Shop OBEX P1 Docs P2 Docs Learn Events
DC Motor Question — Parallax Forums

DC Motor Question

JohnBot115JohnBot115 Posts: 13
edited 2009-12-28 14:55 in Robotics
Hi,

I'm very new to robotics, so sorry for the stupid question. But I don't quite understand how to calculate how many amps a motor will draw, with load.

Lets say I have a 3 to 12 volt DC motor capable of about 520 oz-in. The specs say that at 12 volts it draws no load of 45mA.

Great, at first I thought that I could just get a 12 volt battery capable of producing 1 amp, and I'd be good. But if I intend to put a load of about 500 ounces on it, from my understanding, it will draw more amps. That's ok, but how do I know whether to get a battery capable of handling 3 amps or 7.

So, how does one calculate amps needs? Thanks!

In case you're wondering, here's the motor I'm thinking about purchasing.

http://www.servocity.com/html/4_rpm_gear_motor.html

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-12-26 20:20
    The link you showed doesn't give very much information. They do list a "no load maximum efficiency" current of 160mA and a torque at "maximum efficiency" of over 500 oz-in. It doesn't make a lot of sense to describe "no load maximum efficiency", but I assume that they're describing a load around 500 oz-in. and the current drain would be around 160mA which is well within your planned battery capacity. The only way to know for sure what the motor needs is to get one, put it in a circuit with an Ammeter and measure what it actually draws under various mechanical loads.
  • JohnBot115JohnBot115 Posts: 13
    edited 2009-12-26 21:16
    Thanks so much for the reply. I can certainly do that- put a meter in the circuit and measure it. Guess I just need to make sure I have enough amps available if it needs it.

    Just a quick clarification: If I were to buy a DC wall transformer rated at 12 volts and 3 amps... is there any possibility of burning the motor out? I'd like to buy a transformer that I can hook up to my breadboard for prototyping. That said, I'd like to get a one with some power. Just to have available for projects generally. So 12v / 3 amps seems good. But if that can burn out a motor only rated at 12 volts with a max efficiency rating of less than one amp, maybe I should not?
  • Mike GreenMike Green Posts: 23,101
    edited 2009-12-26 22:39
    First of all, a motor (or anything else) will only draw what current it needs. The issue of "burn out" only arises if something is wrong, either a short circuit or a stalled motor. In both cases, if the power supply is big, it can supply large amounts of current into a short circuit or motor windings where the motor is stalled. An easy solution is to use a fuse or circuit breaker rated at the maximum amount of current that the device is designed to need under normal circumstances.

    One possible disadvantage in buying a power supply rated at much more than you expect to use is that they're often not well regulated and the no-load voltage can rise well above 12V (in your case). That's usually not a problem because it drops quickly to near 12V with any load, but it's something to keep in mind, particularly if you have something else connected to it that can't handle the higher voltage like a voltage regulator that can only handle up to 15V on its input.
  • GWJaxGWJax Posts: 267
    edited 2009-12-27 03:45
    Mike Green said...
    First of all, a motor (or anything else) will only draw what current it needs. The issue of "burn out" only arises if something is wrong, either a short circuit or a stalled motor. In both cases, if the power supply is big, it can supply large amounts of current into a short circuit or motor windings where the motor is stalled. An easy solution is to use a fuse or circuit breaker rated at the maximum amount of current that the device is designed to need under normal circumstances.

    One possible disadvantage in buying a power supply rated at much more than you expect to use is that they're often not well regulated and the no-load voltage can rise well above 12V (in your case). That's usually not a problem because it drops quickly to near 12V with any load, but it's something to keep in mind, particularly if you have something else connected to it that can't handle the higher voltage like a voltage regulator that can only handle up to 15V on its input.

    Very True Mr Green but one thing about DC motor are interference or feed back to a MCU or other circuit. Below see the photo on connecting a non grounded DC motor case to prevent or cutdown on feedback spicks aka Noise control.

    Jax

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    If a robot has a screw then it must be romoved and hacked into..
    motorNoiseControl.JPG
    499 x 125 - 8K
  • JohnBot115JohnBot115 Posts: 13
    edited 2009-12-27 19:14
    Thanks everyone. Glad to hear you can't burn out motors. Hopefully that goes not just for amps, but voltage too. I literally, about 30 min ago, accidentally hooked up just over 12 volts / 1 amp to a small voice recorder / playback chip circuit that is designed for about 3 volts. Needless to say, it no longer works. Good thing I bought enough parts for two circuits.

    The motor in question is rated at 12 volts. I'll start with 9 volts / 1 amp... then move up to 12 volts max 3 amps. We'll see how it goes.
  • FranklinFranklin Posts: 4,747
    edited 2009-12-27 19:41
    Somebody said...
    Glad to hear you can't burn out motors
    This is not true. It is true a device will not use more amps than it needs but a stalled motor thinks it needs more amps than it can handle and will heat up and melt parts if left stalled and powered by a big supply.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • JohnBot115JohnBot115 Posts: 13
    edited 2009-12-27 21:39
    ok, got it. If it's stalled, it can be a problem. But, for example, I can hook up a 3-12 volt motor to a 15 volt power supply and it would be no problem, correct?
  • kwinnkwinn Posts: 8,697
    edited 2009-12-28 03:04
    It might still overheat and cook the windings. At the very least it would run at a higher than rated speed, draw more current, and get hotter than it would at 12V. Even somewhat below the maximum rated voltage it is possible to cook a motor if it is stalled or overloaded.
  • JohnBot115JohnBot115 Posts: 13
    edited 2009-12-28 14:55
    Ok, Makes sense. So, generally as a rule, I'll run the motor at or below it's rated voltage. But if I have a power supply at 12 volts 1 amp or 12 volts 3 amps... I should be fine -it will only draw what it needs.

    But I won't connect it to a power supply of higher than 12 volts. Only problem being, as Mike said, that you have to be careful that the un-regulated power supply does not rise above 12 volts for too long. If I need to, I'll either monitor that with a meter in the circuit, or put in a fuse / breaker.

    Let me know if my understanding is off... and THANKS so much for the replies!! It's really helping.
Sign In or Register to comment.