Question

In a doubly excited magnetic systems, the magnetic torques and forces act in such a direction as to tend to ___________

(a) decrease the field energy at constant currents

(b) decrease the field co-energy at constant currents

(c) increase the field energy at constant currents

(d) none of the mentioned

The question was posed to me in my homework.

This intriguing question comes from Doubly Excited Magnetic Systems topic in portion Basic Concepts in Rotating Machines of Electrical Machines

Answer 1

Right option is (c) increase the field energy at constant currents

Best explanation: Te = ∂Wfld(is,ir,θr)/∂θr = ∂Wfld^1(is,ir,θr)/∂θr

fe = ∂Wfld(is,ir,x)/∂x = ∂Wfld^1(is,ir,x)/∂x

The positive sign in the formula indicates that force/torque acts in a direction as to tend to increase both field energy and co-energy.