Series or Voltage Resonance:

Consider an ac circuit containing resistance of R ohms, inductance of L henrys and capacitance of C farads connected in series, as shown in fig.3.21 (a).

Impedance of the circuit

If for some frequency f applied voltage, XL = XC in magnitude then

• Net reactance of the circuit is zero i.e. X = 0

• Impedance of the circuit, Z = R

• The current flowing through the circuit is maximum and in phase with the applied voltage. The magnitude of current will be equal to V/R.

• The voltage drop across the inductance is equal to the voltage drop across the capacitance i.e. VL = VC.

• Supply voltage is equal to the voltage drop across the resistance i.e. V = VR.

• Power expended = V I watts and

• Power factor is unity.

When this condition exists, the circuit is said to be in series resonance and the frequency at which it occurs is known as resonance frequency.

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