IRU C power source

[ASCTU744]

According to the manuals the C IRU switches over to the APU battery bus when AC power is lost, when DC bus 3 is lost after that the C IRU runs for 5 minutes and then drops off the APU hot battery bus.



In PSX it's possible to iniate the 5 minute timer by shutting DC bus 3 down, but the C IRU actually shuts down after those 5 minutes (while still having the AC source)...

-Nadir


P.S. My schematic shows AC bus 2 as the power source for the C IRU, I suppose that's KLM only?

[Hardy Heinlin]

When DC 3 power is removed, the IRU C disconnect relay is relaxed which causes the 5 minute timer to run.

[ASCTU744]

That's right, but when AC bus 1 (or 2 for the KLM schematic) is still energized (the C IRU still receives AC power), the tripping of the disconnect relay after 5 minutes shouldn't cause the IRU to shutdown. The disconnect relay only trips DC power, but the AC power is still available.

[Magoo]

Interesting KLM has AC BUS 2 supplying power as opposed to the standards L,C,R with 3,1,2.

[Hardy Heinlin]

That's right, but when AC bus 1 (or 2 for the KLM schematic) is still energized (the C IRU still receives AC power), the tripping of the disconnect relay after 5 minutes shouldn't cause the IRU to shutdown. The disconnect relay only trips DC power, but the AC power is still available.

That's the conclusion when looking at the wiring diagram. But I'm not sure whether it is really the case if IRU C is wired with AC bus 1 which, if I recall correctly, powers the battery charger. When a DC bus is unpowered, the cause is usually the loss of the related AC bus which would normally also power the ground service bus to constantly power the battery. When the related AC generator fails, AC power will be generated by the battery and an DC-to-AC static inverter. So when using that backup source, the battery would also discharge. And that's what the 5 minute timer wants to avoid.

[John H Watson]

My schematic shows AC bus 2 as the power source for the C IRU, I suppose that's KLM only?

In another part of the KLM manual, it shows AC Bus 1. Probably just a typographical error.

How are you removing power from DC Bus 3? Via the CB?

[ASCTU744]

In another part of the KLM manual, it shows AC Bus 1. Probably just a typographical error.

How are you removing power from DC Bus 3? Via the CB?

Ah that must be the case, the KLM tech manual says AC bus 1 too.

No, I isolated the BTB and openend the GCB but pulling the CB gives the same result.

[ASCTU744]

That's the conclusion when looking at the wiring diagram. But I'm not sure whether it is really the case if IRU C is wired with AC bus 1 which, if I recall correctly, powers the battery charger. When a DC bus is unpowered, the cause is usually the loss of the related AC bus which would normally also power the ground service bus to constantly power the battery. When the related AC generator fails, AC power will be generated by the battery and an DC-to-AC static inverter. So when using that backup source, the battery would also discharge. And that's what the 5 minute timer wants to avoid.

But aren't the batt chargers still powered when AC and DC bus 3 are shutdown?

[ASCTU744]

https://youtu.be/Vt4VrlEwXJ4

Here is a video with the scenario.

[Hardy Heinlin]

Now I see what you mean. I guess IRU C should remain ON when AC bus 1 is still powered.

[John H Watson]

But aren't the batt chargers still powered when AC and DC bus 3 are shutdown?

Sounds about right. With the DC 3 bus shut down, the Bat Xfer Relay is deenergised. This energises the main battery relay. Auxiliary contacts on the main battery relay put the battery chargers into Transformer/Rectifier mode.



(EDIT: Sorry, that image is a bit small)

[ASCTU744]

I'm still not sure if there is a certain logic in the controller portion (inside the IRU).

Or are both AC and DC power constantly connected to the IRU?

[Markus Vitzethum]

If it helps, we've also had this discussion a couple of years on, if I remember correctly, roughtly the same topic.

https://aerowinx.com/board/index.php/topic,3398.0.html

Markus