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
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.