How Does Coil Resistance Affect Choke Coil Power Factor?

A choke coil is an inductor used to block high-frequency alternating current (AC) while allowing lower-frequency or direct current (DC) to pass. A practical choke coil is not a perfect inductor; it is modeled as a pure inductance (L) in series with a small but non-zero resistance (R). This resistance comes from the many turns of copper wire used to make the coil.

The power factor (cos φ) of such a coil is defined as the ratio of its resistance (R) to its total impedance (Z).

cos φ = R / Z

Where impedance Z = √(R² + X_L²), and X_L is the inductive reactance (2π f L).

The effect of resistance is direct:

• Low Resistance (Ideal / Good Choke): A “good” choke coil is designed to have very high inductance (L) and the lowest possible resistance (R). As R approaches zero, the ratio R/Z also approaches zero. Therefore, a good choke coil has a power factor very close to zero. This is desirable because it means the coil consumes very little real power.
• High Resistance (Poor Choke): If the coil is poorly designed or has a high resistance, the value of R becomes significant compared to X_L. This increases the power factor, moving it away from zero. A higher power factor means the coil wastes more energy as heat (P = I²R), making it less effective as a choke.