Dynamo - Testing Components

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Testing the Armature, testing the Field Coils , Servicing the Commutator.

The Dynamo

The electricity required for the normal running of the bike is derived from the generator or dynamo. This is a rotating device which utilizes the principle of electro-magnetic induction.
Around any magnet there is a field of magnetic influence, and if a wire is caused to move within this field a voltage is set up across the ends of the wire, so that when the ends are joined by an external circuit a current will flow.

In a dynamo there are a large number of moving wires, mounted in coils on a rotor known as the armature which revolves in between and very close to, the poles of an electromagnet. The internal connections of the dynamo are such that the voltage induced in the rotating armature coils is applied to the stationary field coils. So that, in effect, the dynamo provides it's own magnet.

The voltage is "picked off" the armature by means of carbon blocks or brushes which press on to a drum mounted at one end, known as the commutator.
This consists of a number copper strips close together to form a cylinder, with mica insulation between the strips. The commutator also ensures a unidirectional (DC) flow of current.

Obviously, if there is a defect within any of these components then the dynamo will be less efficient or will not work at all.

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Testing the Armature

The armature can be tested for earth faults and shorted turns. One end of a continuity tester (multimeter set to measure resistance, ohms) is applied to earth and the other is touched against each of the commutator strips in turn.
If there is continuity to earth then there is a fault.

To test the armature for shorted turns, clean and service the commutator (detailed below) , this is to ensure that none of the segments are connected with each other.
When the armature is removed from the engine check that there is no continuity between segments.

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Servicing the Commutator

The commutator surface should be clean and bright, slight scratches and discolouring can be removed with wire wool or emery paper and a little metal polish. Ensure all polish etc. is removed. Deep scratches/ridges require the commutator to be turned in a lathe. The mica should be under cut (use a fine hacksaw blade) by 0.2 - 0.4 mm to a maximum width of 0.7 mm.

falsch = wrong richtig = correct

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Testing the Field Coils

Before testing for leakage to earth disconnect the adjustable resistor from the main body.
Like the armature, the field coils are checked with a continuity tester. If there is continuity between DF and earth then there is a short within the field coils. These can only be repaired/replaced by a specialist.
 
To test for shorted turns an Ohmmeter is required. The two probes are connected to positive and negative sides of the field coil, the reading should be between 1.7 to 2.1 ohm. If the reading is below 1.7 ohm, the turns are shorted. If the needle fails to deflect (no continuity) the coils are interrupted.

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