Skip to main content

Low power factor causes, disadvantages

Short answer

Power factor = cos φ = P/S. Causes: induction motors, transformers, discharge lighting, arc furnaces, light loads. Disadvantages: higher current, increased losses, poor regulation, larger equipment, reduced capacity, and penalty tariffs.

Define power factor and explain the disadvantages and causes of low power factor?

Power factor is the cosine of the phase angle between the voltage and the current, cos φ = P/S = kW/kVA.

Causes of low power factor

  • Induction motors, especially when lightly loaded or idling. They draw a large magnetising current.
  • Transformers running at no load or light load.
  • Fluorescent and discharge lamps with inductive ballasts.
  • Welding sets, arc furnaces and induction heating equipment.
  • Rectifiers and other nonlinear loads (distortion).
  • Variation in load, and operating the supply at a voltage higher than the rated value.

Disadvantages of low power factor

  • Large current for the same power. I = P/(V cos φ) for single phase, or P/(√3 V_L cos φ) for three phase. A low cos φ means a larger current.
  • Higher copper loss. Loss = I²R rises as 1/cos²φ in the cables, transformers and generator windings.
  • Poor voltage regulation. The large current causes a bigger voltage drop (IR and IX) in lines and transformers, so the load voltage falls.
  • Larger equipment size and cost. Generators, transformers, switchgear and cables must be rated in kVA, not kW, so they are bigger and costlier for the same useful power.
  • Reduced capacity. The same kVA rating delivers less kW (kW = kVA × cos φ), so a plant cannot supply as many loads.
  • Lower efficiency and heavier tariff. Utilities usually charge a penalty on low power factor (below about 0.8 to 0.9).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Basic Electrical Engineering chapter 7 questions →