Question 17 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is B ### Why Option B is Correct **B) The motor is started as an induction motor.** A large polyphase synchronous motor cannot inherently self-start under synchronous conditions. To initiate rotation, these motors are equipped with **damper windings** (or amortisseur windings), which are shorted conductors embedded in the face of the rotor poles, acting like the bars of a squirrel-cage induction rotor. When AC power is applied to the stator, the rotating magnetic field induces current in the damper windings. This results in an induction motor torque that accelerates the rotor up to near-synchronous speed (typically 90% to 95%). Once near speed, DC excitation is applied to the field winding, pulling the rotor into exact synchronism with the rotating magnetic field. Therefore, the motor uses the induction principle for starting. *** ### Why Other Options are Incorrect **A) Resistance is gradually added to the rotor circuit.** This statement describes the starting procedure for a **wound-rotor (slip-ring) induction motor**. Standard synchronous motor rotors (using salient poles) do not have external variable resistance circuits for the starting damper windings. **C) The starting current is held below the rated current.** Starting a large motor, even when started as an induction motor (using damper windings), requires substantial current to produce the necessary starting torque. Large synchronous motors typically draw a starting current that is several times (often 5 to 7 times) the full-load rated current. While reduced-voltage starting methods (like autotransformers) are used to *limit* the current, it is not held *below* the rated current for practical operation. **D) The field winding is energized for starting purposes only.** The field winding (DC excitation) is what creates the main rotor magnetic poles. It must be energized *after* the motor is nearly up to speed and must remain energized continuously for the motor to operate at synchronous speed and maintain torque. Furthermore, if the field is energized during the initial low-speed start, it would likely impede the starting process and potentially induce dangerously high voltages in the field winding due to transformer action.