LUCAS GENERATOR
(a.k.a. DYNAMO)
It has been said many times that, when Joseph Lucas & Sons. designed the DC dynamo, they did so with neither an excess of copper nor steel.
Just after the turn of the century as motorized transportation was developing, motorists became aware of the dangers of driving about after dark. (The quotation "A gentleman does not motor about after dark." has been attributed to Joseph Lucas, but I cannot confirm if this is true.) Acetlyene lamps were mounted on vehicles and served more to warn other motorists of your approaching, and of limited value to see where you were going.
As maximum speeds increased it became necessary to have more efficient lighting. By this time, Thomas Edison (an American) had already invented the electric light bulb which could be powered by a lead/acid battery. The battery was a DC (direct current) device, so now we need a DC generating device to keep the battery charged. No problem says Lucas, and they began manufacturing DC dynamos for the British transportation industry.
The DC voltage generated by a DC dynamo is governed by three factors, and each factor has limitations.
1) Number of turns of wire on the armature. This value is fixed by the physical size of the armature,and by the size of the wire. The more turns, the higher the voltage. The larger the wire, the higher the current.
2) Magnetic field developed in the field coil. This depends upon the number of turns of wire and the current passing through the wire. The output voltage will rise until the magnetic saturation of the pole piece is reached.
3) The rotational speed of the armature, which is governed by the engine speed.
Generator styles
Although there were a myriad of variations, the most common styles of generators are as follows-
E3H (short) Rated at 48 watts. Recognized by having only one screw holding field coil pole shoe.
E3L (long) Rated at 60 watts. Recognized by having two screws holding field coil pole shoe.
Keep in mind that Lucas generators were designed and built in England, which has a rather cool climate, and that power output ratings are nominal. Output ratings might be de-rated 10% for applications where the generator is mounted to the rear of engine, used in southern temperate climates, or south of the Mason Dixon line.
What's wrong with my generator?
Each time the generator begins rotation, it generates a small voltage using the residual magnetism from the last time it ran. This small voltage is then fed to the field coil, which induces more magnetism. The increased magnetism then creates a higher voltage in the armature. Thus the generators literally lifts itself by it's own boot straps. When the desired voltage is reached, the voltage regulator kicks in to control the maximum voltage output.
Possible problems
Although the DC generator is a relatively simple instrument, there are several things that can go wrong.
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Open winding in armature. This test will require a DVM (digital volt meter). Because the windings are connected in series, you must measure the resistance between each adjacent commutator copper. And because this is a relatively small coil of large diameter wire, you can expect only a fraction of an ohm resistance. You must find the same resistance between each adjacent pair of commutator coppers. If you have an open coil, you will read to total resistance of the remaining coils in series. Look for evidence of overheating and/or thrown solder, as a wire may have become unsoldered from the commutator copper. This is most often caused by overloading the generator, or a defective regulator.
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Armature coil shorted to ground. Check for continuity between the commutator copper to armature body. Continuity will indicate a shorted coil. Most commonly caused by overheating and charring of insulation.
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Armature coil shorted internally to itself. This can only be detected by a "growler" so you might have to take it to a local armature shop.
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Defective field coil. Common problem is overheating due to sticking points on the regulator. The insulation becomes charred and flakes off with time and vibration. Nominal resistance shoud be around 2.8-3.2 ohms. A visual inspection is a good idea and will require unwrapping some of the fabric wrapping. If there is any visual indication of charring, the field coil should be replaced.
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Note- all tests below are done assuming your electrical system is Positive Earth (ground).
Testing the generator
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Remove the generator from the bike.
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Connect a temp. jumper between "D" and "F" terminals.
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Connect one end of another wire to the hot terminal of a battery. Touch the other end to the D-F terminal jumper. The generator should motor smoothly, and in the direction of the arrow on the case. (assuming you have the correct generator for the bike). If the armature rotates in a jerky motion, then you most likely have an open armature winding and the armature must be either replaced or rewound If it rotates very slow, you may have a defective field coil.
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By doing the above test, you will have determined if the generator is healthy, and you will have polarised it for positive earth. Be sure to remove the D-F jumper.
Another test (this can be performed with generator mounted on bike)
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Disconnect the regulator from the generator.
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Connect a temp. jumper between "D" and "F" terminals.
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Connect an analog volt meter between this jumper (negative) and earth (positive).
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Start the bike, but do NOT rev the motor higher than a fast idle.
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Because the output is unregulated, voltage will depend on rpm. You should read maybe 5-10 volts. Do not run over 10 volts for more than a minute or you can burn out the generator.
Flashing the field (this can be performed with generator mounted on bike)
Because the pole shoe is soft iron, it won't hold the magnetism for an indefinite amount of time. This is especially important when using a solid state regulator because they require a generator output of 0.7 volts in order to initialize.
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Disconnect the voltage regulator from the generator.
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Connect a temp. jumper to the "hot" terminal of the battery.
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Touch the other end of this jumper to the "F" terminal on the generator. Maybe touch it several times holding no more than a second. This should be enough to re-establish the field magnetism.
