I have a motor that normally runs at about 0.65A. When stalled, this current shoots up to about 5.5A (according to the datasheet), which is far more than the low current ICs I'm familiar with (e.g. L293D) can handle. Using a >5.5A chip seems to me like overkill, since it will only rarely be needed (and are usually more expensive). Is there any way I could implement a hard current limit (say at 1.5A) that will cut power to the motor when it shoots up to its stall current? Or are there any good motor driver ICs that anyone can recommend?
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1\$\begingroup\$ How often does your motor actually stall? If it's a fairly regular occurrence, might want to get a beefier motor. If that's not an option, there are DC/DC regulators with a current limit built in. What is your power supply to these motors? How many motors? \$\endgroup\$– FullmetalEngineerCommented Jan 11, 2017 at 22:07
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1\$\begingroup\$ I think the simplest thing is to put a 1A fuse in series with each motor, maybe medium blow timing, since you will likely notice when the motor is stalled and stop the car immediately. Since it's not likely to happen very often, you can afford to replace the fuses. \$\endgroup\$– FullmetalEngineerCommented Jan 11, 2017 at 22:21
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2\$\begingroup\$ Or replace the fuse suggested by John Fu(se) by a resettable fuse (polyfuse)? \$\endgroup\$– Wouter van OoijenCommented Jan 11, 2017 at 22:22
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1\$\begingroup\$ It refers to how quickly the fuse opens when subjected to a current that's higher than its rated current. This is an informative article: littelfuse.com/~/media/files/littelfuse/technical%20resources/… \$\endgroup\$– FullmetalEngineerCommented Jan 12, 2017 at 12:34
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1\$\begingroup\$ Your timing will vary - now that I think about it, the medium timing may be too slow to prevent damage to your L293D in the case of a stall. I will add an answer below explaining in more detail. \$\endgroup\$– FullmetalEngineerCommented Jan 12, 2017 at 12:40
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2 Answers
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This is a common issue for large AC power systems. The starting current of a large induction motor can be quite large. So large the NEC has a bunch of special rules to allow larger breakers, thinner wire, and the like, where that would not normally be permitted in any other case.
One of the solutions in the industrial setting is to use an electronic controller to limit startup current - for instance a flux vector drive does this as well as allow fine speed control. Of course that is for AC induction motors.
Methods differ for controlling other motor types, but the same concept could apply. For instance an induction motor will give you plenty of back EMF if you control it properly... a series-wound DC motor won't, and you might just have to have a DC-DC converter be part of your controller which is capable of the startup current it needs at the least voltage it can live with, which would be quite low.
This can be part of a "prevent the motor from overheating" strategy, e.g. on a DC motor limiting current to continuous-safe-for-commutator so it can start moving the moment the user frees it; but also occasionally spiking current to see if the motor can get unstuck on its own.
answered Jan 12, 2017 at 1:20
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I think the simplest thing is to put a fuse in series with each motor. According to the Absolute Maximum characteristics section of the L293D datasheet, it can tolerate 1.2A for 100uS, or 0.1 ms. In this case, I would chose a fast acting fuse that's rated for 33% above the nominal current of 0.65A (per this timing app note by Littelfuse.). That would be 0.85A. Then I would look up some fuse datasheets on digikey to determine how quickly it opens when subjected to a current of 1.2A - which is roughly 140% of your nominal rating - and find a fuse that opens in < 0.1 ms under those conditions.
If you want to get fancy you can chose a DC/DC controller that adds cost and complexity to your RC car hat will probably not stall very often anyway. That would allow you to limit the current more intelligently.
answered Jan 12, 2017 at 12:45