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:
1. Compensated Voltage Control
2. Current-Voltage Control.
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.
COMPENSATED VOLTAGE CONTROL
RB. 106 LUCAS
Fig. 1 shows a section through the RB106, and from this we can trace its build-up and operation.
Two 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. 1-4).
This 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 battery.
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.
Any 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.
Both 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 specified voltage.
Malfunction of the control box can be due to several factors:
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 AIR GAPS.
1. REGULATOR SETTINGS
Disconnect 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.
Start 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·
We 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.
If outside these limits, switch off the engine, and. adjust screw (Fig. 8-1), inwards to raise the voltage, outwards to lover it.
Once 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 charge.
Watch 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)
We 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.
2. OXIDISATION OF THE POINTS
It 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 methylated spirit.
3. AIR GAP SETTING RB. I06/2
Unscrew the fixed contact adjustment
Unlock armature-securing screws
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.
Always reset the voltage setting after cleaning or resetting.
Unscrew the armature securing screws (Fig. 11-1)
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.