Alternator breaker keeps popping

[MikeOH]

1 hour ago, N201MKTurbo said:

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.

Your assumption is that the OVP is part of the field circuit; I didn't think we knew what kind of regulator the OP has.

But, let's say the OVP fails to work for whatever reason...then the output voltage is going to rise until something really bad happens (blown avionics if they are on) but the ALT output breaker isn't going to blow in 5 seconds.  The fact it does trip quickly indicates a pretty solid short somewhere.  Once the breaker opens the field voltage (from the bus) will go away, as well.

Why would the crowbar fry? The whole point is it supplies a dead short so that the breaker opens and stops the current.  If the breaker did NOT open then there's gonna be smoke from somewhere  But, once again, the breaker IS tripping off.

We are getting pretty far afield from helping the OP (see what I did there)

[Ragsf15e]

Yikes, I’m confused, but one thing I’m sure of is I’m glad I’m not a EE!  You guys agree less than lawyers!

So on my F (PP Alt, zef vr), that breaker would only stop the ALT output.  The battery wire comes through the starter solenoid and then directly to the busses.  So in the case of the ALT breaker blowing, the vr is still getting bus voltage (supplied by the battery).  Either the ALT is putting out way too much current for some reason (and popping the breaker) or there’s a short after the breaker and the alternator is trying to put out way too much current, no?

[N201MKTurbo]

28 minutes ago, MikeOH said:

Your assumption is that the OVP is part of the field circuit; I didn't think we knew what kind of regulator the OP has.

But, let's say the OVP fails to work for whatever reason...then the output voltage is going to rise until something really bad happens (blown avionics if they are on) but the ALT output breaker isn't going to blow in 5 seconds.  The fact it does trip quickly indicates a pretty solid short somewhere.  Once the breaker opens the field voltage (from the bus) will go away, as well.

Why would the crowbar fry? The whole point is it supplies a dead short so that the breaker opens and stops the current.  If the breaker did NOT open then there's gonna be smoke from somewhere  But, once again, the breaker IS tripping off.

We are getting pretty far afield from helping the OP (see what I did there)

I'm talking about a short after the regulator. On a J alternator which has a high side regulator which supplies + voltage to one field terminal and the other field terminal has a jumper to ground. Imagine the insulation on the field wire got old and fell apart and the field wire contacted the output terminal. This is unlikely, but possible. If the OVP in the regulator turned on a crowbar transistor or relay, it wouldn't pop its breaker because there would be an almost unlimited amount of current being backfed from the alternator output terminal. It would likely fry the crowbar circuit.

As far as getting back to the original problem, I'm sure that any one of the three of us discussing this problem could figure this out in short order if we were at the plane with the cowl off.

[Ragsf15e]

And come to think of it, if mine had a short after the ALT breaker, that cb might pop, but the short would probably still be there (now shorting the battery).  Something tells me that would be bad, no?

[PT20J]

Fascinating discussion, but I'm not sure we are helping the OP troubleshoot his issue. I think @EricJ's troubleshooting suggestion on the previous page made a lot of sense. If this were mine and it was tripping the big ALT breaker, I'd look for a short at the breaker panel (it's tight back there and mine had little pieces of sheet insulation material that could easily slip out of position separating breaker contacts). Next I'd consider a faulty alternator. Lastly I might question the circuit breaker itself although those are pretty reliable. It's a thermal device and any oxidation on the contacts might increase internal heating and lower the trip point. I might exercise it several times to clean the contacts. Actually, AC43.13 recommends exercising the breakers periodically and I usually do it at annual inspection.

An alternator with a functioning voltage regulator is best modelled as constant voltage source. The voltage is a function of rpm and field current. The regulator adjusts the field current to maintain a constant voltage as the rpm and load vary. Let's keep the rpm constant and add load. Eventually we will get to the rated output -- say for example 60 amps. What happens if we add more load? At some point the field current will be as high as it can go and after that point the voltage will decrease and the current will decrease since with constant load the current is proportional to the voltage. How much more than 60 amps will it put out. It it is well designed, not much. Why? Without getting technical, it would cost more (heavier stator and rotor windings and bigger diodes). So, it's not going to be way over designed.

Suppose we disconnect the voltage regulator from the field and connect the field terminal directly to the battery. What happens? Now the alternator voltage is solely a function of rpm and load. If the load is not the limiting factor, the voltage will increase and that will drive more current through the load. Maybe it will be enough to trip the ALT breaker, but more likely it will fry all the avionics due to the over voltage.

[EricJ]

1 hour ago, Ragsf15e said:

And come to think of it, if mine had a short after the ALT breaker, that cb might pop, but the short would probably still be there (now shorting the battery).  Something tells me that would be bad, no?

Yes, and that's partly why turning the master off is always a potential thing to do if there's smoke in the cockpit, especially if it smells electrical.   Then turn everything off and turn things back on one at a time.   If you can't even turn the master back on without issues, then that's probably the sort of fault you just found.   

[The Other Red Baron]

On 9/6/2024 at 6:35 PM, N201MKTurbo said:

 

As far as getting back to the original problem, I'm sure that any one of the three of us discussing this problem could figure this out in short order if we were at the plane with the cowl off.

That is what is driving me nuts is this shop doesn't seem capable of basic troubleshooting.  If someone told me their computer won't turn on and they just vacuumed around the power cables I wouldn't start by replacing their CPU I'd go check the power cables/power strip/ etc...

[Utah20Gflyer]

On 9/9/2024 at 9:41 AM, The Other Red Baron said:

That is what is driving me nuts is this shop doesn't seem capable of basic troubleshooting.  If someone told me their computer won't turn on and they just vacuumed around the power cables I wouldn't start by replacing their CPU I'd go check the power cables/power strip/ etc...

I’ve found it very helpful to do as much troubleshooting and parts research as possible before engaging with the mechanic.   Generally I tell them what the problem is and what the correct part numbers are if something needs to be swapped out.  
 

For example, I found a broken bolt with a nut attached at the bottom of my cowl during last weeks oil change.  So far I’ve figured out where it came from, ordered the parts and figured out a game plan for making the repair.  Next step is to call the mechanic.  Maybe he knows something I don’t and the plan will change but that’s fine with me.   I’m always looking for the best solution.  
 

I think electrical problems is where most mechanics are the weakest.  The more you can get up to speed with electrical troubleshooting the better off you will be.  
 

When the solution to a problem isn’t just replacing an alternator it can get expensive and frustrating to replace part after part until the actual defect gets fixed by pure luck.  If you understand how the system works and systematically test each part of the system eventually you’ll find the issue and correct it with the minimum cost.  
 

You need to isolate and test each part of the system.  
Ultimately it comes down to

a defective alternator 

a short in the system

a defective breaker

[The Other Red Baron]

18 hours ago, Utah20Gflyer said:

I’ve found it very helpful to do as much troubleshooting and parts research as possible before engaging with the mechanic.   Generally I tell them what the problem is and what the correct part numbers are if something needs to be swapped out.  
 

I'd love to be able to work on our plane but I have a huge knowledge gap which isn't getting any better because we don't have any A&P buddies to sign off on any work that we would perform.  For the longest time I was under the impression we couldn't do anything other than oil change, tire change, etc. but it seems quite a few people do work on their own aircraft then have someone else sign it off.