CONTROL OF D.C MOTORS:
CONTROL OF D.C MOTORS:
(OP)
I NEED TO KNOW ABOUT FIELD WEAKENING METHODS AND ARMATURE VOLTAGE, CAN ANYONE EXPLAIN IT IN GENERAL TERMS I WOULD REALLY APPRECIATE THIS MANY THANKS.
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RE: CONTROL OF D.C MOTORS:
RE: CONTROL OF D.C MOTORS:
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?
RE: CONTROL OF D.C MOTORS:
http://www.reliance.com/pdf_elements/d509.pdf
for field weakening, field pre-weakening, fixed reference point, thresholds, etc.
RE: CONTROL OF D.C MOTORS:
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.
RE: CONTROL OF D.C MOTORS:
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).
Moty