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G8MNY > TECH 17.09.04 15:54l 144 Lines 7370 Bytes #999 (0) @ WW
BID : 11126_GB7CIP
Read: GUEST OE7FMI
Subj: Car Alternators
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 040917/1256Z @:GB7CIP.#32.GBR.EU #:11126 [Caterham] $:11126_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jun 04)
Here is a typical circuit.
OUTPUT RAIL Thick Lead
ÚÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ³ ³ Exciter Rail Regulator LAMP ³
3x 3A ³ ÚÄÄÄ(ÄÄÄÂÄÄÄ(ÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÂÄÄÂÄÄÄÄÂÄÄÄÄÄ0ÄÄÄÄo\oÄÄÄ´
diodes > ³ _³_ ³ _³_ ³ _³_ e\³ e ³ ³ ³ ³ thin IGN ³
_³_ /_\ _³_ /_\ _³_ /_\ PNPÃÄ\³ ³ 1K ³ ³ lead SWITCH ³
/ \ ³ / \ ³ / \ ³ T4 /³ ÃÄ)ÄÄÄÄ´ ³ ³ ³
ÀÄÂÄÙ ³ ÀÄÂÄÙ ³ ÀÄÂÄÙ ³ ÃÄÄÄ/³ ³ ³ ³ 6K8 ³
ÚÄÄÄÄÅÄÄÄÙ ÃÄÄÄÙ ÃÄÄÄÙ BRUSH Û T3 ³ ³ ³ ³ BATTERY ÄÄÁÄÄ
_())) AC1 ³ ³ ³ Slip Ring () 1K ³ ³ ³ ÛÛÛ
³ ³ ³ ³ ³ ³ ³/ \³ ³ ³
³ ÚÄÄÄÄ)ÄÄÄÄÄÄÄ´ ³ ROTARY ³ ÃÄÄ´T2 T1ÃÄÄ´ _³_
³_())) AC2 ³ ³ ³ FIELD ³ ³ ³\e e/³ ³ ////
³ ³ ³ ³ WINDING ³ ³ ³__³ ³
³ ÚÄÄÄÄ)ÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄ´ ()))))) 5V_³_ ³ ³
³_())) AC3_³_ _³_ _³_ ³ /_\' ³ ³
Star / \ / \ / \ () Slip Ring ³ 2K7 4K7
connectedÀÄÂÄÙ ÀÄÂÄÙ ÀÄÂÄÙ Û BRUSH ³ ³ ³
3 phase ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ¿
stator 6x 50A power diodes Ref Comparator _³_
windings. For high current ////
3 phase bridge.
The modern car AC Alternator with its integral rectifiers & Regulator took over
from the old DC Dynamo & intergrated regulator box in the 1970s after the high
current Silicon diode became readily available. The main advantage is a much
higher current output for the size, this is due to the heavy output winding now
being on the stationary outside stator.
WARNING LAMP
Turning the ignition switch on, the dash board lamp puts a few mA of current
through the Rotary field winding (6ê), because the voltage on the exciting rail
is lower than 14.5v, the lamp lights up.
When the engine is running this low level exciting current plus residual
magnetic field in the rotor is enough to build up the generated voltage in the
3 stator star connected windings. This is high enough to overcome the 3 phase
bridge voltage drop (1.4V) & continuously increases the excitor rail voltage
until the regulator cuts in at 14.5V. The lamp goes out.
If the lamp goes open circuit then the alternator may not self excite until
very high engine revs are used. This could cause damage to the alternator or
car electrics if the regulator is a bit slow in it's action!
REGULATOR
The voltage regulator compares the excitor rail voltage, not the actual
battery voltage, as other than the difference in the top 3 diode losses the
voltages should be the same. In this circuit a sample of the voltage is
attenuated to about 5V and compared by NPN T1 & T2 a longtail pair, with a 5V
reference. When the sample is less than 5V, T2 is on & turns on PNP power
darlington to increase the exciting field winding current. In practice there
will be a back emf diode protecting T4. When the sample is above 14.5v the
reverse happens & the excitation is reduced.
All this happens in a fraction of a second, as the engine RPM can be as low as
500 revs (due to belt pulleys 1500 on the alternator) & the alternator
regulator will be at max excitation 12v @ 2A through field winding, giving
about 10-20A only @ 13V to the battery. While a second later the engine could
be at 7000 RPM (21000 on the alternator) & the output would try to rise the
voltage to 360V @ 500A if the regulator failed to cut the excitation current
down to 1/14th of what it was.
ALTERNATOR WHINE
It is common for alternators to have 2 or more pairs of poles on the rotor &
matching sets of load windings. The battery current is fed with pulses of
current from the 3 phase rectifier.
From the above example the AC ripple frequency across the battery at tickover
will be 1500/60 x 2 x 6 = 300Hz rising to 14 times this @ full revs 4200Hz.
So it is easy to see why it takes only a small amount of these communications
frequencies across the battery to affect a Radio/Tranciever etc. The cure is
not to add C across the many Farads of battery, but to put an iron cored choke
(of a few turns) in series with the radio, assuming the radio has a C inside it
anyway.
REGULATOR FAILURE
Regulator failure, either results in no charging, or excessive charging causing
damage/symptoms usually in this order:-
a) Fan belt squealing noises when engine is revved up.
b) Blowing many car lamps.
c) Blowing up the battery.
d) Snapping the fan belt once a new battery is fitted!
A new regulator is a fiddly thing to fit, but may only cost a tiny fraction of
a replacement alternator. If you can find a matching regulator that is!
REMOTE SENSE
On larger alternators it is usual to have remote sense. This can give 2
advantages:-
1/ True Battery voltage ignoring lead losses.
2/ Current limiting to protect alternator.
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
Sense Lead ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³ LAMP ³Battery
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ)ÄÄÄÄ0ÄÄÄÄo\oÄÄÄÄ´Post
³ ³ IGN ³
6K8 6K8 SWITCH ³
D1 ³ ³ ³
T1\³ ÚÄ´<ÃÄ´ ³ ÄÄÁÄÄ
ÃÄÄ´ ³ ³ Battery ÛÛÛ
/³ ÀÄ´<ÃÄ)ÄÄÄÄ´ ³
D2 ³ ³ _³_
3K9 4K7 ////
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄ¿
_³_
////
There are 2 attenuator samples, the normal internal excitor voltage, as well as
a lead sending the actual battery voltage. The 2 attenuators are slightly
different permitting 14.5V on the battery terminals or 15.5V on the internal
sensor.
When there is more than 1 volt drop across the battery lead/high current diodes
(compared to the internal diodes) then D1 will conduct to reduce the alternator
voltage. One Volt drop across the thick battery charging lead may represent the
safe 60A current limit of the alternator.
Using D1 & D2 steering diodes rather than just resistive adding both inputs is
important so that the alternator does not blow up if remote sense lead is
disconnected!
------------------------------------------------------------------------------
Tony, G8TBF says, on some alternators there is an additional output terminal
connected to the star point of the main winding. This is used for automatic
choke - it feeds a heater winding next to a bimetal coil in the (carburettor)
choke assembly. (In effect, it's a regulated feed at half the alternator output
voltage, as long as the engine is running).
On cars with this system, a failure of an alternator rectifier diode messes up
the choke operation!
If one of the positive o/p diodes goes open circuit, the star point voltage
rises significantly & the choke goes off far too quickly. Likewise, if a diode
on the ground side fails, the star poing goes low and the choke stays on way
too long, if it goes off at all...
Why Don't U send an interesting bul?
/QSL
73 De John, G8MNY @ GB7CIP
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