Consider once again the attracted armature relay excited by an electric source as in Fig.4. The field produces a mechanical force Ff in the direction indicated which drives the mechanical system (which may be composed of passive and active mechanical elements). The mechanical work done by the field when the armature moves a distance dx in positive direction is
In this form of expression for the mechanical force of field origin, is the independent variable, i.e. it is a voltage-controlled system as voltage is the derivative of .
In linear systems where inductances are specified it is more convenient to use coenergy for finding the force developed (Eq. (25)). If the system is voltage controlled, the current can be determined by writing the necessary circuit equations.
It is needless to say that the expressions of Eqs (25) and (29) for force in a translatory system will apply for torque in a rotational system with x replaced by angular rotation θ.
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