For maximum power transfer, how must the load reactance relate to the source reactance?

• A. It must be equal to the source reactance
• B. It must be zero
• C. It must be twice the source reactance
• D. It must be the negative of the source reactance

Answer: (D)

Maximizing real power transfer in AC circuits comes from making the load present the complex conjugate of the source impedance. If the source impedance is a real part Rs plus an in-phase imaginary part jXs, the load should be RL minus jXs. In other words, the load reactance must cancel the source reactance, so XL = -XS. This cancellation makes the total impedance seen by the source purely real, causing the current to be in phase with the voltage and allowing the available power to be delivered efficiently to the load. The equal but opposite reactance is what achieves the best match; other options leave a net reactive component that reduces the real power delivered.