(a) air gap is not uniform.
(b) reluctance seen by stator mmf varies.
(c) reluctance seen by rotor mmf varies.
(d) reluctance seen by the working mmf varies.
Answer: (a) air gap is not uniform.
Explanation:
CONCEPT: Reluctance Torque
- Reluctance
torque is produced due to the tendency of a magnetic
system to align itself along the path of minimum reluctance.
- The
rotor moves to a position where the magnetic reluctance is minimum and
the inductance is maximum.
- Reluctance
torque does not require rotor current; it depends on the geometry
of the magnetic circuit.
Fundamental Cause: Non-Uniform Air Gap
- The primary
physical reason for reluctance torque is a non-uniform
air gap.
- A
non-uniform air gap occurs due to:
- Salient
poles
- Rotor
slots
- Variable
magnetic path length
- As
the rotor rotates, the air-gap reluctance changes with position.
Explanation of Torque Production
- Because
of the non-uniform air gap:
- Magnetic
reluctance varies with rotor position.
- The working
magnetic field (MMF) interacts with this varying reluctance.
- This
interaction produces a torque that tends to align the rotor in
a position of minimum reluctance.
- Thus,
the rotor continuously experiences torque as it seeks the least-reluctance
path.
Clarification of Options
- (a)
Air gap is not uniform → ✔️ Root physical
cause
- (b)
Reluctance changes → Effect of (a)
- (c)
Inductance varies → Mathematical consequence
- (d)
Working MMF → Necessary for torque production but not the root
cause
Final Answer
✅ Correct option: (a) Air gap is not uniform .
✔️ Key One-Line Exam Statement
Reluctance torque is produced due to a non-uniform air
gap causing variation of magnetic reluctance with rotor position.