Which one of the following loads has the poorest power factor?

Question

Examoogle · Accepted Answer

Induction furnace

Explanation: An induction furnace possesses the poorest power factor among the listed options because it operates on high-leakage inductance principles, requiring a large magnetizing current to create the necessary eddy currents for heating. While standard induction motors also have lagging power factors, the electromagnetic coupling in an induction furnace is significantly less efficient, leading to a much lower inherent power factor.

Working Principle: The furnace works on the principle of electromagnetic induction. A high-frequency alternating current flowing through the furnace coil creates a rapidly changing magnetic field, which induces eddy currents in the charge (metal). The inductive reactance of this coil is extremely high, resulting in a large lagging reactive power requirement, which depresses the system power factor significantly.

Key Formulas: $PF = \cos(\phi) = \frac{P}{S}$ — Definition of power factor as ratio of active to apparent power; $Q = I^2 X_L$ — Calculation of reactive power consumed by inductive loads

Key Points: Induction furnaces require large capacitor banks to compensate for the very low lagging power factor.; Power factor is typically in the range of 0.2 to 0.5 for furnace coils without external compensation.; Induction motors have better power factors, typically ranging from 0.7 to 0.9 depending on loading conditions.; Transformers operating at no-load have a poor power factor, but it is generally better than an active industrial induction furnace.

Advantages: Uniform heating of the metallic charge; High efficiency in melting processes; Clean and environment-friendly operation

Disadvantages: Extremely low inherent power factor; Requires high investment in power factor correction capacitors; High harmonic distortion due to power electronics

Applications: Steel melting; Non-ferrous metal casting; Heat treatment of metallic components

Additional Info: The power factor of an induction motor improves as the mechanical load on the shaft increases.. Transformer power factor is very poor at no-load due to the magnetizing component, but it improves significantly under load.. An induction furnace is essentially a transformer with a short-circuited secondary, leading to heavy lagging currents.

Answer: D is correct — The induction furnace relies on a high-leakage inductive coil to induce currents in the charge, resulting in a very high lagging reactive power requirement and consequently the poorest power factor among the options.

Exam Tip: In competitive exams, always remember that any device relying heavily on leakage reactance for operation (like induction furnaces or arc welders) is a prime candidate for the 'poorest power factor' category.

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