Points to Remember in Coupled Circuits

Points to Remember in Coupled Circuits

1. Self inductance (L) of a coil is defined as the weber turns (flux linkage) per unit current in the coil. It is measured in Henrys.

2. Mutual inductance (M) between 2 coils is defined as the weber - turns in one coil per ampere current in other coil. This is bilateral. It is also measured in Henrys.

3. Coefficient of coupling (K) = M / √L₁ L₂ when K = 1, M has maximum possible value K can be defined as ratio of actual mutual inductance to the maximum possible value of mutual inductance between two coils.

4. Dot convention in coupled circuit is used to determine the polarity of mutually induced voltage in the coils.

5. For a series combination of two magnetically coupled coils, the equivalent inductance = L₁ + L₂ ± 2M.

6. For a parallel combination of two magnetically coupled coils, the equivalent inductance L₁ L₂ – M² / L₁ + L₂ ± 2M

7. (a) The maximum value of the coefficient of coupling is 100%.

(b) If two coils are isolated magnetically M = 0 and hence K = 0.

(c) If two coils are coupled perfectly K=1.

8. The maximum possible mutual inductance of two inductively coupled coils with self- inductances L₁ = 25 mH and L₂ = 100 mH is given by M = √L₁ L² = 50 mH.

9. If there are three or more magnetically coupled coils, the equivalent inductance can be calculated by inductance matrix.

10. Application of double tuned circuits is in radio receivers. It is also very useful in communication systems.

11. The value of K is more for iron core coupled circuits when compared to air cored coupled circuits.

12. K = M / √L₁ L₂ also K = X_m / √X₁ X₂

13. Critical coupling is given by K_c = Mc √L₁ L₂ = Here Me is called critical value of mutual inductance. At this value of mutual inductance, the output voltage or voltage amplification in double tuned coupled circuits is maximum.

14. When the capacitor is introduced in the primary or in the secondary circuit of a transformer (it consists of two coupled coils), the circuits become selective. The transformer is called a singly tuned transformer.

15. A doubly tuned coupled circuit is one in which the capacitors are introduced in both primary and secondary coils.