Alternator breaker keeps popping

[The Other Red Baron]

Stuff like this is making me want to go experimental...

Over a month ago I had to make an unexpected landing as I was getting low voltage warnings and night was coming soon.  Earlier on that same day I wasn't getting a mag drop on my left mag.  This has happened quite often previously and we thought tightening up the screws behind the ignition switch was fixing it.

I asked the local mechanic if he could have a look and possibly tighten up those screws.  He mentioned all the screws were tight then had an idea.  Popped open the oil door and reached way back behind the mags and kinda shook the wires back there a little bit.  He gave me the reasoning behind it but I couldn't remember.  Something about the frayed wire shielding.  Regardless I was now getting a good mag drop.

I probably should have noticed this earlier but into the flight I noticed I was no longer charging and was getting low voltage warnings.  I landed at the nearest large-ish airport I saw in hopes of getting decent access to a maintenance shop (I'm still kicking myself in the behind for this.)  I explained what happened to the shop and they said they'd check it out when they had time.  At the time I was trying to avoid thinking about the logistics of getting all my crap back home commercially or via rental car.  When they were troubleshooting it I finally noticed the alternator breaker was popped.  I pushed it back in and we all thought I'd be on my way.  Well the breaker kept popping and they didn't have time to investigate further so I got a rental and drove home. 

They assumed it was the alternator, got a new one ordered, put it on, and now it turns out our original alternator is in fact probably fine and they still have no clue what the issue may be.
I feel like they're ignoring the obvious where the previous mechanic that touched it fumbled with some wires and potentially fixed one issue and caused another.  I'm about at my wits end with this.  They have had the aircraft over a month and made zero progress from what it sounds like. 

Would any of you guys have a clue what may have happened so I can guide them in the right direction? I don't know how much they'll listen to me since they seem to have ignored everything I told them at the very beginning.

[BlueSky247]

Sure sounds to me like you've got a short in the wiring. You need to find an A&P that cares enough to do quality troubleshooting on the issue. It's not your job to educate those guys on something like that. Sounds like you need to find a different shop.

[The Other Red Baron]

4 minutes ago, BlueSky247 said:

Sure sounds to me like you've got a short in the wiring. You need to find an A&P that cares enough to do quality troubleshooting on the issue. It's not your job to educate those guys on something like that. Sounds like you need to find a different shop.

I'd love to get someone else to look at it unfortunately I think they're the only shop on the field and I read a review where someone got their on AOG service to the field and the FBO was charging them a $300 fee for not using their shop.  At this point we may need to just bite the bullet and find a mobile service. 

[N201MKTurbo]

Where is it?

[The Other Red Baron]

On 9/4/2024 at 10:43 AM, N201MKTurbo said:

Where is it?

TN

Edited September 11 by The Other Red Baron
Took out airport code, message me if you'd like to know.

[N201MKTurbo]

It sounds like there is an issue with the big cannon plug on the firewall. It has the P leads and the field wire for the alternator. If there is an avionics shop on the field, you may ask them to take a look at it. They are usually better at wiring.

[The Other Red Baron]

Doesn't really look like there's another shop on the field unfortunately.  Left a message with the rotary guys but I'm expecting a "no thanks" from them. Contacting AOG services at this point. 

 

edit: Anyone know if Savvy would be worth it in this situation?

Edited September 4 by The Other Red Baron

[Utah20Gflyer]

If all the wiring looks to be in good order and the charging system is delivering the correct voltage before the breaker trips then my suspect would be the breaker itself.   Breakers often fail when they get old, or sometimes when they get abused.  

[PT20J]

Mags: In order to turn off a magneto, the p-lead must be grounded. The p-lead is a shielded wire to reduce radio interference. The shield can be grounded at the ignition switch end, at the magneto end, or at both ends. In some installations, the shield is grounded at the magneto only but connected to the ignition switch so that the switch can ground out the mag using the shield. If the shield is loose at the magneto end, then it may not be able to ground out the magneto. Wiggling the wire might temporarily improve the connection.

Alternator: You didn't say which breaker tripped. There should be two: A breaker on the alternator output usually labelled ALT, and a breaker for the field usually labelled ALT FIELD. If the ALT breaker trips, there is a short between the alternator and the breaker. There are a couple of things that can trip the ALT FIELD breaker. It could be a short somewhere in the field circuit. Also, some voltage regulators use a "crowbar" circuit to intentionally create a short to pop the breaker and shut down the alternator when the regular senses an overvoltage condition.

[Ragsf15e]

As @PT20J said, is it the big red 60amp alternator breaker or the ~5amp alt field breaker?

