Diagnosing an alternator problem

[chris mitchell]

I seem to have an intermittent fault with my PlanePower alternator, and am not too sure how to approach the problem of diagnosis - tho treatment might simply be to replace it - but it only has about 20 hours on it.

Everything was fine for the first 14 hours. Then last weekend just prior to take off, power checks complete, turned on the pitot heat and the 5A field circuit breaker popped. THe CB would not reset, so we taxied back to park and closed down. Thought we would see what happened on a restart and everything was fine. Flew, landed, closed down.

Next flight everything was fine until the power check when the CB popped with one of the mags being turned back on. THis time I left the engine running, turned the master switch off, reset the CB, then master switch back on - battery then bat plus field - all was fine, repeated the power checks and no problems.

Since then I have also had the CB pop when lowering the flaps, tho not consistently. Two nights ago the CB popped in flight when there had been no alterations to the load ie we had not operated any switches. Usual reset routine and all was well for the remaining 30 minutes of the flight.

I have inspected the wiring and there are no obvious problems with chafing or loose connections. The engine earth strap seems fine. Electrical system monitoring is via a Dynon flightdek 180. Charging amps and volts seems fine, we haven't clearly seen any spikes in either except after starting when the amps are high as I guess would be expected. The charging circuit is protected by a current limiter and there have been no problems here.

So - is the inbuilt regulator dying or is it a dodgy circuit breaker - those seem to be the choices but I'm anxious to know if I might be missing something else? If I replace the CB and all seems well for an hour or two can I really relax?

Thanks

Chris

[MrNomad]

You said: Electrical system monitoring is via a Dynon flightdek 180. Charging amps and volts seems fine, we haven't clearly seen any spikes in either except after starting when the amps are high as I guess would be expected.

In other words, you dumped the D180 log, and the log is set for 1 second increments. Even so, spikes can be very short and not recorded.


Your message said: turned on the pitot heat and the 5A field circuit breaker popped
It also said: the CB popped with one of the mags being turned back on
Your message also said: the CB pop when lowering the flaps

Is it safe to assume that you've added up the total possible load that's going thru the 5A breaker and your safety margin is significant?

Is it also safe to assume that the wire going to/from the CB is adequate for the load? You said you inspected the wire but have you insured that nothing is chafing it such as a throttle cable, causing an intermittent short?

How are the connections to the CB? Any cold solder joints? That?s an unlikely cause but you might as well make sure everything is perfect.

Alternators have been known to spike but the demand on the other side of the breaker should be the delimiting factor. If you've checked all of the above, if you've confirmed that you have not exceeded the load placed on the CB, the simple answer is to swap the CB. Your CB would not be the first unit to fail.

Assuming you have a fully charged battery, you could rely upon batt power & an external batt charger to see if it happens while you turn things on and off on the ground. Make sure the heated pitot cycles on during the test.

[Daver]

I suspect your circuit breakers.

Are they sized too close to the load?

I size my overcurrent protection at least 125% of the full load ampere rating of the load.

You can experience "nuisance" tripping if your load amperes is too close to the trip rating of the circuit breaker.

In the case of motors (like flaps), the motor can draw more than its rated
FLA if its mechanically overloaded. Also, when the motor is started, it draws "in rush" current that can exceed the trip rating of the CB but this should not cause trip because the starting current does not persist long enough.

Just a few things to consider.....

Dave
-9A finish kit

[allbee]

I have the same alternator. On my first flight the 5amp blew. I was greated with the Dynon warnings and bat volts under 9. No radio, orbiting over field. I did a quick stall check and landed.

I then went into the Bobs N. book and he states 20amps for the alt. field. I changed my field to 20 amps and went on with life. sounds like that's what you need to do.

[Mike S]

You are having the problem when adding load to the system.

The breaker in question is the alternator field breaker, which sees additional demand from the alternator when any load in the system is turned on.

I agree it sounds like the the field breaker is probably bad, but you need to also check all connections in the charging circuit, and the load circuits that caused the trip.

Not sure why turning on a mag would cause it to trip, maybe just a coincidence??

Have you contacted the alternator manufacturer??

Good luck.

[allbee]

I would just put a bigger breaker in. I'm not the only one that had trouble with the 5amp breaker on this alternator.

[MrNomad]

The Bigger Breaker Solution is the most obvious, but pls consider two caveats:

Pls check the wiring going to/from the CB. Make sure its the right size. Too small a wire could overheat and could draw too many amps.

Pls don't let anyone talk you into "used" breakers. In all probability, they're worn and near dead. It's simply not worth it. Ask me how much time we wasted on a friend's C150 due to used breakers.

Barry

[frankh]

Originally Posted by MrNomad

Pls check the wiring going to/from the CB. Make sure its the right size. Too small a wire could overheat and could draw too many amps.

Barry

An overheating wire would increase resistance and REDUCE current flow..I.e less amps.

Frank

[airguy]

Originally Posted by frankh

An overheating wire would increase resistance and REDUCE current flow..I.e less amps.

Frank

Unless it's on the final output to the battery - then output current will be restricted, the battery voltage will fall, the regulator (if it's external, internal won't care) will crank up the field coil current, and viola! you blow the field CB.

[chris mitchell]

Thanks for the advice and helpful comments. Just to clarify a few points:

The alternator is internally regulated, supplied by Plane-Power.
The installation is exactly as per Plane Power's recommendations, using the supplied harness for the field circuit and low voltage warning light, with the wiring completed using 18g wire. The 5 amp circuit breaker is also as per Plane-Power. Connections are all crimped using terminals and tools from B and C. The CB was new also from B and C.

As I understand the electrics of the set up, the sole function of the field circuit is to excite the alternator. If the field circuit breaker pops, the alternator output will fall to zero, so that is how alternator can be disabled if it produces too many volts. The internal regulator works by sensing the alternator output and will disable it by increasing the amps flowing through the CB if it senses an over-voltage condition. The B-lead is protected by a current limiter at 60 amps. Have I understood the regulator and CB functions correctly?

If so, I therefore don't understand the need to have calculated any loads on the field wiring or CB, nor to increase the CB rating or wire size. But maybe I'm not understanding the way the regulator works? I couldn't see anything in my AeroElectric connection about uprating the field CB, only about ensuring that the B lead had an adequate current limit of up to 80 amps.

I will go back this afternoon and recheck the wiring for chafe, intermittent shorts etc, and will recheck the earth lead and other connections. I have been very careful (my inspector is renowned for his pickiness) to ensure that all wiring is properly supported, loomed, protected from chafe etc, but no problem with searching again. I'll also run the engine and check the voltage (pickup point is at the current limiter location, as per Nuckolls and Dynon) - what sort of values should I expect? The battery (Odyssey PC680) usually gives around 12.5 volts when I turn it on after a few days in the hangar.

Thanks

Chris