How does a phase difference between voltage and current affect real and reactive power flow?

How does a phase difference between voltage and current affect real and reactive power flow?

• A. Phase difference leads to reactive power; larger difference increases reactive power and reduces real power at a given magnitude.
• B. Phase difference only affects voltage.
• C. Phase difference has no effect on power.
• D. Phase difference increases real power.

Answer: (A)

Phase difference between voltage and current determines how power splits between doing useful work and being stored and returned by reactive elements. The real power is P = VI cos(phi) and the reactive power is Q = VI sin(phi), where phi is the phase angle between voltage and current. If voltage and current are in phase (phi = 0), cos(phi) = 1 and sin(phi) = 0, so all the power is real and usable, with no reactive component. As the phase difference grows, cos(phi) decreases, so the real power for the same voltages and currents drops, while sin(phi) increases, so the reactive power rises. At a phase difference of 90 degrees, real power drops to zero and reactive power reaches VI. So, a phase difference creates reactive power, and increasing that difference (for a fixed magnitude of voltage and current) shifts power from real to reactive. The sign of the reactive power depends on whether the current lags (inductive) or leads (capacitive) the voltage.

Phase difference between voltage and current determines how power splits between doing useful work and being stored and returned by reactive elements. The real power is P = VI cos(phi) and the reactive power is Q = VI sin(phi), where phi is the phase angle between voltage and current.

If voltage and current are in phase (phi = 0), cos(phi) = 1 and sin(phi) = 0, so all the power is real and usable, with no reactive component. As the phase difference grows, cos(phi) decreases, so the real power for the same voltages and currents drops, while sin(phi) increases, so the reactive power rises. At a phase difference of 90 degrees, real power drops to zero and reactive power reaches VI.

So, a phase difference creates reactive power, and increasing that difference (for a fixed magnitude of voltage and current) shifts power from real to reactive. The sign of the reactive power depends on whether the current lags (inductive) or leads (capacitive) the voltage.