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I'm making a robot/car and I need 2 dc motors. I had 2 but they were not strong enough.
It has to fit so it would be nice if the motor is small (like max 50mm length). The motor has to work on 12 volt or lower. My car weights less than 0.5kg. I found out that I need something like 300RPM.
Do somebody know where I can find it?
Thanks!
300 rpm is a very low speed for a DC motor, so you'll have gearing. 5-10k rpm is quite normal for DC motors.
You need to know current power/torque rating of your motors, otherwise you don't know whether any other motor is stronger.
I just used one of them with a speed control I knocked up using TIP31C a 5k pot and 3v battery and it won the day. My friend had a project going linked to his 3d printer. I was asked to get a turntable he had that was surrounded by leds giving much too much brightness to my mind, running almost stopped, as he said he needed it.
I had it running a one rev per minute and every time I turned that pot lower it stopped dead on me. Try as I could it would not run so low as to turn that turntable slower and my suggestion of it being belt driven so when the motor turned five turns the table turned one, meaning he could go slower he refused that idea.
He eventually took it away to try out and came back grinning. Whatever it was doing the one rev per min worked so it was smiles and beers all round.
If you have any ideas why that motor refused to go slower than one rev per min I would be grateful.
BFN
petyoung
I have on my desk a few types of geared motors and bare motors, I did not use them yet. The geared ones are heavy.
After spending hours on the internet I decided it is much better to buy a couple of them and experiment.In my opinion, the transmission is the critical point, rather than the power of the motor, which of course counts as well.
The cheap DC motors that one can buy don't generally have manufacturer's part numbers or available technical datasheets. And there can be several variations of motor with the same physical size.
This Mabuchi web page has useful info about sizes but I suspect most of the cheap motors on the market are chinese copies.
For powering 00 Gauge model trains the N20 size motors are a convenient size and only cost about £2 each. However I have several variations on the theme and {A} it can be difficult to know if the one in my hand today is the same as the one I had last week and {B} whether motor X is more powerful than motor Y.
It occurred to me a few days ago to measure the no-load current of the motors when powered with a controlled voltage (which happened to be 3.3v because that is what I had set my regulator at for another purpose).
I discovered that some of my motors consume 12mA, some 20mA, some 29mA and some 60mA. I did not measure the rotation speed but, in general, I think the higher current also signalled a higher speed.
Is it reasonable to assume from this simple test that the motors with the higher current will have higher torque and, if I was offered other N20 motors could I safely assume that one which consumes (say) 29mA is pretty much identical to the 29mA motors that I already have?
I also have some 130 size motors which are physically bigger. If I was to measure the no-load current (at 3.3v) of one of them what (if any) conclusion could I draw about it in comparison to the N20 motors.
...R
Thanks. I should have mentioned the magnets, and I forgot. And I have no means to know what magnets might be inside the motors. However the N in N20 signifies a rare-earth magnet so I am content to assume they will be similar.
The winding resistance will depend on the length of wire and the diameter of the wire. I have been assuming that the lower current corresponds to more turns and thinner wire.
I need to think more about the impact of the speed - higher speeds mean a higher back-emf. How does the back-emf vary with the number of turns on the coil?
I think your comments do point out that comparisons across different sizes of motor would not be meaningful.
I wonder how practical it might be to build a tiny dynamometer ?
...R
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I wonder if I started off with the wrong question ... maybe I should have been thinking about power rather than torque.
If all of the motors are powered at 3.3v then the 12mA motor consumes about 39mW and the 20mA motor about 66mW. Assuming they are roughly equally inefficient then the higher current clearly implies higher power.
And I am inclined to think that that would hold true for comparisons between completely different types of motor - always keeping in mind that I am not looking for great precision.
I guess motors with rare-earth magnets should be more efficient because they generate more torque and hence achieve their power at a lower speed.
However AFAIK the quality of the commutator - brush interface has a large impact on the efficiency of very small motors.
...R
If all of the motors are powered at 3.3v then the 12mA motor consumes about 39mW and the 20mA motor about 66mW. Assuming they are roughly equally inefficient then the higher current clearly implies higher power.
Yes, in concept but that’s a huge leap of faith. Are these new, clean motors? If not, those differences could just be frictional losses. Small, cheap motors are not known for their excellent shafts, bearings and brushes. If your dealing with an unknown motor, the only thing that would give any useful info is a dynometer. Measure the torque and speed, then you know.
I guess motors with rare-earth magnets should be more efficient because they generate more torque and hence achieve their power at a lower speed.
A definite maybe. They can create a stronger field for a given area. Field strength plus armature current determine the torque. That can be used to generate a higher torque per amp in the same volume but it might be used just to make a smaller motor.
However AFAIK the quality of the commutator - brush interface has a large impact on the efficiency of very small motors.
Absolute fact since cheap small motors use spring wire rather than carbon brushes. I doubt efficiency is even documented for “toy”motors.
There are only a few significant specifications for electric motors. The electric bike websites seem to have a good handle on this.
Electric bike hobbyists have many options for different specs inside basically identical motor casings. With a fixed case size, there are only a few variables to play with. If you want more turns of wire on the coils then the wire must be thinner. Thinner and longer equals more resistance.
The RPM per volt is an important figure. Each motor has a maximum speed for a given input voltage. Poor bearings or poor lubrication will slow it down a little but it is still a good figure to measure. If you are testing many motors on a constant voltage, you can measure RPM with an optical sensor and a dot of paint on the shaft.
But motors with high speed will have less torque for the same case size. If you can also measure speed with a known load that will give you a lot more information. Maybe test the time it takes to wind up a fishing weight on a given pulley size?
Gearing is also important. Geared motors can look very much like pure motors but they bring the thousands of RPM down to a speed that's useful for normal things like wheels and robot arms.
MorganS:
But motors with high speed will have less torque for the same case size. If you can also measure speed with a known load that will give you a lot more information. Maybe test the time it takes to wind up a fishing weight on a given pulley size?
That's not really the case - size (specifically rotor volume) and cooling determines the maximum torque
available, only mechanical strength and bearing ratings limit the speed. Its possible to get enormous
power from a small motor if run at very high speed (the hard part is reducing losses). For instance the
exotic motors from Celeroton: https://www.celeroton.com/en/products/motors.html.
More prosaically cordless drill motors achieve high power density by having a high speed (20krpm or so),
and efficient fan cooling.
Practical DC brushed motors are often limited by the commutator - which will arc and wear too rapidly
at very high speed as well as having high friction losses. Brushless DC motors often run significantly
faster for the same size, have no commutator, and have a lifetime mainly dictated by the bearings.
The link of torque to rotor volume is from the basic physics of the situation and the properties of
copper and iron. The torque depends on the radius times the cylinder area, which is equivalent to
volume. The limits of copper and iron determine the limit for the force per unit area of the rotors
cylindrical surface - iron alloys admit upto a max of about 2.2T of field strength, copper's resistivity
places thermal limits on the current density that react with the field. Moving to superconducting
windings or liquid cooled windings can lead to much greater continuous torque ratings by overcoming
the limitations of copper.
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