I recently ordered a small, high torque electric gearmotor (https://www.pololu.com/catalog/product/994) from Pololu. The gearmotor comparison chart (https://www.pololu.com/search/compare/51) has some notes on the bottom concerning the theoretical nature of stall torques, due to an inability to measure some of the motor’s stall torques without causing damage.
The #994 has no such warning, however. Does this mean that this motor can be stalled without drawing too much current and damaging it?
Due to the way we wish to use this motor it would be very practical to be able to stall it, while it draws power, for 4-5 minutes.
Any thoughts or information would be greatly appreciated.
It may be convenient from a design point of view to stall the motor for several minutes, but is it practical? Even if the motor driver and power supply can survive the excessive load, you may have to plan on replacing the motor very frequently.
As a test, you could connect the motor to a power supply, stall it for an hour or two and check for survival. But in my opinion, this is not a good idea.
Those footnotes are about the gearboxes destroying themselves if you try to stall the combination with the motor (at full power). You should picture a car smashing into a wall: the forces and torques are such that metal will instantly start getting mangled, so it makes no sense to talk about whether it can withstand that for five seconds or five minutes.
Stalling high-performance brushed motors for more than a few seconds is asking for something to melt: you are putting, say, 10W electrically into a small space and getting no power out mechanically, so all of that power is turning into heat.
If you really need to stall a motor, you should do it with a low-power motor and some circuitry to limit the stall current (a power resistor might be enough).
I can confirm with the 25D 1:172 that it will eventually die if you run it around 2.5A (this is of course >6V input since 6v is max 2.2A) It fails with a tooth or two fracturing, and none of the gears look smooshed (though I’m not sure if it’s noticeable when the gears deform.) I think it’s hard for Pololu to give much more accurate answers than the times they can give warnings, because these motors and gearboxes aren’t really sold with precise specs.
We had it running for a few weeks at 1.8A max, so this suggests that the current limit does float around the 2.2A limit they talk about in the warnings. We tended to only need about 1 second of stall current, because our system has a lot of static friction, but I also don’t know how much the system may have been damaged in other ways when I wasn’t working on it.
I would call into question Jan’s argument about stall current not being something you want to think about for more than a few seconds. Certainly it’s not something to talk about for hours or minutes, but when I did some napkin calculations (i assumed 60% iron & 40% steel, heat distributed throughout the motor and the motor being perfectly insulated [not shedding any heat], the 25D motor would take 2A for more than 40 seconds before heating from 20C to 60C. We’re talking about ((2 amp)^2)*2.8 Ohm = 11.2 joules into 82g of metal. It’s conceivable I could have been mistaken, but it takes more than 5 seconds to heat up these motors.
The times you might want to sustain a stall current is if you have say a hand/object holding back the motor for 2 seconds, and you want to start pushing the moment the hand lifts from the system, so you stall it for 2 seconds until the system is able to move. Or more generically, you have a de-ramped resistance and you want to start pushing from the start, even if that might be a stall condition until the de-ramp reduces.
But maybe I was missing the big picture. In the big picture I certainly agree that there’s no sense talking about sustaining stall current for minutes or hours. It might be relevant to talk about whether a certain stall can be tolerated by any gearboxes in the system for many repetitions.
I do not think this is a valid calculation. It assumes some kind of perfect heat transfer from the windings to the motor casing, right? I think, in practice, the motor windings and brushes can melt long before the case gets hot.
Absolutely true that my napkin-calculation does assume a perfect heat transfer, which is not at all valid. [
That said, the whole motor is metal and the windings and brushes shouldn’t melt until 100-160C, and I think should transfer the heat fairly well to the casing because unlike some motor that had a lot of air room between the internals for some structural reason [some bicycle hubmotors come to mine], this motor casing is right around the motor. My calculation also assumes no heat is leaving the motor, which is a conservative assumption because the casing could also be shedding heat.
But you’re right that my calculators were wildly presumptive in assuming perfect heat transfer, but they were also very conservative on the allowable max temp. In that sense and overall their point wasn’t to just contradict you but to sort of make a counterpoint that there is some relevance to asking if you’re allowed to hit stall current for a several few seconds at a time, because empirically and with my crummy calculations I find it doesn’t kill the motor- but we know it can kill the gearbox.
