Electronic – Measuring the resistance of a real inductor

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Real inductors can be represented as perfect inductor plus a series resistance \$R_s\$.

The impedance of this component is \$Z_{L+R_s}=R_s+j \omega L\$ therefore $$\frac{1}{Z_{L+R_s}}=\frac{1}{R_p}-\frac{j}{Z_{Lp}}$$

With \$R_p=\frac{R_s^2+(\omega L)^2}{R_s}\$ and \$Z_{L_p}=\frac{R_s^2+(\omega L)^2}{\omega L}\$

Which means that the same system can be seen as the parallel of a perfect inductor \$Z_{L_p}\$ and a resistance \$R_{p}\$.

I never tried actually, but if I use a RLC bridge or simply a ohmeter to measure the resistance of a real inductor (the same way as if it was a resistor), what do I measure? \$R_s\$ or \$R_p\$? Or I do not measure anything?

If I measure \$R_s\$ will the measurement depend on the frequency of the RLC bridge?

Best Answer

If I measure \$R_s\$ will the measurement depend on the frequency of the RLC bridge?

Generally, your bridge measurement will be larger than an ohmmeter measurement. How much larger depends on the quality of the inductor.

  • Skin resistance of the inductor's wire will increase at higher frequencies.
  • Losses in an iron core usually increase with frequency.
  • B-H nonlinearity of core material affect both inductance and resistance.

Some bridges allow measurements of either series RL, or parallel RL models. The more common setup is series RL.
Note that you can detect core saturation by varying the bridges' AC amplitude. You may find that small amplitude yields high-Q (small series resistance), but increasing amplitude kills inductor Q, and series resistance increases.

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