View Full Version : 737-300/400 TR3 Disconnect Relay
mangatete
18th Mar 2007, 08:01
Why is this relay labeled TR3 when in reality it is disconnecting the TR1 from TR2 and separating the two DC buss-es during the glide slope capture or when the bus transfer switch is switched off. I realise that TR3 can back up TR2 and TR1 via a diode under normal operations however this is separate to the TR3 disconnect relay.
Cheers mangatete.
alexban
18th Mar 2007, 08:15
Tr1 and Tr2 are operating in parallel,are not connected. The Tr3 is a backup for Tr2 (direct) and for Tr1 through the Tr3 relay. The Tr3 relay also is used to disconnect DC busses at GS capture .
mangatete
18th Mar 2007, 08:25
TR1 and TR2 operating in parallel means they are electrically connected together which is true via the TR3 disconnect relay for normal operations prior to glide slope capture, still does not explain why it is labeled the TR3 disconnect relay when it disconnects TR1 from TR2 at glide slope intercept?
InSoMnIaC
22nd Mar 2007, 13:53
The reason why its labeled TR3 disconnect relay is because it is the 3rd Relay influenced by the position of Transfer Bus switch.
ie in Transfer bus switch in Auto causes TR3 relay to close (except on glideslope capture) also arms the automatic switching of the 2 transfer relays that allow the transfer buses to be powered by the opposite gen bus in cause of gen failure.
Transfer Bus switch off doesn't allow automatic switching of transfer relays 1 & 2 and also opens the TR3 disconnect relay (irrespective of other conditions)
That's my logic behind the labelling anyway
mangatete
23rd Mar 2007, 06:54
Sounds fair enough, so what you are saying is TR is an abbreviation for transfer relay in this case, and not as I presumed and abbreviation for Transformer rectifier as used in the labels TR1, TR2 and TR3?
Mangatete
alexban
23rd Mar 2007, 07:09
IMHO ,it doesn't disconnect TR1 from TR2 ,it disconnects DC bus 1 from DC bus 2.
Indeed you may say the TR supply the DC bus,but this is what the FCOM say,it disconnects DC bus1 and 2.
And it is called TR cause it is used to connect TR3 to DC bus 1 if TR1 fails.
mangatete
25th Mar 2007, 06:54
Alexban, When you say used to connect TR3 to DC bus 1, Has there been a change to the system? as the present system uses a diode to provide TR3 as a backup for DC bus 2, and also DC bus 1 when the TR3 disconnect relay is closed. How did the system use to be?
Cheers
alexban
25th Mar 2007, 17:21
No change.When the relay is closed ,the TR3 has acces to DC1 bus,when it is open ,dc buses are separated, and so is the TR3 from DC bus 1.
This is why,if TR1 fails inflight, and you didn't notice,cause the TR3 is back-up ,at G/S capture you're for a surprise as the DC bus 1 will be de-energized.
So the relay connects or disconnects the TR3 from DC bus1- hence the name -TR3 disconnect relay.