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I'm working on a project wherein I have a motor that spins a wheel. The wheel has two hard stops spaced some angle apart. The motor must rotate to one hard stop, change direction, and return to the other hard stop. It's required that the motor apply a force to the hard stop, so a limit switch does not work here. As such, my idea is to run the motor until it stalls, detect the stall current, then reverse the direction before the windings have any chance to heat up meaningfully.

I'll be controlling the motors with a PWM controller that has a 10A nominal output. My question is: is it ok to use this controller despite the fact that the stall current of the motor will exceed this? This question also extends to the power supply I'll be powering it with. Can I spec the power delivery for the nominal power draw of the motor? If the power supply cannot deliver the full power drawn at stall, is it possible to still damage the controller or the power supply? I do not require the full stall torque, I just need some fraction of it.

Thanks

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  • \$\begingroup\$ Depends how OCP is implemented in the controller. Does the datasheet mention anything about it? \$\endgroup\$
    – winny
    Jun 10 at 17:36
  • \$\begingroup\$ docs.google.com/document/d/… . The only excerpt I can find is "Maximum current up to 13A continuous (without heatsink at 25℃) and 30A peak (10 second). Current limiting at 30A". Regardless, I don't need 30A, should I still spec my power supply to be able to deliver this? \$\endgroup\$
    – Jules
    Jun 10 at 17:40
  • \$\begingroup\$ I believe what you are looking for is current foldback, see here: e2e.ti.com/support/power-management-group/power-management/f/… \$\endgroup\$
    – winny
    Jun 10 at 17:51
  • \$\begingroup\$ Consider this potential pitfall: You're reversing motor direction at the endstop. Be aware that reversing motor direction requires a large current surge - likely even more than stall current. \$\endgroup\$
    – glen_geek
    Jun 10 at 17:54
  • \$\begingroup\$ @glen_geek that's a good point. Though the motor is coupled to the wheel which it rotates, so the end stops should bring it to a complete stop. \$\endgroup\$
    – Jules
    Jun 10 at 17:57
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If the motor stalls whatever voltage goes into heating the coil of the electromagnet. This could be estimated if you have the coil resistance and the voltage going into the coil and the power equations.

At the end of the day, it's the voltage (and for PWM the duty cycle) that determines how much power a load receives (because the load will limit it).

Can I spec the power delivery for the nominal power draw of the motor?

Under normal conditions this will be determined by the load (torque) and the speed of the motor (so look at the load curves for the motor) There will also be a loss due to heating.

If the power supply cannot deliver the full power drawn at stall, is it possible to still damage the controller or the power supply?

This implies that the design puts a larger load on the supply than it can handle, what happens next depends on the supply. Some supplys will drop out and go into a limit cycle, some will shut down. Most certified supplys can handle a direct short for a given length of time so consult your power supply manufacturer or documentation for how it can handle shorts.

In addition a larger load than the rated current will drop the voltage on that power supplies rail, which could have consequences for any other load on that supply (like a controller). It would probably not be good for a supply to droop or drop out in any design (unless the probability of that event occurring the probability was very very low)

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  • \$\begingroup\$ Thank you for the reply. I'm not well versed in motor control or power delivery so I'll likely take some time to try to comprehend this. Could you check my understanding here? In short, I should avoid any kind of power draw on the supply that exceeds its rated output, even for short periods? In the case of powering a motor controller, the maximum power that would be drawn from the supply is the maximum draw that the controller can handle (even if the motor could theoretically draw more current), correct? \$\endgroup\$
    – Jules
    Jun 10 at 18:12
  • \$\begingroup\$ I see your problem. Size the power supply for the max power from the motor in a stall condition, plus the inefficiency of the controller. (If the motor in a stall could max out at 100W, I could use a 1000W controller and the max draw would be a little over 100W) \$\endgroup\$
    – Voltage Spike
    Jun 10 at 18:14
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Any DC motor controller should have current limiting built in. Preferably, the limit should be adjustable. A good adjustment range would be 50% to 150% of the motor's safe continuous current rating, depending on the motor design and intended use. Generally the current limiting function would be a closed loop active current regulator. The current regulator would be an inner loop that would have the error signal of an outer voltage loop as its reference. The current loop would supply the current required to satisfy the outer loop up to the set current limit, when limiting, it would provide a hard override on the voltage to prevent the current from exceeding the limit. There are many PWM DC motor controllers available that satisfy that descriptions.

DC motor controllers of the above description can be powered by supplies that protect themself by shutting off when their current limit is exceeded. The power supply should be selected such that the power supply will shut off only if the motor controller fails. The power supply shut-off probably should have a manual reset since shut-off is a sign that the motor controller needs to be repaired or replaced.

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