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Testing windings

  1. May 25, 2017 #1
    I get given a lot of brush-motor tools that are "burning', as well as assorted other damaged electrical items. I'm not an electrical engineer, just a fiddler with some background in physics.

    Now, Jim Hardy and Tom.G have just walked me through testing a 12V alternator, which was found, as far as I can see, to have shorted/low insulation stator windings. I bought an old Megger from eBay for the purpose.

    If I see a "burning' brush motor tool again, what tests can I do on it? The Megger found shorts between the phases on the alternator, but how do you test a single phase motor for a coil short that is not to ground - a ringer?

    The same question applies to transformers - if the short is not to ground, what test gear will find it?

    In other words, I have a multimeter, scope, bench power supply, ESR meter and a Megger insulation tester. What other gear do I need to fully test appliances found in the home and workshop?
  2. jcsd
  3. May 25, 2017 #2


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    The manufacturers Quality Control Department.:rolleyes:

    A "growler" https://www.google.com/?gws_rd=ssl#q=growler+tester Also lists instructions and make-your-own.

    A way to measure Inductance (perhaps an LCR meter. Decent battery powered ones start around $150USD.)

    A variable voltage transformer, commonly called a Variac. Be sure to get one that can be wired to provide output above line voltage. A 10Amp rating is plenty for run-of-the-mill consumer stuff. (Again around $100 to $150) 5Amp if you must keep costs down. The ones with a built-in voltmeter and outlets are handy but you can always add an outlet and voltmeter (and maybe an ammeter.)

    A clamp-on ammeter is handy to have too. If you get one that also reads DC current you can use it on electronic equipment. Since the stuff I do is more often electronic, I have an AC/DC clamp-on without a built-in meter. It has output jacks for a meter or 'scope connection. Works for power-line stuff, electronic equipment and automotive electrical systems troubleshooting.
  4. May 26, 2017 #3
    I vaguely remember growlers from when I was younger (insert joke here). Between your link and Jim Hardy's in my other thread what I conclude is that I should build a time machine and go back to a time when simple articles in popular magazines would show you how to construct mains-connected devices. Our voltage is 240V, but still, we seem to have lost that understanding that once you're an adult, you can be assumed to be capable and responsible.

    My ESR meter does do inductance, but I find the values meaningless without a known good unit to compare.

    Variac - yes. On my list, as is an isolation transformer.

    I do have an AC/DC clamp meter, a cheap UnitT one but it does seem accurate.

    What about testing the field of a brush motor?
  5. May 26, 2017 #4


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    Sorry, don't know of a good way; I've always used the smoke test for that! With practice, you could probably learn ballpark values for different size motors. @jim hardy, with his power background, may well have an answer though.
  6. May 26, 2017 #5

    jim hardy

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    I wish i had a handy field coil tester. It's so seldom i test such motors i never got around to building one. But i've thought about it.

    Since it's the job of a field coil to make flux, it'd be logical to measure how much flux it makes for given current.
    Which takes you right back to basic physics definition of inductance which is # of turns X flux per amp.
    L = NΦ/I
    i suppose an inductance meter would be a natural tool to try.

    But what value to look for? That's the sort of knowledge you'd pick up with practice.
    At first you'd just compare coils to one another.
    In a simple drill or circular saw motor with one could pass a known AC current through both field coils and measure voltage across each.
    If they read the same voltage it'd mean either they are both good or are equally bad.

    An old car battery charger has inside it a current limited transformer that makes a suitable amount of AC voltage .

    I've never done it. But that's the direction i would begin experimenting.
    with precautions of course for working with line voltage.

    If one is already fixing such power tools can we presume him capable ?

    old jim
  7. May 26, 2017 #6


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    Could that be done using a clamp-on ammeter as a field detector while the coil is energized with a low voltage?

    I know the clamp-on I use detects the Earth's magnetic field. For very low DC currents I have to either take two readings turning it around and averaging or orienting it in East-West direction.
  8. May 26, 2017 #7

    jim hardy

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    Worth a try.
    I was thinking of a small search coil, perhaps the coil from a little solenoid, connected to an AC millivoltmeter.
  9. May 27, 2017 #8


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    Going to tool cabinet to retrieve my Klein Tools CL200 clamp-on,


    to see if it'll measure the Earth's magnetic field. :oldcool:

    Edit: Indeed it does. Well, earth's field will induce a few mV when you move it around.
    Last edited: May 27, 2017
  10. May 27, 2017 #9
    As you say, without a manufacturer's schematic that includes expected inductance values, or a known-good unit, all we can hope for is a relative value that builds our experience.

    What about this: http://www.ebay.co.uk/itm/AnaTek-Bl...315165?hash=item2a2ea6e1dd:g:plkAAOSwBLlVDzSA

    Looks exactly what I need.

