RL series circuit

RL series circuit A.C circuit

Ac series circuit differ from dc circuit. In dc circuit we consider resistance only but in case of Ac series circuit resistance(R) ,inductance(L) and Capacitance(C) are taken into account. L & C offer opposition(XL and XC) to the flow of current in ac circuit. The magnitude of current in an ac circuit is affected by the supply frequency because XL=2πfL and XC=1/2πfC are the frequency dependent. In dc circuit ,voltage and currents  can be added or subtracted arithmetically but in case of ac circuit there is a phase difference of 90° between voltage across and current through L or C.

V=rms value of applied voltage

I=rms value of the circuit current

Voltage across R=VR=IR

(VR is in phase with I i.e OA represented by the phasor diagram)

voltage across L=VL=IXL

(VL leads I by 90° i.e AB represented by the phasor diagram)

Taking current as references phasor,

Applied voltage V is the phasor sum of these two drops i.e

circuit current,

I. Phase angle

From phasor current I lags behind the applied voltage V by Φ°.

Applied voltage,

As the Inductive current lags behind the applied voltage .The angle of lag(i.e Φ) is greater than 0° but less than 90°.It is determined by the ratio of inductive reactance to resistance tanΦ=XL/R. The greater the value of this ratio ,the greater will be the phase angle Φ.

II. Impedance

The total opposition offered to the flow of alternating current is called impedance(Z).In series RL circuit,

III. Power

Instantaneous power p=vi


is constant part and whose average value over once cycle is the the same.



is a pulsating component and whose average value over one complete cycle is Zero.

Average power

where V and I are the rms value of voltage and current.


Related terms:

  1. Reactive power measurement in Balanced Three phase circuit
  2. Power factor

For more notes on Electrical Engineering:




Leave a Comment