generator alternator in trainers aircrafts

[baobab72]

Dear All

i have a couple of final questions about the generator/alternator used on trainers which might help to dissipate my doubts once for all, since i guess i had it wrong cos i was under the impresson that the trickle charge from the battery to the VR was fed to the stator and that the rotating armature was actually the component supplying the emf! while if i understand it correctly, the stator consisting of the 3 coils spaced 120° apart and built around the rotor driven by the engine is the one supplying the AC! emf which is induced as the trickle charge is supplied to the rotating armature aka field coil, through the slip rings/brush assembly, generating an electromagnetic field and in which the same armature is spun – Faraday’s principle stating that whenver a conductor is moved into an electromagnetic field a certain emf is induced in the conductor – in this case the source of the electromagnetic field is represented by the rotor while the conductor by the stator! – and the magnitude of the emf is proportional to the relative velocty of the conductor with respect to the electromagnetic field and to the strength of the same field!
Is that a correct statement?
also what is the function of the BATT switch, the ALT switch i guess controls the trickle charge to the VR and then to the rotor through an alternator field relay – in case of an overvoltage the VR will trip the relay, cutting off the trickle charge to the field coil, causing the electromagnetic field to collapse and thus the output of the alternator to drop to zero! but the BATT switch what does it exactly control?
Also how is the emf picked from the stator?
And finally the right sequence for the CB’s is SOURCE –> CB –> switch –> item or source –> switch –> CB –> item?

many thanks!!
Baobab72

[A and C]

You seem to have most of the theory correct but the BAT switch controls the battery relay ( and so connects the battery to the battery bus bar)

The ALT switch controls the power to the alternator regulator, and this controls the alternator output by regulating the current in the field windings.

A bit of a simplistic explanation but I hope it helps.

[The Flying Pram]

You seem to be confusing a dynamo with an alternator. The dynamo has fixed field windings generating a magnetic field, and a rotating armature with a commutator and brushes to produce a direct current suitable for charging the battery.

An alternator has 3 (fixed) stator windings 120 degrees apart which produce 3 phase alternating current. This is converted into DC by a number (normally 6) of diodes linked together and called a rectifier. The magnetic field is produced by the rotor which has a DC voltage supplied to it by slip rings and brushes. This circuit is controlled by the regulator in response to the battery voltage and the demand from the aircraft systems. The following diagram is from a typical car installation which is very similar, the main difference is that it uses 3 extra diodes to provide the supply for the field circuit. This allows it to be completely self contained, and cars don't have any provision to isolate the alternator or battery when running: http://www.familycar.com/Classroom/Images/Charging_AlternatorCircuit.gif

The principal advantage of the alternator is that slip rings only carry a few amps of current and don't suffer the wear that dynamo brushes do. This allows an alternator to be run much faster and so give a better output low engine speeds.

Hope this helps.

[baobab72]

Many thanks to all of you for your prompt replies
however i still have a couple of questions - i rwally want to master the electrical system!!

how is the emf picked from the stator?

the schematic of the C172, at least the one i have access to, a POH, shows the power form the battery being fed to a battery contactor, and then it shows a line going to the MASTER switch split in a BATT portion and in an ALT portion: the ALT portion does make sense to me - ALT to VR to Rotor to generate the electromagnetic field - the BATT portion not much, since it doesn t show, or at least i can not spot it, a clear connection between the BATT switch and the distribution system so my guess is that power form a battery relay controlled by the switch is fed to the bus system and from there is made available to the ALT switch.

I hope the attachement will show!

Also how is the power from the ALT routed back to the battery? the schematic is a bit confusing since it doesn t show the direction of flow of the emf!

Many thanks

Baobab

[phiggsbroadband]

Hi Baobab, in a car alternator charging system the alternator output +ve
terminal is connected directy to the battery. It cannot discharge the battery when the engine is not running, as the silicon diodes inside the alternator are reverse polarity for any battery to alternator current.

One interesting point with car alternators is that they require the current from the ignition light (maybe 0.1 Amp.) to flow into the rotor coils of the alternator to start the alternator producing any voltage at all. After that the regulator starts to work. The rotor only needs about 2 Amps for the alternator to provide the full 60 Amps of current.

