Magnetic circuits – necessity of magnetomotive force (mmf) To establish magnetic flux in a magnetic circuit or air gap, is a magnetomotive force (mmf) necessary?

Introduction:
Magnetic circuits are often analyzed by analogy to electric circuits. Magnetomotive force (mmf) is the driving “cause” of flux, just as electromotive force drives electric current. Recognizing the role of mmf helps in sizing windings, currents, and gaps in devices like transformers, relays, and motors.

Given Data / Assumptions:

• Magnetic circuit with ferromagnetic path and/or air gaps.
• Steady or slowly varying fields (magnetostatics/quasi-static).
• mmf provided by current in turns (N * I) or by permanent magnets (intrinsic mmf).

Concept / Approach:

Flux Φ in a magnetic circuit follows Φ = F / R_m, where F is mmf and R_m is magnetic reluctance. Nonzero flux requires nonzero net mmf, whether supplied by a coil (N * I) or by the magnetization of a permanent magnet (equivalent mmf). Even with high-permeability cores, some mmf is required, particularly to overcome air-gap reluctance.

Step-by-Step Solution:

Verification / Alternative check:

Devices with permanent magnets demonstrate that magnetization provides mmf without external current, yet mmf is still present in the magnetic circuit description.

Why Other Options Are Wrong:

Claiming mmf is unnecessary contradicts the fundamental relation; limiting necessity to air gaps or AC is incorrect, as DC magnetization also needs mmf.

Common Pitfalls:

Confusing permeability with absence of required mmf; neglecting air gap dominance in mmf budgeting.

Final Answer:

True