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Another Alternator Question

 
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kearney



Joined: 20 Sep 2008
Posts: 563

PostPosted: Thu Aug 10, 2017 7:08 am    Post subject: Another Alternator Question Reply with quote

Bob et Al

I have been reading the AEC trying to get smart(er) on alternators and failure modes etc. The "Z" diagrams are quite informative.

As the -10 I am building will have EFI/EI I want to make sure that I understand what happens when electrical things fail. The one failure I am unsure about is the battery contactor.

For example, if I am trundling along at altitude with all my electro-goodies powered up and the battery contactor fails, what happens (assuming I am wired in accordance with the Z13/8 diagram). I know that the OV protection will deal with voltage spikes. Is there anything else to be concerned about such as a current spike (if it exists)?
Inquiring minds need to know.

Les


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PostPosted: Thu Aug 10, 2017 7:20 am    Post subject: Another Alternator Question Reply with quote

On 8/10/2017 10:08 AM, kearney wrote:
Quote:


Bob et Al

I have been reading the AEC trying to get smart(er) on alternators and failure modes etc. The "Z" diagrams are quite informative.

As the -10 I am building will have EFI/EI I want to make sure that I understand what happens when electrical things fail. The one failure I am unsure about is the battery contactor.

For example, if I am trundling along at altitude with all my electro-goodies powered up and the battery contactor fails, what happens (assuming I am wired in accordance with the Z13/8 diagram). I know that the OV protection will deal with voltage spikes. Is there anything else to be concerned about such as a current spike (if it exists)?
Inquiring minds need to know.

Les

You really can't have a current spike, unless you have either a voltage

spike or a resistance drop (load increase).

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kearney



Joined: 20 Sep 2008
Posts: 563

PostPosted: Thu Aug 10, 2017 8:19 am    Post subject: Re: Another Alternator Question Reply with quote

Hi

Thanks for the the quick response. Unfortunately I don't understand what happens inside an alternator when it is instantly disconnected from the battery with the field still energized. Is a high current spike possible in such a situation and if so will if fry my toys? If this is a risk, how do I protect against it?

Cheers

Les


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PostPosted: Thu Aug 10, 2017 10:01 am    Post subject: Another Alternator Question Reply with quote

On 8/10/2017 11:19 AM, kearney wrote:
Quote:


Hi

Thanks for the the quick response. Unfortunately I don't understand what happens inside an alternator when it is instantly disconnected from the battery with the field still energized. Is a high current spike possible in such a situation and if so will if fry my toys? If this is a risk, how do I protect against it?

Cheers

Les

Which architecture are we discussing? Are you talking about your master

contactor? If so, if it disconnects, the field voltage should drop at
the same time, collapsing the alternator's output.

In any case, unless the battery is severely discharged (not a good
starting point for a flight) & is drawing heavily on the alternator at
the time the 'battery contactor' drops out, there should be minimal
variation in alternator load and minimal variation in it's output.

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kearney



Joined: 20 Sep 2008
Posts: 563

PostPosted: Thu Aug 10, 2017 10:26 am    Post subject: Re: Another Alternator Question Reply with quote

Hi Again

My question was generic and not based directly on any of the "Z" diagrams. I just want to know what happens when things go sideways.

Assume a simple situation, battery, contactor and alternator servicing a 20a load on a simple bus. Assume that there is appropriate OV protection per AEC.

What happens if the battery contactor fails thereby disconnecting the battery from the system. For clarity, assume the alternator field is still energized. And everything was working fine prior to the disconnect.

Cheers

Les


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PostPosted: Thu Aug 10, 2017 12:22 pm    Post subject: Another Alternator Question Reply with quote

At 10:08 AM 8/10/2017, you wrote:
Quote:
--> AeroElectric-List message posted by: "kearney" <kearney(at)shaw.ca>

Bob et Al

I have been reading the AEC trying to get smart(er) on alternators and failure modes etc. The "Z" diagrams are quite informative.

As the -10 I am building will have EFI/EI I want to make sure that I understand what happens when electrical things fail. The one failure I am unsure about is the battery contactor.

For example, if I am trundling along at altitude with all my electro-goodies powered up and the battery contactor fails, what happens (assuming I am wired in accordance with the Z13/8 diagram).

Not much . . .

Quote:
I know that the OV protection will deal with voltage spikes. Is there anything else to be concerned about such as a current spike (if it exists)?

I would guess that the most shop-worn topic
in discussion of vehicular dc power systems
is 'spikes'.

Most of what circulates as learned assertions
and worrisome concerns is not based on good
physics or practical design going back 100
years or so.

To lay foundation for the answer to your question,
I'll refer you first to this 2-bit tour of a legacy
qualification protocol for electro-whizzies to
to into airplanes.

http://tinyurl.com/ydctrct5

These tests equal or exceed the stresses known
to be deleterious to performance or hazardous
to longevity for electrical/electronic appliances.

