P-Mag Test Switches, necessary?

[Toobuilder]

Originally Posted by Ironflight

...but there were some fairly compelling technical arguments a few years back on the topic - look up posts by GMCJETPILOT if you have a lot of time to kill!...

Thanks! I'm all over "compelling technical arguments" like a fat kid on cake - particularly if they are contrary to my own behavior. To be honest, I was taught to fly this way and since it did not conflict with my (not insignificant) understanding of alternators, I accepted it.

I?m not sure how charging the field around a spinning rotor is any harder on the unit than subjecting it to the enormous voltage swing created by cranking, and then start of the engine ? but I?ll sure try find out.

[Ironflight]

Originally Posted by Toobuilder

I?m not sure how charging the field around a spinning rotor is any harder on the unit than subjecting it to the enormous voltage swing created by cranking, and then start of the engine ? but I?ll sure try find out.

The problem is not so much with the spinning part of the alternator part of the equation - this seems to kill the internal regulator. Just wanted to head you down the right path!

Paul

[Toobuilder]

Originally Posted by Ironflight

The problem is not so much with the spinning part of the alternator part of the equation - this seems to kill the internal regulator. Just wanted to head you down the right path!

Paul

Well, it's the "spinning" that creates the voltage that the regulator has to deal with... All I was trying to say.

[Ironflight]

Originally Posted by Toobuilder

Well, it's the "spinning" that creates the voltage that the regulator has to deal with... All I was trying to say.

Concur - just wanted to make sure I wasn't misleading you Michael!

[Ted Johns]

I?m not sure how charging the field around a spinning rotor is any harder on the unit than subjecting it to the enormous voltage swing created by cranking, and then start of the engine...

Having an internally regulated automotive alternator spinning at speed, then switching it on (the field input on this type of alternator just turns on the regulator) results in a large voltage / current spike that the output diodes have to deal with. I have the Delco SI type internal regulator schematic, and the regulator sense circuit control speed is limited by loop filtering / slope integration. The field control does not have filtering. Switching this type of regulator on via the "field" control input will result in full current instantly, followed by a ramp down to the correct output voltage. It simply wasn't designed to be switched on at speed.

Some alternators have output diodes robust enough to deal with this spike and not suffer reliability issues. From personal experience I can tell you that some Delco 10SI units do not. This type of regulator - alternator circuit is designed to start producing current from the first revolution. This results in an intrinsic soft start, as the output at starter rpm is minimal.

The regulator IC can suffer from the field current surge as well (I'm told), though I have not personally experienced this failure mode.

As to the "enormous voltage swing created by cranking", the voltage drops to perhaps 8 volts while cranking, and returns to battery voltage as the engine starts and the starter is disconnected. This doesn't result in any stress to the alternator.

If I was designing a regulator for aircraft use, I would ensure that the regulator circuit could not be switched on in a way that resulted in an initial full field output prior to ramping down to the correct voltage. I'd also specify over rated diodes for the alternator, all for the sake of reliability. One hopes B&C and P.P. have done this.

Some of the latest automotive alternators sense the alternator rpm, keep it off line until a certain rpm is reached (reduces the load on the starter), then soft starts the alternator automatically. There are even schemes where the ECU controls the alternator via a serial bus. Perhaps the Vertical Power team will look into that.

Examples of the latest regulators:
L9911.pdf
IRVR101.pdf

[Toobuilder]

Thanks for that Ted. I'm pretty familiar with the Delco alternators though my "car" hobby, but never had to think about switching one on/off (obviously). However, research indicates that the B&C CAN take the switching with no problem - as it is designed for just that. However, I realize that just because the alternator is capable of handling the switching does not mean that that is a good reason to do so on a routine basis, so I'll be starting with the field on from now on.

That said, I think the idea of using the master switch as a Pmag "test" switch can still be valid. If a given pilot's only concern is the switching of the alternator "at speed", then I think the fact that the Pmag testing is done so infrequently, coupled with the fact that very few of us are running purely automotive alternators (apparently, the only ones that can't handle the voltage spike), is still better than adding separate switches on the panel. Even the automotive versions seem to be fairly robust, as evidenced by the fact that I am still using the "mock up" 60 amp ND (same as the basis for the B&C unit) that I pulled from the junkyard. After the bracketry was built, I decided to see if this well used unit produced any power, and it has flawlessly for 300 hours now - and suffering the abuse of switching on after start every flight.