[The Other Red Baron]

13 hours ago, Ragsf15e said:

As @PT20J said, is it the big red 60amp alternator breaker or the ~5amp alt field breaker?

The big ALT (50?) amp breaker.

[EricJ]

4 hours ago, The Other Red Baron said:

The big ALT (50?) amp breaker.

If it's not the breaker itself whatever the fault is should be reasonably obvious in the (large) wires from the breaker to the load buses.   Tripping a properly functioning 50A breaker takes a significant fault that is likely to leave a mark somewhere.   There may be three or so large wires running from the output side of the breaker to the buses supplying other breakers.    If there is an electrical fault tripping that breaker, it'll be one of those wires or connections, or one of those bus bars touching ground somewhere.   It's the sort of fault that I wouldn't want to be flying around with any more than I had to.

[Ragsf15e]

5 hours ago, The Other Red Baron said:

The big ALT (50?) amp breaker.

Yikes.  Get up under the panel and look for burn marks on the buses?

[PT20J]

That breaker is there to protect the wiring between the alternator and the bus. Unless the alternator has been improperly replaced with a larger one, the alternator will not put out enough current to trip that breaker. But, if there is a short somewhere between the alternator and the bus, the battery can supply more than enough current to trip the breaker. 

[N201MKTurbo]

2 hours ago, PT20J said:

That breaker is there to protect the wiring between the alternator and the bus. Unless the alternator has been improperly replaced with a larger one, the alternator will not put out enough current to trip that breaker. But, if there is a short somewhere between the alternator and the bus, the battery can supply more than enough current to trip the breaker. 

I disagree with your assessment. If the field current went to full on and put full bus voltage to the field, it could output enough voltage, and current to pop the breaker. The alternator breaker should never pop with a normally regulated alternator.

[MikeOH]

21 minutes ago, N201MKTurbo said:

I disagree with your assessment. If the field current went to full on and put full bus voltage to the field, it could output enough voltage, and current to pop the breaker. The alternator breaker should never pop with a normally regulated alternator.

Maybe,.... but from the OP it sounds like the breaker trips quickly.  A circuit breaker does NOT immediately trip at its rated current; the greater the overcurrent the faster it trips.  As an example specification for a 60A breaker shows that at 120 Amps it may take as much as 30 seconds to trip!  I think it far more likely that the battery is the source of the current, NOT the alternator just as @PT20J suggested.

 

[PT20J]

1 hour ago, N201MKTurbo said:

I disagree with your assessment. If the field current went to full on and put full bus voltage to the field, it could output enough voltage, and current to pop the breaker. The alternator breaker should never pop with a normally regulated alternator.

If the field goes to bus voltage, the alternator voltage will increase, but the alternator is only capable of supplying so much power and the current will decrease.

[N201MKTurbo]

8 hours ago, PT20J said:

If the field goes to bus voltage, the alternator voltage will increase, but the alternator is only capable of supplying so much power and the current will decrease.

The alternator is capable of making way more power than it is rated for. The current supplied by the alternator is a function of its voltage and the resistance of its load. I=E/R. The load in most cases is the battery which has a very low resistance. 
 

Your statement is contradictory. You said the alternator can supply enough current to pop the breaker into a output short, but cannot supply enough current to pop the breaker in an overvoltage situation.

According to Ohms law, the only way an alternator can reduce its current, is to reduce its voltage.

[PT20J]

1 hour ago, N201MKTurbo said:

current to pop the breaker into a output short, but cannot supply enough current to pop the breaker in an overvoltage situation.

The current to pop the ALT breaker in the case of a short between the alternator and the bus (or more likely, within the alternator itself) comes from the battery, not the alternator. 

Even if the regulator fails and the alternator voltage increased (which would drive output current higher by Ohms law), the over voltage protection circuit would shut down the alternator by (depending on design) either removing the field excitation or crowbarring the field circuit (which would pop the ALT FIELD breaker.)

From the M20J POH

[The Other Red Baron]

12 hours ago, MikeOH said:

Maybe,.... but from the OP it sounds like the breaker trips quickly.  A circuit breaker does NOT immediately trip at its rated current; the greater the overcurrent the faster it trips.  As an example specification for a 60A breaker shows that at 120 Amps it may take as much as 30 seconds to trip!  I think it far more likely that the battery is the source of the current, NOT the alternator just as @PT20J suggested.

 

Correct, the alternator breaker trips within approximately 5 seconds of being pushed in.

[EricJ]

The easy way to sort out whether it's battery or alternator power that trips the breaker is to just turn the master on without the engine running.     If it trips the breaker, the current is coming from the battery and there is likely a short between the breaker and the alternator, or in the alternator (which has been changed and didn't fix the problem).    If the breaker doesn't trip until the alternator is running, then the issue is likely between the breaker and the other distribution buses or the circuit back to the battery.