Note that for a 14 volt airplane, the greatest expected
surges is 20 volts for 1 second and 40 volts for 100
milliseconds. These are what might be termed high-energy
perturbations. They ALWAYS originate with a malfunction
in the ship's power generation (alternator or generator).

The only thing DO-160 calls a 'spike' is the 300v spike
test. While a alarmingly 'high' number, the voltage is
delivered from a high impedance source (50 ohms)
with a duration on the order of 100 microseconds. Relatively
low energy and easily managed with rudimentary design
techniques for the appliance being tested.

The significance of these tests is not so much what
they IS FOUND to be significant risk, but the absence
of a test that the automotive industry calls a "load
dump". The in automotive vernacular, the worst case
"load dump" occurs when an alternator is near or at
rated output where (1) system requirements are small
and (2) the BATTERY is severely discharged and is
demanding a very high charge rate. Under this
condition, disconnecting the battery is like
cutting the rope during a tug-of-war game.

The sudden release of 'tension' causes a high energy
overshoot of the alternator's output that persists
until the alternator's regulator can bring it under
control. The full effects of this overshoot are impressed
on what ever appliances are in operation at the time.

Here's the test setup for evaluating the alternator/
regulator response to an automotive 'load dump'.

http://tinyurl.com/yapuhf2s

DO-160 doesn't suggest testing for this kind of event
because (1) it is poor practice to take off with a badly
discharged battery and (2) even if you do . . . likelihood
of loss of contactor coincident with launching into
the blue with a depleted battery is very low.

the scenario you describe, loss of contactor during
cruising flight is a low probability event in the first
place. If you do, your battery will most likely be in
a high state of charge. Under these conditions contactor
failure will probably not even be noticed until you
shut down the alternator before shutting down the engine
whereupon the panel goes dark immediately.

This would be cause for investigation of the failure
at the end of your current flight. If, as some POHs
suggest, the alternator/battery switch is turned
off before stopping the engine, then you won't know
the contactor is gone until next preflight.

Based on the way airplanes are operated, the automotive
style load dump represents no significant risk. I've
never heard it even mentioned in 50 years of conversation
in airplane-speak. In the big airplanes, taking a battery
off line in flight is discussed in the POH in responses
to certian system failures were the airplane is
expected to continue flight sans battery with no
problems.








Bob . . .


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user9253



Joined: 28 Mar 2008
Posts: 1907
Location: Riley TWP Michigan

PostPosted: Thu Aug 10, 2017 3:29 pm    Post subject: Re: Another Alternator Question Reply with quote

Les, last year my PC680 battery failed suddenly while flying. It turned out to be an internal open circuit. The affect is identical to a master contactor failure. The Dynon D-180 gave a high voltage alarm. I can not remember what it was, but probably not more than one volt above normal. The voltage was definitely not high enough to cause any damage to electrical loads. Analyzing the data from the D-180 showed that the voltage was unstable without the battery connected and varied above and below the set point by a volt or so. Again, it was enough to set off alarms, but not enough to damage my electronics, which are rated for 10 to 30 volts.
Based on my experience, a failed battery contactor (or any open battery circuit) will not cause any damage to any electrical load, including avionics. And a failed battery contactor does not create an in-flight emergency. All of my electrical equipment kept working normally powered by the alternator without a battery. (Except when the engine was idling, the voltage dropped)


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kearney



Joined: 20 Sep 2008
Posts: 563

PostPosted: Thu Aug 10, 2017 6:02 pm    Post subject: Re: Another Alternator Question Reply with quote

Bob / Joe

Thank you for the info it is most helpful. Given that an alternator will continue to function even if the battery goes offline, some of my redundancy concerns have been addressed.

Based on my application (EFI/EI) the Z-12 dual alternator / single battery architecture looks most appropriate.

I have been reviewing each of the various "Z" diagrams trying to understand the logic behind each and how it would operate / meet my needs.

I like the Z12 with dual alternators would provide (*I think*) the best redundancy in the event of a failure of one of the electrical system components.

The only changes to the Z12 I might consider making would be

1. To move the back up alternator field connecton to the endurance bus. My rational for this is that a failure of of the DPDT MASTER Switch would take
the main alternator offline perhaps before the backup could energize. If that happened, there would be no way to energize the backup alternator field. If normal ops had the E-Bus Alternate switch always closed, the backup alternator would be energized automatically from the battery via the EBUS.

2. I would eliminate the always hot bus (I wouldn't use it) and put all my engine EFI/EI circuits and essential avionics on the endurance bus. All other avionics etc would be on the main bus. Some of the EFI/EI components are not switched.

