Alt Breaker Pops when Field Turned Off

[Boilermonkey]

We are having and intermittent issue with the left alternator circuit breaker popping when we turn off the Alternator field switch at shutdown.   No odd indications on the JPI 930.  The system works fine on startup and can run off right, left, both alternators just fine.   The voltage regulator was OH be Maxwell last year when we did a factory reman on the engine.  I assume the alternators we OH or new at that point too.

Any ideas what could cause the CB to pop when energizing the field?  M20M Bravo 1993.

-Jeremy

[Fly Boomer]

2 hours ago, Boilermonkey said:

Any ideas what could cause the CB to pop when energizing the field?

Bad breaker?

[N201MKTurbo]

So, no spike on the ammeter or voltmeter when you shut down the field?

Do you have dual alternators? If so does it do that when you turn off the other one first?

[ZuluZulu]

There’s a diode missing that causes this — ask Don Maxwell, he is familiar with this issue. 

[laytonl]

4 hours ago, ZuluZulu said:

There’s a diode missing that causes this — ask Don Maxwell, he is familiar with this issue. 

Mine occasionally does this too.  Where is the diode located?  Lee

[ZuluZulu]

1 hour ago, laytonl said:

Mine occasionally does this too.  Where is the diode located?  Lee

I lack the knowledge to explain it, that’s why I said to call Don. He told me it’s missing a diode on the field terminal that causes this and it is a very common issue. He sent me the diode and a diagram, but I didn’t understand the diagram and it never got installed. Now I’ve misplaced the diode!

[rgpilot]

A rectifier between positive terminal and ground at the alternator. This will ground the spike that occurs when you turn off the Alt field circuit.

 

[tls pilot]

it is the diode

[LANCECASPER]

On 10/13/2023 at 6:59 AM, Boilermonkey said:

We are having and intermittent issue with the left alternator circuit breaker popping when we turn off the Alternator field switch at shutdown.   No odd indications on the JPI 930.  The system works fine on startup and can run off right, left, both alternators just fine.   The voltage regulator was OH be Maxwell last year when we did a factory reman on the engine.  I assume the alternators we OH or new at that point too.

Any ideas what could cause the CB to pop when energizing the field?  M20M Bravo 1993.

-Jeremy

If you need to send out one or both of your alternators this is the place: http://aeroacc-vny.com/

They did the ALTS on the last Bravo I owned and they did a great job. They have also done mags for me with equal precision and a good turn-around.

[N201MKTurbo]

If the flyback diode is to be effective, it needs to be between the + field wire and ground, not the output and ground. Most of our regulators are high side regulators, so one of the terminals is already grounded. You can put the diode between the two field terminals. The diode must be reverse biased which means the cathode (side with the line) needs to go to the + terminal. There are no markings on the alternator which field terminal is +- because it doesn’t matter to the alternator. You just have to look at how it is wired and hook the cathode to the terminal that is not grounded.

[PT20J]

Here’s the schematic for my 1994 M20J. It must be an error because it shows the diode on the grounded field terminal. I think it should go on F1. Parts manual shows it to be a 1N2483 (alternates 1N5060 or 3720GE).

[Boilermonkey]

Thanks for the advise.  We'll take a look at the wiring.  Wilmar did the engine install and I remember talking about the diode when they were having issues with the voltage regulator.  Maxwell fixed the regulator.  Perhaps the diode is faulty or reversed in the installation.  

[glenn reynolds]

I had exactly this problem. Advanced aircraft in Troutdale or, a mooney service center explained that I was missing a diode on my alternator. The diode goes between the field terminal and the ground terminal on the alternator. It took about ten minutes to install and came with a diagram showing which end of the diode goes on which terminal.  My Alternator has four terminals on the end.  One is the output terminal and this has a large diameter wire and this is the output from the alternator.  The second terminal is ground and this has a braided ground strap which runs to my engine case.  The third terminal has a small diameter wire which is the field circuit.  my field circuit has a couple amp circuit breaker which is what popped on shutdown.  The diode gets installed between the field terminal and the ground terminal (which is of course the same as installing it directly to ground only much closer).  The diode that Advanced Aircraft Services (503-465-2298) mailed me had terminals crimped on both ends and had been heat shrink wrapped so it was a really well done piece.  Luke told me that he ordered a couple so he would have them in stock.  Thus said, I don't actually know the diode rating as it was sleeved in heat shrink.  They sent the diode in an envelope so you can get it super quick.  I have the diagram at the hanger (in log book) so I can add a picture of that here after my next therapy season with the airplane.