Quote:
Originally Posted by martin417
My Electric Circuits theory is a bit rusty, but I am pretty sure that an inductor appears the same as a wire to a DC current. I don't think there can be any advantage gained by using different components in place of the inductor. The purpose of the inductor is to work with the shunt capacitor to make the motor less electrically "noisy". Unless the inductor has a resistive component as well (more than an equivalent length of wire) the only effect of substituting the component will be more electrical noise in your system.
___pause to check the specs____
I just looked at the specs for the 3.9 microhenry inductor listed, and it shows a 2.3 ohm DC resistance. It also lists a max current of 280 mA. at the max listed current, the inductor would drop 0.65 volts. I doubt that .65 volt drop would account for a "significant" difference in pole climb time, but it could make a small difference.
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Your circuit theory cannot be much rustier than mine, however, the current to a brushed DC motor, direct as it is, still varies as the commutator redirects the current to the armature coils. This motor has a three coil armature, so two coils are powered at a time. The coils are inductors themselves, although they are not usually rated that way. One way to think of an inductor is a current sustainer. That is, it tries to maintain the same current flow across voltage fluctuation. (contrast to a capacitor that tries to maintain voltage in current fluctuations) A collapsing field in a 3.9 micro henry inductor will not be able to generate much current flow for this motor. The capacitor will be equally inadequate to maintain much voltage. The result is called a low-pass filter, because only the very high frequency parts of the direct voltage variation will be absorbed by this LC filter. Well into the radio range I'd guess without doing any math.
This brings me to the current rating of the inductor. Based on various other observations, we can be pretty sure that more than 280mA is being drawn by this motor in minibot operation. Probably more than that in FTC robot operation as well. So the mystery becomes, how can such a relatively low-rated component expect to survive these over-current excursions? by duty cycle? by conservative rating spec.? Another spec. I read mentioned a 40 degree C rise for their inductor at rated current. This setup won't allow for very much heat dissipation, especially being enclosed and in as close proximity to a plastic mounting as it is.
The one thing I do know is that replacement by a wire in one case allowed way too much current for the fuse I was using. There may be something else wrong with that motor, but it wasn't obvious visually. Meanwhile, I have replacement motors to use for testing using the thermal limiter device. I will postpone further repair attempts until time allows or more learned commentary here convinces me to have another shot at it.
Thanks for your consideration.