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Revmaster PM alternators

 
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dj_theis



Joined: 28 Aug 2017
Posts: 7
Location: Minnesota

PostPosted: Fri Aug 16, 2019 2:28 pm    Post subject: Revmaster PM alternators Reply with quote

The subject of the Revmaster PM alternators has come up in the past, regarding the regulators used and the nature of how best to wire them. In summary, I think Professor Nuckolls declared there was not enough detail to offer significant, specific advice.

I have a little more detail of a failure that was posted on the SonexBuilders forum and would like to toss this to the AeroElectric group and see what wisdom I might extract.

My interest is related to the fact that I have a Revmaster (R2300), the same design as what is noted in the following story. The Revmaster has a dual PM alternator setup as part of the flywheel assembly.

Two separate failures with very similar circumstances go as follows:
Dual PM alternators with the default, dual regulators. Both planes had both alternators wired to charge full time. One engine did not have any method of shutoff of either regulator (hardwired) and the other may have chosen to always run with both (even though switches may have allowed for single PM operation) One engine had 160 hours running with this configuration and the second had 450 hours. Both had Odyssey PC680 batteries.

The 160 hour plane experienced a difficult time starting the engine while exiting KOSH a few weeks back and after wearing down the battery significantly (my assumption, as it was reported that it took 30 minutes of cranking to finally get started with no note of any ground recharging once is started). Once the plane was started and after about 10 minutes in the air, the pilot noticed a smell of smoke and promptly returned to KOSH. I think the plane was eventually trailered back home (to TX) and upon disassembly, the stator(s) were/ was diagnosed as "fried."

Please review the included drawing. I've added the relays and selection switches but for the sake of this discussion, imagine the switches for PM selection are both left in the "ON" state. I also have the patent of what I think the 2 wire regulator details include and will dig it out but I think it was posted in the past. I've simplified the regulator in my sketch as an SCR controlled by a black box.

An interesting note, the two (20 amp) auto fuses were not blown. I need to verify if one or both of the stators were destroyed and likewise, if all 4 of the PM regulator fuses were OK.

The second plane had 450 hours running both alternators full time during those hours. As noted, same battery (PC680). I don't know if this failure had fuse failures for the regulator, I don't believe so. I'll double check the report but the pilot smelled smoke about 6 minutes after takeoff and once he landed and had everything apart he identified that the stator was toast. Again, I'm not sure if both sides had failed or just one. I'll follow up and check.

The manufacturer (Remaster) recommended not running both alternators at the same time and pointed to this as a likely contributing factor. He also indicated that the PC680 was partially to blame. His comment was that the RVLA batteries draw too much current. Hmmm, why did the fuses not blow then.....

I think I know what the consensus will be regarding the PC680 (not likely a contributor to the debacle). My sense, because the 20 amp inline fuses did not blow, that the stators were not overloaded (else they have the wrong fuse selection)..I've heard that each alternator is capable 18 amps but the detailed specifications are very limited. I plan on checking my engine alternators for open circuit voltage curves and short circuit current. I'm just a little ways away from that...

The alternators are (as can be seen) single phase. I believe the two alternators are in phase (I need to verify if it's possible to wire them so they charge out of phase, i.e. the regulator is half wave so it might be that each alternator could provide a half wave charge pulse, out of phase with the other.

My theory (aside from agreeing that only one system can run at a time) is that running both on a heavily discharged battery allowed both regulators to operate. At some point, the battery reached a "closer to" full voltage level and as might be expected, one of the regulators started opening the charge circuit from the alternator. If this occurred at the wrong time, with the other regulator running, the inductive voltage spike(s) burned a hole in the stator winding insulation at one or more spots. the result, a fried stator.

What do you guys think?

I look forward to hearing opinions.

Dan Theis
Sonex 1362R


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user9253



Joined: 28 Mar 2008
Posts: 1436
Location: Riley TWP Michigan

PostPosted: Sat Aug 17, 2019 5:12 am    Post subject: Re: Revmaster PM alternators Reply with quote

The purpose of a fuse in the alternator output circuit is to protect the battery from short circuits. Alternator output is self current limiting. The quantity of electrons flowing is limited by the ability of the magnets to push them.
There is no inductive voltage spike when a load is removed from the alternator. The voltage will go up because the electrons being pushed by magnets have no where to go. But the voltage is not induced by a collapsing magnetic field.
Alternator windings can fry due to overheating caused by inadequate cooling or due to prolonged high current output. Electrically speaking, there are two separate alternators. But both are contained in one physical enclosure. Operating both alternators simultaneously at full output generates lots of heat which could fry windings. The solution is to redesign the alternator with larger wires and/or better cooling. Or limit the load. Or limit the total output current by operating only one alternator at a time.


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dj_theis



Joined: 28 Aug 2017
Posts: 7
Location: Minnesota

PostPosted: Sat Aug 17, 2019 8:00 am    Post subject: Re: Revmaster PM alternators Reply with quote

Thanks Joe,

“The purpose of a fuse in the alternator output circuit is to protect the battery from short circuits. Alternator output is self current limiting.”

I appreciate your feedback. The short circuit protection meant to protect the battery was not something I had considered as the design intent. I Understand the fixed current model of the PM alternator so what a damaged winding suggests is a weak or less than “robust” design, as you imply.

If you noticed, the ignition circuits are part of the same flywheel assembly and in neither of these cases did the ignition fail. This is evidence to me that the self destructive heat generated by a stator winding was not high enough to propagate the damage beyond the source of the heat (the physical relationship is as the drawing shows, the ignition coils separate the alternator windings).

Now that I think of it, the 450 hour plane did have a previous incident where the ignition coil failed and the smell he sensed when the alternator failed led him to expect one of the ignition coils had failed again. Note, with none of these failures did the engine stop running. The ignition coils are redundant. One feeds the upper ignition and one feeds the lower.

I doubt I can obtain any forensic data on the failed systems but I will give it a shot and see what might be learned.

My goal is to modify (improve) my engine charging system to avoid any of these failures.

Thanks again for your feedback Joe. It helps a lot.

Dan Theis
Sonex 1362R[/quote]


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