AC machines concept check: In both induction motors and synchronous AC motors used in industry, which statement correctly describes the behavior of the stator's magnetic field and overall speed characteristics?

Introduction:
AC motors—both induction and synchronous—are cornerstone machines in electric drives. Despite differences in how their rotors operate, they share a fundamental principle: the stator windings create a rotating magnetic field when energized by a balanced AC supply. This question checks whether you recognize that common feature and avoid confusing it with rotor-specific details or application-dependent speed steadiness.

Given Data / Assumptions:

• Three-phase (or appropriately driven single-phase with auxiliary means) AC supply energizes stator windings.
• Balanced sinusoidal currents produce spatially and temporally varying magnetomotive forces.
• No special starting or control schemes are being considered.

Concept / Approach:
With a balanced three-phase supply, the stator currents are phase-shifted by 120 electrical degrees. The superposition of their magnetic fields yields a resultant stator field that rotates at the synchronous speed Ns = 120 * f / P (f in Hz, P in poles). In synchronous motors, the rotor locks onto this speed via DC-excited field (or permanent magnets). In induction motors, rotor currents produced by slip cause torque, and the rotor runs just below Ns, but the stator field itself still rotates at Ns.

Step-by-Step Solution:

Verification / Alternative check:
Phasor diagrams and space-vector models show a constant-magnitude vector rotating at electrical angular speed ωe = 2 * pi * f * (2/P). Practical tests (stroboscope) confirm the rotating field and resultant torque production principles in both machines.

Why Other Options Are Wrong:

• The operating speed is very steady: Often true for synchronous machines, but induction motors vary with load (slip); it is not a universal statement.
• The stator magnetic field is stationary: Incorrect; it rotates.
• The "squirrel cage" forms the rotor: True for many induction motors, not for synchronous motors; not common to both.

Common Pitfalls:
Confusing rotor behavior (slip vs lock) with stator-field behavior; assuming “steady speed” applies equally to both across all loads; overlooking the role of pole count and frequency.

Final Answer:
the stator magnetic field rotates