Electronic – equivalent inductance


I have a basic doubt in understanding the concept of equivalent inductance. When two inductor coils are in series the equivalent inductance is calculated as

\$ L_{eq} = L1+L2 \pm 2M \$ (depending on the dot convention)

where L1 and L2 are independent coil self inductance and M is the mutual inductance dependent on coupling between the coils k (coupling coefficient). M is calculated as

\$M = k \sqrt{L1. L2} \$

k can take values from 0 to 1, 0 for no coupling and 1 for perfect coupling between the coils.

So if two coils are placed adjacent to each other very far where there is no coupling between the coils then k = 0 and \$ L_{eq} = L1+L2 \$ because M = 0.
If the coils are so far that L2 has no impact on L1 should we still consider L2 for calculating \$L_{eq} \$ ?

Best Answer

If coupling is not present as you mention:

Leq = L1 + L2

If L1 >> L2, Leq ~ L1

However, when L1 is not >> L2, you cannot just ignore L2 since it is still in series with L1.