Peter wrote:
Self excitement works in shunt connected generators. A bit of residual magnetism in the iron gets it started.
The old aircraft generators are like this, we used to have one on the Auster till we changed it for an alternator. I think it was exactly the same part as the generator on our old International Harvester tractor we use to tow the gear out at the glider club…
How do you control the output voltage? One needs 14V/28V to charge a lead acid battery, and this voltage needs to be pretty well stabilised.
A shunt connected generator will output a voltage proportional to the square of the RPM – if I remember my 1970s electrical theory correctly.
Obviously in this age one could take that, say 10V-100V, and generate a stable 14/28V from it, but way back?
I got into the habit of turning the alternator off as the landing light went on on a night final approach
This might be my conservative mind, but it sounds like one of those stupid habits that students make up when they discuss flight procedures without input from an instructor. Quote
Trust me, I’m not a student who makes up stupid habits. I certify and STC approve engine changes in GA aircraft. I doubt that many instructors have the detailed knowledge of the inner workings of the engines, and alternator torque characteristics to develop this understanding, and train it. Some experience in piloting comes from sources other than instructors.
I would expect that airframe manufacturers would rather not suggest turning off an/the alternator in flight, as there’s nothing in it for them to save you operating expense. “Just leave everything on” is easier and more liability friendly for them, just more costly to the owner of the aircraft.
On the other hand, there’s lot’s in it for the informed pilot/owner to save themselves alternator drive coupling cost for certain engine models. A Continental drive coupling is found for US$595 + $300 core charge. The engine parts manufacturing company I worked for for years did a lot of development on PMA replacements for these parts, as there was high demand. From my days flying Cessna 310 and 340, which were so equipped, no one ever told me about the result of idling down final with everything turned on – we just paid the high cost of replacing these couplings periodically – not knowing why. The coupling is required, so as to slip a seized alternator, rather than damaging the engine gear train. The problem is that once you slip one, the engine oil works into where it should not be, and the coupling will not carry the torque of a normally operating alternator any more – and must be replaced. It tries to, and seems to work, until you load it up – just when you really want it to work! We had many come into the shop for replacement over the years
Suppliers love to sell you the parts, so they don’t care if you wear things out early. On the other hand, if you minimize the load being carried by vulnerable drive train parts of the engine, they will function normally, and last a long time.
Flying with the alternator turned off is perfectly fine, as long as you realize that while doing so, you are running on the battery only, so endurance is limited. Certification will require that the basic aircraft demonstrate 30 minutes battery capacity, but that can be optimistic, depending on what equipment is installed, and what you’re willing to turn off at night. Happily, LED lighting is making things much better.
If flying for short duration without the alternator turned on creates undesirable worry, knowing that for moving the switch, you can have it back on, consider the risk of the alternator drive slipping from torque abuse, and permanently failing when you really need it, and least expect it! Then you have no choice but to continue on the battery, and cannot get the alternator back if you want to.
Some things on aircraft are consumable, and I like to fly to “save” them, when doing so is practical.
I’m wondering if you won’t also generate a very high load by first draining the battery and then switching the alternator back on once on the ground, where you won’t exactly be at high RPM either? Or did I misunderstand your procedure?
But your insight may add a data point to our recent discussion on whether the pitot heat should always be on, or just when you expect that you may need it. One less thing to generate load on the alternator.
Yes, anything which reduces higher electrical draw at low RPM is good. Once on the ground following a night landing, I turn off what I don’t need, and then I increase the RPM a bit as I taxi in, and then select the alternator back on. Nothing is perfect, I just do my best.
I use pilot heat if conditions dictate, otherwise I do not, as is my choice for most electrical.
Belt driven alternators are great, in that the “slipping” feature is cheap to replace. The Gear driven are more expensive. For Lycomings though, it’s a double edged sword, as it is usually necessary to remove the prop to replace an alternator belt. That can be a really big job, for an otherwise small task. When I take my flying boat into the far reaches, I carry this type of drive belt, which can be assembled around the pulley, rather than having to remove the prop. I’ve never had to use it, but I’ve heard very good things about it. You’ll sometimes see a spare belt tywrapped around the engine case, but generally these have perished from heat and idleness before they are needed, so when they are, they’re no good anyway.
Peter wrote:
How do you control the output voltage? One needs 14V/28V to charge a lead acid battery, and this voltage needs to be pretty well stabilised.
The self-exciting aircraft generators have a relay-based voltage regulator, I don’t have a wiring diagram of how it worked (and didn’t care all that much – as soon as I got the plane one of the first jobs was (a) ditch the old tractor generator for an alternator and (b) ditch the useless cable-operated drum brakes for hydraulic disc brakes)
Airborne_Again wrote:
But that is hardly a reason to turn the alternator off before shutting down the engine. The few seconds between engine shut-down and master switch off can’t make a pratical (or even measurable) difference to the battery charge.
Agree, sorry for the confusion. I meant this more in a general way. Some do an extensive walkaround, with battery and alternator on (and all lights and pitot heat), then to complain the aircraft has a weak battery. I think anyone should do the walk-around which suits him or her best, with the alternator switch off.
alioth wrote:
The self-exciting aircraft generators have a relay-based voltage regulator, I don’t have a wiring diagram of how it worked
It is controlled with three relays, a reverse current protection prevents it working as a motor, when the output is lower then the battery voltage. Then it has a current relay and a voltage control relay. While a generator is selfexciting the output is controlled by controlling the field current.
Peter wrote:
If you are going to have brushes, you can generate DC. It’s called a “generator”
You mean a dynamo. Both dynamos and alternators are generators.
Pilot_DAR wrote:
Some experience in piloting comes from sources other than instructors.
Thanks for your clarifying post re. your motivations! However, my Aircraft Flight Manual has a line that says:
“WARNING Do not take the alternator off line (either by turning off the Bus 2 Master or by pulling the alternator field circuit breaker) in flight except in an emergency” that is repeated several times…
Now, you’ve generated your own SOP (Standard Operating Procedure) to turn off the alternator on final approach, at night. In that you’ve made a trade-off between safety (redundancy) and maintenance cost. That is a tricky one to make. In your specific instance with your specific instance it might perhaps be justified. Nevertheless, I still don’t quite understand how flight idle on final approach could be an issue as you wrote:
Pilot_DAR wrote:
Power = torque x RPM. When you reduce RPM (to idle) the torque demanded goes way up – and you challenge the drive coupling.
It is a good point that an alternator is going to need more torque at a lower RPM – for a given output power i.e. (because the bus voltage is pretty constant) for a given output current.
At a lower RPM, the regulator will wind up the field current to a higher value, hence a higher torque to turn it.
The Q I see is whether the alternator torque is relevant in the wider picture of alternator failures, including alternator drive belt (or plastic coupling, if used instead of a belt) failures.
Certainly the alternator brush life will be inverse with the field current, because that current is carried by the brushes. But the brushes are easy to inspect and replace.
