1. Applied Electricity and Magnetism
ECE 3318
Applied Electricity and Magnetism
Spring 2017
Prof. David R. Jackson
ECE Dept.
Notes 32

2. Mutual Inductance Two coils are in proximity of each other.
Current reference directions and unit normal vectors are defined on both coils.
(The unit normal vectors are each determined from the corresponding current reference directions, by the right-hand inductor flux rule.)
Right-hand rule for inductor flux: Fingers are in the direction of the current I in the coil, and the thumb gives the direction of the unit normal (the reference direction for the flux).

3. Mutual Inductance (cont.)
Coil 1 is energized
Coil 2 is left open-circuited
Define mutual inductance:
(calculated when I2 = 0)
Note:
For the figure shown, 21 < 0 if I1 > 0.
Hence, M21 < 0.
In general, if coil 2 has multiple turns:

4. Mutual Inductance (cont.)
Coil 2 is energized
Coil 1 is left open-circuited
Define mutual inductance:
Note:
For the figure shown, 12 < 0 if I2 > 0.
Hence, M12 < 0.
In general, if coil 1 has multiple turns:

5. Mutual Inductance (cont.)
A general property (proof omitted) is that both mutual inductance components are always equal:

6. Circuit Law for Coupled Coils
Total flux through coil 1:
Total flux through coil 2:
Faraday’s law:
Faraday’s law: + -
Note:
We use passive sign convention on both coils.

7. Example
Find M12 , M21
Right-hand rule for inductor flux:
For M12:

8. Example (cont.)
For M21:
Bz1 R1 R2 x y
Note:
Bz1 = 0 for  > R1.

9. Example (cont.)
Summary

10. Dot Convention
This allows us to use mutual inductance M without having to visually inspect how the coils are wound.
The dots tell us where to put the + sign for the voltage on one coil, and where the current enters the other coil.
Note:
The dot convention is consistent with the passive sign convention,
as you can see from the following figures.

11. Here is one possible dot arrangement:
Dot Convention (cont.)
Here is one possible dot arrangement:
Coil 1
Coil 2 + -

12. Here is another possible dot arrangement:
Dot Convention (cont.)
Here is another possible dot arrangement:
Coil 1
Coil 2 M + -
Note:
The M here is the negative of the M on the last slide (if the coils are the same).

13. M could be either positive or negative.
Example
Write down KVL phasor-domain mesh-current equations to describe this circuit.
Note:
M could be either positive or negative. + -
Phasor voltage

14. Example (cont.) + -