Question 59 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is B. ### Explanation for Option B (90) The question refers to the magnetic phase differential required between the reference signal (often applied to the fixed stator windings) and the control signal (often applied to the quadrature stator windings) in a **two-phase induction servomotor**. 1. **Principle of Operation:** A two-phase servomotor operates by generating a rotating magnetic field. This rotating field is necessary to produce torque that drives the rotor. 2. **Two-Phase Requirement:** To create a perfectly rotating magnetic field (as opposed to an alternating or pulsating field), two conditions must be met: * The two sets of stator windings (the reference winding and the control winding) must be physically separated (in **space**) by $90^\circ$ electrical degrees. * The AC currents feeding these two windings must be separated (in **time** or phase) by $90^\circ$ electrical degrees. 3. **Magnetic Axes and Fields:** When the reference winding and the control winding are energized with currents $90^\circ$ out of phase, the magnetic field produced by the reference winding leads or lags the magnetic field produced by the control winding by $90^\circ$. This creates the necessary $90^\circ$ **magnetic phase differential** between the reference signal magnetic axis and the control signal magnetic axis, resulting in maximum starting torque and precise speed control typical of a servomotor. Therefore, the standard and required magnetic phase differential for proper operation of a two-phase induction servomotor is $90^\circ$. ### Explanation for Incorrect Options * **A) 45:** A $45^\circ$ phase difference would result in an elliptical or pulsating magnetic field rather than a purely rotating one. This significantly reduces the starting torque and efficiency of the servomotor, making it unsuitable for precision servo applications like automatic radio direction finders. * **C) 135:** Similar to $45^\circ$, a $135^\circ$ phase difference would produce an improperly rotating or elliptical field, resulting in poor motor performance and reduced efficiency compared to the optimal $90^\circ$ condition. * **D) 180:** A $180^\circ$ phase difference means the two magnetic fields are completely opposing each other (anti-phase) or combining linearly along a single axis (depending on the winding configuration). This configuration would prevent the creation of a rotating magnetic field, resulting in zero or near-zero starting torque, and the motor would not function correctly as a true servomotor.