Alternator failure!

[Ringway Flyer]

Ten minutes or so after leaving EGCC yesterday, we had a 'low bus voltage' warning, rapidly followed by 'alternator inoperative' warning. So we called the tower and were given permission to return to the field. As always, the controller was most helpful. After a further few minutes a 'hot' smell started to waft into the cockpit.... But a safe arrival followed, after a few anxious minutes. There was no obvious sign of fire, so we took everything out of the aircraft, having first extracted the fire extinguisher. Then we removed the engine cover. There appeared to be nothing wrong - but the alternator was VERY hot.
Question. Has anyone else experienced this sort of problem? The aircraft is a PA28 Archer, just over a year old.
Whilst I knew that the battery would give us enough power for the radios and transponder for a while, the overheating is worrying....

[Fujiflyer]

I haven't had an alternator fail on an a/c however I have had one fail on a car. I believe they are of similar design. Your problem sounds like the insulation on part of the windings inside the alternator has failed giving rise to a partial short circuit and hence overheating. The other possibility is that one of the diodes in the rectifier (at the alt' output) has failed, again causing a short circuit.

Needless to say you will need to have an overhauled / new replacement - even if the original cause of the fault was repaired I would not trust it because of the excessive temperature it has been exposed to.

In the case of my example the failure mode was different. The regulator went "funny" (lost normal regulation) and caused the output of the alt' to go up to some 20V or so. The alternator then began to overheat producing that strange hot smell. The vehicle electrics seemed to stand it fortunately (for long enough to get home) but I hate to think of the damage that this could inflict of avionics if it ever happened in the aircraft. I don't think that they have much in the way of protection against long term over-voltage although I am not sure.

Glad you managed to make a safe return to the field. I believe that alternators can produce a fair amount of fumes etc when they fail but don't think they can actually catch fire. I am open to correction on this point though. Once the degree of breakdown had spread past a certain point the dissipation would reduce as the exciter current was lost / reduced (this is normally produced by the alternator, not from an external source). Initially though the regulator action effectively ensures that the overheating is rapid by increasing the excitation - it is trying to maintain the 14V or so output (12V system) in the face of the fault.


Fujiflyer

[Thrifty van Rental]

You don't say if you selected the Alternator off, and pulled the field circuit breaker.

Full marks if you did. If not, something to revise for next time! As Fujiflyer says, alternators can fail in several different ways, but in all cases, the failure benefits from being isolated completely. This minimises the risk of fire, or damage to other electrical equipment.

Alternator failures are relatively common in GA types, so this is worthwhile understanding.

Diode failures are not normally instantaneous. Usually, they follow a period during which you will have had a noticeable background whistle on the intercom. If you are suffering in that way, try switching the alternator off momentarily and seeing if the whistle goes away. If it does, expect an alternator failure over the next few flying hours and get it looked at.

Well done Ringway Flyer on a safe landing.

TvR

[bluskis]

As Thrifty says alternator failure is very common. I had double alternator failure on every take off for 10 years until I finally found a maintenance outfit who knew what they were doing.

Meanwhile a reset once the u/c was fully retracted usually solved the problem.

I wouldn't attempt to guess what the specific cause of your problem was as there are a multitude of possibilities, but it is not good it happened in such a new aircraft.

[A and C]

The first thing to do is turn off the alternator as if it is the regulator that has failed and it might we be that the alternator is putting unregulated current into the battery this may well start a fire if left long enough , but it will destroy the battery in a very short time .

Another reason for switching off is to conserve battery power as in the case of a belt falure the regulator will sence this as an under voltage and put as much power as it can into the alternator field to compensate , this will have no effect what so ever on the alternator output if it is not turning but it will drain the battery much quicker.

The failure that started this thread sounds like a bearing failure in the alternator, this can sometimes be picked up before the alternator fails compleatly , the tell tail clue is intermitant and very short term low volts warnings , this is the result of the brushes not contacting the rotor as the rotor bearings are starting to fail.

It takes about 10 min to check the alternator for play in the bearings and if it is not that about another 10 min to change the brushes if they are worn out.

In the case of a major mechanical failure of the alternator it must be switched off to prevent the risk of an eletrical fire , it is quite likely that the belt will break in this case and the belt is likely to end up resting on a hot part of the engine and giving off a smell of burning rubber.
This smell is most worrying when you are in the air , but I have yet to see a belt catch fire.