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TECHNICAL ARTICLES © Morris Minor Car Club of Victoria Inc.
Generator, Voltage Regulator & Cut Out
Owning a Morry seems to involve a never ending series of repair jobs to keep the old car going. On the way home from the Geelong camel races I noticed that the headlights of "Grommet" seemed more yellow than normal. A quick check of the battery showed that it was almost flat. One more thing had gone wrong with my Morry! I decided to try and repair the fault myself and in the process learn how the electrical system works. These notes are the result of my research into the secrets of how generators and regulators work.
A little history.
Perhaps the most important accessory that made the public accept the car as an alternative to horses was the introduction of the electric starter motor in 1912. However, the starter motor created a new problem for the engineers of the time; it required a storage battery to operate. The lead-acid battery was the choice of car manufactures but required a means of being recharged. The battery can only be charged by direct current at a controlled rate to prevent damage to the battery. The obvious solution was to use a generator.
In very simple terms, an electrical current will be induced in a wire when it is passed through a magnetic field. Conversely, a magnetic force will be evident when an electrical current is passed through a wire. Generators work because any coil of wire rotated in a magnetic field produces an electric current.
The conventional generator consists of a rotating shaft, on which is wound copper wire, called an armature. The armature rotates between fixed coils of wire called the field windings. The field windings produce a magnetic field which in turn generates an alternating current in the armature winding. It should be noted that conventional generators can only produce alternating current in the armature. The alternating current is converted to direct current by the commutator. The commutator consists of a series of copper segments separated by insulating material on the end of the armature on which two fixed brushes conduct the current away from the armature.
However, the voltage produced by the generator increases as the speed of rotation (engine rpm) increases. Voltages can be produced that are sufficiently high to cause lead-acid batteries to overcharge, damage accessories and even generate sufficiently large currents to damage the armature winding. In addition, when the engine (and generator) are stopped the current flows in reverse from the battery through the armature winding causing the battery to flatten and possibly damage the armature winding.
The control box consists of two magnetic switches to (a) regulate the voltage output of the generator, (b) control the current from the generator, and (c) to disconnect the battery from the generator when the engine is stopped. How does it work?
It looks complicated but in actual fact is fairly simple!
The generator has two field windings that produce a magnetic field through which the armature rotates. The armature produces the electrical power to recharge the battery and operate the various accessories such as the lights and ignition coil. A voltage is applied across the field windings to produce the magnetic field. The regulator controls the voltage and current delivered to the battery and car accessories.
The voltage regulator keeps the voltage at a constant value and therefore controls the output in accordance with the requirements of the battery and any accessories operating. When the battery is low or power consuming items such as headlights are on, the generator output is near maximum. But when the demand for power is very low, the voltage regulator limits the generator output so as to protect the battery from overcharging and protect the electrical system from high damaging voltages. Click on diagram to enlarge it - and print it for future reference.
The voltage regulator consists of two winding assembled on the same core, a set of spring loaded contact points (armature) and a fixed resistance (see diagram) The voltage or shunt winding consists of many turns of fine wire connected so that the generator voltage is applied across the winding. When the generator output voltage is low (at low engine rpm), the full output voltage is applied across the field windings of the generator. When the generator voltage reaches the value at which the voltage regulator is set, the magnetic pull of the voltage shunt winding of the regulator is sufficient to overcome the armature spring tension, so that the contact points are separated. When the points are separated the generator field current then passes through the resistor. This reduces the current flow through the generator field windings and so decreases the generator voltage and current output.
The resulting reduction in the output of the generator causes a reduction in the magnetic field of the voltage winding and the contact armature is pulled back by the spring. The contacts are closed and the generator field and output increase. This cycle is then repeated and an oscillation of the armature contacts is maintained. The average value of the output of the generator remains the same even though the speed of the generator may be increased.
The second winding consists of a few turns of thick wire wound around the outside of the voltage winding. The winding is designed to protect the generator from being overloaded, for example, when the battery is low and the headlights are in use.
When the generator is delivering a heavy current, the current must pass though the winding thereby increasing the magnetic field and the pull on the armature contacts. The regulator comes into operation at a lower voltage and limits the output of the generator. Simple!
The cut-out is simply a magnetically operated switch connected in the charging circuit between the battery and the generator. The cut-out consists of two windings; a shunt or voltage winding of many turns of very fine wire and a winding of a few turn of very heavy gauge wire. The contacts are normally held open by a spring. When the speed of the generator increases, the voltage across the voltage winding increases until it reaches a pre-set value when the contact armature is pulled down which causes the contacts to be closed. The current from the generator then flows through the few turns of heavy gauge wire creating a magnetic field in the same direction as that of the voltage winding. This increases the pull on the armature so that the contacts are firmly closed and cannot be separated by vibrations.
A lead-acid battery charging rate varies with temperatures. In cold weather a higher voltage is required to charge a battery than in warm weather. A compensation device is fitted to the voltage regulator to allow for the changing temperature characteristics of the battery. The tensioning spring of the regulator contact armature is a bi-metal spring whose tension varies with temperature. By careful design of this spring the charging rate can be changed with temperature.
Having worked out how this ancient electro-mechanical system worked, the next step was to determine why grommet's battery had gone flat.
