Rod Hower wrote:
I agree completely with Rod. I'd like to add one small FYI note here:When I describe the 'size' of the field winding I am actually explaining the size of the field winding wire. What this comes down to is amphere-turns. The series motor has heavy gage wire that handles the same current as the armature (hundreds of amps). If the field has 100 more turns of much smaller gauge wire it still has effectively the same 'amp turns' as the series motor. The field only needs to handle 1/100 the amount of current to create the same field. This is the major advantage of the Sepex motor, you can electrically reverse and control the field with a fraction of the amps of a series system.
Internal motor area for the field winding is usually limited. In a sep-ex system it takes many more turns of much smaller wire to get the same ampere turns. Many more turns means more of the area is used by insulation and less by copper. Also, the small round wire of a sep-ex field is harder to wind compactly than the large rectangular wire of a series field. As a result it's sometimes hard to fit enough wire in some sep-ex motors and the wire is generally run a bit hotter for the same field strength. This can lead to a small efficiency penalty at some operating modes, and I suspect it might be responsible for the reputation of sep-ex systems of not having as good a stall torque as series motors. I think they still have plenty of torque for most EV use, but sometimes it shows up in situations like forklifts trying to start with a full load over a two by four.
-Otmar-
http://www.CafeElectric.com/ Home of the Zilla.
http://www.evcl.com/914 My electric 914