    Or... maybe I could feed pulsed DC into the coil and count the ringing on my scope?
  11. May 27, 2017 #10

    jim hardy

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    Now THAT's interesting.

    Please pardon the following ramble. Old guys just do that.

    I once devised an inductor test that seemed to work
    but it gave results comparable to a much easier to use computerized inductance meter we already had in the shop.
    I mention it because it's sorta similar to your "Ringing" test.
    I established about an amp DC through the coil
    then interrupted it and captured the 'inductive kick" in a capacitor trapping it there with a rectifier..
    Connected to the capacitor i had a Fluke DMM set for peak capture.
    I sized the capacitor for about a hundred volts on the coils i was measuring.
    It would find coils with shorted turns but so did the little computerized inductance tester .

    I do hope you build yourself some sort of tester. Just brainstorm about the properties of inductance and experiment away.

    Another thing i tried is Lissajous patterns on a 'scope.
    Remember that sinewaves are a special math function - their derivatives have the same shape as the undifferentiated one.. That's why for a sinewave voltage you get a sine-shaped current, really cosine because either one is the derivative or integral of the other.
    I built an integrator and produced the time integral of the induced voltage in the inductor. That should be a measure of the flux in the inductor because e=Nturns X dΦ/dt..
    I applied that to the Y axis of my 'scope.
    I measured the current through the inductor with a low ohm resistor and applied that to the X axis of my 'scope. That too should be a measure of the flux in the inductor because Φ = μNIA/Length.<---- observe absence of derivative
    A perfect inductor should give a straight line on the 'scope because flux and current are in phase
    any phase difference between the signals makes the trace more of an open circle.
    I had the luxury of a second winding on my inductors which i used for a zero current "Flux detector" to measure induced voltage. You'd need either a search coil(as i had in that second winding) or a Hall sensor to get your flux signal.

    A perfect inductor will give a straight line for a Lissajous pattern of ( Φ by integrating induced voltage or direct measurement ) versus (current by direct measurement).
    Its open-nes approaching a circle indicates there are other currents flowing besides what you're putting through the windings yourself.
    On my inductors that showed me just what lousy cores they had.
    With core out i got straight lines as expected . With the core partway inserted i got ellipses. as i inserted more iron core the trace got more circular.
    My cores were not laminated and had so much eddy current flowing in them they were suitable for only less than 10hz operation.
    I got about 30 degree phase shift because of eddy currents. (Incidentally that explained their odd temperature coefficient - as they warmed their inductance went up because the hot core's resistivity went up, reducing eddy currents)

    So i'm thinking- since you have a 'scope - and we can now buy Hall flux sensors reasonably
    One of these glued to a popsicle stick should detect flux:
    And the most striking thing i noticed about my inductors was their phase shift when extra current flowed.....
    And since power tools have a laminated core, eddy currents should be low,
    meaning more than a teeny bit of phase shift between measured current and measured flux should be a decent indicator of shorted turns.
    I envision a source of line frequency AC current an amp or less, a current measuring resistor, and a Hall flux detector probe.
    Readout could be either a 'scope, or an analog meter indicating phase shift in degrees.
    Just squaring up and AND'ing two sines gives a PWM proportional to phase difference....... a DC meter reporting average value of that PWM would work.
    You'd soon either become skilled at its use or figure out what's wrong with the idea.
    And if you said it worked i'd buy one.

    old jim
  12. May 29, 2017 #11
    Apologies for the late reply - you've given me lots to think about!

    I was thinking along these lines, based on the properties of inductors as follows:

    1: Flyback, as in the device you made. I made an electric fence energiser based on a 555 timer that could use this effect...
    2: Inductance, as measured by a commercial meter.
    3. Flux, as per your Hall sensor idea. This seems the most 'life-like' test, as magnetic flux is the goal with transformers and stators.
    4. Resonance - a tunable LC oscillator, L being the device under test? Needs a variable cap, though.
    5. Ringing - I found this:


    It appears the timer pulses the coil, and the ringing is used to clock a shift register.

    The ideal properties of a coil tester would be:
    Battery power, for portability
    Standalone use, ditto
    Quick go/no go results

    Off to the bench!

  13. Jun 5, 2017 #12
    Right, I've breadboarded (several times) the circuit above and can't get it to work. The 556 stays high and pulses low for about 2ms, which I think is right, but the first op-amp output resultant from this only dips a volt or so (does not go fully low). The collector of the transistor stays high and thus no pulse is transmitted to the test leads. They stay low. The 4017 outputs a zero regardless of what the test leads are connected to.

    I've subbed in a 2907A general purpose PNP for Q1 and 1N4004 diodes - are these OK?

    I'm sure it's something silly, but what?

