c172 alternator
 

Dear All
i am not sure i am posting in the right section of the forum, anyway i am about to do my BFR and probably i will do it in a C172. As i was reviewing the systems, i came across the electrical one, and one doubt crept into my mind: how does the alternator work? does it have a stationary field coil supplied with a trickle charge from the battery and a rotating armature driven by the engine via a drive belt and coupled to a commutator turning AC into Dc or is the armature stationary and the coil rotating? In case of an overvoltage in excess of 31.5Vdc, a breaker will trip, discontinuing the trickle charge from the battery to the field coil, causing the electromagnetic field to collapse and then voltage of the alternature to drop, taking the alternator offline as reflected by a decrease in Volt ( to 0?) and by a discharge on the ammeter as the load placed on the battery is increased?

Many thanks for your replies.

Baobab72

C172 alternator
 

First of all, many thanks for your reply
Second, probably it will be a new C172 not the one with Garmin 1000 but a new model.
Third, i have no clue if my instructor knows about how a generator works, and i am almost positive that he will not go that far into details, however i am fascinated about the systems of an aircraft, whatever it is, and i do like to understand them in details.
So to sum it up, the armature is stationary and it is connected to the commutator converting AC into DC, while the field coil is driven by the engine through a belt and it is excited by the trickle charge from the battery?

Baobab72

I don't think this has been mentioned but typically the regulator in the charging circuit is a separate device and not integral with the alternator as in the car.

From current 172R manual, hope it helped :)

Strictly speaking, there is no armature in an alternator. Instead, there is a rotor with laminated-iron poles and a continuous coil terminated in slip-rings. these can be cylindrical or end-face (a disc surrounded by a concentric ring)

the rotor is surrounded by a stator,-again, laminated-iron, with multiple poles upon which the output-coils are wound..

By energising the rotor ,it will induce a current in the stator windings. the energising input is regulated to give the desired output-voltage at normal operating speeds and to disconnect the machine when output-voltage is less than the connected battery....with a blocking-diode fitted, the main output is safely left connected directly to the battery,as no back-flow can take place.

I assume that in aircraft applications it is disconnected via the master switch and thus fully isolated.

Normally, the battery is used to provide initial excitation current and in solid-state circuits it can be arranged to be the "earthy" side of the warning-lamp. as the machine starts generating, the output opposes the battery-voltage and the light goes outdue to the battery and generator-voltages reducing the current-flow.


in summary, the generating-coils are stationary, the magnet (electromagnet) revolves. output is AC , normally rectified by a diode-pack....regulation of field-current can be solid-state or electro-mechanical.

IMHO, the former is more reliable and i think that early 20th century equipment belongs in the museum.;) (ducks incoming)

alternator c172
 

many thanks for your replies.

Hopefully this time i get it right.


The alternator represents the primary source of DC power when the engine is running and it basically consists of a stationary armature and of a rotating field coil driven by the engine through a drive belt and trickle charged from the battery through the ACU and an alternator contactor field, generating an electromagnetic field: as the coil revolves around the stationary armature, it induces as a result of an electromagnetic induction phenomenon a certain amount of induced current into the armature then converted into a direct current flow by rectifier, and the magnitude of which depends upon the strength of the electromagnetic field and thus of the trickle charge and upon the rotational velocity of the field coil with respect to the armature.
The alternator is used in conjunction with an ACU or Alternator Control Unit housed in a power junction box also referred to as the Master Control Unit or MCU located on the forward left side of the firewall and designed to provide a constant 28.5VDC supply to the Main Bus system, by increasing the trickle charge and thus the strength of the electromagnetic field as the load on the electrical system is increased or as the engine spools down or by decreasing it as the load on the electrical system is decreased or as the engine spools up.
In addition the ACU will provide a low voltage indication should the main bus voltage supply drop below 24.5VDC; over-voltage protection should the voltage supply to the main bus exceed 31.5VDC forcing an ALT FIELD CB open, cutting off the trickle charge to the field coil, causing the electromagnetic field to collapse and thus the output of the alternator to drop as reflected by a drop on the voltmeter and by a discharge indication on the ammeter as the battery copes with the extra load and reverse current protection.
Alternator operation is controlled by the ALT half of the split rocker switch that when turned ON it will energize the ACU and the alternator contactor field, restoring the trickle charge to the field coil and thus flashing the electromagnetic field back.

anything to review, or add to it?

Thanks

Baobab72

One key thing is that an alternator generates AC voltage.

There is a separate box that rectifies this to convert it to DC. This is usually solid state and sometimes it is built into the alternator housing, sometimes it is a seperate box or integral with the regulator which controls the voltage.

I think you said this but wanted to make sure.

Dynamo = Fixed field-coils in the casing, rotating armature has multiple windings the terminations of which are diametrically opposite segments of a commutator.....each coil is positioned so that brushes contactthe segments just as the field -poles induce current on the armature......the output is DC.

the alternator is, in some senses an inside-out dynamo.-as previously stated, the armature's GENERATING FUNCTION is replaced by a stator (the casing or carcase of the machine) The rotor,as it's name suggests, rotates inside and concentric to, the stator.....often it contains residual magnetism which can "kick-off" the generation,when spun. Primarily, it uses a single winding and although it still needs slip-rings and brushes to energise the spinning rotor (field coil equivalent) the contact is continuous, therefore, unlike a dynamo, the ring/brush arrangement has a very long service life, whereas the dynamo's segments wear the brushes more rapidly. (old trick, blow conductive dust out with a blast from an airline and it sometimes revives a machine which had been "tracking-down" through excess deposits )
In all cases the ALTernator output is ALTernating current and is rectified either internally or externally (diode-pack) (a 3-phase bridge, iirc ) and a regulator which senses machine output or battery-voltage and regulates the rotor-field accordingly. (there is a slight difference between the machine and battery due to wiring/connector/battery internal resistance) Temperature can also be compensated for as it helps extend the service life of the battery.

Lucas, AC Delco, Paris-Rhone ,Ducellier and many other Alternator manufacturers use the nomenclature above (stator / rotor) I believe theirs are the correct names.

C172 alternator
 

Thanks once again for the replies.
So, to sum it up, the armature is the stator and is outside, and the field coil is the rotor driven by the engine via the drive belt and it spins inside the stator-armature?
The resulting current is AC which is then converted by a solid state rectifier into DC.
Are those correct statements?

baobab72

Would LH work better than BA, or would you recommend AF/KL?

(sorry, couldn't resist)