Synchronising generators

[Carpass]

On a multi engined aircraft how does a GCU know when the generator it's controlling/monitoring is phase synchronised with the others, and ready to come on line? Is there a seperate synchronising unit which watches all the generators? I've heard that on the DC10, you sometimes have to advance or retard the throttles to get the generators matched. Any help much appreciated thanks

[Blacksheep]

The generators may only be connected together when they are at the same frequency and phase relationship. First, consider a situation where both generators are at exactly the same frequency AND exactly out of phase. The voltage difference between them is at a maximum and remains in this state indefinitley. The generators cannot safely be connected together as the circulating current would be excessive. Something is going to melt!

Next, consider both generators at exactly the same frequency and exactly in phase. There is no voltage difference between them and the circuit breakers can safely be closed to connect them together.

Then consider two generators at slightly different frequencies, say one at 400hz and the other at 398hz. The two will drift in and out of phase at a rate equal to the difference between their frequencies. In this case at 400-398 = 2hz. (This is a beat frequency and, if you are a musician, eliminating the beat frequency is how you tune your instrument) The auto synchronisation circuit in the GCU monitors this beat frequency and closes the circuit breaker when the difference voltage is at a minimum. Once they are connected together in parallel the individual frequency control systems for each generator maintain synchronisation, ensuring that both generator speeds remain matched.

If there are more than two generators to be paralleled, then the third and any subsequent generators are matched to the system in the same way by their respective GCUs.

Really old systems had no auto-synchronisation, they were manually parallelled with the assistance of synchronisation lamps connected across the busses. We closed the GCB switch when the lamps were dark, indicating that both generators were in synchronism. Life was more exciting in those days. Especially in the REALLY old days before CSDU's had quill drives!

I hope this simplified explanation is useful...

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Through difficulties to the cinema

[Jetlagged]

I cant seem to find the info. Is it the GCR or Load Controller that jogs the freq on a 747 sync bus ?.

[DoctorA300]

Jetlagged,
On B747 classic and DC10, it is the load controlles that controls the freq by speeding up or slowing down the csd.

Blacksheep,
You said that the GCU/loadcontollers keep the gen freq´s syncronised. As I understand it, once the generators are conected in parellel they will always be syncronised, but the generator which phase comes ´first` will take the load.

Brgds
Doc

[CONES R US]

Doctor A300, i see from your signature that you've never worked on a 146 or ATP then..... ;)

[DoctorA300]

Cones,
I´m Licesed on the RJ and 146 , hence my signature
Brgds
Doc

[CONES R US]

then maybe you should turn your sarcasm detector up a notch or two :p

[DoctorA300]

Cones,
It´s fully up , it´s just not in Britmode null :cool: :cool: