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PMAG overvoltage response ?

Larry DeCamp

Well Known Member
A recent thread referenced a dual Pmag failure due to an internally regulated OV event. Pmags run of internal generated power above 900ish RPM and I verify that on every run-up. The reported event implies buss power is still applied to the Pmag brains in parallel with internally generated. This would make OV control the single point failure in a dual Pmag config. Does anyone know what is fact ??
 
There was a major overvoltage event, with P-Mags, thread here in the past. The P-mag electronics were dead, but the mags kept running.

George Meketa
 
OV protection ?

OK, who has any stats on EXTERNAL OV protection ? I have B&C for both of my of B&C alternators (RV3 and RV4) Any failure pireps to consider ?
 
It should not ever happen in a properly designed electrical system that has over voltage protection......

Don't let OVP make you think you are invincible from all possible OV causes. If your battery suddenly becomes disconnected from your electrical system (battery failure, wire to battery failure, master relay failure, master switch failure, etc..) you will be at the mercy of an alternator load dump (https://en.wikipedia.org/wiki/Load_dump). When a load is suddenly removed from the electrical system (in this case, a battery being charged), the alternator's regulator (or OVP) can't react fast enough to compensate for this immediate load removal. The temporary side effect while the regulator adjusts is up to 100+V spike for 400ms.

I had a load dump happen to me last year. Luckily I was on the ground getting ready to take off. The only thing that kept my engine running was my Slick magneto.

Some aviation electronics use TVS diodes to protect themselves against this transient voltage surge. I do not think PMag's do. This situation should be preventable by adding a TVS diode close to the fuse/CB that gives power to the PMAG's. The TVS diode in theory should short out the circuit and blow the fuse/CB before the surge makes it to the PMAG.

Just some food for thought....
 
A recent thread referenced a dual Pmag failure due to an internally regulated OV event. Pmags run of internal generated power above 900ish RPM and I verify that on every run-up. The reported event implies buss power is still applied to the Pmag brains in parallel with internally generated. This would make OV control the single point failure in a dual Pmag config. Does anyone know what is fact ??

There was a major overvoltage event, with P-Mags, thread here in the past. The P-mag electronics were dead, but the mags kept running.

George Meketa

Looking at the thread that I think is being referenced....

It seems to imply that the PMags external power circuitry was killed by the OV event but that the internal power and the firing electronics kept working until below the minimum RPM required for the internal power to run the PMag.

It is my understanding that the internal and external power isolation on the PMag is simply 2 OR?d diodes.
 
The plane had dual P-Mags and kept running while in the air, despite a nasty high voltage event. Does this mean they should be trusted? Not by me.

The thread I mentioned was shut down after my comments. While I am in favor of electronic ignition and electronic fuel systems, a lack of true isolated and redundant system design is a problem that is going to eventually get someone here killed.

A link should be posted to that OV thread. There is a lot of things one can learn from it.

George Meketa
RV8, 1600hrs.
 
Has anyone contacted E-mag and asked them for details?

I have seen P-mags erroneously blamed for everything from damaged cylinders to bad primer.
 
Time out

I promoted this conversation because it was implied that Pmags could fail in an OV event. No one has brought any facts to the conversation. So, as suggested, I will consult Brad on Monday and relay the info.
 
Emagair ( Brad ) Discussion today

Reference the report on VAF regarding fried Pmag electronics due to an OV event. Per phone con with Brad today, 114 Pmags have two independent sources of secondary coil excitation and will continue to run if either is compromised. The PRIMARY source is powered from the ship's buss. The secondary is the INTERNALLY generated source that takes over above the defined approximate rpm. The primary circuit board will tolerate up to 60vdc before terminating its participation in the ignition function. The best evidence that the above is true was the engine kept running to a safe landing with the primary boards fried.

The Pmag will function like a NON IMPULSE magneto without ship's power. So the only way to know if the primary function is compromised is to start the engine on each Pmag independently.

Brad makes no prediction regarding an OV level that might compromise the existing two layer independence. Tom is the guy to call if you are knowledgeable enough to explore this subject ( such as TVS diodes etc).
 
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