What is it about field weakening methods that you want to know? Are you interested in SCR field control or MG set control or what? The methods of field control are similar to that of any DC bus voltage control. Are you interested in digital controls or analog controls? The old mag-amp stuff was different altogether. What specifically do you want to know about?
Maybe my basic understanding could possible give a basic answer since I am not "fogged" by extraneous knowledge. I only hope I am correct.
In a DC motor the armature voltage controls speed and the armature current controls torque. The stator can be a fixed field strength by different means such as either a fixed voltage or permanent magnets. Or, the stator could have connections to the outside world where you can vary the field strength by varying the stator voltage.
Anyone care to make correcctions to my simple answer?
well ...i think i can help you with some equations from which you may understand field weakening for separatly excited dc motors you got three main equations :
field circuit eqn.:v(f) = I(f)*r(f)
I(f):field current
r(f):feild resistance
i neglected the reactance for steady state operation
armature circuit eqn.:v(a)=E(b)+I(a)*r(a)....(2)
v(a):armature voltage
E(b):back e.m.f
I(a):armature current
r(a):armature resistance
E(b)=k*N*F.....(3)
k:cnstant
N:speed of motor in R.P.M
F:flux
from 2 & 3 you can get that:
N=( v(a)-I(a)*r(a) )/( k*F )
thus from the above eqn. you can control the motor speed by controlling ( F ) the field which is called field weakening process or by controlling V(a).
i hope this could help you enough.
DC motor with shunt field (stator winding powered from external circuit) runs at constant torque within control range (i.e. control range 1:10 yield constant torque from 10% nominal speed to 100% nominal speed). The control range is given in motor data sheet.
By controlling (reducing) field current, motor will operate beyond nominal speed in constant HP mode (torque decreases proportionaly to speed increase). IBESHIR showed the math to explain the field weekening phenomenon. Reliance Electric and some othe manufacturers will offer means of field control, but you have to make sure motor can withstand the top speed (otherwise you have to limit it), and that the machine will not stall with decreasing torque (application oriented).
