On 15/09/2019 15:44, Dave Plowman (News) wrote:
Pretty well all generators need DC applied to the field windings before
they will generate. Some older dynamos could start up on residual
magnetism then use their own output for the field - but not all.
Alternators often can't.

When we tested dynamos in old cars we would wire the output to the
field and test the voltage between output and ground at low revs, till
we saw no more than 18V. As you say this depended on residual magnetism
in the field to get it booted up (a term which I would not have used at
the time or before 1970).

This was the period when older (often 6V) vehicles would be positive
earth. To change the system to negative earth it was necessary to flash
the field with 12V before it would generate.

AJH