To use a series wound DC motor as a generator you need to energise the field winding with DC. This willrequire as a minimum access to the connection between the two windings. #you can then control the output voltage from the armature by varying the current through the field winding.

See:https:
//www.electricaleasy.com/2022/09/construction-and-working-of-dc-generator.html

for a description of how this type of machine works. You may be able to make it self exciting, Google “Flashing the field” (not in the field that may bring up different results:-) )

EDIT: If you have two STATOR windings ignore the one producing a weak field. It is likely a compensating winding. This may be needed once you have basic working if it is a large generator. It is connected in series with the armature winding to reduce sparking at the brushes.

Robert.

Probably a target for people to shoot down. In a series motor, all the current goes through the field. If you try to run it as a parallel generator you need a source of full current at low voltage. In a parallel wound dynamo the field is usually lots of turns of much finer wire. I’ve never heard of a series wound dynamo, I suspect it’s output voltage would be all over the place.

I can’t see why it shouldn’t work.  What are you driving it with?  Seems a lot of faff to generate 30W electrical power.  An alternative would be a beefy permanent magnet commutator motor, the kind of thing that Jason described building into a “faux cast iron” scale housing recently.  Those come in power ratings easily over 30W and don’t need a field supply.