I have an existing setup with a moderately not-small DC motor to which a spool is attached, and the spool pulls a wire against a spring to move an arm up to a stop. The goal is to be able to switch the arm between two positions from some controller. So the motor is spending most of its time either off, or energized but stalled (against the stop). The arm only has to stay in the pulled position (and the motor stalled) for 20 seconds at a time. But we need to turn it on and off a couple hundred times a day, 2 days a week.
The motor is a brushed DC motor salvaged from a disabled person electric cart. It is rated 24V 30A. I measured the coil resistance and it is approximately 0.8 ohm. From this and my basic understanding of motors I deducted that the stall current is Vsupply / 0.8 (because stalled it's a resistor), and that should also be the maximum current ever running through it (because the inductance goes against current change).
The existing working system uses a 12V car battery, which provides enough starting torque to pull the arm against the spring and keep it (stalled) against the stop. The motor is switched on and off using relays on an Arduino shield (3 relays share the load).
So what I tried to do next is replace the battery with a mains AC to 12V DC power supply rated for 15A (= 12V / 0.8ohm). This is cheap chinese crap from ebay. It worked at first, but after a few evenings of use, it failed. The PSU kind of works under low loads (just the Arduino), but when I energize the motor the voltage drops to 8-9V. I can't see anything blown inside, so I assumed it might have been slightly under-rated for the motor (if it outputs slightly more than 12V the motor would have pulled slightly more than 15A).
So I purchased another similar 12V PSU rated for 30A. It also worked fine at first, but failed after a short while.
So my first question is: Is my math OK? Is the maximum current ever going through a brushed DC motor equal to Vsupply / Rcoil?
If so, should a cheap switching power supply be able to provide that kind of power, or is there some kind of technology incompatibility? Or is it just a quality problem? If so where to get a decent one and what would be the cost?
If my idea just can't work (I'd love to know why), can you suggest alternative solutions?
The car battery solution is really annoying us because we need to constantly attach and detach the battery charger we borrowed (which also can't be returned). So any solution using mains power would help immensely. The only constraint is it would have to be relatively cheap, and not involve too much DIY (I can modify an off-the-shelf PSU to add some output filtering, I don't have time or skills to design and build one from scratch).