INTRO into Small Motors,...and their commutator actions. - Page 1
Forum section: electronics / beginners

author: RJSV
author_rank: Super Contributor
timestamp: March 11, 2022, 08:17:27 pm
content: Hello,Most small PM motors around, like in toys, and those little 'ink' accessories in printers, all have that 'Three working with Two, in the moving parts, here is some simple concepts:Second figure has the typical triangle or DELTA shape, that's your 3 coils, actually literally in series, around.But I wanted to mention, first, the 'nice', 2-Step sort-of SINE WAVE wanna-B...As you work out, that 2 to 3 type interface, notice the pattern shifts perfectly (looking) for a half-way point, in the resulting drive; that's directly caused by having coils resulting IN SERIES, thus rough estimates of being 'half-power' step (loosely).Most interesting, this Commutator method, applying simple 2 polarity plus and minus, has 'dead spots' where ine coil is, obviously, short-circuited, momentarily while the copper spring contact rubs, a bit flat, even, across the 3 contact surfaces (metal).

author: Benta
author_rank: Super Contributor
timestamp: March 11, 2022, 08:29:40 pm
content: So the question is?

author: RJSV
author_rank: Super Contributor
timestamp: March 11, 2022, 08:39:47 pm
content: The subtle thing (those are the worst questions).WHATS UP, exactly, waveform-wise, as that dead spot in the waveform is not going to be an 'ideal' SINE.You have a REAL, tough constraint, here, by shorting any particular coil...Especially if a 'naturally' expressed waveform 'might' be off a few degrees, from the commutator shorted center...In that vein, I've heard of so-called 'shaded poles' for inducing a particular motor to always favor turning in one direction (AC motor).But would be interesting, if there is LAG or LEAD that gets 'corrected', via brute force (shorting does that).Thanks

author: gcewing
author_rank: Regular Contributor
timestamp: March 12, 2022, 02:17:34 am
content: The "dead spot" probably isn't quite as dead as you think. The coils have inductance, so it takes time for the current to fall to zero during the shorted period, and to build up in the opposite direction afterwards. The current waveform through each coil is probably rather more smoothed out than your stepped diagram suggests.Also, a stepped waveform that has a zero period can be a better approximation of a sine wave than one that doesn't. Think about rounding a sine wave to a set of discrete steps. There will be some period of time for which the value is closer to zero than anything else.

author: RJSV
author_rank: Super Contributor
timestamp: March 12, 2022, 04:10:36 am
content: Thank you, possible, that the more SINE looking response, of current flow, helps; I'm wondering if that switching convieniently was result, of history...in other words might not have been done that way, directly deliberately, just used often after (that) smoother current flow.That makes for less RF, and perhaps motor wear, on brushes. Especially, the 'feature' of not needing to 'care' about aspects of an (arcing) brush or commutator graphite block; the contact can, by nature, be 'shorting', between contact plates, usually.The 3 coils exhibit a usual, 3 phases, 0°, 120 °, and 240 °.The thing NOT checked here, involves the simple order, of the three motor coils;...that might be CW, clockwise, 'A', 'B',. 'C', ;. OR CCW around the motor axis, for C,B, A order, around, on the motor coil wind mounts.Sounds very academic; Send an IMPULSE (DC step), in, to a coil. The first thing, a magnetic field is expanding, helping limit current and slowing waveform rise. But I've also considered, might be a 'ring' of some sort, 'on top' of the slower curve.Such as 2 MHz or time constant similar. (?)Plus, each dc PM Motor I'm dealing with, has 4 uFd cap, on dc input.General, region, is 10 revs per second, not much more as an audible 'whine' sets in.A 30 revs per second motor, before any gear train, that's going to be 1800 RPM, a respectful speed.If my est is right, might be about a '4 mSec' exponential rise, with '2 mhz' ring...You could, reasonably see, on scope, about 8 or so little rings, on the incoming (5 V) pulse, due to fast switched rise time. Of course, part of that faster signal is due to slower capacitor, at 4 uF.IF... I had any estimates correctly.

author: amyk
author_rank: Super Contributor
timestamp: March 12, 2022, 05:08:50 am
content: They only have 3 coils because 2 wouldn't make for self-starting and any more would be too expensive.If you are into motor theory, you may enjoy reading:https://scholar.archive.org/work/sgzr55esbnh2vipz5oayhmndfm/access/ia_file/crossref-pre-1923-scholarly-works/10.1049%252Fjiee-1.1914.0040.zip/10.1049%252Fjiee-1.1916.0086.pdf

author: RJSV
author_rank: Super Contributor
timestamp: March 13, 2022, 03:59:54 am
content: Right now, I'm thinking, about what, literally, is a magnified 'rise-time', really exacting, and, with HUGE variations, came up with a 'ramp', going up during a 'Metal Slide' action.The figures are with:30 revs per second is 1800 RPM,Around the 3 piece commutator, that's,360 degrees, in 33 mSec. orsimplify to 330/33 is about 10 degrees, per mSec.If, by a stretch, approx 1 degree of coverage, while 'sliding' contact; that's 100 uSec and whatever frequency that's corresponds to.That is, an assumption, that at very tentative contact, over very small starting contact, will not be as fully conductive, due to small cross-section etc.At any rate, you have THAT rising edge, impulse into the coil, plus some small capacitance, between windings.I'm thinking, some sort of 'damped' ringing, perhaps only a minor amount, (on scope).

author: RJSV
author_rank: Super Contributor
timestamp: March 15, 2022, 09:09:25 pm
content: It's maybe, still not small enough scale, but figure 'its got to be somethan.'Figuring, by the time you start out dividing a 100 micro-meter, or Micron, that even with good commutator metal, conductance gets low, at 20 Microns...(not sure)!With diagram, includes a 'rise time' curve, at 100 percent equaling 100 uSec. Very roughly.The point being; Doesn't Rise Time translate, to a frequency content, even tho it's a one-time impulse.And that being so, there 'could' be some potential 'Ring', there.Just asking, because, PM motors are suprisingly complex, or 'esoteric', even.(I've been looking at how the usual, 3 rotor coils act, to cancel the 3 currents ..good luck with that...).