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I know nothing about motors. I have DC motor (sealed in a casing) that uses 6v/12v 1a (mains switching power supply) with 2.1x5.5mm DC connector. It has variable speed controller, reversible direction, and can operate at between around 5-100rpm.

The only available spec states it can be used with powersources of 6v to 12v. Presumably too high a current could damage it though?

I've used it safely with P9 batteries but they go flat too quickly.

What batteries do you think it's safe to use this with? car batteries? Lawnmower batteries?

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  • \$\begingroup\$ Could you please share a link to the specifications, or at least a part number? \$\endgroup\$
    – user17592
    May 15, 2013 at 14:00
  • \$\begingroup\$ Car battery is slightly over 12V, which might be a problem. You can certainly get 6V lead-acid batteries. \$\endgroup\$
    – pjc50
    May 15, 2013 at 14:52
  • \$\begingroup\$ PJC50 is right, if the car battery is more that 12V then more that 1A will flow. There will be some tolerance in the motor though, especially if its large, and if its draining batteries that quick then I'm guessing it is. \$\endgroup\$
    – Tim M
    May 15, 2013 at 14:57

2 Answers 2

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You don't need to worry about the current input, the resistance of the motor (which can be obtained using V = IR : 12V = 1A*R => R = 12 Ohm ) will limit this. What ever 12V battery you use will only supply 1A at that resistance (V = IR again). A larger battery will last for a longer amount of time because it has more stored charge (measured in Amp Hours). Just make sure you don't use to much voltage. If you use a higher voltage than stated you WILL be allowing to much current to flow (the motor will spin faster but will burn out sooner). Which battery you want to use will depend on how long you want to use it for. For example: if you buy a 20 AH battery @ 1A it will run for 20 hours.

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  • \$\begingroup\$ There is some misinformation in this answer about DC motors. See my answer below. \$\endgroup\$
    – Eric
    May 15, 2013 at 19:16
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The other answer that said you don't have to worry about current is correct but not for the right reasons. On a DC motor, from the perspective of the equivalent circuit, the current draw is going to be a function of the internal resistance and also the back-emf. The back-emf is a voltage of opposite polarity as your battery that will work to reduce the effective voltage across the motor windings. The back-emf is dependent upon the speed of the motor (among other things) so it will change as your motor speed changes. So you can't just divide voltage by current to find resistance.

Really, your current draw is going to be dependent on the load that your motor is turning. The higher the torque, the more current your motor will draw. This is independent of the capacity of the battery because the motor will only draw as much current as it needs.

If your voltage is higher than 12 V it may or may not cause a higher current, depending on the load. In general the only thing that a higher voltage will do is increase the speed of the motor. As long as the load torque remains the same, the current will remain the same. However, if the load is a fan or a pump or something similar where the torque increase when the speed increases, then your current draw will be higher if the voltage increases.

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  • \$\begingroup\$ Thanks for the additional info Eric. I get the basic idea. \$\endgroup\$
    – JohnH
    May 15, 2013 at 22:12

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