The main characteristic of
the plain shunt connected generator is that the output rises with increasing
speed and if this were uncontrolled irreparable damage would be caused to the
generator and the battery.
The systems currently used to
control this output are:
Compensated Voltage Control
It will be remembered that
battery terminal voltage varies with the state of charge, so if we control the
voltage of the generator at a specific level i.e. terminal voltage of a fully
charged battery, then the pressure differential between the generator and the
battery will be greatest when the battery is discharged allowing a high
current to flow, reducing as the battery becomes charged, until theoretically
the pressures will be equal and no current will flow,
If the battery was in a very low state of charge, the current flow could be extremely high, most probably higher than the generator could safely deliver, and the armature windings would be damaged. Because of this, we have to use some form of COMPENSATION.
At low speeds, the generator
output is less than the battery voltage, so an automatic switch or cut-out is
incorporated in the circuit to prevent the battery being discharged through
the armature winding.
RB. 106 LUCAS
1 shows a section through the RB106, and from this we can trace its build-up
soft iron cores (Fig. 1-1) are
mounted on an 'L' shaped soft iron frame (Fig. 1-2). (For simplicity,
the frame is shown in two parts)
An. ‘L’ shaped armature (Fig.
1-3) carrying a contact Point (Fig. 1-5)
is mounted above each core on a spring blade (Fig.
contact is so positioned to line up with a stationary contact (Fig. 1-6)
insulated from the frame. Another spring blade (Fig. 1-7) is attached to the vertical arm and lines up with fine
threaded screws (Fig. 1-8), which
can be adjusted to vary the force required to move the armature.
You will see that the L/H points are held in the closed position by the spring blade (Voltage Regulator) and the R/H points are held open by the spring blade (Cut-out).
In Fig. 2 we can see the circuit from generator
terminal (D) to the frame, through the regulator contacts, and back to the
generator terminal (F); this means the generator field is connected and there
will be an output from the generator. To regulate this, a shunt coil (Fig.
2-1) is wound round the core with one end connected to the frame (dynamo
potential) and the other to earth. When current flows through this coil,
magnetism tends to pull the armature down against the spring; at a given
voltage it will overcome the spring and the O & F connection will be
broken. Because the generator output now falls, current through the shunt coil
will also fall, reducing the magnetism until spring tension closes the
contacts again. This cycle is repeated approximately 60 - 100 times per
second, which gives a steady control over generator voltage.
The controlled voltage can
easily be set to the required level by using the screw (Fig. 2-2) to adjust the spring tension. If we break the circuit
when the field current is passing
considerable arcing will take place across the regulator points, also the
generator field will be slow to collapse. To prevent this, a resistance
(Fig. 2-3) is placed in parallel
with the points.
The operation of the
'cut-out' (Fig. 3) is similar to
that of the regulator, except that the points are spring loaded open, and the
magnetic pull draws them together. A similar shunt Coil (Fig.
3- 1) is wound around the cut-out core with one-end connected to the frame
and the other to earth. Spring tension is set so that the points close when
the generator output is Just above nominal terminal voltage of the battery and
open again to disconnect the battery as the generator output falls.
Fig. 4 shows a constant
voltage control circuit where Output from the 'D' terminal goes to the
regulator frame, the cut-out points are pulled together, and current is
passing around the heavy series winding (Fig.
4 - 1) on the cut-out bobbin and on to terminal 'A' which is connected to
The purpose of the series coil is to add strength to
the shunt coil. Once the contacts have closed, current passing to the battery
along the series winding strengthens the magnetic field and prevents the
contacts 'bouncing' or 'chattering'. Also, when generator output falls below
battery voltage, the current reverses in the series winding, causing the
magnetic field to collapse quickly and open the cut-out points.
The generator voltage then builds up until it reaches
the pre-set level when the regulator points operate.
