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Andy A

Generator whining over intercom

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I think I have narrowed down the whining over our intercom to the generator.  The whining increases with an increase in power.  When I disconnect the breaker, the whining stops.  However, I can also hear the blinking of the strobe lights when they are turned on over the intercom.  I understand the problem if probably a grounding issue, but, I don't really know how to go about fixing it.  Right now my plan is to remove the generator, make sure all the connections are clean, and reinstall.  Any other recommendations?  Its a 2014 CTLSi.

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Does it have a Garmin GPS? If so try turning off the GPS circuit breaker. Some of the early airplanes had issues with generator noise coming in through the GPS.

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I noticed today that the generator is grounded to the same bolt that the avionics busbar is connected to.  I think all of the avionics and wires on the cabin side of the firewall are connected to a busbar on the copilot's side (no pictures of this).  The busbar is connected to the firewall by a bolt that passes thought the firewall which is used as a grounding point.  This bolt can be seen in the first two pictures and has two black connectors with wires attached.  These two black wires are the ground wires.  One wire goes to the negative side of the battery, the second wire connects to the steel frame where the motor mounts are located, the generator is also attached to this bolt (picture 3).  I also noticed the other side of the airplane has the same steel frame and bolt with nothing attached (picture 4).  I am thinking about disconnecting my generator ground from this bolt and connecting it to the empty bolt located in the fourth picture.  Does anyone think this will accomplish anything or will it still have the whining because its all attached to the same steel frame?  Also, someone mentioned to me that I might be able to use a plastic isolation washer to place between the 2 grounds.  Anybody ever heard of this?

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Won't really do anything. Use a larger capacitor and make sure grounds are pristine.

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Please elaborate.  What kind of capacitor and where does it go?  The grounds look pristine to me but I don’t really know what I am looking for.  I appreciate any advice you have, but I don’t have an A&P background.  Thanks again.

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I use kemet ALS40A104KF025. Chanik here on this forum was the first to recommend it. Since then, I had passed this along to rotax, sportcruiser, flight design, and dynon avionics. You'll hear about this part recommendation if the original isn't cutting it.

You replace the old capacitor with it.

In the older rotax design, it is almost a necessity to install a capacitor that is 3x the minimum recommendation by rotax (this is coming from them in classes as well as field experience) to absorb electrical noise. The souce of the noise is the regulator recitifier's SCRs, or silicon controlled rectifiers, which are not properly noise suppressed. They act like gates that are constantly being slammed open and closed, and they are the common failure point as they are also switching very large loads (HEAT!). MOSFETs would have been a lot quieter... but also significantly more expensive (and I mean by orders of magnitude... SCRs are pennies, mosfets of this size are probably dollars to tens of dollars).

I believe the 912i still uses a similar design, so they would also benefit greatly from a larger capacitor.

Now, again, I want to drive home the point of grounds.

Your electrical system is a lot like a water pipe system. It would be annoying if your municipal water system pressure constantly fluctuated. This is what your electrical system is doing. A capactor acts as an accumulator, which in the water equivalent, would be like having a water tower on your front lawn to absorb the pressure fluctuations.

Corroded positive connectors would act like restrictions in the pipe, lowering your pressure, sometimes enough that some devices won't function.

Now, ground connections are a little bit harder to use this analogy with, so i need to set you up first: imagine how an old water meter works. Water flows, pushing against a mechanical device. When you close the valves in your home, the pressure is still there (just like voltage, it's still there), but there's also pressure pushing back. The wheel doesn't move, so the meter doesn't run. When you open a valve, you lower the pressure on the far side, and water moves, acting on the wheel and moving the meter. If you have a restriction in your piping, the water flows slowly even if you have all your valves open, and the wheel moves slowly. It's because that restriction causes a pressure rise before it.

When you have a dirty ground, it's like having a restriction. It raises the voltage in between the dirty ground and your device. So power won't flow as well, and some of it will dissipate as heat instead of being used to power your device.

So point being, do a ground voltage drop test for your electrical system and see if you have a dirty ground. It shouldn't be higher than a few tens of milivolts, and when you turn on all of your electronics, you shouldn't see large swings in voltage. The larger the swing, the more it will translate to noise. The swings should never put it over 100 millivolts, but the less variation, the better!

The regulator rectifier puts out noise and does cause some backfeeding on the ground, which is what the big capacitor is for. Your battery absorbs large power swings, the capacitor absorbs the small, rapid pulses that translates into audible sounds.

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One more question.  Where is the original capacitor located?  Is it a pretty easy thing to swap out?

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The capacitor is up above the battery and just behind the square engine mount tubing. It probably has some fire sleeve over it and two wires coming out off the end. Two wire ties hold it in place. I carry them if you need one. They aren't expensive. I have found a few with broken connectors at the base of the capacitor. These are polarity sensitive so make sure the right wire goes back on the right terminal. If you swap them as soon as the engine starts up it will fry it.

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I've heard people in the past talk about discharging a capacitor before removing it.  Does anybody know if this needs to be done to this capacitor?   Do I need to do anything else special to it before removing it?  Or just remove the wires, install the new one, double check to make sure the polarity is correct and then I'm done.  I just ordered the capacitor off of Amazon for about $30.

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