbon-zinc will only give you 1100 mAh of output, and that common rechargeables (NiCd/NiMH) are potentially all over the map when it comes down to output. In reality, even standard AA cells can have different outputs based on the manufacturer - you should check the datasheets for the cells from the manufacturer when possible.

Why is knowing this important? Well - how long do you want your motor to run? The only way to have any sort of handle on this is to know these details:


  • Current draw of the motor (running and stall) - noted in amps (A)

  • Current supply capability of the battery - noted in amp-hours (Ah)



Knowing both of these, you can get a pretty good idea of how long your motor will run for a given battery size. So - how do you figure that out? Fairly simple math.

We already know the current draw of the motor (at least, by the given specs - your actual implementation may have a higher or lower current draw, and is something you need to determine as well):

No load: speed 7600 RPM, current 40mA.

At maximum efficiency:
speed 5928 RPM, current 143mA, torque 12.1 g-cm, output 0.74W


Now - I am going to make a grand assumption that the above is at 9 volts. Unfortunately, the specs do not indicate that this is so, though they do give a clue that the motor runs nominally at 9 volts. The truth is your current consumption will change depending on the voltage you run the motor at, so keep that in mind when you do your actual measurements.

Anyhow - we see from the above that under "no load" the current draw is 40 mA; likely, your motor will have a load on it - so they do say that at its maximum efficiency it draws 143 mA of current. That means, though, the motor is still turning - we don't know that its stall current is, but it is likely higher than 143 mA; potentially much higher, but probably not in this case of such a small motor. You could probably safely double that number to get an idea - but it would be better to directly measure it by applying the appropriate voltage to the motor while keeping the shaft motionless, and measuring the current with a multimeter placed in series with the motor, set to measure current...

So - we'll just work with the numbers as is. We'll assume that you've made a perfect system, and are operating the motor at it's maximal efficiency. So - it draws 143 mA. How long will the motor run with the given battery? Here are our numbers again:


  • Battery (6 AA alkaline cells for 9 volts): 2700 mAh

  • Motor: 143 mA



Conveniently, everything is expressed in "mA" - so we can just divide them:

2700 (mAh) / 143 (mA) = 18.88 (hours)

So - in a perfect world you can run the motor for 18+ hours on one set of cells (running constantly, at it's most efficient loading). Not bad! Of course, in the real world this isn't a true number: You may not always be running it at its maximum efficiency, you probably aren't going to run it constantly, your batteries may have different ratings, the batteries themselves will change voltage as they are discharged, your load may change (depends on what you are doing), etc.

You need to keep all of these things in mind; note that if the motor was stalled, and we make the assumption that in stall conditions the current consumption doubles (it might triple! who knows?) - your run time is halved. Once again, fairly simple math. Also, none of this takes into account anything else those batteries might be running, or might be flowing through - for instance, if you are controlling the motor using an h-bridge, and the battery is used exclusively for driving the h-bridge/motor combo, the h-bridge itself will consume some current (look at the datasheet of the h-bridge for the specs on what current consumption it has too!). If the battery is being used to control other things, more current consumption.

Finally - also note that if you throw more battery at the problem (to increase current output - let's say you added a second 6 cell pack in parallel, to allow for 5400 mAh of output) - you might be fighting a circular battle if the battery pack is being carried by the device (like say for a robot vehicle, for instance): Adding extra weight in the form of a battery will mean your motors have to work harder, which can increase their current draw, which may not be made up for by the new pack...and around and around you can go (just be aware of this - always measure, measure, measure - you might just need more efficient motors, or a change in some other part of the design to lighten the load).

I hope this post helps you understand the importance of having all the details and specifications in front of you for your project. You can "back of a napkin" some things, but at a certain point you need to sit down and work out things with real numbers (otherwise, you might simply end up throwing a lot of money at a problem wondering why it isn't working out how you think it should).

:)

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

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