Motor question
Ugha
Posts: 543
I have a·few questions about normal DC motors (especially the cheap ones).
First, which is more likely to reduce the life of a motor, continous usage with low or no load for a long duration,
or short usage sessions under heavy load?
How bad is stall (load to high for it to turn) for a motor?
The motor I have will have its life cut to 1/4th normal if 5 volts are used instead of 3.
Can PWM be used to "make up" for having too high of a supply for a motor and extend its life?
Sorry for the newbie questions, I've never worked much with normal motors before.
First, which is more likely to reduce the life of a motor, continous usage with low or no load for a long duration,
or short usage sessions under heavy load?
How bad is stall (load to high for it to turn) for a motor?
The motor I have will have its life cut to 1/4th normal if 5 volts are used instead of 3.
Can PWM be used to "make up" for having too high of a supply for a motor and extend its life?
Sorry for the newbie questions, I've never worked much with normal motors before.
Comments
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
Post Edited (Leon) : 12/13/2008 4:01:03 PM GMT
Two things wear in a motor: The bearings and the commutator/brush system. Each of these wears when the motor turns, but the wear on either doesn't particularly depend on the load. So running it continuously, loaded or unloaded, will wear it out faster than running it intermittently, loaded or unloaded.
Series motors, and (generally) only series, tend to overspeed when run unloaded. This will cause accelerated wear. But if you control the speed somehow, then series motors will wear no faster than shunt or compound motors. If your motor would have its life reduced by a modest overvoltage, it is probably a series motor, which would run hot, and overspeed, with overvoltage.
Shunt motors will conduct large currents through the armatures when stalled, or nearly stalled -- so the brushes and commutator will wear a little (not much) faster when very heavily loaded but still turning. At stall, these motors will run hot -- perhaps hot enough to melt the armature connections. This would be undesirable. But some motors are designed for service at or near stall.
Bottom line: Run them intermittently, run them only with load, and don't let them stall or overspeed. They'll last a long time.
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· -- Carl, nn5i@arrl.net
One more stall-related question...
If the duration of stall is very brief (a half second to two seconds) would that be a big enough impact to run hot and increase
wear?
After the stall, no operation will take place for several seconds to several minutes, then followed by normal operation.
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
If you have something that is running 100% of the time 24/7, it requires a 'full duty cycle motor' which just costs much more. If you are reversing direction on a DC motor, the cheap motors may not have good enough brushes for it. They tend to adapt themselves to one direction and that is all. If you are burning out motors due to stall, you may just be using too small of a motor in the first case.
Regarding series wound, shut wound, and so forth...
With the advent of neodium magnets, such configurations are more common in larger, more powerful motors. They are trivial to most minature applications as permanent magnets are used. Most of the wear is in the brushes. The shaft and bearings are usually quite durible.
At 5 volts, I suspect that all the 'overvoltage wear' is in the brushes. You may need to look for a better, more rugged motor in terms of brushes. The copper enamel wire that is used for either a 3 volt or 5 volt motor more than likely has insulation that can easily withstand 50 volts or more. It is just easier to product more than adequate insulation on wire than it is to taylor insulation to small differences.
And a good motor will have cooling. If the motor offers no cooling, it is meant for only occasional use or a toy.
One alternative if you want good motors for a cheap price is to get a 12volt or higher motor. With the exception of drive motors removed from CD/DVD players, it is hard to locate quality 5 volt motors these days. You might find replacement tape cassette motors in some instances, but they are getting rare.
If you really need to overcome stall, a geared motor is a much better option. No mention was made of what RPM you are using, but the faster an electric motor turns, the cooler [noparse][[/noparse]due to internal cooling] and smoother it usually can run. The brushes are happier too.
There is an exception. Stepper motors can really provide a lot of torque at a snail's pace. If you look inside a 3.5" floppy disk drive, there is a little stepper that I suspect you could run at 5 volts and at a crawl. Sorry no specs.
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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
Does it require using diodes to recognize when the motor is producing an opposing voltage?
Current sensing is well documented on the Web. Google is your friend.
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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
Can you show us a practical method of measuring the back EMF by your method?
Just curious....it would seem removing the supply periodically would really upset the smoothness of mechanical power delivery.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html
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· -- Carl, nn5i@arrl.net
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
It is a crafty method of measuring motor speed with some limitations as to getting full power out of the motor and taking a reading with a forward/reverse capability.
