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| Toggle switch for small aircraft charging system in case of failure |
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| ITman496:
--- Quote from: richard.cs on April 16, 2020, 07:29:08 pm ---If you route the stator in on twisted pair then the noise shouldn't really be to much of a problem. The AC is low frequency, and whilst it will have switching harmonics from the regulator the energy is quite limited - it's impulses when the SCRs turn on and the repetition rate and therefore the overall energy content will be low. I'd expect it to mess with poorly shielded audio but not to have enough high frequency content to cause radio problems - SCRs are just not very fast devices. You could put the short circuit after the rectifier. It'll obviously stress the rectifier diodes a bit more but the current won't be much different to charging a dead battery. That would allow you to have a simple changeover switch with alternator output on the moving contact, battery one side and short circuit the other. You would need it to be an ordinary break-before-make switch and a permanently connected Zener, or better a TVS diode (basically a pulse rated Zener) on the alternator output to clamp the voltage for a few tens of milliseconds as the switch moves. This is exactly the same as your proposed load dump, just with a zero Ohm dump resistor. The power resistor would get you slightly lower (perhaps 20%, dependent on engine revs and chosen resistor value) rectifier currents when dumping at the expense of increased engine load. With or without a power resistor, a Zener or TVS to cover the switchover is required - 10 ms of switching time is forever as far as insulation breakdown and the semiconductors are concerned. Thinking about it a bit more this is probably the approach I would take, with a zero-Ohm dump. It's simpler than switching both output and stator, and the rectifier should cope just fine. PS: you can replicate the behaviour of a high power Zener with a low power Zener and a big transistor, though at >100 W it might not be that sensible. If you really want a load dump then it should probably be a resistor. --- End quote --- See now, this is why I'm glad I came here. You know exactly what you're talking about. How high are the currents going to be do you think, when it comes to the shorting the regulator output? The "lighting coil" as it is called is stated to have a rating of 120W. So presumably a normal current of 10A or so. I assume it will short circuit higher, though? I want to make sure my wiring and my switch are up to snuff, so to speak. Would hate to have it melt down! I'm also a little worried about the DC current but it's pulsed anyway so in theory that means I can use the AC ratings on the switch.. Now the question is, what TVS diode to use. Do they come in mounts that are easy to deal with? I assume thermals are irrelevant because its a pulsed load. I've never worked with TVS diodes before.. What voltage would be good? Presumably I want the reverse standoff voltage to be somewhere in the 18-24V range so it never tries to conduct while the system is running normally? |
| richard.cs:
The way it normally works is that the stator is approximately constant current. As it's designed for use with a battery that current will probably be slightly more than the expected maximum load current of a lightbulb or two to allow for charging, and the short-circuit current might be 20% more or so than the normal current. As a very rough guide a typical 12V system might make 20 V or so at tickover (and not manage full charge current) and 100 V or so at max rpm. If it were designed for 10 A output it might have a 9 Ohm or so stator reactance at whatever frequency corresponds to max RPM, so with 14 V across the battery you get a current of around (100-14)/9 = 9.6 A. At half revs the open circuit voltage will be half so 50 V, but the frequency is also halved so the reactance is halved and the current is similar: (50-14)/4.5 = 8 A, so the output current is fairly flat with rpm until low revs where the battery voltage becomes significant. Calculating the short circuit current of this example, at max rpm it's (100-0)/9 = 11 A and half revs is the same, (50-0)/4.5 = 11 A. This is a simplification which ignores a few points, e.g. that at low revs when the reactance is low the resistance becomes significant, eddy current losses in the core, etc. but it makes the basic point that short circuit current is usually only modestly more than the normal running current. If you're unsure of the rated current then it would be fairly simple to measure the short circuit current with a suitably rated meter. TVS diodes are normally rated in slightly silly units, peak power for a some particular test waveform. This is a little