After learning what is the mutual inductance and dot convention, we will move on how to calculate the energy in a coupled electric circuit.

We can call an electric circuit as a coupled circuit if the circuit has a mutual inductance from two coils or inductors.

Make sure to read what is electric circuit first.

## Energy in a Coupled Electric Circuit

We saw that the energy stored in an inductor is given by

(1) |

We now want to determine the energy stored in magnetically coupled coils.

Consider the circuit in Figure.(1).

Figure 1. The circuit for deriving energy stored in a coupled circuit. |

We assume that currents *i*_{1} and *i*_{2} are zero initially so that the energy stored in the coils is zero.

If we let *i*_{1} increase from zero to *I*_{1}while maintaining *i*_{2} = 0, the power in coil 1 is

(2) |

and the energy stored in the circuit is

(3) |

If we now maintain *i*_{1} = *I*_{1} and increase *i*_{2} from zero to *I*_{2}, the mutual voltage induced in coil 1 is *M*_{12} d*i*_{2}/d*t*, while the mutual voltage induced in coil 2 is zero, since *i*_{1} does not change.

The power in the coils is now

(4) |

and the energy stored in the circuit is

(5) |

The total energy stored in the coils when both *i*_{1} and *i*_{2} have reached constant values is

(6) |

If we reverse the order by which the currents reach their final values, that is, if we first increase *i*_{2} from zero to *I*_{2} and later increase *i*_{1} from zero to *I*_{1}, the total energy stored in the coils is

(7) |

Since the total energy stored should be the same regardless of how we reach the final conditions, comparing Equations.(6) and (7) leads us to conclude that

(8a) |

and

(8b) |

This equation was derived based on the assumption that the coil currents both entered the dotted terminals.

If one current enters one dotted terminal while the other current leaves the other dotted terminal, the mutual voltage is negative so that the mutual energy *M**I*_{1}*I*_{2} is also negative.

In that case,

(9) |

Also, since *I*_{1} and *I*_{2} are arbitrary values, they may be replaced by *i*_{1} and *i*_{2}, which gives the instantaneous energy stored in the circuit the general expression

(10) |

The positive sign is selected for the mutual term if both currents enter or leave the dotted terminals of the coils; the negative sign is selected otherwise.

We will now establish an upper limit for the mutual inductance *M*.

The energy stored in the circuit cannot be negative because the circuit is passive.

This means that the quantity 1/2*L*_{1}*i*_{1}^{2} + 1/2*L*_{2}*i*_{2}^{2} − *Mi*_{1}*i*_{2} must be greater than or equal to zero,

(11) |

To complete the square, we both add and subtract the term *i*_{1}*i*_{2}√(*L*_{1}*L*_{2}) on the right-hand side of Equation.(11) and obtain

(12) |

The squared term is never negative; at its least, it is zero. Therefore, the second term on the right-hand side of Equation.(12) must be greater than zero; that is,

(13) |

Thus, the mutual inductance cannot be greater than the geometric mean of the self-inductances of the coils.

The extent to which the mutual inductance M approaches the upper limit is specified by the *coefficient of coupling k*, given by

(14) |

or

(15) |

where 0 ≤ *k* ≤ 1 or equivalently 0 ≤ *M* ≤ √(*L*_{1}*L*_{2}). The coupling coefficient is the fraction of the total flux emanating from one coil that links the other coil. For example, in Figure.(2),

(16) |

and in Figure.(3)

Figure 3. Mutual inductance M_{12} of coil 1 with respect to coil 2. |

(17) |

If the entire flux produced by one coil links another coil, then *k* = 1 and we have 100 percent coupling, or the coils are said to be *perfectly coupled*. Thus,

The

coupling coefficientkis a measure of the magnetic coupling between two coils; 0 ≤k≤ 1.

For *k* < 0.5, coils are said to be *loosely coupled*; and for *k* > 0.5, they are said to be *tightly coupled*.

We expect *k* to depend on the closeness of the two coils, their core, their orientation, and their windings. Figure.(4) shows loosely coupled windings and tightly coupled windings.

Figure 4. Windings: (a) loosely coupled, (b) tightly coupled; cutaway view demonstrates both windings. |

The air-core transformers used in radio frequency circuits are loosely coupled, whereas iron-core transformers used in power systems are tightly coupled.

## Energy in a Coupled Electric Circuit Example

Consider the circuit in Figure.(5). Determine the coupling coefficient. Calculate the energy stored in the coupled inductors at time t = 1 s if v = 60 cos (4 t + 30◦) V.

Figure 5 |

*Solution:*

The coupling coefficient is

indicating that the inductors are tightly coupled. To find the energy stored, we need to obtain the frequency-domain equivalent of the circuit.

The frequency-domain equivalent is shown in Figure.(6). We now apply mesh analysis. For mesh 1,

(1.1) |

For mesh 2

or

(1.2) |

Substituting this to Equation.(1.1) yields

and

In the time-domain,

At time t = 1 s, 4t = 4 rad = 229.2◦, and

The total energy stored in the coupled inductors is

Figure 6 |

Have you understood what is Energy in a Coupled Electric Circuit? Don’t forget to share and subscribe! Happy learning!

*Reference: Fundamentals of electric circuits by Charles K. Alexander and Matthew N. O. Sadiku*

## Leave a Reply

View Comments