ALT FIELD CB vs ALT CB
 

Hello folks

for the geeks outhere i was just wondering what is the practical difference between the ALT FIELD CB controlling the the field current to the field coils of the alternator and controlled by the VR and the ALT CB between the ACU and the bus network?
In case of an overvoltage shouldn t both CB's pop?
Maybe the diagram is misleading, maybe what is shown as an ALT FIELD CB is not a real CB but just a relay operated by the VR?
What would cause the ALT FIELD CB to pop without the ALT CB popping?

Many Thanks

Baobab

I am sure the answer must be implementation dependent.

AFAIK the reason for a CB in the field current is

- to enable the pilot to prevent the alternator from generating any power, but without having a BIG (up to say 100A rating) switch in its output line

- to protect the field circuit in case of a shorted field winding

Neither CB should pop on an overvoltage condition, so long as the alternator is basically working.

If there is overvoltage protection (how common is that?) that should be implemented inside the voltage reg, which should open-circuit the field winding when some preset figure (e.g. 35V on a 28V system) is exceeded.

Some overvoltage protection is by means of a crowbar circuit which will deliberately trip the breaker.

Hi Baobab, The Alt CB protects the output of the Alternator (60A.), and the Alt Field CB protects the Alt Field current of say 2 Amps.

I think you are confusing the role of the Regulator... This only has control of the field current (max 2 Amps.) So is only handling very small currents.
The Alternator current does not go anywhere near the regulator!

The output of the Alternator is capable of many Amps, and could be wired with thicker wire.

The highest current on the aircraft is the Starter current, maybe 300 Amps, and as such requires a very thick cable.

It could be interesting if you could colour-code your diagram with the size of wires/cables used. So for instance the Battery to Battery Contactor, to Starter Contactor, to Starter, should be in thick cable.
Also the Ground Service Plug to Reverse Polarity Contactor to Battery Contactor can carry this Starter Current.

Pete

Baobab, that's a GTE question......

Yes, but that is used on the "client" side, not on the supply side. One could have a crowbar after each "client" circuit breaker but it would be a bit of overkill. AFAIK crowbar circuits are only used to protect the avionics. For the other circuits, a short spike of overvoltage is not too bad, and continued overvoltage will trip the CB.

(but me too am sure G could improve on my words)

No, it's as I describe. Either built into the voltage regulator, or a separate device.

When overvoltage is detected, a relay contact closes which literally shorts out the field supply. This then trips the breaker (or blows the fuse) and removes the field voltage, thus removing the overvoltage.

Wow! That seems like a dangerous setup to me - a minor fault could trigger the complete electrical system to go down (after exhausting the battery). At the least, dimensioning of the CB's should be very closely looked into.

Is this a common installation on US-an engines (of which I admit knowing next to nothing) or is it particular to certain makes/models?

It's just one way of doing things. Some voltage regulators simply remove the field voltage in a case of over voltage, but you've often got to cycle the breaker to reset it, so in practice it is no different.

At least with the crowbar circuit, you'll see the tripped breaker when you investigate the fact that you are no longer charging.

I don't see how it is dangerous particularly? After all, a flat battery is probably better than one that has been boiled by over voltage, and your avionics shutting down because the battery has started running down is better than them being permanently fried!

I agree about protecting the avionics - I did point out I expect this to be done in the client side, i.e. on the other side of the bus bar, didn't I.

But (for just two examples) flying a C152/C172, where I understand the flaps are electrically operated on most models or perhaps even on all, I'd be rather disturbed to see the flaps go inop due to a probably minor electrical fault.

I'm confused. Why would the flaps go inop because an overvoltage had tripped the alternator offline.

The crowbar only trips the alternator field breaker, nothing else.

When the event occurs, you'll get either or both an LV warning indication and an alternator warning indication. You'll also start seeing a discharge on the ammeter.

So you'd cycle the breaker for the alternator to see if that sorted it out. If it didn't, then you're in the same position you'd be in if your alternator had failed - which is to shed non-essential electrical load and get yourself on the ground.

Well, I must admit there's some logic in your story. Having considered the opposite arrangement as self-evident, I needed some time to get used to an alternate idea.

Allow me to repeat an earlier question: Is this a common installation on US-an engines or is it particular to certain makes/models?

over-voltage protection is a certification requirement in CS23 and later versions of FAR23. On all systems that I've come across (Lamar, Electrodelta etc) the over volt system will interrupt the field supply, not trip the breaker. Pilot action is always required for an over-voltage reset -it should never automatically clear the fault. Older Pipers used a separate relay as the protection device. Most later Cessnas have the protection within the voltage regulator although early 28v aircraft have a discrete over-volt unit mounted just above the park brake handle. You very rarely find an over-volt protection on an aircraft with a generator but normally always with an alternator.

Just to add to the discussion on the Robin HR100/210 there is a 225amp fuse in the positive feed to the starter and beyond - I've not seen this on other aircraft.

Not common, but it does exist! The original poster asked for reasons why the ALT FIELD breaker may trip and I offered another possibility.

If you google aircraft overvoltage crowbar, you'll get plenty of hits and schematics.

My sincere gratitude for the patient and, err, how was that, edifying? , discussion! I am learning again!