Alternator field breaker tripping

[Dterhaar]

I have a 61' PA-18-150 that was totally restored. I bought it a couple months ago with only 3 hours on it since restoration. The issue is that about 75% of the time, when cranking the engine, it trips the Alt field circuit breaker. The breaker resets fine and charges for the rest of the flight. It only has ever tripped while the starter is engaged. For a quick work around I have been pulling the breaker before I start it but know that someone must have something wired wrong and I need to get it figured out. I now have about 25 hours on it with the same issue.

Any ideas on where to start looking would help.

Thanks

 

[behindpropellers]

First thing I would check is to find the regulator MFG/PN and get the manual and make sure they have the correct size circuit breaker installed.

 

[wireweinie]

What charging system is installed?

Check the field breaker and see if ANYTHING besides the regulator is connected.

Web

 

[Dterhaar]

Its the B&C LR3 B-14 regulator with the B&C L-40 40 amp alternator. It has the recommended 5 amp breaker installed on the field. Tomorrow I will check if there is anything else connected to the breaker.

 

[wireweinie]

Do you have the diagram for the reg?

Check that the reg is grounded, either through the mounting screws or with a ground wire to the case stud AND terminal 7. Then check that the field breaker is connected to terminal 6. At the reg, check that terminal 3 (sense) is connected to the bus through it's own breaker or fuse (no jumpers to other terminals). If you have the optional battery temp sensor, disconnect it for a couple of flights.

Web

 

[PerryB]

Web beat me to it, but inadequate grounding was what came to mind for me.

 

[skywagon8a]

The issue is that about 75% of the time, when cranking the engine, it trips the Alt field circuit breaker. The breaker resets fine and charges for the rest of the flight. It only has ever tripped while the starter is engaged. ......... someone must have something wired wrong

??????????????Turn key to start position. Alt field circuit breaker trips????????? Only 75% of the time??????????????????? Why only 75% when it should be never? Why not 100%?
Is the engine grounded to the airframe? Is the engine only grounded to the engine mount because there is paint in the mount to fuselage bolt holes? Is the current to the starter returning to the battery through the alternator?
Try using a long jumper cable connected directly to the starter or engine case with the other end connected to the battery ground terminal. Bypass everything for the return flow. Does the breaker still trip?

If this works, try removing a bolt which holds the engine mount to the fuselage. Clean all of the paint out of both bushings. Now what?

 

[mike mcs repair]

I vote ground wire needed between Engine and firewall screw stud also...


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[wireweinie]

Well, if we're taking bets, lol.

My money says wiring problems with the regulator. Been an ongoing issue since rebuild.

Who's runnin' the board and holdin' the cash?

Web

 

[sjohnson]

Does the B&C setup have an overvoltage clamp? If so, an OV spike could be randomly tripping it. As above, check grounds and connections, and also check that there is a snubber diode installed across the solenoid start terminal and ground, and that it works.

 

[kase]

I had same issue several years ago. It was a poor crimp on field wire.

 

[mike mcs repair]

Does the B&C setup have an overvoltage clamp? If so, an OV spike could be randomly tripping it. As above, check grounds and connections, and also check that there is a snubber diode installed across the solenoid start terminal and ground, and that it works.

Yes, it has crowbar overvolt, but that pops a different breaker the overvolt 2 amp


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[cubflier]

Yes, it has crowbar overvolt, but that pops a different breaker the overvolt 2 amp


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Mike - Not meaning to be disagreeable here but when the crowbar circuit in the LR3B or C is activated it creates a dead short between terminal 6 and ground which pops the 5 amp breaker.

Jerry

 

[wireweinie]

As per the B&C manual, when the crowbar shorts, the 5 amp breaker pops. This shuts down the reg and allows the low voltage light to remain on as it draws power from the 2 amp breaker and grounds through regulator terminal 5.

Web

 

[sjohnson]

Yes, it has crowbar overvolt, but that pops a different breaker the overvolt 2 amp

I found this arrangement sort of surprising, so I looked up the instructions for the LR3:

"OVER VOLTAGE WARNING - the 5 AMP alternator field breaker will open causing alternator to be shut down. "

I believe the 2 amp CB is for the undervoltage lamp and the test circuit.

