banner graphic
Airport Webcams
Who's Online
17 registered (dls1981dustin, studentSA, Don Tedrow, Robert114, flynhi, California Flyer, Jake@Bevan, Bigdoggh, Shannon, Cotsboe, RGilland, Kentucky Captain, Rod Man, ytodd, vettdvr, BAS172, Henry M.), 115 Guests and 1 Spider online.
Key: Admin, Global Mod, Mod
Newest Members
JBinAlabama, Tdnp, Vzff, claycarter, Josh-D Davis
8766 Registered Users
Top Posters
Bargain Bob 153579
vettdvr 7587
Cessna Dude 6628
Rusty Rudder 6563
California Flyer 5499
Willie 4451
Clyde Cessna 3717
Joeman434 3574
Don Tedrow 3040
Peedie Montgomery 2867
wrecksum 2704
Ward Holbrook 2436
XP Driver 2394
November X-ray 2168
ytodd 1933
oilwell1415 1906
Jim_1 1856
Nightowl 1826
Nintendo Pilot 1726
Glenn Darr 1708
Bigdoggh 1704
RodneyHooverCFI 1645
KevinMcP 1634
Pilawt 1624
EdW 1413
Shuswap 1302
Pilot110 1293
Showboatsix 1267
JD Casteel 1178
Henry M. 1166
Top Posters (30 Days)
Bargain Bob 2294
vettdvr 68
Joeman434 63
Don Tedrow 47
Viper_96 47
Rusty Rudder 45
N5479R 44
November X-ray 39
Joe K 31
Peedie Montgomery 30
Kentucky Captain 30
bpost58 25
California Flyer 24
Wayne R 22
Willie 20
Curious1 19
JD Casteel 18
Clyde Cessna 18
Flubber4.0 17
Bigdoggh 16
Newpilotoldplane 15
Kinhop 14
Shuswap 14
Bimota 13
Ernani 12
DeputydogK9 12
Rockhead 11
Showboatsix 11
C172Flyer 10
Jim_1 10
Topic Options
#63666 - 01/13/12 02:27 AM help needed with electrical system!!!
diego72 Offline
New Member

Registered: 10/01/10
Posts: 4
Loc: Nice, France
Dear All
i have a couple of questions that i would like the gurus amongst you to shed some light upon.
as far as i know in a C172 or in any light aircraft 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 a 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.

the armature consists of a stotor or casing.

is that a correct statement and even in earlier models, is the voltage regulator the same as the ACU just a different wording?

the ammeter provides a readout of the charging or discharging rate of the battery: when the engine is running and the alternator is online charging the battery - i guess to compensate for the current drawn from hot wired items? - the ammeter shows some kind of deflection towards the + side of the dial, as the load is increased, the needle swings towards the 0 position as the emf available to the battery decreases until it becomes excessive and then the needle swings to the - minus side, is that correct?

many thanks


#63691 - 01/13/12 11:24 AM Re: help needed with electrical system!!! [Re: diego72]
Henry M. Online   content
Gold Pilot

Registered: 08/23/11
Posts: 1166
Loc: Leander, TX
Sounds reasonable time. My plane has a generator and the low power light remains on until the engine revs enough for the generator to provide enough voltage.
1964 C172E


Cessna172 Tribute
Special thanks to:
Thank you for making this site great
Fredericksburg TX '15 Countdown
September 25-27, 2015 T82
Today's Birthdays
1980Cutlass, 19Steve61, cyendrey, DaveIndenver, jheldt, johnhuth, LBaltzley, Tyler
Member Map
Where in the world are the Cessna 172 Club Members?

View the map

Helpful Links
100LL Price Finder
AD Directory
Aging GA Aircraft
Aircraft Directory
Airplane Report
Airport Facilities Directory
Aviation Weather
C172 History
Crash Records
DUAT Voyager Planner
Flight Aware
Flying Tools (Files)
Google Earth
Ground Speed Records
National METAR Map
Sky Vector
Uvalde Flight Ctr
All the Webcams
Shout Box

Weather Lookup
Airport code:
(separate multiple codes with spaces)

List of stations