Hello, I recently got a DC Motor (Item #1592) and performed some tests on it. For the most part it performed according to specs: powered from a benchtop DC supply at 3.7V, I measured a no-load speed of ~23krpm and a no-load current of ~350mA.
However, the specs say the stall current should be 4A when powered at 3V, scaling up to 8A at 6V. I expected a stall current around 5A at 3.7V, but this motor only pulls 1.6A when I manually stop the motor from turning.
The stall current does scale with the power supply voltage, more or less as expected.
Another interesting thing I noticed is that the motor will not turn at all when powered with less than 2.7V. But the 1592 motor is rated to be powered as low as 1.5V. This makes me think maybe I got the wrong motor in a 1592 package.
I would appreciate any help you could offer to help me understand this discrepancy.
Just after making this post I figured out why the motor wouldn’t turn on for supply voltages less than 2.7V. That’s the limitation of the motor driver chip I’m using.
I also realized that the driver chip applies 0.7V less than the input to the output. So the numbers I gave in my first post were a bit off.
Neither of those issues affect my main problem with the stall current though. Here are the more accurate numbers for that:
With 3V applied to the terminals of the motor itself, the stall current is 1.6A. At 6V the stall current is 3.6A. This is vastly different than the specs.
Sorry for the confusion and thanks again.
I imagine that it would be quite easy to burn the brushes and commutator on a dirt cheap toy motor like that one, raising the contact resistance and lowering subsequent current draw and performance. They are certainly not designed to be forcibly stalled.
A much better test is to monitor the actual no load motor current, sampling at 1 kHz or higher, as the motor starts up (it will draw the stall current for the first couple of milliseconds). Try that with a new one, though.
Stalling DC motors like this could cause thermal damage on the order of seconds (or less), and the resistance of the motor changes as it heats up, so even in a sudden stall condition the motor might only draw the theoretical stall current for a very brief period. To avoid damaging your motor, we generally recommend keeping applied loads such that the motor never has to draw more than around 25% of it’s theoretical stall current.