Author Topic: Toggle switch for small aircraft charging system in case of failure  (Read 7796 times)

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Offline ITman496Topic starter

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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.

The charging system is a shunt regulator on a single phase stator and permanent magnets on the flywheel.  It's a 4 wire regulator.

The theory, as far as I can tell for how it works, is the regulator directly dumps into the battery after passing through the rectifying diodes in the regulator, and the regulator has SCRs connected across the stator winding BEFORE the rectifying diodes, and it will intermittently shunt the current of the stator winding to short them out when the voltage at the battery starts to rise too high.  As load increases, it shorts them less, and as load decreases, it shorts them more often, until zero load where theoretically it's holding the winding shorted as to not charge anything.

My question is this.  I'm building an airplane, and I know from past experience with friends with dirtbikes/motorcycles/atvs/snowmobiles, which all have this charging system, that regulators commonly fail, and their failure mode is to start just pumping power into the 12V system, overcharging the battery, boiling it over, and frying whatever is on the 12V system if it cant take voltages north of 17V or however high it decides to go.

My ability to disconnect the regulator has two benefits. 

One, on the ground, I can easily turn it on and off and see, hey, I'm charging when its on, and not when its off, due to the voltage dropping from 13 something to 12 something and then rising back up again.

Two, if the regulator fails, I can turn it off and just cruise home on battery.  The capacity of the battery is such that it can run the aircraft even with the backup electric fuel pump switched on for around 2-3 hours.  It's oversized for the demand because I need the CCA for the starter motor.  It's a big boy.  But when the aircraft is flying, normally, the only electrical demand is the flight computer, which draws 1.5 amps.  The fuel pump is mechanical (crank case pressure pulse driven) and the backup draws 1.6 Amps.

If I can't turn a failed charging system off, then my issue is under my seat, in my wooden airplane, is a battery that's having up to 130W (I have no idea what the overcharge curve of an AGM lead acid battery is.  How does the current absorption work as you go over the limit?) being shoved into it.  This turns an OK turn and head back for home situation into a land ASAP situation, which is far more annoying.  Also, I'd rather like to not find out just how high the unregulated voltages can go, and fry my flight computer.

So what I am asking here is...  How can I disconnect the charging system safely while not damaging it due to the aforementioned testing idea.

So far I've devised 3 methods.

A:  Disconnect positive wire from regulator from battery, let it do whatever.
Pros:  Simple, the wire for it even passes right where the switch would be located.

Cons:  Nothing to sink the rectified voltage from the stator into.  Voltage skyrockets, I've heard upwards of 100V p2p can be sourced from those stator winding at high RPMs!

B: Short stator winding on engine with toggle switch, cut it off at the source.
Pros:  As this is how the regulator works anyway, it shouldn't harm anything, right?

Cons:  Unnecessary parasitic load on the engine.  It won't be 130W, as while the stator can supply a relatively constant amperage across the rpm range, the voltage will rise and fall with rpm, and if you short it out, you reduce the voltage across it and the back EMF winds up reducing the resistance on the motor.  I think.  That's what I read, anyway.  The idea being that if your regulator on your motorcycle shorts the winding out, its not putting the full load of the electrical system on the engine, just a little bit.  (The numbers I saw were that a 400w charging system on a motorcycle will experience 60W of heat generation when the regulator is completely nullifying the output due to the battery being fully charged)  But the switch that shorts it will still have to flow considerable current.  How much does a stator winding supply at short circuit?

C: Run one stator winding wire through the switch before it goes into the regulator input.
Pros:  Completely open circuits the stator, removing its ability to generate any load at all, can't overcharge the battery, can't load down the engine, can't get hot.

Cons: Open circuit voltage of the winding can skyrocket north of 100V on some engines at high speed, or so I've read.  Is this bad?  Does the winding care?  The other catch I can think of is when you reconnect the winding back to the regulator input.  If you are unlucky, you'll close the switch contacts right at the highest peak voltage, exposing the regulator/rectifier to incredibly high voltages, blowing it out.

D: Do nothing. I'm overthinking this problem.  Get rid of the switch and just move on.

Does anyone have any experience with any of this?
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Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #1 on: April 16, 2020, 05:36:11 am »
Embedded here is an example circuit of how the system works that I found on google.  The example is 3phase, however, and mine is single phase, but you get the idea.

