Maximum Power Transfer Theorem:
This theorem is utilized to determine the value of load, for which source shall transfer the maximum power. Before applying the maximum power transfer theorem, we apply Thevenin theorem across load terminals. Now there are following three cases for load to receive the maximum power.
Case I : Load is Pure Resistive
Now we have to determine load resistance RL so that maximum power is transferred to it.
Load current iL :
iL = VTH / (RTH + RL ) + j XTH
Power delivered to load RL.
PL = | iL |2 × RL = ((VTH) 2/ (R TH + R L) 2 + (X TH) 2) × RL
Condition for maximum power
dPL/ dRL = 0
(V TH )2 = {(R TH + RL )2 + (X TH )2 } - R L 2 (R TH + RL ) / {(R Th + R L )2 + (X TH) 2 } = 0
(RTH + RL )2 + (XTH )2 = 2 RL (RTH + RL )
⇒ (RL) 2 = (R TH) 2 + (X TH) 2
or,
= | ZTH |
So load resistance RL should be equal to the magnitude of the internal impedance of circuit.