If you are talking about generators, you’d be right. They don’t require a field to generate power.
It depends on the generator.
In almost any “real world application” where you actually want to end up with some known voltage, you have to have a regulator which varies the field current.
The field can come from the fixed part or the rotating part.
Setting aside decades-old aviation-speak, an alternator is the same thing as a generator, except that an alternator (now using the car terminology) has slip rings and generates AC (it has to because there is no way in the known universe of generating DC by a rotating machine) which is then rectified with silicon diodes to get DC. A generator also generates AC (it has to because there is no way in the known universe of generating DC by a rotating machine) but it uses a commutator to do the rectification, using the crude means of swapping the coil connections around as it goes around. An alternator is a slicker implementation because it uses slip rings for the field current, and they last much longer than generator brushes because (a) they carry much less current and (b) they sit against a smooth surface.
Generators tend to be used where long past history has used them, or where the thing doubles as a starter motor. You can use an alternator as a starter but it needs some electronics to synthesise the multi-phase AC. This is actually done all over industry (google “AC drives” etc) but aviation is decades behind.
Metroliner, Citations – call for alternators on together with the battery and off together with the battery
w_n were these the baby models? All turbines have generators.
there is no way in the known universe of generating DC by a rotating machine
DC homopolar
Of course, but I thought nobody has yet found a way to make a monopole. It’s right up there with ways of interacting with gravity (without using large masses), levitation, etc.
I googled on “DC homopolar” and it’s interesting. I wonder what the catch is?
Archie wrote:
I highly doubt this? What then does the regulator do in case of an overvoltage condition? How can it shut down the alternator to protect your 1000s of $$$ avionics? If you are talking about generators, you’d be right. They don’t require a field to generate power.
Yes, Alternators found typically found in aircraft (with external regulator) will switch off when you switch is the field (this is the ALT switch). On some aircraft it is always on, unless as Archie wrote there is an overvoltage condition.
I doubt @Flyer59 Alternator was damaged by leaving it on. On some aircraft it is automatically engaged when the master switch is switched on. I can’t see a reason for damage at the point either. Alternators just do fail. Maintenance can prevent most failure’s, yet some failure will happen without warning / possibility to inspect.
Leaving the alternator on (if possible to switch off) is not a good practice when the engine is off, mainly becuase the field wiring gets maximum excitation. It will draw about 2 to 4 Amps depending on alternator, so you are wasting a lot of battery capacity for no good reason.
Peter wrote:
I wonder what the catch is?
Brushes, lots of them.
Silvaire wrote:
DC homopolar
Interesting! I hadn’t heard of these before. Thanks!
Jesse wrote:
Leaving the alternator on (if possible to switch off) is not a good practice when the engine is off, mainly becuase the field wiring gets maximum excitation. It will draw about 2 to 4 Amps depending on alternator, so you are wasting a lot of battery capacity for no good reason.
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.
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.
I admit I have not read the minute details of the previous posts (been away on holiday) but the only reason I can think of for turning off the alternator field current just prior to engine shutdown is that it avoids a possible high-going spike on the bus as a result of the voltage regulator going crazy, as it finds the alternator output voltage falling and goes to increase the field current to maximum.
I have never seen evidence of such a major defect in a voltage regulator but one needs to be aware that
Interesting! I hadn’t heard of these before. Thanks!
Neither had I but I should have qualified my earlier post with the word “brushless”. If you are going to have brushes, you can generate DC. It’s called a “generator” 
Most Cessnas have a split master switch, one for the alternator, the other the master. The Cessna Pilots Association (who have zillions of members) had an article on this subject recently. Their conclusion is the engine should be started and settled ONLY THEN turn on the alternator switch (presumably turning on the field coil). Then the check is the ammeter should go from a slight discharge to a good positive charge the instant the field coil is exited by turning the alternator switch on. After a short period the positive charge should start to drop, you are good to go.
If the positive charge is bigger than normal and stays there then it’s likely the starter motor is still engaged (that’s what the article says). I guess this check does the same job as the red starter warning light in G reg (and possibly other) aircraft.
For the close down they say it’s better to turn off the alternator just before shutting down. I cannot remember the detail of their argument but in a nutshell they say it must be better off in case anything else causes spikes during the shutdown.
It’s seems to be logical to me so I’ve added it into my check-lists.