EDIT: However I would like to note that my regulator does not get an ignition circuit power source.  Fed from its own output.  Also, it is old, and this is an old design, so the regulation is likely not an IC, but a zener that triggers the SCRs when the voltage is too high.  It is not.. very exact regulation.

« Last Edit: April 16, 2020, 05:49:50 am by ITman496 »
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Offline Nusa

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #2 on: April 16, 2020, 07:23:15 am »
In general, the answer would be A. As for cons, what do you think will happen in the zero-load case? And how is that a problem?

However, if you've already got a proper fuse or circuit breaker panel, the answer may be D. You can pull the breaker or pull the fuse to kill the circuit.

If it's not a fused circuit, it should be, like everything else electrical in an aircraft. You can't afford to have the wire itself be the weakest link.

« Last Edit: April 16, 2020, 07:30:44 am by Nusa »
 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #3 on: April 16, 2020, 08:14:55 am »
My worry for A is that with nothing to sink the load into, the voltage coming out of the regulator will spike to whatever the p2p voltage of the stator winding is. Which may be above the rating of certain components in the regulator.  I've seen evidence of this when friends have ran ATV's without batteries in them.  It tends to melt the wiring harness from the surging voltages shooting through, the lights tend to run a lot brighter and pulse at idle.

It does have a breaker, but that will not really help because the issue is it not regulating.  When a regulator fails, its typical failure mode is to simply do nothing.  Which leaves the stator to dump freely through the rectifying diodes and charge and charge and charge the battery until it boils over and there's nothing left.  The charging current will never exceed the usual current you'd see through the circuit, it just will never stop.

Unless anyone has a better idea, I'm going to simply put a toggle switch that is normally always switched on that links the output of the regulator to the battery.  I'll then put a red flip cover on it so it can't be disturbed and is actively held down, conducting.  If the computer sees the voltage rise above 15 or so volts, it will throw an alarm, and if I determine its overcharging, I'll pull the cover up and flip the switch to isolate the rectifier.  Then I'll go and land and replace the unit, since at that point its already broken and it doesn't matter if it breaks more.

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Offline Nusa

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #4 on: April 16, 2020, 09:01:16 am »
If you don't have a complete circuit, there is no load. It's current that melts things, not voltage. So your anecdote about the ATV isn't terribly relevant. Yes, the charging circuit needs the battery to act like a capacitor and smooth the voltage, so no surprise the voltages are too high and pulsing. Visible pulsing at idle is simply because the frequency of the AC being converted to DC is very low when engine speed is low. It pulses at high speeds too, but it gets too fast to see with the human eye.

Most aircraft-type breakers are also intended to be used as switches, in which case you've already got what you're asking for. If you have a different kind of DC breaker, then never mind. I never said the breaker/fuse should pop in this instance, merely that one should be present for safety reasons.
 

Offline David Hess

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #5 on: April 16, 2020, 11:37:48 am »
My worry for A is that with nothing to sink the load into, the voltage coming out of the regulator will spike to whatever the p2p voltage of the stator winding is. Which may be above the rating of certain components in the regulator.

The rectifier diodes can be driven into reverse breakdown and damaged if the output is left open.

There used to be kits to convert car alternators to 120 volts which could work to power hand tools which used universal motors.  But the result was often a failure of one of the rectifiers in the alternator due to high voltage.

 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #6 on: April 16, 2020, 12:48:36 pm »
My breaks unfortunately are pop up only and push to reset type, I can not pull them manually.  Everything that matters has to also have a switch.  But that's besides the point.

That's interesting David!  It makes sense though, those windings can sure put out a lot if you let them!  I had no idea that was ever a thing.  I guess I know why it continues to not be one..
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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #7 on: April 16, 2020, 01:55:49 pm »
What does the generator AC voltage do when presented with a capacitive or inductive load? What does the waveform look like on a scope?
Two, if the regulator fails, I can turn it off and just cruise home on battery.  The capacity of the battery is such that it can run the aircraft even with the backup electric fuel pump switched on for around 2-3 hours.  It's oversized for the demand because I need the CCA for the starter motor.  It's a big boy.  But when the aircraft is flying, normally, the only electrical demand is the flight computer, which draws 1.5 amps.  The fuel pump is mechanical (crank case pressure pulse driven) and the backup draws 1.6 Amps.

