Fitting an alternator to C90 query

[Lodems]

Am about to fit a new B&C alternator to my Continental C90 to replace a failed ancient generator. I would like to run the engine occasionally whilst I sort out the new wiring setup. Is there any danger to the new alternator if it's not electrically connected? I know regulator stators can be damaged by a sudden open circuit but can't see how I would be risking alternator damage by an engine run before connection. Am I right?

[Flyingmac]

Are you changing the voltage regulator?

[Lodems]

Fitting alternator to C90

Flyingmac
Yes. I'll be fitting the full set of bits including the solid state regulator and the surge diode etc.

[Genghis the Engineer]

I fitted that unit to an 0-200 a while ago. It worked well - eventually. However, it wasn't the straight swap that they would have you believe. If you would like to send me an email address, I can send you a copy of the worksheet and wiring diagram for our eventual good solution.

G

[pulse1]

I did this for an 0-200 and we also had initial problems. we eventually realised that the regulator was getting too hot and was shutting down. A simple air duct cured the problem and it has been trouble free for 5 years or so.

[The Flying Pram]

Is there any danger to the new alternator if it's not electrically connected?

Whilst I have not had direct experience of aviation alternators, you do not want to be running any automotive alternator without a battery connected. It's the main power diodes that don't like being open circuit. However, as I understand that the field circuit on light aircraft can normally be switched on/off by the pilot (unlike self contained car types), then you may be OK if this is not energised. There is still a risk of self excitation from any residual magnetisation, but unlikely unless the engine is run at high revs. I would be inclined to earth the field terminal to prevent any chance of this.

I now await being shot down in flames from those more knowledgeable...

[Jan Olieslagers]

Not claiming any authority ( I have tinkered a good bit about my Rotax 912, especially the electrics, but never touched the US'an stuff) I think the T/S may have it right: if no electrical connections are made, nothing can go wrong. What can and will go wrong is to generate even the slightest bit of field (as suggested in the previous reply), that will unavoidably create output and that has to go somewhere. So I think one can be on the very very safe side of things by shortcircuiting both the field and the load connections of the alternator, either with a bit of (thick!) wire or, perhaps even more prudent, with a low resistance, a car bulb or so. But there ought to be no need for such extreme prudence. I think

[The Flying Pram]

By short circuiting both the field and the load connections of the alternator, either with a bit of (thick!) wire or, perhaps even more prudent, with a low resistance, a car bulb or so

I was tempted to say that, but didn't want to push my luck! I have experimented quite a bit with car alternators (which are basically the same technology). I once had to jump start my car after finding the battery had failed completely open circuit whilst driving - predictably a week after the warranty had expired. I deliberately left the side lights on, to provide a resistive load, and no damage ensued on the return journey. Diodes have both a forward and a reverse maximum voltage, and exceeding this is asking for trouble. With no load on the output even a small amount of field magnetism can produce surprisingly high stator voltages, and consequently the risk of damaging the diode pack.

[Lodems]

Alternator to C90

The tech guys at B&C say it's ok to run the engine with their alternator electrically disconnected so I hope they are right!

[Jan Olieslagers]

Do not hope. Count. Get them to promise to pay for the damage if it does go wrong. If they're really confident they won't object.

[The Flying Pram]

If it does go wrong

Which may not be apparent straight away. Would you want the risk of the diode pack failing some time later during a flight? I've seen electron microscope pictures of semiconductor junctions which have been subject to excessive voltage, and it's not a pretty sight! More worryingly they may continue to work seemingly O.K. only to fail completely at a later date. Ask your engineers to find out the maximum forward AND reverse voltage rating of the rectifier diodes - if they can't (or won't) then ground the main +ve terminal. With no field excitation, even residual magnetism won't induce sufficient stator current to cause overheating, but at the same time neither will there be any open circuit voltage rise...