The maintenance facility I used to work for as an employee has one guy full time on starters, generators, alternators, magneto’s etc. That guy is also helping me if it is busy, and I will help him when he is busy. I still use that facility to perform most of my avionics installation work.
That facility is Vliegwerk Holland on EHMZ. Their component shop does maintenance of components on most aircraft which are maintained by maintenance facilities in the Netherlands.
(note: discussed this post with Peter)
They can do the 500h “overhaul”. That is not a specialist job.
Cessna alternators are 1960s Ford alternators. For the 28 V aircraft they just changed the winding. Prehistoric stuff.
achimha wrote:
They can do the 500h “overhaul”. That is not a specialist job.
Their is a difference between a 500 hours inspection and overhaul. Basically all alternators follow the same principle, this is why it stupid to no perform maintenance on them. Sure one brush set last longer than other, they do wear out, especially on heavily electrically equipped aircraft. Therefore it is a good idea to perform maintenance whatever the manufacturer says. Replacing a worn rotor due to brushes that have been worn completly have ruined the slip rings is more expensive than doing a 500 hours inspection.
Replacing a worn housing due to damaged bearings is also more expensive then having the bearings cleaned and relubricated at a 500 hours inspection.
Testing on the test bench will show if the alternator produces the correct output at the right RPM. This is something you can not do accuratly on an aircraft. This test will show partially shorted winding or defective diodes.
Two other things to keep in mind:
- Leave the alternator switch OFF when the engine is off (walk around / maintenance etc) Having the switch on will have the regulator producing maximum field current, which can burn in slip rings, and wastes battery capacity for no good reason.
- After installation or adjusting belt presure, be sure to also tighten the long bolt which is used as a hinge. Quite some housings become damaged due to this bolt being loose.
vic wrote:
Rewinding a three phase alternator is dead simple compared to the more complicated DC rotor. You may want to look up the companies in the links. It is every day job to rewind all sorts of electric gear, ignition coils included. Vic
Rewinding these is not allowed under EASA, but would also not be worthwhile. Apart from that stator issues are quite rare.
you are right, stators almost never fail, same goes for DC or AC stators. I meant rotors for rewinding, legal for aircraft or not, who knows ? Worthwile, well, depends on what you pay for a replacement, dunno .
How often do you check your car alternator ? It is exactly the same design and should have a lifetime as long as the car, so why is there this habit of breaking down alternators on aircraft ?
And I can´t believe there are regulators on aircraft that send full voltage to the rotor while the engine is stopped. How stupid is that ??? Never seen that on bikes or cars. You only see low current through the charging bulb for exciting the field on 3 phase alternators, a set of diodes will prevent reverse current .
FWIW I have used Trafford Rewinds in Manchester, UK, for rewinding all kinds of DC and AC motors, right down to the smallest ones.
Whether it is economical is often debatable.
Last time I used them they were able to source me a motor which was exactly identical and cost the same as it would cost to have it rewound.
I too would think what can possibly go wrong with the stator on an alternator? It doesn’t move. However, the enamel can fail and you get shorted turns.
The diodes in the rectifier do fail fairly often.
vic wrote:
you are right, stators almost never fail, same goes for DC or AC stators. I meant rotors for rewinding,
Rotors fail even less then stators when it comes to windings. Rotor failure is often where the slip rings are gone, because wire and or springs where used instead of brushes 
This can easily be prevented by performing 500 hours inspection, which is more then a brush check.
vic wrote:
How often do you check your car alternator ? It is exactly the same design and should have a lifetime as long as the car, so why is there this habit of breaking down alternators on aircraft ?
Had quite some failures on cars as well, twice with a modern car, a little more often with older cars. Nowadays when I buy an oldtimer car I would overhaul the alternator or generator and go for a solid state regulator for reliablity. Never had a failure on my motorbikes though.
vic wrote:
And I can´t believe there are regulators on aircraft that send full voltage to the rotor while the engine is stopped. How stupid is that ???
Well test for yourself, you will find a 2 to 4 amp current draw. Also note that the voltage warning on most alternators is stupid, as described previouse, they just sense is the regulator gets voltage from the battery (regulator power input) if so, then it must be ok. You can easily test this on older Cessna’s, put on both master and alternator, with engine off, you will see the voltage light is OFF, and will go ON when you switch the alternator switch off.
Peter wrote:
Whether it is economical is often debatable.
Often only when you don’t have an alternative. Then it is the best you can get.
Peter wrote:
However, the enamel can fail and you get shorted turns. The diodes in the rectifier do fail fairly often.
True, however these kind of failures can happy any time. You often will not notice them directly, that is why a test on a test bench is usefull, as it measures output performance. Most other failures you WILL notice are due to ignoring maintenance (worn brushes to the extend that the springs or the wire has damaged the rotor)
And I can´t believe there are regulators on aircraft that send full voltage to the rotor while the engine is stopped. How stupid is that ???
That’s just a consequence of the regulator not knowing anything about engine RPM.
It’s a bit like my EDM700 flashing the master warning annunciator when I lean at idle. It thinks I am “going flying” with oil temp below 91F (the preset lower oil temp limit).
Actually I wonder how many regulators are going to make a smooth transition to regulating, if you turned the field current ON and started the engine and let it go up to say 2000rpm. It is initially getting the max possible field current (limited only by the resistance of the field winding) and it will be getting that same current as the alternator output is shooting up as the engine revs up. It would not surprise me if there was an overshoot on the bus. Another reason to have the avionics master switch OFF during starting.