annoying, but basically corresponds to an amount of energy that they can absorb, better datasheets give this in Joules in addition to the peak power though it can be estimated from power specifications. The event is assumed to be adiabatic, so fast that no energy is dissipated from the TVS, it just gets hot according the the energy dissipated and its own heat capacity. As a rough guide a mechanical switch might take 10 milliseconds to change over. Note, you will definitely want a switch with a snap action, not a cheap slide switch or whatever, to make this reliably fast. If your TVS voltage were 20 V say (this is the voltage at which they're rated not to leak much) and it's many-hundreds-of-amps clamping voltage were 28 V (normally you only get these two points, and maybe a zener voltage at a few milliamps) then we could guess that at 10 A it might drop around 25 V. 10 A * 25 V = 250 Watts, 250 W * 0.01 seconds = 2.5 Joules, so now you have an energy spec to aim for. You can get them in leaded, diode-like packages. A common current waveform (due to the near-constant voltage the power waveform is similar) specified is 1 us rising exponential, 1000 us falling. Let's approximate it to a simple sawtooth 1 ms wide. A TVS rated to 1000 W with this waveform is really rated to about 0.5*1000*0.001 = 0.5 Joules, so you probably want a 5 kW rated TVS for your 2.5 Joules. Looking at a random manufacturer's datasheet for a 5 kW 15-20 V part: https://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_5kp_datasheet.pdf.pdf I shall pick out 5KP16A. It's rated to leak near-zero-current at 16 V, behaves as a poorly specified Zener around 19 V, and clamps to 26 V at 200 A. This particular one is 9 mm long so could be soldered across the back of the switch or anywhere else convenient. I'm not recommending that specific part, though it's probably fine for your needs, more trying to explain the selection process. |
| SkyMaster:
--- Quote from: ITman496 on April 16, 2020, 05:27:52 am ---Hello! I'm trying to figure out the best way to have a toggle switch to temporarily disable the charging system of a small 440CC engine. --- End quote --- Isn't the alternator Field that you want to disconnect (or is it short to ground?) in order to disable the alternator? In a light aircraft, the alternator switch is switching the Field of the alternator; there is no switching done on the output of the alternator. Now, with a 440 cc engine, what is your aircraft? :) |
| Nusa:
--- Quote from: SkyMaster on April 16, 2020, 10:23:42 pm --- --- Quote from: ITman496 on April 16, 2020, 05:27:52 am ---Hello! I'm trying to figure out the best way to have a toggle switch to temporarily disable the charging system of a small 440CC engine. --- End quote --- Isn't the alternator Field that you want to disconnect (or is it short to ground?) in order to disable the alternator? In a light aircraft, the alternator switch is switching the Field of the alternator; there is no switching done on the output of the alternator. Now, with a 440 cc engine, what is your aircraft? :) --- End quote --- That would be true if he had a modern alternator with field coils. He's got permanent magnets on the flywheel to create the field. Sometimes called a magneto, even today. Although in aircraft that term usually refers to units dedicated to firing the spark plugs. |
| ITman496:
--- Quote from: richard.cs on April 16, 2020, 08:54:54 pm ---... --- End quote --- Thank you again so, so SO much for not only the advice, but the education. People like you are why I love the internet so much. I feel a lot better about this. I will do a bit of research and figure out what TVS I need. A question, can I parallel them or is that a bad idea? I figure if I get one like you listed, putting two in wouldn't hurt? --- Quote from: SkyMaster on April 16, 2020, 10:23:42 pm --- --- Quote from: ITman496 on April 16, 2020, 05:27:52 am ---Hello! I'm trying to figure out the best way to have a toggle switch to temporarily disable the charging system of a small 440CC engine. --- End quote --- Isn't the alternator Field that you want to disconnect (or is it short to ground?) in order to disable the alternator? In a light aircraft, the alternator switch is switching the Field of the alternator; there is no switching done on the output of the alternator. Now, with a 440 cc engine, what is your aircraft? :) --- End quote --- It's an ultralight! More specifically, a Minimax 1100/1030R hybrid, with a kawasaki 440A 2 stroke engine! I unfortunately can't just disconnect the field winding, as much as I wish I could, as it is not a field winding driven alternator, but a permanent magnet on the flywheel moving past a single phase coil. Can't turn that off no matter how hard you try. ;) |
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