In electrical engineering, a synchronous motor can be operated at which of the following power factors under normal conditions?

Introduction / Context:
Synchronous motors are important machines in electrical engineering, widely used in industry for high-power applications and for power factor correction. This question checks your conceptual understanding of how a synchronous motor operates with respect to power factor. Knowing whether it can only run at one type of power factor, or can be adjusted, is crucial for both examinations and practical electrical system design.

Given Data / Assumptions:

We are given:
- Several possible power factor conditions: lagging, leading, unity, any power factor, or zero power factor only.
- We consider a standard synchronous motor operating with appropriate excitation and load.

Concept / Approach:
A synchronous motor runs at constant synchronous speed determined by supply frequency and number of poles. Its power factor can be controlled by changing the field excitation:
- Under-excitation usually leads to lagging power factor.
- Normal excitation gives approximately unity power factor.
- Over-excitation can make the motor draw a leading current and operate at a leading power factor, similar to a synchronous condenser.
Therefore, by properly adjusting the field excitation, a synchronous motor can be made to operate at lagging, leading, or nearly unity power factor as required by the system.

Step-by-Step Solution:
Step 1: Recall that the stator of a synchronous motor is connected to the AC supply, and the rotor is excited by DC.
Step 2: Understand that changing the rotor (field) excitation changes the magnetising current drawn from the supply.
Step 3: For under-excitation, the motor behaves like an inductive load and operates at lagging power factor.
Step 4: For normal excitation, the motor operates close to unity power factor.
Step 5: For over-excitation, the motor behaves like a capacitive load and operates at leading power factor.
Step 6: Therefore, a synchronous motor can operate at any desired power factor (lagging, leading, or unity) within design limits.

Verification / Alternative check:
Electrical machines textbooks and laboratory experiments on synchronous motors clearly demonstrate V-curves, which show how stator current and power factor vary with field current. These curves confirm that the power factor can be varied from lagging through unity to leading by changing the field excitation, making the machine extremely flexible in reactive power management.

Why Other Options Are Wrong:
- Lagging power factor only: Incorrect, because over-excitation can produce leading power factor.
- Unity power factor only: Wrong; unity is only one operating condition among many possible.
- Leading power factor only: Also incorrect; under-excited operation gives lagging power factor.
- Zero power factor only: Not correct; the motor is not limited to purely reactive operation and normally supplies real power to mechanical loads.

Common Pitfalls:
Students often confuse synchronous motors with induction motors. Induction motors typically run at a lagging power factor, while synchronous motors are special because they can be tuned to the desired power factor. Another confusion is thinking that leading power factor is only for capacitors, but a synchronous motor with over-excitation can also behave like a synchronous condenser and supply reactive power, improving system power factor.

Final Answer:
Correct option: At any power factor (lagging, leading or unity)