First off you'd need to add 2 more connections. Adding just "a third connection" will not provide correct polarity for both S and S-P connection of the 4 coils.

Secondly, the S and S-P configuration of the fields on the series motor gives you 2 modes with a 2 to 1 difference in field strength. Typically this is too wide of a swing for "good" performance in both modes (strong field and field weakened).

There are motors which are designed to be operated in this manner with a 2 to 1 difference in field strength. They have a bit too strong of a field in S mode and a bit too weak field in S-P mode. Your motor obviously was not designed for this compromise so in your altered field mode you're likely to be way off and performance would suck or the fields would over heat or commutation would arc.

Cheers,

major

 

thanks major. much appreciated.

 

thanks for posting the photos.

Can you tell by looking--is that coil wound "6-in-hand", that is to say, 6 wires held together and coiled together as one? Or possibly more than 6 ? i couldn't really tell from the photos. Is it flat or round wires?