Nature of Electricity
Drift Velocity & Electron Mobility
Heating Effect of Electric Current
Magnetic field of current carrying conductor
Magnetic Flux Density
Resistance Variation with Temperature
Temperature Coefficient of Resistance
Theory of Electrical Potential
Capacitor and Capacitance
What is Capacitor?
Single Phase Power
Single Phase Power Equations
Three Phase Power
Single Phase Power Equation for Purely Resistive Circuit
Let's examine single phase power calculation for purely resistive circuit. Circuit consisting of pure ohmic resistance is across a voltage source of voltage V, is shown below in figure(a).
Where, V(t) = instantaneous voltage
Vm = maximum value of voltage
ω = angular velocity in radians/seconds
According to Ohm’s law,
Substituting value of V(t) in above equation we get,
From equations (1.1) and (1.5) it is clear that V(t) and IR are in phase. Thus in case of pure ohmic resistance there is no phase difference between voltages and current i.e. they are in phase as shown in figure (b).
From single phase power equation (1.8) it is clear that power consist of two terms one constant part i.e.
and another a fluctuating part i.e.
whose value is zero for the full cycle. Thus power through pure ohmic resistor is given as and is shown in fig(c).
Single Phase Power Equation for Purely Inductive Circuit
Inductor is a passive component. Whenever AC passes through inductor it opposes the flow of current through it by generating back emf. So applied voltage rather than causing drop across it, needs to balance the back emf produced. Circuit consisting of pure inductance across sinusoidal voltage source Vrms is shown in fig (d).
We know that voltage across inductor is given as,
Thus from above single phase power equation it is clear that I lags V by π/2 or in other words V leads I by π/2 , when AC pass through inductor i.e. I and V are out of phase as shown in fig (e).
Instantaneous power is given by,
Single Phase Power Equation for Purely Capacitive Circuit
When AC passes through capacitor it charges first to its maximum value and then it discharges. The voltage across capacitor is given as
Single Phase Power Equation for RL Circuit
A pure ohmic resistor and inductor are connected in series below as shown in fig (g) across a voltage source V. Then drop across R will be VR = IR and across L will be VL = IXL.
These voltage drops are shown in form of a voltage triangle as shown in fig (i). Vector OA represents drop across R= IR, vector AD represents drop across L=IXL and vector OD represents the resultant of VR and VL.
is the impedance of RL circuit.
From vector diagram it is clear that V leads I and phase angle Φ is given by,
Thus power consist of two terms, one constant term 0.5 VmImcosφ and other a fluctuating term 0.5 VmImcos(ωt - φ) whose value is zero for the whole cycle. Thus its only constant part that contributes to actual power consumption.
Thus power,p = VI cos Φ = ( rms voltage*rms current*cos Φ) watts
Where cos Φ is called power factor and given by
I can be resolved in two rectangular components IcosΦ along V and IsinΦ perpendicular to V. Only Icos Φ contributes to real power. Thus Only VIcos Φ is called wattful component or active component and VIsin Φ is called wattless component or reactive component.
Single Phase Power Equation for RC Circuit
We know that current in pure capacitance leads voltage and in pure ohmic resistance it is in phase. Thus net current leads voltage by angle of Φ in RC circuit. If V = Vmsinωt and I will be Imsin(ωt + Φ).
Power is same as in the case of R-L circuit. Unlike R-L circuit power factor is leading in R-C circuit