If I can't turn a failed charging system off, then my issue is under my seat, in my wooden airplane, is a battery that's having up to 130W (I have no idea what the overcharge curve of an AGM lead acid battery is.  How does the current absorption work as you go over the limit?) being shoved into it.  This turns an OK turn and head back for home situation into a land ASAP situation, which is far more annoying.  Also, I'd rather like to not find out just how high the unregulated voltages can go, and fry my flight computer.
Have you considered using a LiFePO4 or NiMH battery instead? Weight really matters on aircraft, hence why lead acid is rarely used in those applications.
Unless anyone has a better idea, I'm going to simply put a toggle switch that is normally always switched on that links the output of the regulator to the battery.  I'll then put a red flip cover on it so it can't be disturbed and is actively held down, conducting.  If the computer sees the voltage rise above 15 or so volts, it will throw an alarm, and if I determine its overcharging, I'll pull the cover up and flip the switch to isolate the rectifier.  Then I'll go and land and replace the unit, since at that point its already broken and it doesn't matter if it breaks more.
What about have the switch redirect the generator to a dummy load?
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Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #8 on: April 16, 2020, 02:09:35 pm »
What does the generator AC voltage do when presented with a capacitive or inductive load? What does the waveform look like on a scope?

I'll have to get back to you on that as I am soon to get the engine running but not quite yet.

Presumably whatever a permanent magnet ac motor, single phase would do when presented with that, which I'm not sure.  A little outside my area..

Have you considered using a LiFePO4 or NiMH battery instead? Weight really matters on aircraft, hence why lead acid is rarely used in those applications.

I was actually considering a LiFePO4, but I could not get definitive answers on how they would cope with the charging system of a small engine like this.  I heard that they do not appreciate the large pulses of single phase charging systems, and much prefer the 3 phase ones.  Also, this regulator overcharging issue would go from thats annoying to catastrophic with one of those, so this issue is even more important.  Finally I'd need to get a new, more expensive regulator because lead acid ones float too high for the LiFePO4 cells. Not done any research into NiMH cells, however.  I kind of forgot they existed..

The thing that swung me to lead acid was simply cost and at the end of the day, a couple pounds difference at this scale.  It's no car battery, that's for sure, only a little AGM.  The cost and charging system retrofit savings made it worth it.

What about have the switch redirect the generator to a dummy load?

I was thinking about that, too.  Just have to find one, I suppose.  Maybe a chunky aluminum finned power resistor bolted on the other side of the floor into the air stream?  Or perhaps an incandescent light?
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Offline BradC

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #9 on: April 16, 2020, 02:57:43 pm »
My worry for A is that with nothing to sink the load into, the voltage coming out of the regulator will spike to whatever the p2p voltage of the stator winding is. Which may be above the rating of certain components in the regulator.

You are asking for a way to disconnect the charging circuit in the case of rectifier/regulator failure, and you are worried about damaging the regulator?

I'd be more concerned about an unrestrained stator generating enough voltage to cause insulation breakdown in the windings.

I'm with Mike. Switch it over to a dummy load small enough to be viable and big enough to prevent huge inductive spikes. A couple of well calculated high power resistors somewhere in the prop airflow should do the job and weigh bugger all.
 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #10 on: April 16, 2020, 03:12:59 pm »
My worry for A is that with nothing to sink the load into, the voltage coming out of the regulator will spike to whatever the p2p voltage of the stator winding is. Which may be above the rating of certain components in the regulator.

You are asking for a way to disconnect the charging circuit in the case of rectifier/regulator failure, and you are worried about damaging the regulator?

I'd be more concerned about an unrestrained stator generating enough voltage to cause insulation breakdown in the windings.

I'm with Mike. Switch it over to a dummy load small enough to be viable and big enough to prevent huge inductive spikes. A couple of well calculated high power resistors somewhere in the prop airflow should do the job and weigh bugger all.


Agreed.  Also, the reason for the disconnect without damaging the regulator was to allow me to disconnect it on demand, on the preflight run-up of the engine on the ground before takeoff to test to make sure its working.  But in retrospect, that's kind of dumb because I can see that its working by the voltage rise on the system.  If the battery is 13V or higher, it HAS to be working.

