banner graphic
Airport Webcams
Who's Online
27 registered (Teg916, JD Casteel, Jimbot, RickG, Henry M., Joe K, flynal, Will, rusty1964, Flubber4.0, Swazi Flyer, oilwell1415, Ghost, CanadaAirPilot, T6ZERO, Bio2, Viper_96, dshirey, Pilot110, IDontFly, vettdvr, Don Tedrow, Joe Jet, Joeman434, Dave W, chinnook, Don Phillips), 129 Guests and 6 Spiders online.
Key: Admin, Global Mod, Mod
Newest Members
SpreadFlying, Pipedream, Alwayssunny, NordicDave, Aaron Odell
7392 Registered Users
Top Posters
Bargain Bob 122490
vettdvr 6855
Cessna Dude 6628
Rusty Rudder 5782
California Flyer 4997
Willie 3763
Clyde Cessna 3409
wrecksum 2704
Don Tedrow 2426
Joeman434 2348
XP Driver 2280
Ward Holbrook 2269
Peedie Montgomery 2172
oilwell1415 1906
ytodd 1883
Nightowl 1826
Nintendo Pilot 1711
Glenn Darr 1708
RodneyHooverCFI 1645
Lee T. Hargic 1633
November X-ray 1629
Jim_1 1577
Pilawt 1538
Bigdoggh 1532
EdW 1315
Shuswap 1208
Showboatsix 1124
Propduster 1102
Pilot110 1056
Awkward Bird 1048
Top Posters (30 Days)
Bargain Bob 2464
Willie 146
Joeman434 125
Peedie Montgomery 91
Mike492 58
vettdvr 56
R Chisler 55
Flubber4.0 48
N5479R 42
November X-ray 40
Rusty Rudder 37
SunDog 36
JD Casteel 35
Viper_96 34
Kentucky Captain 33
Teg916 32
Jim_1 28
Pilot110 28
EdW 26
3hawks 25
NightHawk XP 25
Desert Hawk 25
Curious1 25
IDontFly 23
Don Tedrow 19
RickG 19
Renegade 9 18
Kinhop 18
Donboy 17
Henry M. 16
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
Silver Pilot

Registered: 08/23/11
Posts: 993
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
No Birthdays
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