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