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I am building my first arduino uno project to control a small motor, using the following guide as a reference http://learn.adafruit.com/adafruit-arduino-lesson-13-dc-motors?view=all

It says the following:

The motor that comes with Adafruit Arduino kits does not draw more than 250mA but if you have a different motor, it could easily draw 1000mA, more than a USB port can handle! If you aren't sure of a motor's current draw, power the Arduino from a wall adapter, not just USB.

I have a motor with the following specifications:

Operating voltage 1.5 - 5Vdc.
Rated load 8 g.cm.
Speed@rated load - 13000rpm
Current@rated load - 1A.
No load current - 350mA

What would happen if the motor couldn't draw enough current - would anything break, or would it just not run at maximum speed?

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    \$\begingroup\$ It all depends on how much the usb port can supply - anything could happen between, at one end of the scale everything works fine to the other end of the scale with your PC crashing \$\endgroup\$ – Andy aka Jul 7 '13 at 15:32
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Your motor draws 1 A at "rated load". The stall current could be significantly more, as Brian pointed out in a comment. As the warning you quoted says, that exceeds what a normal USB port can be counted on to supply.

When you exceed specs, there is no way to know exactly what will happen. Desktop computers usually connect the USB power line to the internal 5 V supply with just a polyfuse in between. If the motor trips the polyfuse, then the voltage could collapse for a while, probably resetting whatever you have connected to that power. Other motherboards may actively shut off the power for a while. It is unlikely that a computer is designed so that a short on a USB port can damage it, but that of course can't be ruled out.

If you want to use that motor, use a external power supply rated for the current. Playing games trying to get away with something usually just ends up without a working result and lots of wasted time.

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  • \$\begingroup\$ Minor correction : the motor can easily draw much more than its rated current, if the load on it is higher than the rated load - all the way up to the stall current which could be 3A or more. This only reinforces Olin's very valid point. \$\endgroup\$ – Brian Drummond Jul 7 '13 at 18:25
  • \$\begingroup\$ @Brian: Good point. Answer fixed accordingly. \$\endgroup\$ – Olin Lathrop Jul 7 '13 at 19:37

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