banner graphic
Airport Webcams
Who's Online
27 registered (CerveloRod, Shuswap, elevatorman, lawman67, Vit, Mark Y., Willie, Herterman, Sunday Pilot, Joeman434, Darth Beaver, Ward Holbrook, Cezna172, oilwell1415, vettdvr, Rikochet, Shifty, TxHawk, lantajohn, Rusty Rudder, BenC, flynhi, GregInMI, Desert Hawk, N5479R, Kinhop, Pilawt), 125 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Newest Members
Specs101, xchox, GLM_Phil, CO Skyhawk, Mike G
6751 Registered Users
Top Posters
Bargain Bob 105710
Cessna Dude 6628
vettdvr 6043
Rusty Rudder 5187
California Flyer 4836
Clyde Cessna 3342
Willie 2878
wrecksum 2704
Ward Holbrook 2235
Don Tedrow 2138
XP Driver 2133
ytodd 1836
Nightowl 1826
Nintendo Pilot 1689
Glenn Darr 1670
oilwell1415 1661
RodneyHooverCFI 1645
Lee T. Hargic 1631
November X-ray 1589
Peedie Montgomery 1462
Pilawt 1448
Bigdoggh 1440
Jim_1 1330
EdW 1223
Joeman434 1138
Propduster 1072
Showboatsix 1038
Shuswap 1026
Awkward Bird 1021
Renegade 9 933
Top Posters (30 Days)
Bargain Bob 3328
Joeman434 176
vettdvr 163
Peedie Montgomery 108
Willie 88
magman 76
Don Tedrow 56
Jim_1 54
JD Casteel 49
Rusty Rudder 47
Pedals2Paddles 46
Underdog 39
Shuswap 37
Luvrv8 37
Nintendo Pilot 29
Curious1 28
flyhayes 27
N5479R 27
Henry M. 26
Tommygun53 25
cadcap 25
BNB-SP 24
Ward Holbrook 23
Pilawt 21
Singh 21
Pilot110 21
Desert Hawk 21
Teg916 20
Renegade 9 19
Dave W 19
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

diego72

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

Registered: 08/23/11
Posts: 869
Loc: Austin, 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.
_________________________
Henry
1964 C172E

Top


Cessna172 Tribute
THIS MONTH'S SPONSOR
Special thanks to:
You!
Thank you for making this site great
Strother '15 Countdown
June 18-21, 2015 KWLD
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
AirNav
Airport Facilities Directory
AOPA
Aviation Weather
Barnstormers
BaseOps
C172 History
Cessna
Controller
Crash Records
DUAT Voyager Planner
DUATS
FAA
FltPlan
Flight Aware
Flying Tools (Files)
Google Earth
Ground Speed Records
National METAR Map
Sky Vector
Trade-A-Plane
Uvalde Flight Ctr
All the Webcams
Shout Box

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

List of stations