Question 64 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is A. **Explanation for A (Correct Option):** The primary rotating magnetic field (RMF) in a three-phase induction motor is established when a balanced three-phase alternating current (AC) voltage is applied to the three separate but spatially offset stator windings. Due to the phase difference (120 electrical degrees) in the currents and the physical displacement (120 mechanical degrees) of the windings, the resulting magnetic fluxes combine to create a resultant magnetic field that rotates at synchronous speed. This rotating magnetic field is essential for inducing current in the rotor and subsequently generating torque. **Explanation for Incorrect Options:** **B) interaction of the magnetic field caused by the induced current in the squirrel-cage bars with the magnetic field of the stator:** This interaction is what *produces the mechanical torque* that turns the rotor, but it is not what *establishes* the initial rotating magnetic field itself. The RMF must exist first to induce current in the squirrel-cage bars. **C) laminated steel core and aluminum conductors in the rotor:** This describes the *construction* of the squirrel-cage rotor (designed to minimize eddy currents and provide a path for induced currents), but it does not explain the *establishment* mechanism of the rotating magnetic field, which occurs in the stator. **D) current induced in the rotor windings:** The current is induced in the rotor (squirrel-cage bars) *because* of the primary rotating magnetic field cutting across them. Therefore, the induced current is a *result* of the RMF, not the cause of its establishment.