Exactly BH.
As we know it would be generalising to say all GA aircraft are the same, they are far from it.
Something like a Cherokee has no control of the alternator apart from the switching, and control, of the field voltage.
ie, no voltage on the field winding means no output... this is what the Alt switch does. The alternator switch does not control the high current output from the alternator through a contactor... most installations don't have such a beast.
The Overvoltage relay, sometimes contained within the regulator, does the same. It just turns off the field voltage which in turn kills the alternator output.
Some salient features of a typical GA electrical system.
The battery contactor is controlled by placing an EARTH at the control, via the Master Switch.
The alternator switch controls the FIELD voltage on the alternator circuit.
There is often no protection on the Alternator output, it does not go to the bus via a fuse or CB.... more likely through an ammeter shunt though.
As mentioned in the previous post, - the battery must have sufficient voltage to operate the Master Contactor and then also be sufficient to provide alternator field.
Turning the Alternator switch off in flight may not necessarily guarantee it will come online again.
Some charging systems can be a little fragile and can do something akin to PIO when the alternator field 'chases' the bus voltage. This could be more likely at higher rpm. Basically a series of high voltage surges as the regulator struggles to come to terms with a very active and lively alternator output voltage. I have experienced the Overvolt relay operating and killing the alternator whenever it the alternator was switched on.
If it persists then the obvious thing to do is bring the engine back to idle before trying again. Switching (shedding) some electrical load may be required also.
EDIT.. Sometimes load may need to be applied, especially if the battery is at a high state of charge already. With very little load on the system, a fully charged battery and a high output voltage from the spun up alternator the bus voltage can very easily rise a few points. An Overvolt relay set at say 14.2 volts could easily trip and open the field. (End Edit).
Although the term alternator implies they are an AC device they are not in a typical aircraft installation. The output is DC as the diode pack is self contained in the rear of the alternator.
Now a completely different subject..
The Ignition switch is independent of the charging circuit.
It does do a number of functions though, some of which are not always understood.
The Ign switch places an earth on the relevant magneto, via the P lead, to turn the mag off.
The switch also takes bus voltage and applies it to the start contactor when the switch is in the 'Start' position. At this position there is also other switching which normally applies an Earth to the right magneto. This is via an external link screwed across the terminals.
The link can be opened if the intention is to have both mags live during the start... if for example the engine has two impulse couplings fitted.
Normally only half the spark plugs are firing during a typical piston engine start. Often the top plug on one bank and the bottom on the other.
So it can be seen that while the switch is held on 'Start' it could be that the engine may only be firing on a couple of cylinders... if the bottom plugs are fouled or flooded.
That is why they will often pick-up when the switch is released.
Last edited by baron_beeza; 22nd Jun 2012 at 00:20.