It reinforces the idea that at stall, there is no rotation and therefore no back-EMF being generated, so current is only limited by the resistance of the armature winding. If all you want to do is protect against stall, measure the current with a linear Hall effect sensor or shunt resistor setup.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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Notwithstanding, I guess a fuse (for "stall detection") is out of the question?
I once tried using a hall sensor on a motor driver and wasn't happy with the results, too small a variance in the sensors output. Instead I used an Anologue Devices AD628 differential amp tha can be used with a shunt resistor to read the voltage across the resistor and amplify the difference, it has programmable gain and adjustable output levels to suit your needs. With adjustable gain, you can set up the output to swing a great deal (several volts) with moderate load on the motor, and with a stall you'd see a significant output swing. For stalls I suggest using this chip feeding an A/D or comparator with a trim pot to set your trip point.
Does back EMF measurement require amplification, like current measurement? If not, a comparator feeding directly to an I/O pin that is appropriately calibrated seems a much easier wiring job. And the software seems less complicated as well.
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How do you like my name change?
aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
1. Back EMF tells you how fast the motor is running. A DC motor with permanent magnets can be used in reverse as a dynamo. By spinning the motor mechanically it will generate a DC voltage. If you use PWM it will run the motor during it's active phase and then while it is not driving the motor there is a voltage that the motor is generating that is related to it's speed as if it were a dynamo.
2. Current measurement tells you how much current the motor is drawing or more correctly how much load it is handling. If the motor is stalled by jamming it then this is an overload and without any back EMF the motor looks just like a dead inductive load. This simple method is used to determine if the motor has stopped as in the case of a door opener, when the door as fully opened or closed then the motor will be stalled, the current draw increases sharply and the circuitry will turn off power to the motor or reverse it if necessary.
Nobody wants fuse (other than permanent fault style thermal fuses) because it necessitates maintenance. A polyfuse is a possibility though.
Leon is correct when he says that amplification isn't needed because several volts will be generated and you will probably need to divide the voltage and filter it with a small capacitor.
In saying all this it is way easier to actually hook up a motor and try it yourself, why miss out on the fun.
*Peter*
If you are hovering over it on the bench then yes, you could just use a fuse.
The HB-25 supposedly has protection circuits and should not rely upon a fuse for normal operation which would include motor stalls. If it does rely upon the fuse then it wouldn't go any further afield than my bench with me hovering over it.
*Peter*
This link http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html·and this comment [font=Arial, sans-serif]It has been a while since I learned about this kind of stuff but isn't this method measuring EMF and not Back EMF?[/font]
Is not the opposite true? At stall would not the current be at maximum? Therefore would not·the Back EMF be at the maximum? (as long as the supply was alternating or pulsating)
Current is maximum at stall because there is no back EMF opposing the applied voltage.
Incidentally, the idea of measuring the back EMF by momentarily disconnecting the supply ought to work OK with permanent-magnet motors, but not with other types.
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· -- Carl, nn5i@arrl.net
When we measure back EMF (voltage) we are measuring the voltage being generated BACK from the load (motor) rather than from the source (power-supply). This voltage can only be generated if the motor is spinning so that it acts like a DC generator. I remember doing a +12V design once where I would operate a grinder motor with a heavy wheel and the design called for an emergency stop switch that cut the power to the control board. Well, surprise surprise, guess what happened if the grinder was spinning when the emergency switch was hit and cut the power? Yes, the grinder motor would continue to generate enough power to keep the board and other motors running for a few seconds. Solution was simply to put a heavy-duty blocking diode in series with the motor and driver MOSFET. BTW, there was no way to shunt the motor as that would cause it to decelerate rapidly and the grinder wheel would unscrew and fly across the room with such speed and force that really curled your hair, thinking "Gee, that WAS a close one".
*Peter*
This is about a crummy hobby motor. This·is·not about production lines in plants without "facilities technicians" and plane trips around the world on a mission from·God·owing to a poorly designed control circuit.· If the simple stuff can be so poorly designed that the motor fuse goes flop-pot without good cause then·the stall-detecting, EMF-sensing, gold-plated horse-apple that's aeons past a rotten old fuse doesn't stand a chance anyway.
·
*Peter*
I apologize Ugha for taking your post off on a tangent
Thanks guys
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle