Hello! A few questions about the electrical system. Should tr1 and tr2 have the same amp reading while the dc cross tie relay is closed? On my ngx there is a significant difference between tr1 amp and tr2 amp as if the dc cross tie relay is opened. This is with the bus transfer switch in auto and not in afds app mode with the glideslope captured. Also I noticed that the ac amp meter shows a reading for ground power. Is that per the real aircraft?

Current readings for the two engine IDGs and the APU generator may be read on the AC ammeter.

- From the FCOM 6.20.7

 

Judging by this, I would say you're right, and the GND PWR does not have an Amp meter installed. Which does strike me as weird, but possible.

 

 

TR1 receives AC power from transfer bus 1. TR2 receives AC power from transfer bus 2. TR3 normally receives AC power from transfer bus 2 and has a backup source of AC power from transfer bus 1. Any two TRs are capable of supplying the total connected load.

Under normal conditions, DC bus 1, DC bus 2, and the DC standby bus are connected via the cross bus tie relay. In this condition, TR1 and TR2 are each powering DC bus 1, DC bus 2, and the DC standby bus. TR3 powers the battery bus and serves as a backup power source for TR1 and TR2.

- From the FCOM 6.20.9

 

Again, I think you may be on to something here.

 

EDIT:

However, having looked at 6.20.11 for a while, I think I'd have to come to a different conclusion. It's been a while since I had electricity as a course, so this may be completely off the bat.

The way everything is connected, I think TR1 and TR2 could be providing different AMPS, depending on the situation.

You are correct. Tr1 and tr2 amp could be different if the cross bus tie relay is open. However in my example it was closed or should have been closed unless it failed open. I did not check for any failures. If closed tr1 and tr2 both power dc bus 1, dc bus 2 and the dc standby bus. After some more testing it seems like tr1 amp is always 48 no matter what.

 

Sent from my PC36100 using Tapatalk 2

 

 

On the NG's the Bus Power Control Unit (BPSU) monitors AC loads (and whole wack of other things) through Current Transformers. There's one of these very early on in the AC external power circuit. It's basically a clamp-on amp-meter.

 

I'm lookin in the WDM's right now....never really thought about the TRU loads but I'm pretty sure any two given TRU's should show a balanced load

 

The BPCU monitors these

parameters of external power:

* Current

* Voltage

* Frequency.

 

 

EDITED FOR CORRECT INFO!

Right, I have been asking some more knowledgeable guys than myself about this.

While TR1 and TR2 could, in theory, provide different loads in quite a lot of aircraft configurations, TR1 and TR2 will automatically be balanced.

Also, while the AMPs of the ground power are monitored, this is not something you can do yourself, as a pilot on the flight deck.

The GND PWR setting on the metering panel will not show an Amp rating. The only place on the aircraft where you can see the Amps for external power, are on the GPU panel itself.

Ya you're right about the AC External Power load metering Thomas. There's nothing going to the P5-13 module regarding external power 'cept a tee off B phase line from the external power BPCU.

During normal operation when the DC BUS TIE RELAY is closed the TRU #1 and TRU #2 are parallel connected to the DC BUS # 1 and DC BUS # 2 so the Load,s should be even. The DC BUS TIE RELAY will open in the following condition's;

- The bus transfer switch is not in AUTO position,

- The AC transfer bus # 1 or AC transfer bus # 2 has no power.

- The Autoflight system is in the approach mode with glideslope capture.

 

 

 

Mark.

Boeing 737-6/7/8/900(ER) Ground Engineer.

NGX values seems pretty "fixed" and sometimes does not match changes in the loads. This appens if you simulate TR failures for example. It is a bit deep in the system simulation and maybe that it was overlooked.

Not too much users will see those things.

Set up TR1 failure tonight just to see the result.  After the TR UNIT light illuminated I selected TR1 on the DC meter selector.  DC volts was 0 as was amps.  In this case with the bus transfer switch auto and both autopilots and flight directors off, it should have had some volts as the volt reading is off DC bus 1 which should have still been powered.  Hopefully they will include a full electrical system simulation with sp2.

Sorry but, if TR1 failed, how do you expect a positive reading on it?

The TR unit readout on the meter is there to find if something is malfunctioning, obviously, if it failed it will read 0.

Different thing for the DC bus 1 wich will be powered via the cross tie relay except for dual approach when it will open and DC BUS1 will fail. (and from my test, nothing happened on the ngx)

Remember that if TR1 fails TRU light comes on, if TR2 OR TR3 fails the light will be off, unless you land or both TRU2 and 3 fails.

About SP2 I really doubt they will add more deep details to the system, inside there is already a lot of work, not perfect at all, but to fix, simulate all the things it will take a lot of time and money. I think they hopefully fix something wrong, but will not waste more time in deep simulating more on this product.

But, if I'm wrong, it will be better for us

With the dc meter selector at tr1, amps shows tr1 amps and volts shows volts for dc bus1. So with the cross bus tie relay closed I expect 0 amps and normal volts as dc bus 1 is still powered. On the ngx amps and volts were both zero. Please dont think im just being overly critical of the ngx as in my opinion it is by far the best simulation available. They went to the trouble to simulate failures so they may as well get it right. I rarely program failures so most of the time its a darn near perfect simulation.