Which one is NOT the cause of the decrease in power factor?

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Examoogle · Accepted Answer

Operating synchronous motor with over excitation

Explanation: A synchronous motor operating with over-excitation acts as a synchronous condenser, which draws leading reactive power from the supply. This behavior improves the overall system power factor rather than decreasing it, making it the exception among the listed causes.

Working Principle: In an induction motor, a large air gap increases the magnetizing current requirement, which is purely reactive and lags behind the voltage, thus lowering the power factor. Operating at part load reduces the active power component while the magnetizing component remains relatively constant, lowering the ratio of active to apparent power. Conversely, an over-excited synchronous motor provides leading reactive VARs, which compensate for lagging reactive power in the system, effectively raising the power factor.

Key Formulas: $PF = \frac{P}{S} = \frac{P}{\sqrt{P^2 + Q^2}}$ — Power factor definition as ratio of real to apparent power.; $Q_{net} = Q_{load} - Q_{comp}$ — Net reactive power calculation with compensation.

Key Points: Induction motors are inherently low power factor loads due to their reliance on magnetizing current.; Synchronous motors under-excited act like inductors, while over-excited ones act like capacitors.; System voltage drops often increase reactive power demand in induction machines, further depressing PF.; Part load operation is a primary cause of poor power factor in industrial induction motors.

Advantages: Over-excited synchronous motors provide dynamic voltage support.; Synchronous condensers allow for adjustable power factor correction.

Disadvantages: Synchronous motors require a DC excitation source.; Higher initial cost compared to static capacitor banks.

Applications: Industrial power factor correction stations.; Voltage regulation in transmission lines.

Additional Info: Standard industrial practice is to maintain a power factor near 0.9 to 0.95 to minimize losses.. Option A: Decrease in voltage increases slip and current in induction motors, causing higher lagging reactive demand.. Option B: Increased air gap increases reluctance, requiring more magnetizing current and thus lower power factor.. Option C: At part load, iron losses and magnetizing current remain high relative to output, dropping the PF significantly.

Answer: D is correct — Operating a synchronous motor with over-excitation causes it to draw leading current, which improves the system power factor instead of decreasing it.

Exam Tip: Always remember the V-curves of a synchronous motor: the minimum armature current point corresponds to unity power factor; to the left is lagging (under-excited), and to the right is leading (over-excited).

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