0
\$\begingroup\$

I'm running a 250W worm gear motor rated for 25A at 12V (300W).

The power supply is a 12V lawnmower battery that can supply a lot of current. An MCU drives a relay H-bridge, the gear motor drives a shaft. The motor turns the shaft a half-rotation in either direction until it hits a physical barrier and cannot move, then the motor is expected to stall. A current sense resistor detects the stall and the MCU shuts off the H-bridge. There's a 100ms delay upon detecting the stall before the H-bridge is shutoff, otherwise the MCU was falsely detecting a stall on the inrush current.

After the first 100ms stall on the physical barrier the motor failed. It appears to be a short circuit inside the motor. It no longer draws current and is non-functional even when directly connected to a power supply with no physical resistance. Perhaps upon stall the motor drew more than the 25A current it was rated for and burned up the internal circuits?

enter image description here

enter image description here

\$\endgroup\$
5
  • \$\begingroup\$ Yes, you've almost certainly burnt out the motor windings. There might be some kind of internal fuse that you can replace, but I image it's just the windings themselves. You can stall some motors like this, but it really depends on the motor. Also you're more likely to destroy the gearbox doing this sort of thing (think shock loads as it rams into the end stop). \$\endgroup\$
    – Jon
    Jun 14 at 10:39
  • 1
    \$\begingroup\$ Can you fit some limit switches? \$\endgroup\$
    – Jon
    Jun 14 at 10:43
  • \$\begingroup\$ @Jon a limit switch is an option but I’d prefer a simpler design, like a power drill with a good gearbox that can handle the shock load. I’m trying to figure out what kind of current limiting circuit would be appropriate, or are you saying I may just need a different motor? \$\endgroup\$ Jun 14 at 12:17
  • \$\begingroup\$ It is rather unusual for a motor to burn out from being stalled for only 100 mSec. Many motors are rated to survive stall current for 25 seconds or more. In some cases I've seen up to 60 seconds. I would double check to see if your limit switch shutoff is working correctly to prevent future mishaps. \$\endgroup\$ Jun 14 at 12:38
  • \$\begingroup\$ @MathKeepsMeBusy I did some testing and didn't find problems with the MCU code to switch the motor off upon detecting the stall state from the current sense resistor. But on your suggestion I added an extra condition to shut the h-bridge off on a timer as a backup regardless of the current sense resistor. This guarantees the motor will never run for more than 1 second whether stalled or not \$\endgroup\$ Jun 15 at 2:15

1 Answer 1

0
\$\begingroup\$

I confirmed with the manufacturer, the stall current is 160A and without current limiting stall will immediately burn the motor windings.

I think the solution is an analog circuit for limiting current below the rated current of 25. I intend to replace the relays with power MOSFETs and use a circuit like this:

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.