3. Eliminate the alternator loadmeters & hall sensor. I believe the EFIS screens will provide current info.

4. Eliminate the MAIN VOLTS WARN light. In this case the EFIS should provide bus voltage info.

I would also forgo use of the Vertical Power VP-X (as originally planned) as it wouldn't be adding a lot of value above and beyond using circuit breakers.

Does the above seem reasonable? I'd appreciate any comments / suggestions about this or anything I may have missed.

Cheers

Les


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user9253



Joined: 28 Mar 2008
Posts: 1907
Location: Riley TWP Michigan

PostPosted: Thu Aug 10, 2017 7:18 pm    Post subject: Re: Another Alternator Question Reply with quote

Your alternator will most likely continue to operate without a battery. But that should be confirmed in your aircraft.
1. About the backup alternator: Read Bob's recent post about operating both alternators simultaneously:
http://forums.matronics.com/viewtopic.php?t=16767029
Keep in mind that the E-Bus diode will drop the voltage somewhere between 0.5 and 1.2 volts. The voltage regulator should be monitoring alternator output voltage, not the voltage after it has been reduced by a diode.
2. Some electronic ignition manufactures recommend connecting their devices directly to the battery.
3. An EFIS will display current but a sensor (shunt or hall effect) is still required.
4. Yes, an EFIS will display voltage and sound an alarm if out of spec.
5. I agree with your decision about VP-X. Also consider fuses. They cost less, weigh less, never fail to open with high current, can be replaced easily without putting the aircraft out of service.


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PostPosted: Mon Aug 14, 2017 8:46 am    Post subject: Another Alternator Question Reply with quote

At 09:02 PM 8/10/2017, you wrote:
Quote:
--> AeroElectric-List message posted by: "kearney" <kearney(at)shaw.ca>

Bob / Joe

Thank you for the info it is most helpful. Given that an alternator will continue to function even if the battery goes offline, some of my redundancy concerns have been addressed.

Based on my application (EFI/EI) the Z-12 dual alternator / single battery architecture looks most appropriate.

I have been reviewing each of the various "Z" diagrams trying to understand the logic behind each and how it would operate / meet my needs.

I like the Z12 with dual alternators would provide (*I think*) the best redundancy in the event of a failure of one of the electrical system components.

Keep in mind that Z12 has been in existence for decades
and the failure modes pretty well sifted both in the
OBAM and TC aviation communities on what must now be
thousands of aircraft.

Quote:
The only changes to the Z12 I might consider making would be

1. To move the back up alternator field connecton to the endurance bus. My rational for this is that a failure of of the DPDT MASTER Switch would take
the main alternator offline perhaps before the backup could energize.

???? milliseconds is too long ???

Quote:
If that happened, there would be no way to energize the backup alternator field.

Why does the bus voltage dissapear when the
main alternator quits?

Quote:
If normal ops had the E-Bus Alternate switch always closed, the backup alternator would be energized automatically from the battery via the EBUS.

Not recommended. If you want to simplify Z12, eliminate the
endurance bus. The notion behind the endurance bus is to
(1) provide multiple power pathways to equipment needed
for comfortable continuation of flight to airport of
intended destination while (2) minimizing loads on a
limited energy source. The 20+ amps capability of the
standby alternator should alleviate all concerns for
energy limitations . . . conservation goals of the
e-bus are, therefore, moot.


Quote:
2. I would eliminate the always hot bus (I wouldn't use it) and put all my engine EFI/EI circuits and essential avionics on the endurance bus. All other avionics etc would be on the main bus. Some of the EFI/EI components are not switched.

Not recommended. If you've got smoke in the cockpit,
you'd like to take the electrical system max-cold
without killing the engine. Electrically dependent
engines are best powered directly from the battery
bus via their own control switches.


Quote:
3. Eliminate the alternator loadmeters & hall sensor. I believe the EFIS screens will provide current info.

ALL ammeters of ANY form are not flight instruments.
They serve diagnostic purposes for ground maintenance.
If you've done a proper load analysis, then you KNOW
that the alternator(s) are not overloaded in any
flight condition described in your Plan-B as dictated
by FMEA. Hence, anything the ammeters have to say
is of no value for managing the outcome of a flight.

If I were building the simplest possible a/c with
solid failure tolerance, there would be no ammeters
or voltmeters . . . only the active notification of
low voltage. In the case of Z12, the only time you
would see a low volts warning in flight is if the
aux alternator were overloaded whereupon you reduce
loads to turn out the light . . . and truck on.


Quote:
4. Eliminate the MAIN VOLTS WARN light. In this case the EFIS should provide bus voltage info.

Just how you get ACTIVE NOTIFICATION of low voltage
is optional as long as you have it.

Quote:
I would also forgo use of the Vertical Power VP-X (as originally planned) as it wouldn't be adding a lot of value above and beyond using circuit breakers.

Agreed.


Bob . . .


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