A coil having an inductance of 100 mH is magnetically coupled to another coil having an inductance of 900 mH. The coefficient of coupling between the coils is 0.45. Calculate the equivalent inductance if the two coils are connected in series opposing.

• A. 1135 mH
• B. 932 mH
• C. 1270 mH
• D. 730 mH

Answer: (D)

M = k√(L₁L₂)
M= 0.45√(100*900)
M = 135 mH

L_eq for opposite connection of coils = L₁ + L₂ - 2M
L_eq = 900 + 100 - 2*135
L_eq = 730 mH