#3 generator

D

dkittrell

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Dkittrell
I've been provided a puzzle by a commercial pilot. Is there an AP who can help provide the answer?

#3 generator trips offline every time you pass through Fl 28,0. At lower altitudes it was producing over 100kVA, but it trips offline every time you climb through a similar altitude. It’s been written up over and over. What's the problem?

Thanks

D Kittrell
 
dkittrell said:
I've been provided a puzzle by a commercial pilot. Is there an AP who can help provide the answer?

#3 generator trips offline every time you pass through Fl 28,0. At lower altitudes it was producing over 100kVA, but it trips offline every time you climb through a similar altitude. It’s been written up over and over. What's the problem?

Thanks

D Kittrell
Click to expand...


First post.....Welcome to POA...

Technical questions like you want answered cost 4.99 a minute and payable in cash...;):lol:
 
Well, first of all congrats for getting a 100kVA generator off the ground and into the sky. As I figure it, 100KVA operating at nominal aviation DC 28 volts is capable of producing 3571.43 amps. That is one big-azz generator. :D

Next, the answer to your question is a complex case of the dielectric breakdown voltage as it relates to pressure altitude. There is an elec coronal inception, or streamer formation on some part of the generator which finally arcs and trips the breaker as you increase altitude where the breakdown voltage becomes lower.

If the aircraft has pressurized generators, I would investigate the pressure system for leaks, or tears in a bladder or hose. Or possibly it is not pressurized, in which case I would investigate the generator internals for gaps which are lower than the specified tolerance for the service altitude that they are rated.

The general rule of thumb is that the air gap between conductive fields @ STP to prevent breakdown is about 3 million volts/meter(I'm doing this from memory, so go easy on the math). This is increased by about 1.5% for each 2500' elevation up to 30,000' and about .8% increase in dielectric distance for altitudes above 30,000 each 2500'. There is of course, inter dependencies on the shape, size, and purity of the conductors, but I think we can ignore that as the problem likely is in the stator area where the field is charged, assuming it is an alternator and not actually a generator. If a generator, we would look at the brushes and the commutator.
 
docmirror said:
Well, first of all congrats for getting a 100kVA generator off the ground and into the sky. As I figure it, 100KVA operating at nominal aviation DC 28 volts is capable of producing 3571.43 amps. That is one big-azz generator. :D

Next, the answer to your question is a complex case of the dielectric breakdown voltage as it relates to pressure altitude. There is an elec coronal inception, or streamer formation on some part of the generator which finally arcs and trips the breaker as you increase altitude where the breakdown voltage becomes lower.

If the aircraft has pressurized generators, I would investigate the pressure system for leaks, or tears in a bladder or hose. Or possibly it is not pressurized, in which case I would investigate the generator internals for gaps which are lower than the specified tolerance for the service altitude that they are rated.

The general rule of thumb is that the air gap between conductive fields @ STP to prevent breakdown is about 3 million volts/meter(I'm doing this from memory, so go easy on the math). This is increased by about 1.5% for each 2500' elevation up to 30,000' and about .8% increase in dielectric distance for altitudes above 30,000 each 2500'. There is of course, inter dependencies on the shape, size, and purity of the conductors, but I think we can ignore that as the problem likely is in the stator area where the field is charged, assuming it is an alternator and not actually a generator. If a generator, we would look at the brushes and the commutator.
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Yeah, that IS one mother generator, isn't it?

The general rule of thumb where I learned it in a practical matter drawing the arc to see if the HV supply on the CRT in a TV set was good was about 10 kV/inch. 39 (.37, for the nitters amongst us) works out to about 400kV/meter so I think your rule ought to be about 0.3 MV/m for the arc, not 3. But again, we are talking thumb rules and not absolute precision (TLAR rule "That Looks About Right.")

So, what sort of gap would be indicated on a 28 volt generator/alternator to engender such a problem? Well, 28/10000 of an inch ought to do it, or about 3 mils. That's an AWFULLY tiny gap to worry about, so I think maybe we have to look somewhere else. And, I'm not at all sure that the rule of thumb is linear. I've used 3 mil mylar for insulations up to a couple of hundred volts without worrying, so maybe we'd be chasing our tails to look for an arc at this low of a voltage.

What else happens at FL280? Damned if I know without knowing the airframe it is hung on. But if it is the #3 generator, we've got a few choices from the 727 on up. Me? I'd see if some heating load or some other heavy draw (boots, deice surfaces, etc.) didn't kick in crossing that altitude.

And why wouldn't the generator/alternator be on a bus-sharing circuit of some sort where all the generators feed one big bus and then get partitioned out to share the load over all three power sources.

Defective protective circuit (breaker, OV/OC circuit, etc.)? Possibly. Can't think why altitude would be a factor, but then again, we don't have the shop manual here to tell us how that all works, do we?

