(a) to increase the stored energy at constant flux.
(b) to decrease the stored energy at constant flux.
(c) to decrease the coenergy at constant mmf.
(d) to decrease the stored energy at constant mmf.
Answer: (b) to decrease the stored energy at constant
flux.
Explanation:
CONCEPT: Direction of Force / Torque in Electromechanical
Energy Conversion
- In
an electromechanical energy conversion system,
electromagnetic force or torque is produced due to the
interaction between the magnetic field and the mechanical
system.
- The direction
of the developed force or torque is governed by energy
conservation principles.
- The
system always tends to move toward a position where the stored
magnetic energy is minimum, when flux (or flux linkage) is
kept constant.
Energy-Based Interpretation
- When flux
(or flux linkage ψ) is constant, the electromagnetic force or torque
acts in a direction that:
- Decreases
the stored magnetic energy.
- This
represents the natural tendency of the system to reach
a stable equilibrium with minimum stored energy.
Coenergy Interpretation (Alternative View)
- Alternatively,
if the magnetomotive force (mmf) is kept constant:
- The
force or torque acts in a direction that increases the magnetic
coenergy.
- Both
interpretations are physically equivalent and lead to the
same force direction.
Force Expression (Energy Method)
- The
electromagnetic force can be expressed as:
Where:
= electromagnetic
force
= stored magnetic
field energy
= mechanical
displacement
= flux linkage
(constant)
Explanation of the Relation
- The negative
sign indicates that the force acts in a direction that:
- Reduces
the stored field energy.
- As
the displacement increases,
the stored magnetic energy decreases, provided the flux
linkage remains constant.
- Hence,
the electromagnetic force naturally drives the system in that direction.
Torque Form (Rotational Systems)
- For
rotational systems, a similar relation holds:
Where:
= electromagnetic
torque
= angular
displacement
Final Conclusion
- At
constant flux (or flux linkage):
- Electromagnetic
force/torque acts to decrease stored magnetic energy.
- At
constant mmf:
- Electromagnetic
force/torque acts to increase magnetic coenergy.
- This
principle explains the direction of motion in motors,
relays, solenoids, and other electromechanical devices.
✔️ Key One-Line Exam Statement
Electromagnetic force acts to minimize stored magnetic
energy at constant flux and maximize coenergy at constant mmf.