vlittle
Well Known Member
I had a new PP 60A internally regulator alternator fail at first engine run. I worked closely with Dick at Plane Power to get the problem repaired and as it stands now, I have a 100% fully functional alternator.
The rest of the story....
At first engine run, my alternator field breaker tripped. I reset it, but it tripped again. After a few minutes of running the engine and checking aircraft systems, I shut down and dumped the SkyView data log to a memory stick.
At home, I analyzed the data and found that the PP alternator was putting out 60 amps for a few seconds after start-up, presumably battery charging current plus normal loads. The main battery was run down by ground testing to about 11.5 volts prior to first start, so this was expected. As the voltage rose to about 14V, the alternator field breaker tripped offline.
After a few static diagnostic tests, I called Dick at Plane Power who walked me through some voltage measurements and concluded that everything measured nominally with the master on and engine off. He was concerned that the main battery could be malfunctioning and that was contributing to the field breaker tripping.
I sent the data log to Dick to show him that the alternator was tripping off-line at about 14 volts and we agreed that I should charge the battery fully and try again. In the meantime, I had to swap out the prop governor, so it would be a week or so before I could try the new engine run.
In the meantime, I pulled the alternator and rigged up a bench test: I clamped the alternator in the soft jaws of my vise, connected a battery and a field circuit with a breaker. Using an metric 19 socket and a 1/4" bolt as a mandrel holding the socket, I spun up the alternator using a variable speed 1/2" electric drill.
I monitored the alternator output with a digital voltmeter and an oscilloscope to look for missing phases that would indicate a diode problem. As I spun the alternator, the voltage came up and all of the phases were apparent on the scope. As I approached 13.8 volts, the field breaker tripped. I tried this several times to get a better read on the trip voltage, then I heard a 'tick' from the alternator and the smell of smoke!
Then... everything worked! The alternator output rose to 14.2 volts and the battery was being charged nicely, with no field breaker tripping.
Subsequent testing on the airplane showed that the alternator was now operating properly and charging the battery at 14.5 volts. (Note: on the aircraft, there is about a 0.25 volt drop in the field circuit --wire, switch and breaker -- hence the differences in measured voltages from the test circuit and the airplane... i.e. just add 0.25 volts to the test circuit measured voltages to get the airplane voltages.)
Was I happy?... yes and no. The primary electrical system was working properly, but the events of the bench test (letting the smoke out) worried me. I called Dick and he sent me a new regulator/OVP/brush assembly and I volunteered to send him the old one back for failure analysis.
I received the new assembly and before installation, I tested it in my lab to measure a 17.3 volt over voltage trip point. I then installed it in my aircraft and ran the engine up, with normal operation.
I then tested the removed regulator/OVP/brush assembly in my lab. I could not detect an over voltage trip anywhere up to 20 volts so I concluded that I had probably burned out the OVP protection circuit during my previous bench tests.
Lessons Learned:
1- It's not always your fault.
2- Just because it's new doesn't mean it works.
3- Just because it's expensive doesn't mean it's better.
4- Just because you get it working doesn't mean it's working properly.
5- Data logs are invaluable for system debugging.
6- Systematic measurement beats wild guesses.
I pass this information along as a 'Lessons Learned" tutorial in the hope that others may benefit.
The rest of the story....
At first engine run, my alternator field breaker tripped. I reset it, but it tripped again. After a few minutes of running the engine and checking aircraft systems, I shut down and dumped the SkyView data log to a memory stick.
At home, I analyzed the data and found that the PP alternator was putting out 60 amps for a few seconds after start-up, presumably battery charging current plus normal loads. The main battery was run down by ground testing to about 11.5 volts prior to first start, so this was expected. As the voltage rose to about 14V, the alternator field breaker tripped offline.
After a few static diagnostic tests, I called Dick at Plane Power who walked me through some voltage measurements and concluded that everything measured nominally with the master on and engine off. He was concerned that the main battery could be malfunctioning and that was contributing to the field breaker tripping.
I sent the data log to Dick to show him that the alternator was tripping off-line at about 14 volts and we agreed that I should charge the battery fully and try again. In the meantime, I had to swap out the prop governor, so it would be a week or so before I could try the new engine run.
In the meantime, I pulled the alternator and rigged up a bench test: I clamped the alternator in the soft jaws of my vise, connected a battery and a field circuit with a breaker. Using an metric 19 socket and a 1/4" bolt as a mandrel holding the socket, I spun up the alternator using a variable speed 1/2" electric drill.
I monitored the alternator output with a digital voltmeter and an oscilloscope to look for missing phases that would indicate a diode problem. As I spun the alternator, the voltage came up and all of the phases were apparent on the scope. As I approached 13.8 volts, the field breaker tripped. I tried this several times to get a better read on the trip voltage, then I heard a 'tick' from the alternator and the smell of smoke!
Then... everything worked! The alternator output rose to 14.2 volts and the battery was being charged nicely, with no field breaker tripping.
Subsequent testing on the airplane showed that the alternator was now operating properly and charging the battery at 14.5 volts. (Note: on the aircraft, there is about a 0.25 volt drop in the field circuit --wire, switch and breaker -- hence the differences in measured voltages from the test circuit and the airplane... i.e. just add 0.25 volts to the test circuit measured voltages to get the airplane voltages.)
Was I happy?... yes and no. The primary electrical system was working properly, but the events of the bench test (letting the smoke out) worried me. I called Dick and he sent me a new regulator/OVP/brush assembly and I volunteered to send him the old one back for failure analysis.
I received the new assembly and before installation, I tested it in my lab to measure a 17.3 volt over voltage trip point. I then installed it in my aircraft and ran the engine up, with normal operation.
I then tested the removed regulator/OVP/brush assembly in my lab. I could not detect an over voltage trip anywhere up to 20 volts so I concluded that I had probably burned out the OVP protection circuit during my previous bench tests.
Lessons Learned:
1- It's not always your fault.
2- Just because it's new doesn't mean it works.
3- Just because it's expensive doesn't mean it's better.
4- Just because you get it working doesn't mean it's working properly.
5- Data logs are invaluable for system debugging.
6- Systematic measurement beats wild guesses.
I pass this information along as a 'Lessons Learned" tutorial in the hope that others may benefit.
