One issue I frequently see is that people start to exchange components without measurements. There are MANY single point failures which will result in low voltage. Relative inexpensive parts are often overlooked, such as switches, circuit breakers, over voltage protection.
GOOD measurements are very important. That means measuring voltages at different points when the system is ON / under load:
Also check all wiring and be sure to measure ground leads with a bonding tester. Do not replace parts before the above measurements are carried out. The testing above should not take to long and will give you a good indication of what is wrong.
I had no end of alternator & regulator issues some time back on one of my C152’s, at the end of the day the resolution of the issues was to replace all the electrical cables forward of the firewall.
Thirty years of living in a hot engine bay being exposed to heat, engine oil, exhaust fumes and rain is not an enviroment that is conducive to reliable performance from electrical cables.
GA CBs are a really crap implementation. They are thermal and since the heat dissipation in a resistor is I^2 * R, it’s obvious that the thing is going to run hot at any significant % of the rated trip current. Then you have to make the thing work over some plausible ambient temp range, bearing in mind that it will be mounted close to other CBs which also run, ahem, hot! It’s a stupid design. They also gradually wear out from the constant heat cycling and eventually they fail. The only salvation is in the form of modern aircraft usually drawing far less current than they might have drawn 30 years ago.
One difference between car and aircraft alternators is the presence of circuit breakers. I had repeated electrical failures, each one followed up by changing something (alternator, regulator, overvoltage detector, all the wiring) and still the failures happened. Until one day, failure in flight, I flicked, (not pushed) the field breaker. Bingo! the breaker operated normally until it decided to go open circuit and being of the push only variety could not be cycled. Of course after landing it worked perfectly. I even diverted over the Baltic because of this £0.20 piece of aviation rubbish and worse persuaded a colleague to hand carry an alternator to Helsinki! (interesting x-ray discussion).
Im using the FORD 95A alternator on my P210.
They are notoriously bad. (Had owned my P210 for 0:05 flight hours when the first one broke)
Replaced it with an OH unit from Kelly. Charged ok, but had a serious background noise problem. Diodes on the rectifier plate was broken from day 1. Unit 1 week outside warranty when we discovered where the noise was coming from.(Im not buying anything more from Hartzell/Kelly)
Installed a used unit that has worked pretty well, but has now started showing signs of wear.
So I bought all the required parts(in the US) for an local repair of the “new” alternator. The first shop that opened it up saw the ceramic coating on the rectifier assembly wire joints, and refused to repair. Coating to hard to remove they claimed……
Not keen on shipping the Alternator to the US just to redo the soldering/ceramic coating on the new rectifier board.
I have about $30M worth of experience developing brushed machines
How does one express experience in USD?
ive decided on my route forward to build in the maximum reliability into the charging system. The old suspect Ford 60amp alternator has 560 hours on it since its rebuild. I can have it rebuilt again for £450. I’ve decided not to and have ordered a National Air Parts 70 amp brand new unit for $1100 (£700). This unit replaces the Ford unit and needs only a logbook entry (N reg).
As for the voltage regulator or to give it its correct name the Alternator Control Unit I’m not sure the old Lamar unit has been affected by the alternator issues so that is being replaced with a Zaftronics more modern unit.
All the wiring has been checked including the battery earth and confirmed voltage drops are within tolerance. So, that’s the best I can do for a reliable setup.
Silvaire wrote:
I’d be interested to know if there’s test data to support this assertion. Brushes do wear in proportion to current, but the slip ring being stationary would not negatively affect the situation.
You can see discollored spots. Apart from that you are wasting quite some energy without a reason
Jesse wrote:
Do not switch on alternator field with engine off during walk around / maintenance, it will wear brushes.
I’d be interested to know if there’s test data to support this assertion. Brushes do wear in proportion to current, but the slip ring being stationary would not negatively affect the situation. I would’ve guessed the limited time involved with walk around / maintenance would make this a theoretical issue, not worth considering in the real world even at max brush current.
(I have about $30M worth of experience developing brushed machines, with small brushes carrying hundreds of amps each. Its an interesting area)
Peter wrote:
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.
Valid point!
vic wrote:
Question is where is this going to.
Peter wrote:
My field current is 2.7A with engine off.
Yes, typical field current is between 2 A and 4 A depending on alternator.
vic wrote:
As far as I know the 3 phase alternators no direct battery current is fed into the field coil =rotor winding.
This is untrue, it is, on models with external regulator, such as used in aviation, battery voltage is regulated by the regulator feeding the field winding.
Car alternators, often have an internal regulator. In this situation limited current is fed to the field winding, enabling the alternator. The alternator output is fed back into the regulator, as it also requires more current to get it going. On a alternator with internal regulator, you have to apply field only during start, you could disconnect that wire if you would want to, the alternator would stay working ok, as it is supplying it’s own voltage for the regulator. This is just like you’re saying. Don’t forget this feed point for the internal regulator is in the end the same point as with an external regulator.
When you disconnect field wire on an alternator with external regulator, the alternator will instantly switch off.
You can not switch off an internal regulator alternator other then by stopping the motion (kill the engine). On an external regulator you can switch on and off the alternator as
you please.
vic wrote:
As only field current at a few amps is sent into the rotor the brushes should last a very long time, a few thousand hours typically. I am very eager to learn why this should not be the case with your familiar aircraft power source as opposed to car types. Vic
You WILL NOT get anywhere near thousands of hours on a typical general aviation alternator. This suggestion is not a good idea, and I would recommend everyone with an alternator to do 500 hours inspection.
Differences between a car alternator and aviation alternator do excist.
Do not switch on alternator field with engine off during walk around / maintenance, it will wear brushes. On a car or motorcycle this is not applicable due to the internal regulator system.
Lower air pressure is also an issue and will introduce more wear. One of the reason that some relays are gas filled, and some magneto’s are pressurised to prevent unwanted sparking. HF tuners on older airliners are also pressurised to prevent sparking.
vic wrote:
We are running a 3000 W DC , four brush generator on the Yak, no diodes anywhere.
Generators have their own advantages and disadvantages, they require voltage and current control, and typically have only a small RPM band in which they operate well, the RPM range of an alternator is much larger. The DC generator has the advantage that it is selfexciting.
