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The developed electromagnetic force and/or torque in electromechanical energy conversion system acts in such a direction that tends to ___________
increase the stored energy at constant mmf
decrease the stored energy at constant mmf
decrease the co-energy at constant mmf
increase the stored energy at constant flux
decrease the stored energy at constant mmf
Quick Trick: Remember the fundamental principle: Physical systems always move toward a state of minimum potential energy. At constant mmf (current), torque acts to reduce stored field energy, whereas at constant flux, torque acts to increase co-en...
Remember the fundamental principle: Physical systems always move toward a state of minimum potential energy. At constant mmf (current), torque acts to reduce stored field energy, whereas at constant flux, torque acts to increase co-energy.
Recall the Mechanical Force Expression in Magnetic Fields
The electromagnetic force (F) in a magnetic system can be expressed in terms of field energy (Wfield) or co-energy (Wco). Under constant MMF (current i), force is given by F = -(dWfield / dx) when keeping MMF constant, or F = +(dWco / dx) when keeping MMF constant.
Analyze the Physical Tendency of the System
The negative sign in F = -(dWfield / dx) at constant MMF indicates that developed force or torque acts in a direction that decreases the field energy Wfield (stored energy). This minimizes stored potential energy in the magnetic field.
Match with Options
Since force acts to minimize stored energy when MMF is constant, the developed force/torque tends to decrease the stored energy at constant MMF.
A: Incorrect, because torque acts to decrease (not increase) stored energy when mmf is kept constant. C: Incorrect, because force tends to increase co-energy (not decrease) at constant mmf. D: Incorrect, because at constant flux, torque acts to decrease stored energy or increase co-energy.
B is correct because the developed electromagnetic force acts in a direction that decreases the stored field energy when the system is constrained at a constant mmf.
Always pay attention to whether MMF (current) or flux (linkage) is held constant in energy balance equations. Remember: dWfield is negative for constant MMF, meaning field energy decreases along movement.