 

[cubflier]

The OV test circuit is actually on pin 2 and independent of the UV test lamp circuit/2 amp breaker. OV test is just a direct route to the positive terminal on the voltage comparator that signals crowbar to latch.

Jerry

 

[Dterhaar]

I did some testing tonight and found a weak crimp (pulled it very lightly and the wire pulled out) on the 2 amp breaker.
Tested pin #3 voltage at 0 v (due to weak crimp)
Tested 0.4 Ohms from directly on (-) battery to pin #7
Tested 0.4 Ohms from directly on (-) battery to engine case
Tested bus voltage at 12.1 v
Tested pin #6 voltage at 12.1 v.
Tested 0.2 Ohms from pin #7 to firewall.

Engine case has large grounding strap to lord mount bolt (motor mount) but no ground directly from case to firewall, but the Ohm test seems it is grounded??
There are no other loads on the 5 amp field breaker, the wire goes directly to pin #6.
There are no wires on Pins #1 and #2.
There are no other wires on pin#3, goes directly to 2 amp breaker.

A bad crimp on the 2 amp breaker doesn't seem like it would trip the 5 amp field breaker occasionally while cranking the engine??
Now that the crimp is fixed I will do more testing tomorrow.

What is a snubber diode and how do you test it?

 

[BC12D-4-85]

Does that mount bolt-ground strap assembly consistently contact the engine casing? Where and how? If you remove that low impedance strap you'd still see a battery/frame to engine ground through other conductive paths. The other paths may not be sufficient to carry the levels of starter current.

Gary

 

[wireweinie]

I did some testing tonight and found a weak crimp (pulled it very lightly and the wire pulled out) on the 2 amp breaker.
Tested pin #3 voltage at 0 v (due to weak crimp)
Tested 0.4 Ohms from directly on (-) battery to pin #7
Tested 0.4 Ohms from directly on (-) battery to engine case
Tested bus voltage at 12.1 v
Tested pin #6 voltage at 12.1 v.
Tested 0.2 Ohms from pin #7 to firewall.

Engine case has large grounding strap to lord mount bolt (motor mount) but no ground directly from case to firewall, but the Ohm test seems it is grounded??
There are no other loads on the 5 amp field breaker, the wire goes directly to pin #6.
There are no wires on Pins #1 and #2.
There are no other wires on pin#3, goes directly to 2 amp breaker.

A bad crimp on the 2 amp breaker doesn't seem like it would trip the 5 amp field breaker occasionally while cranking the engine??
Now that the crimp is fixed I will do more testing tomorrow.

What is a snubber diode and how do you test it?

View attachment 48145

The bad crimp on terminal 3 could cause issues as the regulator senses bus voltage from that terminal. That was a good find.

I would definitely get a ground strap from the engine case to the airframe. From what I can see in the pic, the ground path passes through several spots that may introduce high resistance. Is the existing ground braid on clean metal or on paint? Where it's attached at the lord mount bolt, is that painted? The motor mount itself is attached to the airframe through bolted connections. If you go with the ground strap from case to airframe, you'll avoid all of these problem areas.

The snubber diode is usually called a spike diode. It's installed on any item that uses a coil to operate, such as relays. In a nutshell, whenever a coil is DE ENERGIZED, the magnetic field it was producing, collapses and induces a spike that's always reverse polarity of the original voltage. And since it's reverse polarity it can go back through other electronics and cause damage. The spike diode is intentionally installed in the reverse polarity position. This way, when the spike is generated, the diode forms a short circuit to it and prevents it from traveling beyond the unit it's installed on. Jst check to see if there is a diode installed on the control terminals on your master and start relays. If they were installed correctly they should look 'normal'. If they were installed incorrectly, when power is applied, there is a pop and the diode breaks in half.

Web

 

[BC12D-4-85]

Always test meter probe to probe values before using them to test components. Subtract them for true values.

GAry

 

[skywagon8a]

I did some testing tonight ......

Engine case has large grounding strap to lord mount bolt (motor mount) but no ground directly from case to firewall, but the Ohm test seems it is grounded??

