"half-rpm" indication from Lightspeed

[elippse]

Here's something to consider about alternator interference getting into low-level signals. The alternator is a three-phase device which uses diodes to commutate and rectify the outputs of the three windings. The crossover point where one winding's voltage is dropping off while the next one is increasing is at 87% of peak at 60 degrees of the sinusoid. There is a period where the voltage on the windings is below the battery voltage, and during that period of time the battery is supplying current to the load. Let's say that during the peak of conduction the battery is charged to a peak of 14.4V, then when the AC is decreasing the diodes turn off at 13.6V. Now the average voltage being seen by a voltmeter on the battery will be 14V; an o'scope will show the variations. If the diodes have a 0.5V drop each, then the peak voltage on the windings will be 14.4V + 2 X 0.5V or 15.4V. Then the diodes conduction will cease at 13.6V + 2 X 0.5V or 14.6V. This, of course, assumes that the altrernator windings have zero resistance. Now on a sinusoid the 14.6V level would be at 71.5 degrees, 18.5 deg. after the peak. The next diode will start to conduct 23 deg. later when its sinusoid is 18.5 deg. before the peak. So the diodes will pump current into the load and the battery for 37 deg. then be off for 23 deg. during each 60 deg. conduction cycle. Those are 62% duty cycle on and 38% duty cycle off. If the load is 20 A, the alternator will put out a little over 32 A avg. to supply the load and recharge the battery. That current flows from the alternator TO the battery. During the time the battery supplies the load current, the current flows FROM the battery. So what we have is a very high back-and-forth current in the conductors from the alternator to the buss and to the battery; it's not a nice, smooth DC flowing in one direction. Those high, alternating currents can electro-magnetically induce voltages in nearby conductors carrying low-level signals. That's where you get the "alternator whine" in your radios! So my advice is to segregate low-level signals from high current signals as much as possible. Don't run them parallel for any distance. Strobes, too, have high transient currents. And please, please, attach a heavy ground conductor from the alternator ground-lug to the engine; do not rely on the mounting bolts!

[Paul Eastham]

Update: it is not the alternator, nor seemingly any interference. The problem still happened today with alternator off, even with everything electrical off.

I also tried another sensor spacing (this time increasing more toward .06"), no change.

Also tried power cycling the lightspeed, no luck.

Twice today I started the engine ("cold" at 70 degrees) and it happened for about 1.5 minutes. I then shutdown, turned everything off and went through the start checklist again -- didn't recur. The same sequence happened again again a few hours later in the day. Sure is tempting to think it's some sort of thermal issue, although things don't get that hot in 60 seconds, especially with the cowl off. The front of the crankcase is only warm after a 3 minute run like that.

Paul

[dan]

Swapping the coils is quick & easy. Why not rule that out?

[Walt]

Originally Posted by Paul Eastham

I re-wired the connector very carefully using crimped pins. (open barrel pins, unfortunately -- hard to find a DB-15 that takes the good ones)

I would go back and get rid of any cheap DB connector you installed. A soldered butt splice for the signal lines would be the best option. If you use DB connectors in aircraft they should always be the machined pins.

[Norman CYYJ]

Have you swapped the computer control boxes yet to see if the trouble follows the box? If it does you will know where the problem is.

[Ted Johns]

Paul,

Did you check the magnet polarity?


Ted Johns
RV7 plans preview

[Paul Eastham]

Originally Posted by Ted Johns

Paul,

Did you check the magnet polarity?

Yes. Also magnet strength seemed similar.
After a two-hour flight without any problems yesterday, running hard and high OAT, I taxied to the fuel island and filled up. Restarted the engine 5 minutes later, and there's the problem again. I'm really starting to doubt the thermal theory.

Hopefully when Klaus is back from Oshkosh I can convince him to send me a loaner box to try swapping out. He has been reluctant in the past but now I have a lot more data that seems to rule out most other parts of the system.

I haven't tried swapping the coils yet Dan, though I will soon. Anything is possible, but the RPM reads correct even with both coils disconnected, so it's hard to imagine how a bad coil would cause this particular symptom.

Thanks for all the ideas, everyone! We'll get it figured out soon...

[Ted Johns]

Originally Posted by Paul Eastham

Yes. Also magnet strength seemed similar.
After a two-hour flight without any problems yesterday, running hard and high OAT, I taxied to the fuel island and filled up. Restarted the engine 5 minutes later, and there's the problem again. I'm really starting to doubt the thermal theory.

Hopefully when Klaus is back from Oshkosh I can convince him to send me a loaner box to try swapping out. He has been reluctant in the past but now I have a lot more data that seems to rule out most other parts of the system.

I haven't tried swapping the coils yet Dan, though I will soon. Anything is possible, but the RPM reads correct even with both coils disconnected, so it's hard to imagine how a bad coil would cause this particular symptom.

Thanks for all the ideas, everyone! We'll get it figured out soon...

Wow. I suppose at this point I would wait for the system to go into this half speed mode and then hook up a signal generator to the sensor input. If the ignition still was in half_X mode, you would know the box was at fault. Intermittents like this are really a PITA.

It seems like you have debugged this enough though. Klaus really ought to send you a loaner.

Ted Johns
RV7 plans preview

[AlexPeterson]

Paul, just to be clear, what are your minimum and maximum gaps? I.e., when the crankshaft is pushed fore and aft. You mentioned you increased it to .060". Is that with the crank fore or aft?

If the crank is getting a little to far away from the sensors, perhaps a dive with no power to push the crank back might tell you something.

Also, do you have two systems?

Sure seems like a flakey top or bottom hall effect sensor or a marginal magnet proximity problem.

Did you do the clay thing on both the top and bottom sensor? I know that I installed my pickup board using the little metal tabs, but that the top and bottom sensors were at just a little different radius.

For one ignition, each one turn of the crank should cause 4 Hall effect events (two magnets, two sensors). It really appears that one of the sensors is not reliably picking up.

[Paul Eastham]

Thanks for everyone who chimed in with ideas. To follow up (and hopefully close this thread forever : ), Klaus sent me a loaner controller box. After about 4 hours, 3 flights, and at least 8 starts, the problem has not returned. Fingers crossed!