Let's do some more thinking. Unless of course it is a Lucas generator (the Prince of Darkness) and then all bets are off :yes:

Jim
 
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weirdjim said:
Yeah, that IS one mother generator, isn't it?

The general rule of thumb where I learned it in a practical matter drawing the arc to see if the HV supply on the CRT in a TV set was good was about 10 kV/inch. 39 (.37, for the nitters amongst us) works out to about 400kV/meter so I think your rule ought to be about 0.3 MV/m for the arc, not 3. But again, we are talking thumb rules and not absolute precision (TLAR rule "That Looks About Right.")
*************************************************

Let's do some more thinking. Unless of course it is a Lucas generator (the Prince of Darkness) and then all bets are off :yes:

Jim
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Oh man, you made me go look it up. Seems my memory isn't as bad as you would have us believe:

http://hypertextbook.com/facts/2000/AliceHong.shtml

I just used my mnemonic of "3MM". :rolleyes:

It is not linear, which is why I mentioned the delta starting around 30,000' or so. I thought it was 10,000M, but meh - higher than I go so whatev.

I can't believe you used the "L" word. No soup for you!!!!!! :yikes:
 
Unlikely that it's a 28 volt DC generator. More probably a 115 volt three phase AC generator.

The trip might be due to fuselage expansion under pressurization differential, moving some conductor or component and causing a short or disconnect. Or some temperature-related thing. Some systems had a current differential detection system that would knock the generator offline if the amperage leaving the generator wasn't the same as the amperage arriving at the electrical bay via the three conductors. Any short to ground anywhere between the generator and bus relay would create that differential.

Dan
 
FL 280 is about where RVSM airspace starts. I don't suppose it has something to do with that.
 
Dan Thomas said:
Unlikely that it's a 28 volt DC generator. More probably a 115 volt three phase AC generator.

Dan
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Hmmmm, I didn't think of that, but of course very large comm aircraft do have 115 AC service. So, at 100KVA that would be supporting only about 850 amps. Anyway, no matter the voltage rating, 28 or 115AC, I still think we're looking at a dielectric problem. Although intermittent from an open(not a short), or an unbalanced load would do it, I don't think it would be so consistent.

I don't know anything about the extra loads required to handle RVSM. It's hard to believe that would also trip a gen supply, but possible.

The key to me is that it trips every time while passing through approx FL280. Depending on humidity, and pressure alt this was my clue.
 
Greg Bockelman said:
FL 280 is about where RVSM airspace starts. I don't suppose it has something to do with that.
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Something shifting as the hull pressurizes makes the most sense. I would assume that the load on the generator is sensed and can be monitored by the crew so it should be obvious whether or not a "normal" load is causing the trip. But it really seems unlikely that something that comes on at around 28,000 ft would overload a generator, most loads have overcurrent protection at currents far below what the generator can put out.

And if it is a hull flex / pinched wire it might be possible to produce the effect at a lower or higher altitude by varying the cabin pressure differential.
 
Wow! Now that's what I call a healthy response to a thread, and to think it's my first post at that.

Thanks, guys. I didn't know the answer to the question but I was dead set on making sure I didn't let this buddy of mine just stump me altogether. I was always taught that if you don't know the answer to something, get resourceful and find someone who does.

Your time is much appreciated and glad to be on the forum. Feels like home already.

Dave K.
 
Dave

Thanks for posting the question - I love the discussion.

Assuming this is an actual problem, not a hypothetical, please be sure to let us know what is (eventually) found.

And, of course, welcome!
 
gismo said:
And if it is a hull flex / pinched wire it might be possible to produce the effect at a lower or higher altitude by varying the cabin pressure differential.
Click to expand...

Exactly. That is why I don't think cabin pressure has anything to do with it.
 
Greg Bockelman said:
Well, I didn't think there was enough information from the beginning to give a meaningful diagnosis.
Click to expand...

Bingo, the generator need not be in a pressurized environment, it isn't unheard of however. Is the title of "generator" 100% accurate or is it an alternator? Is it engine driven, APU driven, hydraulically driven or is it a RAT?

Ect etc etc...
 
weirdjim said:
So, what sort of gap would be indicated on a 28 volt generator/alternator to engender such a problem? Well, 28/10000 of an inch ought to do it, or about 3 mils. That's an AWFULLY tiny gap to worry about, so I think maybe we have to look somewhere else. And, I'm not at all sure that the rule of thumb is linear. I've used 3 mil mylar for insulations up to a couple of hundred volts without worrying, so maybe we'd be chasing our tails to look for an arc at this low of a voltage.
Click to expand...