The good book said to check the generator fan belt tension and the wiring. That was all okay. Next step was to remove the wires from the 'D' and 'F' terminals and join them together (connect the field windings directly to the generator output). The voltage of the generator was then measured at terminal 'D' whilst increasing the speed of the engine - the meter reading should rise rapidly and without fluctuating to about 15 volts at 1000 rpm. Well that checked out - according to the good book the generator was okay.
The regulator can be checked without removing it from the car. The wires are removed from terminals 'A' and 'A1' and connected together - this simply enables the engine to be run by supplying power from the battery whilst isolating the generator and regulator from the car accessories and battery. A voltmeter is connected to terminal 'D' and 'E'. The engine speed is increased slowly until the needle flicks and then steadies. This should occur at about 16 volts (refer to the manual for the actual voltage as it is temperature dependant). What this test does is measure the voltage output of the generator when it is just sufficient to operate the voltage regulator when the system is not under load. The test is made when the voltage winding is cold and must be completed within 30 seconds. This tested okay in the case of "Grommet". As a matter of interest Bosch regulators are measured when the system is at full operating temperature!
The cut-out is checked by measuring the voltage at 'D' when the contacts close. It should be about 13 volts. Gromet's cut-out setting was fine!
Finally, I removed the control box from the car and tested it with a variable voltage supply. I connected a 12V 5W globe between terminals 'F' and 'D', a voltmeter between 'A' and 'E' and the variable power supply between 'D' and 'E'. As the voltage increased so did the light from the globe. A sudden change of the light from the globe to a dull glow indicated that the voltage regulator contacts had opened and the power was flowing through the resistor. This occurred at 16.2 volts which was within specification.
I could only conclude that the generator was faulty - but the fault was only evident when heavy currents were flowing. So I again checked the generator output with all lights on by connecting a wire between terminal 'F' and 'D' and measuring the voltage at 'D'. The test allowed the generator to run at maximum output without any voltage or current regulation. It showed that the voltage output was barely 11 volts proving a crook generator. The good book recommended cleaning the generator commutator with petrol and carefully polishing with fine glass paper. A nicely polished commutator and new brushes seemed to cure the fault.
Reconditioning old Control Boxes.
Whilst I was playing with the control box from "Grommet", I decided to try and restore an old one from my box full of spares. The Morris Minor Workshop Manual recommends that you clean the regulator contacts with emery cloth or a fine carborundum stone. The cut-out points should be cleaned with fine glass paper. On the other hand, Bosch recommends that the contact points should only be cleaned with a fine file as dirt left behind after cleaning with an abrasive paper will cause burning.
The only description I could find on removing the points, cleaning and resetting the points came from the official BC Morris Major Workshop Manual. I don't intend to explain the procedure as it is lengthy but it is certainly not a difficult task for the DIY mechanic. The only difficulty for most people would be to obtain an accurate volt meter. I found that digital voltmeters were not satisfactory when testing the control box in the car as, when set to the DC range, they appeared to have a very fast sampling time and gave wildly fluctuating results. This was probably due to "noise" induced by the commutator and voltage regulator contacts. However, if the regulator/cut-out is removed from the car and adjusted using a variable voltage supply; a digital meter is ideal to measure the voltages.
Three Bobbin Regulators.
The two bobbin control box as fitted to the Morris Minor has a distinct disadvantage compared with a three bobbin box. The three bobbin control box has separate bobbins for regulating the voltage and current; the third bobbin operates the cut-out switch.
With a two bobbin control box, as you adjust the voltage regulator you are also altering the current limit cut-out. It is therefore more difficult (impossible!) to adjust the setting so that both the voltage and current setting are optimised. In other words, the setting is a compromise. A three bobbin control box, with separate bobbins for voltage and current regulation, does not suffer from this fault. Apparently, BMC continued to use the two bobbin control box to save the additional cost of fitting three bobbins boxes.
A design fault of the Morris Minor that can cause the generator to burn out should be mentioned. The 'E' or earth terminal of the control box also connects to the earth terminals of the windscreen wiper motor and the interior lamp. If the earth 'E' on the control box becomes disconnected from the chassis and the interior light or wiper motor is left on (with the engine not running), it is possible for current to pass through the shunt winding of the cut-out in the reverse direction causing the cut-out points to close. This will cause current to flow from the battery through the generator and burn out the armature winding. If you are restoring a Morris Minor it is a good idea to change the wiring so that the interior light and windscreen wiper motor have separate earth's to the control box. Better still, fit an alternator to the engine!
If your car has a lot of electrical gadgets and widgets consuming heaps of electrical power, then seriously consider swapping the generator for an alternator.
The generator cannot cope with the modern heavy electrical demands of modern cars. The output is too low, it can only operate in one direction, the high centrifugal forces acting on the rotating armature windings prevent high speed operation and commutation is not completely reliable when heavy currents are generated.
In contrast the alternator is light and has a high output, can operate in either direction, can operate at very high speeds and the brushes only carry a very small current to the field windings.
The alternator is able to achieve these excellent characteristics because the heavy rotating armature winding of a generator is replaced by a fixed armature winding in the alternator. It uses a rotating magnetic field in an assembly called a rotor and a fixed armature winding called the stator windings. The rotor assembly is light and designed to rotate at high speeds. The alternator produces only alternating current which must be rectified to direct current. This is accomplished by the use of solid state diodes. With a higher crankshaft pulley ratio, the alternator can be driven at a speed sufficient to charge the battery when the engine is idling and yet continue to operate with complete safety at maximum engine rpm.