L = L 1 + L 2 + L 3
( b ) When three coils of inductances L 1 , L 2 and L 3 are connected in parallel and the coefficient of coupling K = 0 as in parallel , then
L = 1 / L 1 + 1 / L 2 + 1 / L 3
If coefficient of coupling K = 1 , then ( i ) In series ( a ) If current in two coils are in the same direction , then L = L 1 + L 2 + 2M ( b ) If current in two coils are in opposite directions , then L = L 1 + L 2 – 2M ( ii ) In parallel ( a ) If current in two coils are in same direction , then L = L 1 L 2 – M 2 / L 1 + L 2 + 2M