IF I am understanding you correctly, That is your trouble. The lord mount bolt does not touch nor connect to anything metallic on the engine mount, it is isolated by the rubber of the lord mount. You are grounding the engine case to the engine case. An Ohm test would indicate grounded since the path can travel though any metallic connection from the engine to the fuselage. It could be through a shield on any number of wires in your installation or???????

 

[cubflier]

While you are fixing your ground issue and the bad crimps on your voltage regulator could you get the pin 4 voltage at the regulator and the field wire voltage at the alternator and post it. After properly connecting pin 3 on the VR take a small jumper wire and momentarily jump pin 6 to pin 2. Your 5A breaker should instantly pop.

From there you may also look under your seat at the master and starter solenoids for proper and secure wiring. Check the voltages across the contacts and coil leads. The interesting part about your problem is the symptom of the tripping of the OV circuit in the VR while starting. That's why looking under the seat may uncover more discombobulation.

Good luck - Jerry

 

[Dterhaar]

So I added another ground cable from the engine case to the firewall this morning. The engine case to (-) battery terminal went from 0.4 Ohms to 0.1 Ohms.

Tested it and it still trips the field breaker when starting. This time I counted and the breaker tripped 3 out of 10 starts but another clue I found was that the breaker only trips when releasing the key after starting.
I got B&C on the phone today and they believe that the voltage spike after starting is causing the regulator to think there is an over current issue so it trips the field breaker to try and correct it. He suggested possibly a new key switch and also offered to adjust the regulator with more of a over current time delay free of charge, great customer service.

I also checked and I do not have a diode on the starter solenoid. I assume this may also help. Where can I find one and what part number do I need?

 

[wireweinie]

Use diode 1N4004. They are generic and easy/cheap to find. Crimp a #10 red ring terminal on each end.

Assuming that you have relays with two small terminal posts, figure out which post is 'positive' and which is 'negative'. On your master relay the positive side is the one with the short jumper connecting it to the hot battery lead and the negative is the wire leading back to the master switch. Install the diode onto these posts in REVERSE polarity. Negative lead on the diode goes to the positive post on the relay. Positive lead on diode goes to the negative post on the relay.

On the starter relay, find the terminal post that jumpers directly to ground (negative) and the terminal post that leads back to the starter switch (positive). Install a diode in REVERSE polarity, just like on the master relay.

Contact me if you need more info.

Web

 

[cubflier]

You might also try disconnecting the lead to the starter from the starter solenoid so you can engage the stater solenoid by itself and see if the problem still exists.

Jerry

 

[sjohnson]

...This time I counted and the breaker tripped 3 out of 10 starts but another clue I found was that the breaker only trips when releasing the key after starting.
...

Tripping when you release the switch is pretty good evidence that it is an OV transient caused by the solenoid. I wish I had thought of it earlier.

Web has the answer on the diode.

I wouldn't mess with the regulator until you install the diode. Debug one thing at a time.

 

[supercub1999]

Why si the alternator on for the start? I have my field switch off until after I start, than watch the volts increase when I flip the switch on.

 

[sjohnson]

Why si the alternator on for the start? I have my field switch off until after I start, than watch the volts increase when I flip the switch on.

A perfectly sound procedure. It doesn't eliminate the OV transient, but it does reduce the load on the battery during start.

For me, I prefer eliminating yet another thing to remember.

Whether you turn off the field or not, installing the diode eliminates the OV transient (generated by the solenoid), and is kinder to any electronics that you may have left on during start, and the OV clamp circuit.

Update: I should add that my plane doesn't have a separate field switch, only a circuit breaker. Others I've recently flown are similar. I assume that pulling the CB each time would be hard on it, but I don't really know.

 

[BC12D-4-85]

Somewhere way back I was told it was easier on alternator brushes to float on whatever is caused by current than rub without. Millions of vehicles start and run with the alternator connected so it seems to work ok. I'd be more concerned about any sudden voltage/current spike after starting. Ideas?

Gary

 

[wireweinie]

The only guys I've known that always leave the generator/alternator off when starting, learned to fly big round engines. I always assumed it had something to do with their start procedures. Never seen an issue with the typical charging system on a small aircraft being 'on' when starting. If you feel better leaving it off for startup, go for it. Won't hurt anything.

Web