Actually, an alternator does not output 14 (or 28) volts DC to the battery bus - the REGULATOR does. The alternator will produce a 3-phase AC sine wave that may see voltage peaks in the 60-90 volts region and sends that to the bridge rectifier in the regulator which converts it to DC, smooths it with a filter capacitor, and regulates it to the required output voltage. So the gap (while still very small, your logic is still sound) may actually be several times that required for 28VDC.
 
airguy said:
Actually, an alternator does not output 14 (or 28) volts DC to the battery bus - the REGULATOR does. The alternator will produce a 3-phase AC sine wave that may see voltage peaks in the 60-90 volts region and sends that to the bridge rectifier in the regulator which converts it to DC, smooths it with a filter capacitor, and regulates it to the required output voltage. So the gap (while still very small, your logic is still sound) may actually be several times that required for 28VDC.
Click to expand...

I think most people understand that an alt produces sine AC power, and is either regulated, or rectified and regulated.

DC generators(actually a dynamo) would be the exception, and a 100KVA DC generator would be really rare.
 
Move the generator to a different hole and see if the squawk follows.
 
docmirror said:
I think most people understand that an alt produces sine AC power, and is either regulated, or rectified and regulated.
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I wish I could agree with you there - but in my experience "most people" don't even understand how a pencil sharpener works. The level of common sense in the average person is at a sorry state, but that's a whole 'nuther thread.
 
airguy said:
I wish I could agree with you there - but in my experience "most people" don't even understand how a pencil sharpener works. The level of common sense in the average person is at a sorry state, but that's a whole 'nuther thread.
Click to expand...

Heh.. ya got me there... Now that I think back to some kids in intro to DC guess you are keyrekt. :yesnod:
 
airguy said:
Actually, an alternator does not output 14 (or 28) volts DC to the battery bus - the REGULATOR does. The alternator will produce a 3-phase AC sine wave that may see voltage peaks in the 60-90 volts region and sends that to the bridge rectifier in the regulator which converts it to DC, smooths it with a filter capacitor, and regulates it to the required output voltage. So the gap (while still very small, your logic is still sound) may actually be several times that required for 28VDC.
Click to expand...

That ain't right either. The regulator controls the alternator's field current, which governs the strength of the magnetic field, which controls the output of the alternator. More input means more output. The regulator senses the system voltage, and when load is added or subtracted, that voltage will change and the regulator varies the field current to get the voltage back to the correct setting. As the RPM of the alternator changes the output voltage will also change, and the regulator makes adjustments for that, too.

The diodes rectify the AC to make DC, as you said, but the regulator does not control the output directly. That sort of thing is limited to small power supplies in many electronic devices and on some small IC engines like riding lawn mowers and such. It generates too much heat and wastes power in aircraft or automotive systems.

This is the diagram as used in automobiles. The only differences in the airplane are: 1. The regulator is turned on by the ALT switch in the cockpit, connected to the regulator's "S" terminal, instead of the alternator's stator doing the job. 2. Note that the alternators output goes directly to the battery in the automobile; in the airplane, it goes to the bus via a breaker. It charges the battery via the bus, ammeter and master contactor.

alternator-regulator-wiring-diagram-chevrolet-car.jpg


Dan
 
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airguy said:
Actually, an alternator does not output 14 (or 28) volts DC to the battery bus - the REGULATOR does. The alternator will produce a 3-phase AC sine wave that may see voltage peaks in the 60-90 volts region and sends that to the bridge rectifier in the regulator which converts it to DC, smooths it with a filter capacitor, and regulates it to the required output voltage. So the gap (while still very small, your logic is still sound) may actually be several times that required for 28VDC.
Click to expand...
First of all, it's the rectifier not the regulator that converts the 3 phase AC to DC (with some ripple on it). Second, if the rectifier's output is limited to 28V (the regulator plays an important part in that), the highest voltage to "ground" (i.e. airframe) from any of the three phase AC leads is around 30 V.

And FWIW, even if you could draw an arc (the flashover voltage is highly dependent on air pressure and geometry) the current for an arc in air is hardly likely to exceed a few amps unless you're talking about gaps in the micron range.
 
gismo said:
First of all, it's the rectifier not the regulator that converts the 3 phase AC to DC (with some ripple on it). Second, if the rectifier's output is limited to 28V (the regulator plays an important part in that), the highest voltage to "ground" (i.e. airframe) from any of the three phase AC leads is around 30 V.
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I believe that's what I said - the rectifier IN the regulator ...

airguy said:
and sends that to the bridge rectifier in the regulator which converts it to DC.
Click to expand...

Yup, that's what I said all right...

We can nitpit this to death, but the point is still valid - the peak voltage of an alternator when sampled at the 3-phase connection prior to the rectifier WILL BE higher than the aircraft bus voltage any time it's producing power. How much higher depends on the strength of the field current and the RPM of the alternator.

gismo said:
And FWIW, even if you could draw an arc (the flashover voltage is highly dependent on air pressure and geometry) the current for an arc in air is hardly likely to exceed a few amps unless you're talking about gaps in the micron range.
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Really? Better not tell any arc welders that, you'll put an entire industry out of business.
 
airguy said:
I believe that's what I said - the rectifier IN the regulator ...