The problem that would now arise is that if the battery
was in a low state of charge, the pressure differential would be too great,
and the heavy current flow could damage the armature. To prevent this, a
series coil (Fig. 5 - 1) is wound
around the regulator bobbin in the circuit from the cut-out series coil to the
'A' terminal; this means that all the current passing from the generator to
the battery passes through these compensating turns, so adding to the magnetic
field of the shunt coil. The heavier the current, the stronger the magnetic
field and the sooner the regulator points open. This lowers the operating
voltage of the dynamo and restricts the current flow to a safe limit.
extra load - e.g. lights, heater, wipers, etc., - must also be catered for,
and extra turns, called load turns (Fig. 6-1), may be added
to the regulator series coil and taken out to A1, which is connected to the
ignition switch, lights, and accessories fuse.
shunt coils consist of many turns of' fine wire, the resistance of which
varies with changes in temperature. As temperature increases, resistance
increases, and due to the reduction in current flow, magnetism will be
lowered, and therefore the voltage required to operate the contacts would
increase. To counteract this, each armature tension spring has a bi-metal
strip fitted (Fig. 6- 2). these cause the spring tension on the
armature to fall as temperature rises - and so maintain control at the
Malfunction of the control box can be due to several
1. Incorrect electrical settings.
2. Oxidization of the points.
3. Incorrect air gaps.
N.B. THE REGULATOR SETTINGS SHOULD ALWAYS
BE CHECKED BEFORE DEALING WITH THE CONTACT POINTS AND
A and A1 leads, this will disconnect the battery from the generator and take
the series windings out of circuit.
Connect a voltmeter between the regulator frame or 'D' terminal and earth. Join the A and A1 leads to provide an ignition feed.
the engine and run up to charging speed: the voltage reading will increase
until the setting point of the regulator is reached and there should then be
no further Increase· If the voltage does not conform to specification for the
particular model, turn screw (Fig.7-1) inwards to increase the voltage, or
outwards to lower it, then re-check the reading·
must now check the operation of the cut-out. Leaving the voltmeter connected
as before, connect in ammeter between control box ‘A’ and the
disconnected leads, Switch on the headlamps, start engine, and slowly Increase
speed. The voltage reading will rise steadily, and when the contact points
close the voltage will drop back and then rise again. The point reached just
before the drop back should be between 12.7 and 13.3 volts.
outside these limits, switch off the engine, and. adjust screw (Fig. 8-1), inwards to
raise the voltage, outwards to lover it.
the cut-In voltage is correct, the reverse current should be checked. Leaving
the ammeter and voltmeter connected as before, and with headlamps still on,
run the engine at charging speed, ensuring that the ammeter is indicating a
the ammeter carefully as you slowly reduce engine speed. The ammeter should
register a slight discharge - of approximately 2 to 5 amps before the cut-out
points open - before return to zero
CIRCUIT VOLTAGE DROP (SUPPLY LINE)
should now check the supply line from dynamo to battery for high resistance.
Remove the 'D' lead at the dynamo, and connect the ammeter into the circuit.
Start and run engine, increasing speed until ammeter indicates 10 amps.
Connect voltmeter between ‘D’ terminal of dynamo and the battery Insulated
terminal, and the voltmeter reading should not exceed 0.75 volt.
OXIDISATION OF THE POINTS
is Important to note that the regulator points are made of tungsten, and
should be cleaned with carborundum stone or silicon carbide paper, but the
cut-out points are made of silver, and should only be cleaned with fine glass
paper. All dust should be removed, preferably with a cloth soaked in
AIR GAP SETTING
the fixed contact adjustment
Insert 0.021" feeler gauge between armature
and core face, Press armature down squarely against the gauge and re-tighten
armature fixing screws (Fig. 10-2)
Leaving gauge in position, screw the fixed contact
down until it just touches the moving contact, and tighten locknut.
reset the voltage setting after cleaning or resetting.
Unscrew the armature securing screws (Fig.
Press the armature down on the core face and
re-tighten the securing screws. Bend the stop arm (Fig. 11-2) to give a
gap of 0.025" to 0.040" to the armature tongue, with the armature
held down. Release the armature and set insulated contact arm (Fig.11-3) to
give a contact "follow-through” of between 0.010” to
0.020" when armature is pressed against core face.