How to Calculate the Phase Angles in RLC Circuits

Written by petra wakefield
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How to Calculate the Phase Angles in RLC Circuits
. (John Foxx/Stockbyte/Getty Images)

An RLC circuit contains a resistor, an inductor and a capacitor. RLC circuits are a type of alternating current circuit, where the magnitudes of the voltage and current follow the pattern of a sine wave. Phase angle indicates the difference between the voltage and current waves -- voltage and current have the same wave pattern across a resistor, but the voltage wave is 90 degrees ahead of the current wave for an inductor and 90 degrees behind for a capacitor. When an inductor and capacitor combine, as in an RLC circuit, the phase angle is somewhere between -90 degrees and 90 degrees. To calculate phase angle, you must know resistance, inductance and capacitance, as well as frequency or angular frequency.

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  1. 1

    Calculate angular frequency if you know the frequency. Multiply frequency by 2pi = 6.28 to get angular frequency. If the frequency is 50 Hz, for example, 6.2850 Hz = 314 Hz.

  2. 2

    Multiply the angular frequency by the inductance to get the inductive reactance. If inductance is 0.50 henries, for example, (314 Hz)*(0.50 H) = 157 ohms.

  3. 3

    Divide 1 by the angular frequency times the capacitance to get the capacitive reactance. If capacitance is 10 microfarads, for example, 1/(314 Hz)*(0.000001F) = 318.5 ohms.

  4. 4

    Compare the inductive and capacitive reactances. If they're equal, the phase angle is 0 degrees.

  5. 5

    Subtract the capacitive reactance from the inductive reactance if they are not equal. For example, 157 ohms - 318.5 ohms = -161.5 ohms.

  6. 6

    Divide the result by the resistance. If the resistance is 300 ohms, for example, -161.5 ohms/300 ohms = -0.538.

  7. 7

    Take the inverse tangent of the result to get the phase angle. For example, tan^-1(-0.538) = -28.3 degrees.

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