Power in AC CircuitPublished on 24/2/2012 and last updated on 24/8/2018
Suppose, in a three phase system, the power at R phase is PR , at Y phase is PY and at B phase is PB, then total power of the system would be This is simple scalar sum, since power is a scalar quantity. This is the season, if we consider only single phase during calculating and analyzing of three phase power, it is enough.
Let us consider, network A is electrically connected with network B as shown in the figure above. Let us consider the expression of voltage wave farm of a single phase system is Where V is the amplitude of the waveform, ω is the angular velocity of propagation of the wave.
Now, consider the current of the system is i(t) and this current has a phase difference from the voltage by an angle φ. That means current wave propagates with φ radiant lag in respect of the voltage. The voltage and current waveform can be represented graphically as shown below The current waveform in this case can be represented as Now, the expression of the instantaneous power, [where Vrms and Irms is the root mean square value of voltage and current waveform] Now, let us plot the term P versus time,
It is seen from the graph that,the term P does not have any negative value. So,it will have a nonzero average value. It is sinusoidal with a frequency twice of system frequency. Let us now plot second term of the power equation, i.e. Q. This is purely sinusoidal and has a zero average value. So, from of these two graphs, it is clear that, P is the component of power in AC circuit, which actually transported from network A to network B. This power is consumed in network B, as electric power. Q on the other hand does not really flow from network A to network B. Rather it oscillate between network A and B. This is also component of power, actually flowing into and out of the inductor, capacitor like energy storage elements of the network. Here, P is known as the real or active part of the power and Q is known as imaginary or reactive part of the power. Hence, P is called real power or active power, and Q is called imaginary or active power. The unit of active power is Watt, whereas the unit of reactive power is Voltage Ampere Reactive or VAR. We have already considered, where, S is the product of root mean value of voltage and current i.e. This product of RMS value of voltage and current of a system is referred as apparent power is Voltage Ampere or VA. So, This can be represented in complex form as, Again, the expression of the real power is where ɸ is the angle between voltage and current phasor. So, So, here in the expression P, cos ɸ is the factor which determines the real power component of a apparent power S. This is why the term cos ɸ in the expression of real power is called power factor. For both positive and negative value of ɸ, cos ɸ is always positive. This implies, whatever may be the sign of ɸ, i.e. whereas, the current is lagging or leading in respect of voltage, real power is always positive, that means it flows from sending end (Network A) to receiving end (Network B). We have also shown the same in wave form of real power, earlier. Now if current is leading the voltage, the angle between voltage and current phasor is negative, taking the voltage phasor as reference. In that case the reactive component of the power is negative,