Yup, that's what I said all right...

We can nitpit this to death, but the point is still valid - the peak voltage of an alternator when sampled at the 3-phase connection prior to the rectifier WILL BE higher than the aircraft bus voltage any time it's producing power. How much higher depends on the strength of the field current and the RPM of the alternator.



Really? Better not tell any arc welders that, you'll put an entire industry out of business.
Click to expand...

The arc produced by a welder is a different beast
 
airguy said:
I believe that's what I said - the rectifier IN the regulator ...



Yup, that's what I said all right...

We can nitpit this to death, but the point is still valid - the peak voltage of an alternator when sampled at the 3-phase connection prior to the rectifier WILL BE higher than the aircraft bus voltage any time it's producing power. How much higher depends on the strength of the field current and the RPM of the alternator.



Really? Better not tell any arc welders that, you'll put an entire industry out of business.
Click to expand...

The rectifiers aren't in the regulator. They're in the alternator case. Alternators with built-in regulators still have their diodes apart from the regulator. The only connection between the alternator's output and the regulator is a small sense line to determine output voltage.

That output voltage drop across the diodes is only a volt or two. Not a big deal.

Dan
 
Dan Thomas said:
The rectifiers aren't in the regulator. They're in the alternator case. Alternators with built-in regulators still have their diodes apart from the regulator. The only connection between the alternator's output and the regulator is a small sense line to determine output voltage.

That output voltage drop across the diodes is only a volt or two. Not a big deal.

Dan
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A minor technical point, I'll grant that one. Doesn't change the basic point that the voltage is going to be higher in the coils that in the bus - nor does it make it any more realistic than bus voltage to arc. Either way it's a very unlikely failure scenario (though not unheard of).

Skylane81E said:
The arc produced by a welder is a different beast
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Not given the conditions in the context of the statement. Of course I agree the conditions are different - which was my point exactly - you can't compare apples to giraffes.

In any case - let's get back to the point of the thread - this is not likely to be an internal arcing issue, and it's certainly not a welding arc. I'm going with an APU tripping offline as the turbine powering it is shutdown while climbing through FL280. Notice that the OP does not say it trips a breaker, or shorts, or goes under/overvoltage - it "trips offline". It also provides a good deal more power at lower altitudes, apparently 100kva (requiring 142 shaft horsepower at a common 93% genset efficiency) at sea level, but obviously still operates up to the area of FL280, which pretty much limits it to a turbogenset.

Which twin turbine aircraft would use APU bleedair for cabin pressurization, which might require too much power from the APU to allow continued genset output above that altitude?
 
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It's a major point, at its simplest all the regulator does is turn off the alternator when the voltage gets too high and turn it back on when it gets too low.

And the voltage difference will only be that caused by the fairly minor voltage drop across the rectifier and conductors between the coils and the bus.
 
airguy said:
In any case - let's get back to the point of the thread - this is not likely to be an internal arcing issue, and it's certainly not a welding arc. I'm going with an APU tripping offline as the turbine powering it is shutdown while climbing through FL280. Notice that the OP does not say it trips a breaker, or shorts, or goes under/overvoltage - it "trips offline". It also provides a good deal more power at lower altitudes, apparently 100kva (requiring 142 shaft horsepower at a common 93% genset efficiency) at sea level, but obviously still operates up to the area of FL280, which pretty much limits it to a turbogenset.
Click to expand...

But in the OP's case it probably isn't a 12 or 24 volt system. It's likely a 115 volt three phase AC system. Arcing at altitude becomes a real hassle, since air is a dielectric and less of it means less resistance per unit distance between poles. Altitude causes magneto flashover, for instance, so most high-altitude piston airplanes will have mags pressurized off the turbos.

In the large-aircraft AC system, there will be some transformer-rectifiers to feed a few low-voltage DC items like standby gyros and the radios and to charge the battery for that system. Pretty much everything else runs off the 400 Hz AC. Higher voltage means lower current per watt consumed, so conductors are smaller and lighter and cheaper.

Dan
 
HPNPilot1200 said:
What aircraft type? What is turning the #3 gen? Is it connected to a CSD?
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Good thinking. As some have hinted, if this is on an airliner class airplane, it would probably be a 115vac 400hz generator summed with others and feeding an AC Bus. DC would be produced with converters downstream, or off a battery (for the Emergency DC Bus, for example). To get 400hz, you have to run your generator off a constant speed drive (16 poles at 6400 rpm, for example), and many times, the CSD is the real culprit behind an apparent generator problem.
 
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