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Understanding generator / alternator disconnect requirement

 
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kenryan



Joined: 20 Oct 2009
Posts: 424

PostPosted: Wed Nov 12, 2014 9:17 am    Post subject: Understanding generator / alternator disconnect requirement Reply with quote

The wiring diagram in the Rotax 914 installation manual shows the R,B+,L and C wires from the integrated generator's regulator-rectifier are all disconnected, via relay 58 when the master is turned off. For the external alternator it shows the B+ as being connected directly to the battery, while the IG (and L) wires are disconnected when the master is turned off. The text notes that the integrated generator's regulator-rectifier must be disconnected to avoid running the battery down. The text does not mention any such requirement for the external alternator. Neither the text nor the diagram say whether or not relay 58 is supplied by Rotax with the engine. All this raises the following questions for this neophyte:

1. Is relay 58 included with the engine?
2. Does the external alternator need to be disconnected to avoid drawing down the battery?
3. What do the indicator lamps for each of the charging devices actually indicate?
4. Are the indicator lamps considered optional?
5. If installed, can the indicator lamps be LEDs?
6. Are the L and IG wires on the external alternator only for lighting the indicator lamp or do they serve some additional purpose?

I know it's a lot of questions, but none of them seem to be answered in any of the Rotax documentation.

Ken


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h&jeuropa



Joined: 07 Nov 2006
Posts: 634

PostPosted: Thu Nov 13, 2014 11:31 am    Post subject: Re: Understanding generator / alternator disconnect requirem Reply with quote

Hi Ken

Welcome to the Rotax forum. The other really good places to get Rotax information are Rotax-Owner.com (Rob is very knowledgeable on 914) and of course the factory website, Rotax-aircraft-engines.com.

I'll try to help you with your questions.

Relay 58 is not included with the engine. You can get suitable relays from B&C Specialty or Stein Avionics or many other places.

The external alternator is a conventional automotive style alternator and really doesn't need to disconnected. However, circuit breakers 43 and 45 will disconnect it and many prefer to start their engine with the alternator off line.

Indicator lights 36 illuminate when their respective alternator is not operating. Funny I hadn't noticed they both have the same number 36!

In an experimental aircraft you can do whatever you like. Probably you have other means of indicating the operation of the alternators such as voltmeters and or ammeters. But the lights don't add weight and do give you indication of what is happening.

Yes, you can use LEDs. Be sure they have current limiting resistors. In both cases, the L lead is where the negative lead of the LED connects. I have a LED on my 914 (no external alternator).

Others should answer about the external alternator, but I believe it is internally regulated which means the IG wire is used to sense system voltage which in turn is used by the regulator to control the alternator. I could be mistaken in this so check with others.

The regulator 13 has proven to be temperature sensitive. Be sure you provide cooling air to it, mount it on a metal surface if possible and put a temperature indicating strip on it to be sure it stays below 80C. You should have the same strips on the ignition modules. I also drilled a small hole in the fin and connected the ground wire there rather than using one of the mounting bolts to get a more secure physical connection.

Using LED lighting for position and strobes, you may not need the external alternator. In my Europa I have 914, two screen EFIS, GNS430 nav-com-gps, transponder, audio panel , xm weather and AeroLED lights. In cruise the built in alternator puts out less than 10 amps.

AeroElectric Connection has a pretty good electrical system design, Z-16 for a Rotax.

Hope this is helpful.

Jim Butcher


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