Sounds like a series wound dc motor, because only series motors run away like this when lightly loaded. Which means that the load may have been reduced in some way, with a commensurate reduction in the field winding excitation. Therefore I suspect, since you have replaced both the motor and controller, that there may be some sort of 'brake device' fitted to this machine, to keep it loaded when there's no horse on board, and that it may not now be working. The simplest 'brake' would be a fan, or blower perhaps. Also, speed control is often done with these motors by shunting the motor armature with a suitable variable resistance, and mxslicks advice to check all the wiring is good to field windings and to the armture's pot shunting connections particularly, is correct.
Alan
edited to remove a double negative.
[This message has been edited by Alan Belson (edited 08-09-2005).]
