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G8MNY > TECH 20.11.20 00:30l 157 Lines 7892 Bytes #999 (0) @ WW
BID : 13267_GB7CIP
Read: GUEST
Subj: Car Alternators
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 201119/2224Z @:GB7CIP.#32.GBR.EURO #:13267 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Sep 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is a typical circuit.
OUTPUT RAIL Thick Lead
ÚÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
³ ³ ³ Exciter Rail Regulator LAMP º
3x 3A ÚÄÄÄ(ÄÄÄÂÄÄÄ(ÄÄÄÂÄÄÄ)ÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÂÄÄÂÄÄÄÄÂÄÄÄÄ0ÄÄÂÄÄo\oÄĶ
diodes _³_ ³ _³_ ³ _³_ ³ _³_ e\³ e ³ ³ ³ ³ thin ³ IGN º
/_\ _³_ /_\ _³_ /_\ _³_ /_\ PNPÃÄÄ\³ ³ 1k ³ ³ lead ³ SWITCHº
³ / \ ³ / \ ³ / \ ³ T4 /³ ÃÄÄ)ÄÄÄÄ´ ³ ³ ³ º
³ ÀÄÂÄÙ ³ ÀÄÂÄÙ ³ ÀÄÂÄÙ ÀÄÄÄÄÁÄÂÄÄ/³ ³ ³ ³ 6k8 Other º
ÀÄÄÄ´ ÀÄÄÄ´ ÀÄÄÄ´ ³ T3 ³ ³ ³ ³ Loads º
0.1êÚÄÄÄÄÄ´ ³ ³ BRUSH Û ³ ³ ³ ³ ÄÄÐÄÄ
_()))) AC1 ³ ³ ³ Slip Ring O 1k ³ ³ ³ ÛÛÛ
³ ³ ³ ³ ³ ³ ³/ \³ ³ BATTERY º
³ 0.1êÚÄÄÄÄÄ)ÄÄÄÄÄÄÄ´ ³ ROTARY ³ ÃÄÄ´T2 T1ÃÄÄ´ _º_
³_()))) AC2 ³ ³ ³ FIELD ³ ³ ³\e e/³ ³ ////
³ ³ ³ ³ WINDING ³ ³ ³__³ ³
³ 0.1êÚÄÄÄÄÄ)ÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄ´ ()))))) 5V_³_ ³ ³
³_()))) AC3_³_ _³_ _³_ ³ 6ê /_\' ³ ³
Star / \ / \ / \ O Slip Ring ³ 2k7 4k7
connected ÀÄÂÄÙ ÀÄÂÄÙ ÀÄÂÄÙ Û BRUSH ³ ³ ³
3 phase ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ¿
stator 6x 80A 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. Other advantages are better regulation
& output at low revs (tick over), enabiling a smaller light weight low capacity
starting type battery to be used.
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 & 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. T4 & T3 are protected from back
emf with a diode across them or to earth. 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 the 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 4 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.
BATTERYÄÄoÄoÄÄ(((()ÄÄÄÄÂÄÄ Rig C
Fuse ÍÍÍÍÍ === 1000uF
> 10mH ÃÄÄ
< 0.01ê _³_
////
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/replacement 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 sensing ignoring the lead losses.
2/ Output current limiting to protect the alternator.
..... up to 2.4V of Drop ........>
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 16.9V 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. 1.4V 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£ÿ'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 point goes low & the choke stays on way too
long, if it goes off at all...
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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