If you have the POH for the cessna 172, see section 7-24, fig. 7.7 for a circuit diagram. You will see that the output terminal of the alternator is directly coupled though the alt circuit breaker to the main bus bar. This is again linked to the battery when the Battery Contactor is energised.
The other terminals of the alternator are the field (rotor.) and the -ve ground (which is directly bolted to chassis earth.)

Pete

[Big Pistons Forever]

Baobob72

I think you are over analyzing the issue. Personally I tell my students to not worry how the alternator actually works to produce electricity. Instead I make make sure they understand what to monitor from the cockpit to ensure the electrical system is working properly to understand the purpose of the actions in the emergency procedures section of the Pilot Operating Handbook.

In particular I emphasize awareness of where the ammeter needle is and whether it makes senses. For example a fairly high charge rate would be normal right after starting in a cold day but cause for serious concern if noted during cruise flight.

BTW a little known fact. If the charging system fails in flight all power will obviously be provided by the battery. If the pilot fails to notice this thr first thing to fail will be the transponder as it needs full voltage to function. So if you are cruising along and ATC suddenly says they are no longer getting a transponder return on you then I would start by looking at the ammeter and see if it is showing a discharge.

[BackPacker]

If the pilot fails to notice this thr first thing to fail will be the transponder as it needs full voltage to function.

Is this universally true for all brands and models of transponders? I can see no reason why a transponder in general would be more susceptible to a low voltage (12v battery instead of 14v alternator) situation than other pieces of electronic equipment such as your NAV/COM/GPS. Particularly as there is a certification requirement that your battery capacity should be sufficient to run all essential electronics for half an hour or so.

Only when the battery is nearly depleted would the voltage rapidly get below 12v, and keep dropping. Various equipment will stop functioning and the order will indeed be determined by their susceptibility to a low voltage situation. But by then things will happen so fast that practically speaking the order in which they will fail doesn't matter anymore.

Monitoring the ammeter or load meter and knowing what they should read in certain phases of flight is essential though. No disagreement from me there.

[baobab72]

Hi thanks everyone for your answers
very informative!!
for phiggsbroadband, with reference to the electrical system schematic for the 172, it shows a line going from the MASTER switch to the ACU with an INPUT label at the control unit so i guess that what it shows is the field current being supplied to the VR of the control unit; then it shows the alternator with 3 terminals: one G that as you said is GROUND, the electrons must flow in a closed pattern, so i guess showing the grounding post between the alternator and the airframe; an F that i guess is FIELD showing the field current to the rotor regulated by the VR to maintain a constant regulator output; and a B terminal showing i guess the alternator output to the BUS network through the ALT CB.
A different line from the ALT CB through the ALTERNATOR FIELD CB to the BATT MASTER switch is also drawn, without showing the direction of the current flow, so i assumed it was showing the field current to the VR, although from your explanation it looks like that line is showing that whenever the BATT switch is ON power is supplied to the bus network through the ALT CB. Shouldn't the ALTERNATOR FIELD CB be between the ALT MASTER switch and the VR path though?
Also there is no line between the alternator and the battery showing that the battery is being recharged by the alternator? is that to unclutter the diagram or simply i am misinterpreting it?

MANY THANKS
Baobab

[cockney steve]

Why would you need a separate lead straight to the batt.? when both are online, they're connected by a common bus....if the battery's a lower voltage than the alty output, the alty will push current "back" down the battery, thus charging it, whilst simultaneously pushing "forward" to meet any load. the "field" (rotor) current directly affects the alty output and ensures the battery is topped and a stable voltage is maintained to the busbar.

consider a depleted battery as "load" a fully -charged one as a "reserve-buffer".

If the alty croaks whilst it's on-line, the output volts will drop below busbar volts...the battery will then try to push current down to the alty,but is thwarted by the diode-bridge(rectifier) a cunning arrangement that "collects" all the "positive waves" of the AC output , and all the "negative waves" and integrates the two, so you have "lumpy dc" effectively it's a one-way valve allowing current out.

Usual failure-modes:- Open circuit field (1 coil-set) = v.low output charge-light may glow dimly. all coils out or field out = no charge

reg. stuck "full field" output is totally proportional to engine RPM and can reach ~ 50V It'll boil your battery, violently spraying acid everywhere and also any lamps switched on will shine VERY bright for a few seconds until the filament vaporises......but you're lucky, you have an isolating-switch and you can take it off line and isolate it....unlike the poor motorist!

hope this helps.