I'll instead use a SPDT switch and route the positive output of the regulator to a power resistor pack under the aircraft to take the edge off the pulses.
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Offline richard.cs

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #11 on: April 16, 2020, 04:05:52 pm »
Personally I would go for a shorting switch before the rectifier. Output voltage remains low so prevents diode or insulation damage and the load on the engine will be much lower than with a resistor connected. This kind of stator is usually designed to have high output voltage compared to the load and high series inductance so they behave pretty much as a current source (it's a nice trick - open circuit voltage and impedance both scale linearly with RPM so it's constant current independent of revs). They're specifically designed to be regulated by shorting so it does them no harm. Yes there's the I2R loss in the winding, but that current won't be a great deal higher than the normal charging condition.

Using a load resistor draws more power from the engine (a shorted stator looks largely reactive) and gives you the problem of needing to safely dissipate that power somewhere without a fire risk.

There is one advantage to an opening series switch though -  it protects you from shorted regulator or rectifier discharging your battery which is a failure mode that hasn't been discussed. An optimal solution is probably a DPDT switch with one pole wired to open the battery connection and the other wired to short the stator. You need it to be break before make or (perhaps better) have a shunt zener on the rectifier output to soak up the voltage transient during switching.
 
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Offline David Hess

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #12 on: April 16, 2020, 04:37:02 pm »
That's interesting David!  It makes sense though, those windings can sure put out a lot if you let them!  I had no idea that was ever a thing.  I guess I know why it continues to not be one..

Back then there were fewer options.  The flux-switched inverters available back then only put out a square wave and not a modified sine waves and they were pretty heavy compared to just modifying an alternator.  Today we have cheap and powerful high frequency inverters.

What I might do is use a switch to open the output but also include a big power zener diode to limit the voltage to 24 volts or something.  Big stud mount power zener diodes used to be available but no longer.  That leaves a more complex power shunt regulator although it is possible to configure a common voltage regulator like an LM317 as a shunt regulator by shorting its output; use an LM338 instead for higher power capability although I am not sure it will be high enough.
 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #13 on: April 16, 2020, 05:37:54 pm »
Personally I would go for a shorting switch before the rectifier. Output voltage remains low so prevents diode or insulation damage and the load on the engine will be much lower than with a resistor connected. This kind of stator is usually designed to have high output voltage compared to the load and high series inductance so they behave pretty much as a current source (it's a nice trick - open circuit voltage and impedance both scale linearly with RPM so it's constant current independent of revs). They're specifically designed to be regulated by shorting so it does them no harm. Yes there's the I2R loss in the winding, but that current won't be a great deal higher than the normal charging condition.

Using a load resistor draws more power from the engine (a shorted stator looks largely reactive) and gives you the problem of needing to safely dissipate that power somewhere without a fire risk.

There is one advantage to an opening series switch though -  it protects you from shorted regulator or rectifier discharging your battery which is a failure mode that hasn't been discussed. An optimal solution is probably a DPDT switch with one pole wired to open the battery connection and the other wired to short the stator. You need it to be break before make or (perhaps better) have a shunt zener on the rectifier output to soak up the voltage transient during switching.

Interesting. I'll have to raid the parts pile and see if I can find anything like that, I know I have a few stud mount components. 

Right now the plan is to use a dual throw switch to redirect the regulator output to a ~50W 10 Ohm power resistor mounted under the plane on the bottom skin in the air stream for cooling.  Just enough to take the edge off the high spikes, and assuming this switch is only thrown in regulator failure, it should.. be okay?  The idea is that it only has to last until I get back home and not damage the stator in the process.

I do like your idea to have it short the stator instead though.  The only annoying thing with that is that I now have to route a very noisy high frequency AC line through to the middle of my plane, past all the guts and electronics, to the switch.  I suppose I could use a relay up front, but now we're getting very complicated again and I'm trying to avoid that.

That's interesting David!  It makes sense though, those windings can sure put out a lot if you let them!  I had no idea that was ever a thing.  I guess I know why it continues to not be one..