Since the battery would supply current to a fault on either side of the breaker once the master is on, if that doesn't reveal something getting hot somewhere then the issue may be the breaker itself.

[N201MKTurbo]

2 hours ago, PT20J said:

The current to pop the ALT breaker in the case of a short between the alternator and the bus (or more likely, within the alternator itself) comes from the battery, not the alternator. 

Even if the regulator fails and the alternator voltage increased (which would drive output current higher by Ohms law), the over voltage protection circuit would shut down the alternator by (depending on design) either removing the field excitation or crowbarring the field circuit (which would pop the ALT FIELD breaker.)

From the M20J POH

We are talking about a situation where there is a fault somewhere that is putting bus voltage on the field. this takes any overvoltage protection out of the system.

I know what you are saying. that the ALT breaker is there in case the alternator output wire gets shorted to ground, it will keep the plane from going up in flames. 

I was just trying to figure out what a mechanic could do sticking his arm through the oil door and wiggling some wires. If the plane had a low side regulator, he could have shorted the field wire to ground putting putting bus voltage to the field. What else could he have done? Some of the older Mooneys had an alternator filter on the firewall. He could have pushed some shield wire onto one of its terminals. I think if there was a short to ground of the alternator output wire, the breaker wouldn't wait 5 seconds to pop. There would be hundreds of amps going through that breaker.

It would be interesting to see what the ammeter reads when the breaker is pushed in.

[MikeOH]

1 hour ago, N201MKTurbo said:

We are talking about a situation where there is a fault somewhere that is putting bus voltage on the field. this takes any overvoltage protection out of the system.

I know what you are saying. that the ALT breaker is there in case the alternator output wire gets shorted to ground, it will keep the plane from going up in flames. 

I was just trying to figure out what a mechanic could do sticking his arm through the oil door and wiggling some wires. If the plane had a low side regulator, he could have shorted the field wire to ground putting putting bus voltage to the field. What else could he have done? Some of the older Mooneys had an alternator filter on the firewall. He could have pushed some shield wire onto one of its terminals. I think if there was a short to ground of the alternator output wire, the breaker wouldn't wait 5 seconds to pop. There would be hundreds of amps going through that breaker.

It would be interesting to see what the ammeter reads when the breaker is pushed in.

My understanding is that putting bus voltage on the field will result in the alternator attempting to increase its output voltage, which would INVOKE the overvoltage protection in the system; either by opening the field circuit, or deliberately shorting the output (crowbar) to trip the ALT output breaker.

Your low side regulator short to ground would result in the overvoltage and the OVP circuit crow-barring the output.

Depending upon the short circuit current of the battery and the system resistance (limiting the current) it is possible that a short from the alternator output to ground could take 5 seconds to trip a 60 Amp breaker (at 240 amps the range is 1 to 5 seconds):

And, agree that an ammeter to see what's really going on would be useful!

[Falcon Man]

In regards to SAVVY Aviation - what you will pay them for joining and getting assistance will likely be your best option to figure out your situation.

I have used them for years and it's one of the best investments I have made in aviation.

Do you have an engine monitor? 

[N201MKTurbo]

2 hours ago, MikeOH said:

My understanding is that putting bus voltage on the field will result in the alternator attempting to increase its output voltage, which would INVOKE the overvoltage protection in the system; either by opening the field circuit, or deliberately shorting the output (crowbar) to trip the ALT output breaker.

Your low side regulator short to ground would result in the overvoltage and the OVP circuit crow-barring the output.

Depending upon the short circuit current of the battery and the system resistance (limiting the current) it is possible that a short from the alternator output to ground could take 5 seconds to trip a 60 Amp breaker (at 240 amps the range is 1 to 5 seconds):

And, agree that an ammeter to see what's really going on would be useful!

So, once again, if there was a fault that but bus voltage on the field, it would bypass any overvoltage protection. It couldn't stop the field current if it was coming from somewhere else. And if it tried to crowbar it, it would likely fry the crowbar circuit.

This is an M20E I don't think the early models had any overvoltage protection. Most of them came with generators and if they have an alternator, it was a retrofit. If it was a retrofit, all bets are off on how it is wired.

My old M20F had an aftermarket alternator. it was a Prestolite with a Prestolite regulator mounted on the engine side of the firewall. I seem to recall it had a low side regulator. Pulling the field breaker would normally open its NPN output transistor and stop any charge unless it had a shorted output transistor, in which case the alternator would be full on and there would be nothing you could do about it. That's why they don't do that anymore.