    EDIT: I'm 'away from bench' at the moment, but have I confused the transistor connections? I would call the one at 1 o'clock Emitter, then collector at 5 o'clock, then of course base at 9 o'clock.
  14. Jun 5, 2017 #13

    jim hardy

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    555's take a huge gulp of current when the output switches.
    Try a 10 uf tantalum immediately adjacent its power supply pin, or a big aluminum electrolytic and 0.1 ceramic?

    A small one across R6 might help too.
  15. Jun 5, 2017 #14
    Thanks, Jim. It was actually a loose connection from IC2 to ground - a case of "et tu, breadboard?"

    The circuit works well on the primaries of switch mode transformers, showing a 9. It doesn't like the secondaries of same or my old car ignition coil, although the latter scored 2 on the secondary. It looks to be suitable only for higher impedance/Q windings without bulky iron cores.

    Do you think the design could be optimised for lower impedance/inductance applications, ie can it be made more sensitive? I noticed the ringing could be scoped on the secondary if the primary was being rung. Also, a good winding will ring (according to the scope) a good 20-30 times, suggesting the 4017 could be chained to another, giving an output count from 0-19, to allow range to measure lower inductance coils as well as higher.
  16. Jun 6, 2017 #15

    jim hardy

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    Good Find .

    Next question - why doesn't it propagate the rings you see on your 'scope ?

    That's a good one.

    Here's my first thought. Usually my first thought turns out wrong, but here it is.

    TL082 is operating outside its recommended range of input common mode voltage
    snips from your schematic and TL082 datasheet


    It's very confusing because later in datasheet (table 6.5) they show typical common mode range equal to V+ .
    Note with just a 9 volt supply, recommended input range of (V- plus 4) to (V+ minus 4) is only between 4 volts and 5 volts. That circuit has it at 8.4 volts.

    So i take that to mean some amps might work and others might not.

    I'd try about 4K in series with that diode to bias the inputs close to mid supply, and reduce R9 tenfold to bring them closer together.
    Then see if it counts more rings .

    That'll show whether we understand that part of the circuit.

    What do you think ?

    old jim
  17. Jun 6, 2017 #16

    jim hardy

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    Probably a good idea to add diode clamps from node R8-R9-C4 to V+ and V- , to protect TL082's inputs from the inductors

    old jim
  18. Jun 6, 2017 #17

    Ignition coil secondary (good, I think). Reading: 1 ring


    Field coil of vacuum cleaner motor (good). Reading: 4 rings.


    Small wall adapter SMPS transformer primary (good). Reading: 9 Rings.

    I also held the test leads like brushes across the rotor commutator, and turned. Each rotor coil gave a reading of 4 or 5.

    So as you see, it's more useful than I thought. What I really need is some known good and known shorted coils of the same type to calibrate it. I played around as you suggested with the TL082 voltages, managed to reduce them but couldn't get any decent ringing off the ign coil.

    I suspect the failure of the ign coil to ring is something to do with either the sheer size or the fact that the negative side is common to primary and secondary.

    I did try connecting a neon and 33k resistor in parallel with the coil secondary, then touching 6V to the windings. As I disconnected, the neon flashed briefly. Crude, but effective? No good for the primary, though.

    My inductance meter, however, gives a reliable reading of -30.6 uH for the ign primary and 30.0 uH for the SMPS secondary, both of which didn't read well on the ringer. Not sure why the minus sign but it may serves as a benchmark for others. Perhaps, amongst the eBay devices, breadboard builds and my Heath-Robinson approach I have all I need to test any coil?!
  19. Jun 6, 2017 #18

    Last one: Current-voltage characteristic curve for the ign coil secondary, using the scope's component tester. Horizontal line is open circuit, vertical is short. The very slight upward angle suggests high resistance, the looping a phase shift (?).
  20. Jun 6, 2017 #19

    jim hardy

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    I wonder if your ignition coil has shorted turns in secondary. How does its primary ringdown look ?

    Can you perhaps get a junk one from an old lawnmower or weedeater ?
    Q of that one is mighty low.

    Great scope shots !

    Ignition coil primary lissajous will be interesting. The more circular the higher the ratio X/R
    Those secondaries are tiny wire because the sparkplug current is small. That might explain low Q. Primary on other hand has to handle a few amps.

    Interesting work you're doing here !

    old jim
  21. Jun 6, 2017 #20

    Ign coil primary - a score of 2 on the ringer.

    Scope Lissajous is a near-vertical line, as for a short, but shorting the leads 'twitches' the line slightly thinner, so there is a slight circularity.

    The coil was a cheap one I bought online for use with my homemade electric fence energiser. It's done lengthy service sitting in the chicken house, pulsing away open-circuit (very seldom did it get a 'fox' path to ground). It could well have some damage.
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