Back then there were fewer options.  The flux-switched inverters available back then only put out a square wave and not a modified sine waves and they were pretty heavy compared to just modifying an alternator.  Today we have cheap and powerful high frequency inverters.

What I might do is use a switch to open the output but also include a big power zener diode to limit the voltage to 24 volts or something.  Big stud mount power zener diodes used to be available but no longer.  That leaves a more complex power shunt regulator although it is possible to configure a common voltage regulator like an LM317 as a shunt regulator by shorting its output; use an LM338 instead for higher power capability although I am not sure it will be high enough.


I went on ebay and they seem to be.. not awfully expensive? 
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Offline richard.cs

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #14 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.
 
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Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #15 on: April 16, 2020, 07:56:09 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.

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?
« Last Edit: April 16, 2020, 08:48:16 pm by ITman496 »
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Offline richard.cs

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #16 on: April 16, 2020, 08:54:54 pm »
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.
 
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Offline SkyMaster

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #17 on: April 16, 2020, 10:23:42 pm »
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.


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?

 :)
 

Offline Nusa

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #18 on: April 16, 2020, 10:52:37 pm »
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.


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?

 :)

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.
 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #19 on: April 16, 2020, 11:00:50 pm »
...

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?

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.


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?

 :)

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.   ;)
KD2CHS
 

Offline richard.cs

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #20 on: April 17, 2020, 08:21:28 am »
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?

Yes, you can parallel them but they won't really share the dissipation very well and you should assume one (random which but probably the same each time) will absorb all of the energy. If you want multiples as a backup and each can individually cope with the expected energy then this is fine. If instead you want to increase the energy absorption ability it's better to make a series string of lower voltage ones.

As a side-note the usual failure mode for semiconductors at low to moderate overloads is short-circuit. They overheat and re-diffuse into a single conductive lump of silicon. As very high currents bond wires can blow open like fuses, but that wouldn't be expected on TVS diodes which are made for huge peak currents and certainly not in your application.
 
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Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #21 on: April 17, 2020, 08:34:05 am »
Fascinating!  I really had no idea about the mechanical aspect of a silicon meltdown but it makes a lot of sense the way you explained it.

I will put two in parallel just for the sake of it since they are small, and I got a 5 pack.

Do you think this setup is testable or will it damage the regulator to short it out like this?  I suppose it probably won't because, well..  Looking at the schematic I found before, it uses the SCRs to short the stator winding through the bridge rectifier anyway.  So they should be used to this treatment.
KD2CHS
 

Offline richard.cs

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #22 on: April 17, 2020, 08:59:24 am »
It should do no harm to test it, from the stator's perspective it's the same as normal regulation, and the rectifiers will see only a modest current.

You're doing the project I've wanted to do for a long time, but the rules here are very different which makes it much more difficult, plus there's the question of space. Got any photos?
 

Offline ITman496Topic starter

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #23 on: April 17, 2020, 09:52:42 am »
It should do no harm to test it, from the stator's perspective it's the same as normal regulation, and the rectifiers will see only a modest current.

You're doing the project I've wanted to do for a long time, but the rules here are very different which makes it much more difficult, plus there's the question of space. Got any photos?

Sure do!  Keep in mind its a little rough still because I inherited the built from someone else and am going over it.  The fabric will be coming off next week and replaced and repainted with new stuff.  Also, these photos are in no particular chronological order, covering the span of 2 years. Hence the different looking engine, different instrument panel, windshield, no windshield.. etc.  The First picture is the most current.


Ignore the electronics in the food container.  That's just temporary engine datalogging for testing stuff while I continue to build the actual electronics.


Keep in mind that I did not have the ailerons/flaps installed for the photo because they were still strapped to a trailer at the time  ;D

 









« Last Edit: April 17, 2020, 09:58:23 am by ITman496 »
KD2CHS
 

Offline fcb

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Re: Toggle switch for small aircraft charging system in case of failure
« Reply #24 on: April 17, 2020, 11:27:09 am »
I would just have a warning alarm that monitors the battery for prolonged over and under voltage, if the warning goes off I'd land and investigate/disconnect the alternator belt/wiring - I guess you fly with a small tool-pouch!
https://electron.plus Power Analysers, VI Signature Testers, Voltage References, Picoammeters, Curve Tracers.
 


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