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-   -   PMag wiring (https://vansairforce.net/community/showthread.php?t=202896)

carlrai 01-16-2022 03:05 PM

PMag wiring
 
1 Attachment(s)
Question for the brain trust regarding wiring a two PMag system:

If using an ACS keyswitch, can the P leads of the PMags be switched through the ACS switch as with magnetos with the breaker protected power/test circuit wired to separate momentary contact switches?

Should be simple to wire up and is the first choice. Just want to make sure the keyswitch doesn't interfere with anything.

Or would using toggle switches for P lead and Power/test circuit be the best solution? (As suggested in another thread here?)

Attachment 21059

Thanks

Ron B. 01-16-2022 03:19 PM

My friend's had Stein make his panel. He has a magneto and P-Mag wired thru a key switch as you suggested. Cannot see where there could be an issue with both P-Mags thru a key switch.

DavidHarris 01-16-2022 04:50 PM

Advance Flight Systems built my panel. It has a key switch for the P-leads (Kill terminals) of the dual pMags, as for ordinary magnetos, as you propose. They are wired to the ACM power with #20 wires.

The Pmags also require a power connection, and require switches to verify their internal alternators are working during preflight even when the power connection is turned off. My panel has a pair of switches with internal circuit breakers for this purpose. It's required in the Pmag installation manual, but took me a little while to understand, because they don't do anything unless the pmag alternator isn't working.

Be certain to run both of these power wires from the switches to main power independently. I shared a wire, and had a marginal crimp on the shared wire that led to intermittent loss of backup PMAG power, leading to the engine stopping during stall testing during phase 1 testing, when the RPM was too low for the pMag alternators to function. The crimp was good almost all the time, making it very difficult to troubleshoot.

rvsxer 01-16-2022 04:57 PM

I designed many panels at SteinAir using an ACS keyswitch to ground the P-Mags. The only addition was usually a pair of spring-loaded-on toggles to interrupt the electric power for checking the PMA function during run-up. There is a "P-Lead" connection on the P-Mag for this.

MacCool 01-16-2022 05:07 PM

1 Attachment(s)
My Pmags are wired that way too, so my startup and pre-takeoff runup is done the same way I've done it for 50 years with magnetos...with the exception of an additional checklist item...main power to each Pmag so I can check its intrinsic alternator.

1700 rpm, check left ignition, check right ignition, then additionally over to the Pmag switches and turn off power to first left Pmag, then right pmag and make sure they still run when their power is shut down.

BobTurner 01-16-2022 05:23 PM

Make sure the jumpers on your key switch are configured for ‘start on both’.

rv8ch 01-17-2022 12:37 AM

Another opinion - besides looking like the aircraft that you trained in, why use a keyswitch? They are expensive, seem to fail regularly, and you'll still need to add toggle switches to test power on the pmags.

I used a pair of honeywell 3-position toggle switches - one for each pmag - and they work great, easy to wire, and it's really clear what's happening when I test my pmags during runup. Off - On - On w/12v I have a separate toggle for the "maintenance" function of the pmags needed to time them. And of course a big fat start button.

Nice thing about "homebuilt" is we get to do our own thing. :)

carlrai 01-17-2022 07:22 AM

Quote:

Originally Posted by DavidHarris (Post 1582573)
Advance Flight Systems built my panel. It has a key switch for the P-leads (Kill terminals) of the dual pMags, as for ordinary magnetos, as you propose. They are wired to the ACM power with #20 wires.

The Pmags also require a power connection, and require switches to verify their internal alternators are working during preflight even when the power connection is turned off. My panel has a pair of switches with internal circuit breakers for this purpose. It's required in the Pmag installation manual, but took me a little while to understand, because they don't do anything unless the pmag alternator isn't working.

Be certain to run both of these power wires from the switches to main power independently. I shared a wire, and had a marginal crimp on the shared wire that led to intermittent loss of backup PMAG power, leading to the engine stopping during stall testing during phase 1 testing, when the RPM was too low for the pMag alternators to function. The crimp was good almost all the time, making it very difficult to troubleshoot.

Thanks for your input Just what I needed.

carlrai 01-17-2022 07:28 AM

Switches
 
Quote:

Originally Posted by rv8ch (Post 1582645)
Another opinion - besides looking like the aircraft that you trained in, why use a keyswitch? They are expensive, seem to fail regularly, and you'll still need to add toggle switches to test power on the pmags.

I used a pair of honeywell 3-position toggle switches - one for each pmag - and they work great, easy to wire, and it's really clear what's happening when I test my pmags during runup. Off - On - On w/12v I have a separate toggle for the "maintenance" function of the pmags needed to time them. And of course a big fat start button.

Nice thing about "homebuilt" is we get to do our own thing. :)

Not my choice on this. I prefer toggles myself and as many multi function switches as possible to minimize panel space.

Thanks Mickey

GalinHdz 01-17-2022 07:40 AM

1 Attachment(s)
Quote:

Originally Posted by MacCool (Post 1582576)
My Pmags are wired that way too, so my startup and pre-takeoff runup is done the same way I've done it for 50 years with magnetos...with the exception of an additional checklist item...main power to each Pmag so I can check its intrinsic alternator.

1700 rpm, check left ignition, check right ignition, then additionally over to the Pmag switches and turn off power to first left Pmag, then right pmag and make sure they still run when their power is shut down.

Exactly how I set mine up. Mine has a Normal Closed push button and a circuit breaker for each eMag. In addition to the regular P-lead check, I check the power by pushing each button during engine run-up. It is part of my checklist.

Attachment 21086


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