Connect with us

induction...

Discussion in 'General Electronics Discussion' started by BGB, May 14, 2015.

Scroll to continue with content
  1. BGB

    BGB

    154
    11
    Nov 30, 2014
    some recent observations imply my understanding of induction seems weak.

    at first, I used some smaller diodes in a circuit, expecting the current from the inductive flyback to to be small relative to that driving the load (say, one drives 1A into a coil, maybe seeing 50 or 100mA going back up the diode, with the diode existing mostly to limit voltage spikes).

    like, I figured it would have been more like a small capacitor, throwing a small amount of current, which then quickly gives out as the coil is satisfied (basically with the coil and diode in parallel).


    then I observed some diodes getting rather hot (dunno how much heat is ok for diodes), and observed that the flyback current was equal to the current going in (a little over the rated values for the diodes, I was using 1A diodes and measuring around 3A of flyback, with switching at around 6kHz at 50% duty cycle).

    simulators seem to support this, and as well that reducing the duty cycle seems to reduce the voltage but seemingly increases flyback amperage relative to input amperage:

    estimated (simulator output implies this, but doesn't give average values, and the graphs are difficult to decipher in this case), say:
    12v input at 10% duty cycle gives around 1v across the coil, but if there is 100mA input, there is around 1A of flyback current.

    I am guessing as duty cycle goes down, total input current decreases, and at some point this largely goes away (no voltage means no current).


    some of this was noticed when trying to run an automotive alternator turned into a 3-phase motor, for example, got it to spin up with these settings (my variable power supply was only able to supply 4.4v 4.5A in this case):
    field coil: 50% duty cycle, measured 1.5v across field coil, 1A/1A flyback;
    phase coils: 9% duty cycle, about 0.4v across the coils, and apparently around 3A per phase (or around 9A of flyback from the phase coils);
    got motor spinning at around 100 RPM, with very little torque (I suspect the lab supply is a bit weak for this, and a motor this size may require a fair bit of power to be, well, powerful).

    this seemed like some rather crazy measurements, but I guess they are more sane in retrospect.

    I mostly had fiddled with the duty-cycles and similar until I had got it to spin, also determined that I may need to beef some things up before moving to higher-power testing (I prefer to minimize fried components).

    side notes:
    the field coil was wound with what looks like 20 AWG (as a single big coil with steel teeth wrapping around the sides from the top and bottom);
    the phase coils looked like about 5 turns or so of 14AWG wire, with each phase existing as 4 sub-coils wound in series, and then the 3 phases are connected to a central common wire.


    thoughts / comments?...
     
  2. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    The current in a flyback diode will initially be the same as the current in the inductor at the time it is turned off. It will then decrease as the energy in the inductor is dissipated. The rating of the diode must be greater than the voltage across and the current in the inductor.

    Bob
     
    Arouse1973 likes this.
  3. BGB

    BGB

    154
    11
    Nov 30, 2014
    voltage rating, not hard, as most of the diodes I have seen have fairly high voltages (1000v), but the current rating is insufficient in this case (hence probably why they were getting hot, and in these cases I was seeing more than the rated current through them).


    as for continuing breifly and then dissipating:
    yes, this is what I would expect.

    however, I think with PWM, the switching is fast enough that the inductor doesn't have time to dissipate, and I suspect the current goes around in a circle (sort of like the coil is a merry-go-round, and the PWM pulses are kind of like occasionally hitting it to keep it going).

    I am guessing the values I see on a multimeter are likely the averages over time, as the multimeter doesn't really see the kHz pulsing.


    I am guessing the reason I am seeing a higher amperage through the diodes than from the input, is because the input current is in brief large pulses (followed by no current flow), which average out to a smaller value, whereas the current through the diodes is basically the flow for all the time where there is no pulse, and the current is running in a circle.


    at least in the simulator, the current through the coil is a fairly continuous value, with a small sawtooth pattern. the coil in the simulation is 100mH, no idea what the values are for the coils in an alternator (nor how to measure it, most examples show using an oscilloscope or similar, but all I have here is an $5 multimeter...).

    do know with the field coil, if I pull the wire (with no flyback diode), I get an arc for what looks like around 1/4 second or so (when it had been running around 3 amps or so). also, with no flyback diode, it was able to blow up a power transistor...

    the phase coils don't seem to have any (particularly noticeable) arc when disconnected (so I suspect they have much less induction).
     
  4. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    What is it you are trying to do? If you supplying a PWM to a coil with a flyback diode across you basically have build a buck-boost converter with a shorted output.

    Bob
     
  5. BGB

    BGB

    154
    11
    Nov 30, 2014
    I am mostly trying to understand the specifics.

    in particular though, it is for when using an alternator as an electric motor, trying to get a better idea what all is going on (could be relevant for using it efficiently, or figuring out how things might be done differently in terms of winding the coils, *).

    the PWM would be for controlling the power going through the coils, which controls:
    how much power is used;
    how strong the magnetic field is;
    ...

    well, and if the duty cycle is too high, the power supply voltage drops significantly (and it just rocks back and forth rather than spinning). however, if the duty cycle is too low, then it doesn't turn either (no real power to the motor, and no real voltage across the coils).

    field coil resistance seems to be around 5.0 ohm, and phase coil resistance seems to be around 0.06 ohm.


    *: for example, many people who have converted alternators into motors have gone and replaced the 14 AWG phase windings with 18 or 20 AWG windings, I am assuming to get a stronger magnetic field while using less current.
     
  6. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Oh, it is motor control, I missed that somehow.

    Bob
     
  7. Ratch

    Ratch

    1,089
    331
    Mar 10, 2013
    Are you writing a blog or asking a question? I don't see any question marks in your above discourse. In fact, the whole post seems to be rambling, disjointed, and discombobulated. What, specifically, are you asking about? It does very little good to rattle on about how you plan to apply, what? --diodes, coils, capacitors, motors .... A schematic sure would be helpful, too.

    Inductance is the ability to store electrical energy in the form of a magnetic field.

    Ratch
     
  8. BGB

    BGB

    154
    11
    Nov 30, 2014
    yeah.

    I suck at expressing thoughts in an organized manner.

    I naturally tend to think of things visually, and have to try to express the things I see as words (well, some of my thoughts are also text, so it is mostly a mix of text and images).


    as for the topic, it is basically a thing of, "do I understand what is going on here?..."
    and "why exactly are there more amps in a coil with such a low voltage, when seemingly the input amperage is lower than the amperage which is traveling through the coils?".


    I will need to go draw up a schematic for this.
     
  9. BGB

    BGB

    154
    11
    Nov 30, 2014
    here is a partial diagram, which shows most of the relevant coil-related electronics.
     

    Attached Files:

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-