The electromagnetic torque T_em or the electromagnetic force F_em provided by an electromagnetic convertor, can be expressed as being the partial derivative, depending on the rotor or the relay armature's position and on the magnetic co-energy, the latter being expressed according to the currents which flow through the differrent windings of the electromagnetic convertor.

For a rotating convertor we can write :

For a lineary convertor:

The W_cmag magnetic co-energy can be calculated by integrating the fluxes expressions of the different windings depending on the flowing currents. For a n convertor we can write :

• in the case of a rotating convertor:
• in the case of a lineary convertor:

In the writing of the flux-currents relations it is, in some cases, required the notion of magnetic circuit (as well as the notion of reluctance which is associated to it), because this notion allows the direct deduction of y_k(i₁,i₂,...,i_n,q_m) or y_k(i₁,i₂,...,i_n,x_m) relations depending on the geometry of the studied device and on the permeability of the materials it is made of.