Wear on the
is the most common cause of dynamo failure.
Normally the brushes wear first, but as they become smaller the spring
holding them in contact with the commutator weakens.
This leads eventually to continuous sparking - known as
- between brushes and commutator.
Arcing causes rapid wear on the commutator, pitting the surfaces of the segments. In bad cases the solder between the commutator segments and windings melts, causing output to drop and finally to cease.
output eventually drains the
- because the
taken from it by various components is not replaced by the generator.
Warning of a drop in generator output is sometimes given by the
light, which glows faintly.
If the problem is in the brushes or
, remove the dynamo for further checks and, possibly, repairs.
Checks are made easier if the dynamo is clamped securely in a vice
the vice jaws with wood so that the unit is not damaged.
To check the brushes on most modern dynamos, remove the endplate on which they are mounted.
However, some, especially older types, have windows cut in the casing through which you can reach the brushes. The windows are usually covered by a removable metal
around the casing.
The dynamo is generally held together by two long bolts through the length of the casing.
The bolt heads protrude from the endplate, and have screwdriver slots. Unscrew them to remove the endplate. You may have to give the endplate a light tap with a soft-faced hammer to get it off, or gently prise it free with a thin-bladed screwdriver.
Most dynamos have a lug under the endplate to locate it correctly. If yours does not have one, scratch some alignment marks on the endplate and casing, to aid reassembly.
Always clean inside the casing and around the
before reassembling the dynamo. Use a soft brush to remove any dust or dirt from the armature. Be sure, too, that the end
and front plate are clean, and that the ventilation holes are clear.
often has an insulating sheath fitted where it passes through the endplate: do not forget to replace it.