Alternator Balance

[AndyFromCB]

Hello,

A question to all Bravo owners. How balanced are your alternator draw? Mine are about 10 amps apart. Is that normal? When you answer can you also let me know what your serial number is so I know what voltage regulator you're using.

Andy

[AndyFromCB]

Also, coming from a four cylinder aircraft that was pretty rough to begin with so when checking right and left mags I didn't notice much more of a shake, is it normal for my Bravo, which is normally turbine smooth with both mags on to drop about 100 rpm on a single mag and get pretty rought vibration? Worse on the right mag than the left one. Where is John Green when you need him ;-)

[carusoam]

Astel,

How many hours on each mag since they have seen a rebuild or overhaul?

Best regards,

-a-

[AndyFromCB]

Hi,

90 hours since overhaul. It's not unbearable but there is definatelly more fibration than my Arrow's Lycoming IO-360

Andy

[jetdriven]

Check the mag timing.  If it is a smaller number than spec (say 23 BTDC instead of 25) it will have excessive mag drop.

[Awful_Charlie]

Simple answer to the alternator load question - I have no idea!

My left alternator warning light doesn't go out until about 1200RPM with both online, one or two hundred lower if I knock the right one offline.  However, either one on it's own seems to cope with all the load I can throw at it, so just took it as the way it happens to be.

Mag timing should be 20 dBTDC, and my engine maybe a little rougher on a single mag, but not significantly so.  Do you have an engine analyser to ensure it is running on all siz cylinders when on one mag?  With one cylinder out it is certainly uncomfortable (fouled plug was the cause in the one experience I have of this)

Ben

[AndyFromCB]

Thanks Guys,

I'll check my engine monitor to plug issues tomorrow when I go out for a spin.

I finally understand the alternator issue in an older Bravo, or more like the voltage regulator issue. Basically there is two channels in the regulator, one for left alternator, one for right alternator, but really only one, the master is online, driving both channels.So if you're left is the master (depends on how you have it plugged in), that voltage regulator channel is controlling the field output to both regulators. The right channel will only kick in on its own if either the left regulator channel dies or gets disconnected via the alternator field button. The overvoltage protection will first kick off on the slave and then if still overvoltage, it will kick off on the master channel.

So if your right alternator is weaker than your left alternator by much (I know they are supposed to be the same, but having now dealt with this BS for about a month before squaring it all away, I resistance across the field windings varried by about 15% between 5 different ES4009 alternators we tested), it will actually make less power/voltage than the left one. The opposite can be true as well, making one alternator go over voltage while the other is still way under since the voltage sense pickup is not at each alternator but in a common location, see below.

BTW, the older Bravo's pick up sense voltage from the announciator panel circuit breaker, if that ever pops, your voltage regulators go overvoltage and then go offline, nice, isn't it. So much for redundancy ;-)

So the trick is to figure which alternator is stronger, which one is weaker, connect the stronger to the left channel on the voltage regulator (it's the master channel), adjust each voltage regulator's channel to its proper alternator to produce 28.2 volts max. Then the stronger alternator will kick off 28.2, the weaker one below that, hence the lower current draw. Now, if you switch the plugs at the regulator and the difference between alternators is major enought, you'll get the stronger alternator producing over voltage but actually the weaker one's field will pop and then since the voltage regulator will work in one channel only, everything will work again.

Basically, while we have "dual" regulators and dual alternators, they are not balanced. That would require a balance controller with shunts are each alternator. I think certain aircraft have that kind of a controller but not a 1990 Bravo.

Only took a month to track all this down. I actually took the regulator apart, looked at and identified all the parts and paths, plugged in into eletronics simulator and that's how we figured out the issue. If the alternators are way apart, you can have a voltage regulator that works great on a test bench and will trip up in the aircraft. Sometimes you just have to adjust them in the aircraft.

So now the question is why the two alternators are way apart. I tested the field wires and both seem to have the same resistance.

Andy

[johnggreen]

Astelmaszek,

Appreciate that you noted I was gone.  Been busy and fortunately off to Virginia to see new grandson.

You have asked a question with almost endless possible answers.  I'd rather track down any problem other than the magneto ignition system on an aircraft engine.  Now, remember, I'm not a real mechanic.

My Bravo drops abut 165 rpms on each mag if the mixture is leaned; more if rich.  The adjustment of the fuel system at low power is almost as difficult as the ignition.  As long as it works correctly at full power and cruise power, I'm not too concerned.

So, I'll add my two cents worth.  The mags need to be rebuilt about every 400 hours.  Wires and plugs will test ok and still be deficient in operation.  Timeing is always critical.  And don't forget the mixture.  Try checking your mags with the mixture leaned and see if there is a difference, if not then go back to the ignition system.

If push comes to shove, overhaul the mags, check the timeing (is it timeing or timing?), replace plugs and wires.

Also, do a mag check at cruise power, in the air with the engine leaned.  See if everything is working "up there".  A voltage leak, i.e. crossfiring, no matter how small the leak, can cause real havoc.

As far as the Bravo engine running smoothly, i was noticing today coming back from VA, it really is almost turbine like.  I believe the smoothest engine I have ever run and I've owned 32 airplanes at last count.

JGREEN