Looks like I wasn’t wrong 
Various people here know the process; whether we will see posts from the industry is a good Q
IMHO it is an EASA Major Mod. You will need an EASA Part 21 company, and some money.
Under FAA it is a Field Approval, which is a similar idea.
An FAA STC would not be directly usable on an EASA-reg.
I would contact a couple of avionics shops, because you will need to use one anyway.
In the past I tended to find that EASA-reg owners are unwilling to post details of how something Major was done. Maybe they just don’t know, or maybe some other reason.
Exploring standby alternator options on my UK registered Commander 114B running IO540.
I will have a spare vacuum accessory pad come free if I remove the vacuum pumps.
The B&C website does not have an STC for my aircraft. They do have for Bonanza, Piper and Cessnas.
What are my realistic options for a standby ?
Anyone installed standby alternators in EASA aircraft please share info ?
Thanks.
That’s an interesting point. You can wire up an alternator field winding both ways and I am pretty sure I’ve seen both. But looking at the TB20 wiring I think you are right; one end of the field is grounded (the PB1N wire) and the voltage regulator is feeding the other end (PB1G). Why PB1G is called “Bat” on the Lamar, beats me. It looks like the drawing swaps the Bat and Exc labels on the Lamar reg. And a google on Lamar reg wiring found no examples with these two labels so they look like a Socata invention, and have been swapped.
So a short between that field wire and the +28V bus would make the alternator go to max output But this short is less likely than a short to the airframe, generally.
Peter wrote:
A short to GND on the field will massively overvoltage everything
Why? One of the ends of the field windings is permanently grounded, if you ground the other end, the one used for control…then alternator output will be zero…
Perhaps a typo and you mean “short to bus voltage” ?
Is there no risk that voltage spikes or malfunctions of the main alternator can damage the standby alternator?
Not seen that so far, and it’s been installed for about 5 years.
The two alternators are in effect ORed together via diodes, because their rectifiers work as a diode. So one can’t feed the other. But their voltage regulators are permanently bus-connected. Although, not checking the wiring, I think the backup alt’s regulator is bus connected only when that switch is turned ON.
most aircraft electrical systems cut-off pwr supply to the alternator field if nominal voltage is exceeded by 5-7%
Yes; the Lamar main voltage reg has a trip on it.
A short to GND on the field will massively overvoltage everything
The real issue with spikes is the moment of starter motor disengagement.
Airborne_Again wrote:
Is there no risk that voltage spikes or malfunctions of the main alternator can damage the standby alternator?
I guess that depends on the type of failure…most aircraft electrical systems cut-off pwr supply to the alternator field if nominal voltage is exceeded by 5-7%. This will prob99 stop the spike.
Exceptions could be:
-Very sharp, short-term spike
-Controller fault
-Some short to bus voltage (not to gnd) in the field circuit.
Ask me how I know about the latter…
If worried about that, the system does have a couple of breakers that will isolate sby alternator and its controller from the main bus, so in theory you could do it your way, but it is designed to be permanently connected.
OTOH if the unchecked spike is bad enough to damage sby alternator or its controller then perhaps having backup power will not matter that much, since there will not be any electrical system left alive to be powered up on the aircraft…
@Peter, in your writeup you say that the outputs of the two alternators are connected together. Is there no risk that voltage spikes or malfunctions of the main alternator can damage the standby alternator?
Yes indeed; I think a test like this needs to be run for quite some minutes. Or at least let the battery decay to around 24V (with main alternator field OFF of course) and at say 25A that will take a few mins. And then turn on the backup alternator field and see hpw well it holds up the bus voltage.
Of course if your main alternator fails for real then its field must be OFF because it consumes a few amps!
If you do not change electrical load, the rate of discharge will diminish to zero as battery voltage decreases slowly to around 26V, so flight can be maintained indefinitely form that point of view (just in case you are thinking of ferry tanks for your 20-hr flight ;)
