G8MNY  > TECH     21.01.22 08:00l 147 Lines 7102 Bytes #199 (0) @ WW
BID : 58827_GB7CIP
Subj: Old Car Dynamos
Sent: 220121/0805Z @:GB7CIP.#32.GBR.EURO #:58827 [Caterham Surrey GBR]

By G8MNY                                         (Updated Sep 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is a simplified circuit.
             ͵ ͻ
  RECTIFY \ <     OVER CURRENT    ĿVOLTAGE  fuse                   
    Make                              BOOST Coil               Fuse   
             0o \oo\oĶ
                   100    OVER-                Ign IGN   Ign          
   RECTIFYĿ      \< VOLTAGE             Lamp CCT   SW         
    RELAY        \o RELAY                              Battery 
  (I coil)          Ĵ  BREAK                                      __
                                                 OVER-              ////
        WIRE       WIRE      RELAY   RELAY
            __             (12V coil)            (16V)
           /   \  DYNAMO )                     
          Armature       ) Field               
Commutators\___/0.1      ) Winding             
 & Brushes             )  4                 
                 __                      __
                ////                     ////

The DC dynamo was the original generator, it is very similar to a motor in
construction, but with electromagnets for the magnetic field. They use an
external control box to reduce the exciting field magnet to regulate the DC
output. The relays in the control box generally have a voltage coil as well as
a current coil.

Dynamos on bicycles are actually permanent magnet AC Alternators & not
"Dynamos" at all.

Dynamo batteries are designed for higher voltages & withstand more over-
charging than other batteries. Basically the battery has far more lead & water
in it, & is about twice a heavy as a modern one.

In the 70s all Car Dynamos were superseded by higher capacity Alternators with
integral Rectifiers & Regulators, they have many advantages over dynamos,
e.g. output at low revs, good regulation, resulting in only starting work for
the battery & hence smaller batteries.

On turning the ignition switch on, the dash board lamp puts a few mA through
the armature & field winding. Because the dynamo has not self excited it's
output voltage is well below the battery voltage & the lamp lights up.

When the engine speed is enough (1500 RPM) the low level exciting current plus
residual magnetic field in the rotor is enough to start the build up the
generated voltage.

When the dynamo voltage reaches 12V the RECTIFY RELAY operates, connecting
the dynamo to the battery. If this fails to happen after say the dynamo has
been replaced or worked on, the field winding should be flashed to the battery
for a second or so to refresh the residual field.
N.B. Dynamos can be used on +ve or -ve earth systems, only the residue field &
rotation direction determines which!

When the engine RPM drops below excitation drop out speed (800 RPM) the RECTIFY
RELAY releases because there is a 2nd thick high current winding that will
cancel the relay operate condition when the dynamo draws reverse current by
acting as a motor.

There are 2 types of regulator a 2 relay & 3 relay. They are really much the
same in operation. When the dynamo voltage reaches 16V the OVER-VOLTAGE relay
momentarily operates breaking the current in the dynamo field winding. This
reduces the dynamos output to the point were this relay is constantly breaking
the current to maintain the voltage. On breaking the field winding current, the
back EMF is dumped into a resistor (e.g. 100 3W) to stop sparking.

Regulators with 3 relays have one for OVER-CURRENT with just a thick high
current winding, like the OVER-VOLTAGE RELAY this also breaks the dynamo's
field winding current to limit the current to a safe level say 20 Amps for that
dynamo rating.

When the headlamps are on, it is common practice to boost the dynamo output
voltage to allow for wiring losses. This is done by supplying the headlight
power via a few turns of thick wire wound around the OVER VOLTAGE RELAY. The
OVER-CURRENT RELAY may also have a current boost headlight winding to push the
rating up to say 28 Amps at night, when it is assumed the operating temperature
is cooler for the dynamo.

If there is no charging (e.g. ign lamp stays on).

With ignition OFF, disconnect the 2 Dynamo connections. Using a car headlamp
bulb from battery to :-
1/ Armature terminal, Lamp should light brightly (same as an earth)
                                                         If not Brushes faulty.

2/ Turn dynamo around slowly (hand crank engine), lamp may flicker but never go
   out!                              If not Brushes/Commutator/Armature faulty.

3/ Field winding. Lamp should dimly light.
                        If not check connections. If brightly check for shorts.
If all OK dynamo should be OK. Now use the headlamp bulb to earth & the 2 leads
off the dynamo test the regulator with the ignition ON, connect lamp to:-

4/ The thick Lead, Lamp should not light. If it does RECTIFY RELAY contact
                              welded closed! Disconnect battery & break apart.

5/ With lamp still on thick lead, manually operate RECTIFY RELAY. Lamp should
   light fully.                        If it does not check terminals & wiring.

6/ Connect lamp to thin lead, manually operate RECTIFY RELAY. Lamp should light
   fully.    If it does not check for dirt OVER VOLTAGE RELAY contact, & if
             3 relay type the OVER CURRENT RELAY contact. Also check wiring.

7/ With the lamp still on the thin lead, manually operate both RECTIFY & OVER-
   VOLTAGE RELAYS. Lamp should go out (or very dim). If not check adjustment
   of OVER-VOLTAGE RELAY contact.

8/ If 3 relay type repeat 7 but with RECTIFY & OVER CURRENT RELAYS operated.
   Result should be same.

If 4-8 are OK the regulator it should be working. Reconnect the dynamo & retest
to see it you have cleared an intermittent or dirty connector fault.

A/ RECTIFY RELAY spring adjustment, should be set to operate @ 12.5V & release
   @ 12V. 

B/ OVER-VOLTAGE RELAY contact adjustment should be set to give 16V at full revs
   on a charged battery with no loads (other than ignition).

C/ OVER-CURRENT RELAY if fitted, with a low ohm/clip on DC ammeter in circuit.
   Connect a separate heavy load across the battery (not existing headlights!)
   to hold the battery voltage down (eg. a 30A load of 6x 5A headlamps). Rev up
   & adjust relay contact for the maximum current for the dynamo rating
   (normally stamped on casing).

Why Don't U send an interesting bul>

73 De John, G8MNY @ GB7CIP

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