Just how does a dynastart work?
The basic principle of a dynastart is that it has two sets of field windings which are taped together into one physical coil. One of the field windings has few turns of thick sectional copper, like a starter motor. The other has many turns of round wire like a dynamo field coil. The armature is wound like a starter motor armature.
When the unit is first turned on it acts like a two pole series wound starter motor. Current flows from the battery through the armature and through the heavy strip-wound field coils. The physical reaction to the magnetic field causes the armature to spin.
The resistance of the field windings is very low, as is that of the armature. This means the unit will draw a large current which would not be possible if the armature and fields were wound like a traditional dynamo. This part of the design is ideal for a high torque output, required for starting the vehicle.
When the car is running, the dynastart acts like a dynamo and this is when the second winding in the field coil comes into play. As comprehensively explained in how do dynamos work, there is some residual magnetism left in the pole shoes which has a weak magnetic field. As the engine is spinning, the wires in the armature pass through these lines of magnetic flux produced by the pole shoe. This causes a voltage to be induced into the wires in the armature. There is a third brush on the dynastart which supplies the light field winding, and it charges like a dynamo. The output from the armature still goes through the heavy field windings to the output terminals of the dynastart terminal block but, as there are few turns of thick wire, this does not have much resistance and does not reduce the output by any appreciable amount.
We recommend having your dynastart fully reconditioned because modern varnishes, insulations and wires are far more effective and reliable than the materials used in the original manufacturing process.
Our experience has shown that the breakdown of the insulation on the field coils is a major fault. Overtime the wires, insulations and varnishes start to break down and this causes shorting between turns within the field coils, and sometimes to earth.
This occurrence is more prevalent in a dynastart than a dynamo because acting as a starter motor puts high currents through the windings, which generate more heat. This becomes more acute if turns short out on the field coils - or the battery is low, causing the dynastart to be carrying current for longer when it is trying to turn over.
We do not rewind the armature as standard as we find very few problems with the armatures on dynastarts - other than commutator ware - but we can rewind the armature if found necessary.