I know that stalling DC motor can cause overheat because of excessive current. If i will limit the current to the rated current of motor, is there still issue to damage the motor ? to be more specific I have 24v worm geared motor and I want to stall it for 3-5 seconds with rated current. I think the gears will get warm, but how critical will it be for the motor.
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\$\begingroup\$ A powered 'stalled motor' still exerts torque - quite capable of stripping gears. The windings of the motor get hot due to I^2R loss. \$\endgroup\$– JIm DeardenCommented Jun 10, 2015 at 15:46
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1\$\begingroup\$ The gears won't get warm because they're not moving, but the motor will because it's passing current like JIm said. If the motor has a fan built in (most do), then the rating probably assumes that the fan is running, which it's not in your case. That's probably what'll kill it: lack of proper cooling. \$\endgroup\$– AaronDCommented Jun 10, 2015 at 15:54
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1\$\begingroup\$ Note that some motors rely on spinning to provide sufficient cooling when running. And Jim is right, I wouldn't stall any kind of gear motor intentionally, unless the gears themselves were designed for it and made of super awesome alloy steel. Got a part number for your motor assembly? \$\endgroup\$– user39962Commented Jun 10, 2015 at 15:58
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2 Answers
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It depends but it should be okay. The main failure mode in the motor is due to the resistive heating of the windings when current is flowing through them. Since the resistance does not change with speed, the heat generated will be the same whether stalled at rated current or at full speed with rated current.
One issue to consider is that in most DC motors only two coils are energised at any point in the cycle (the six step pattern for 3-phase motors). If the motor is rotating this means that each coil only operates at a 66% duty cycle. When stalled two coils will conduct 100% of the time so the heat generated will not be spread evenly around the rotor. This will also apply to the current flowing through the brushes. Depending on the motor design it may be worth derating the current because of this.
Also note that if the motor relies on forced cooling you will clearly have to derate the current due to the loss of airflow.
Regarding the gear this will not have any issues. The torque output from the motor is directly proportional to the current and since you are limiting this to rated, the gear will be fine provided it is designed for the rated torque of the motor.
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\$\begingroup\$ Rated current at speed produces power to the load and motor losses will be small. Rated current at stall produces no mechanical output power and the motor overheats. \$\endgroup\$– Andy akaCommented Jun 10, 2015 at 16:46
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\$\begingroup\$ "the heat generated will be the same whether stalled at rated current or at full speed with rated current." Incorrect. As motors generate a back emf when turning. At full speed the current is much less than when stalled. \$\endgroup\$ Commented Jun 10, 2015 at 16:47
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1\$\begingroup\$ @Jim Dearden - you are correct if the motor is attached to a fixed voltage and the shaft speed varied, but the question is about what happens if the current is controlled so that it is the same as the rated current when stalled. In that scenario the voltage would have to be reduced because there is no back EMF, but it does not change the fact that resistive losses are only dependent on the current. \$\endgroup\$– JonCommented Jun 10, 2015 at 17:16
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4\$\begingroup\$ @Andy aka - in both cases if the motor current is the same and it flows through the same winding resistance (this does not change with rotor speed), the I^2 * R heating will be identical. \$\endgroup\$– JonCommented Jun 10, 2015 at 17:37
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\$\begingroup\$ Agree with Jon : current limiting to rated current implies reducing the voltage - this reduction compensates for the lack of back-EMF. \$\endgroup\$– user16324Commented Jun 10, 2015 at 19:17
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More info is needed on the motor, and what you are driving it with, but generally.... Yes, you can stall a smaller DC motor briefly (3 to 5 seconds is fine), at its rated full load current. The windings will heat up, and there will be some excessive heating of the commutator, but that usually doesn't affect the service life. By smaller motor, I mean under 3.75KW / 5 HP. Over that, there will possibly be more serious issues with the commutator and winding damage if the locked rotor condition is sustained.
The most important thing, is to be sure that you will be stalling it at maximum rated current or under. Most motor controllers allow for much more than 100% rated current.
