(ET) Shunt-wound motor question

["Ken Muldrew" <kmuldrew ucalgary ca>]

My E8 has a PM motor so this question is irrelevant for that, but I have 
an old EZ-GO golf cart with a shunt-wound motor that is similar (I think) 
to the E15 so perhaps the motor people here can help (I'm afraid I don't 
know that much about motors).

The old control system on this cart consisted of 3 big resistors that 
would be switched in series with the armature (at least I think that's the 
way it worked, everything but the resistors is missing from the cart). Now 
my limited understanding of DC motors suggests that once the motor was up 
to speed with the batteries connected directly to both the field and 
armature, that additional speed could be attained at the expense of torque 
by weakening the field current so that the armature would draw more. If 
that's correct, then my further supposition is that the field windings in 
a DC motor will draw much less current than the armature. So my question 
is whether I could use one of the inexpensive scooter motor controllers to 
weaken the field under conditions where the motor is up to speed and the 
load is light (e.g. cruising on a flat road). The obvious danger is that 
if this controller was ever activated under the wrong conditions, such as 
startup, then the armature would overdraw and burn up. Probably there are 
more dangers and this is a bad idea, so I'd rather learn about them from 
the group rather than by destroying components.

Ken Muldrew.