The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: Peter Gee on 24.10. 2018 19:09

Title: Field Coil F or D Identification and Testing Question
Post by: Peter Gee on 24.10. 2018 19:09
Looking at a field coil from the brushes end,  which is the F and the D wire? Mine are unmarked, but they exit the coil itself left and right when looked at with the commutator cap removed.

There is an ohm test of F and D with F giving 6 Ohms constant and D giving a fluctuating Ohm reading which is also said to test segments of the armature - not quite sure how... would a fluctuating reading on segment A followed by no fluctuation on the next segment B show an armature fault? If not is there any way to test an armature using a normal good Ohm meter?

Lastly, in order to bench  test the dynamo output by driving the armature, am I right in thinking the regulator AND battery must also be correctly hooked up to the  dynamo "as if it were on the bike"?


Peter in Kenya
Title: Re: Field Coil F or D Identification and Testing Question
Post by: trevinoz on 24.10. 2018 21:30
The field leads do not have any set identity.
One lead goes to the F terminal, the other to earth or frame.
Which is which depends upon the rotation of the generator.
I have found that usually bringing the leads straight up through the plate, connect the left one to the frame and the right to F. This is with the leads at the bottom looking at the generator.
This should give left hand rotation, but if the armature has been rewound it may not.
Trial and error is the way to go.
Testing the armature is either by growler or "drop testing" where a low voltage is applied across the commutator and the voltage drop across each adjacent pair of segments is read.
They should be approximately the same for every pair. And the same polarity.
The windings should also be tested for short to earth with an ohmmeter or megger.
To bench test, bridge the F & D terminals and drive in the direction of rotation. You should get an output far in excess of 6 volts, depending on the speed.
You may have to flash the field before you start. Just connect a battery to the bridged terminals and frame in the correct polarity. The generator should rotate in the correct direction. If not, reverse the field connections.
It is always neater to bring the leads out as described. If you have to reverse, cross them under the plate.