Electrical – Common Mode Chokes


Consider the following :
enter image description here

If we need to figure out the input impedance seen by the source, it will be equal to –> Zin = jw(30u) -jw(27u)+1k+jw(30u)-jw(27u).
The first two terms are due to the drops across the first inductor, where the first is the drop due to its self-inductance and the second term is the drop due to the mutual inductance. The same is for the second equivalent inductor. My question is why the drop due to the mutual inductance is negative, how do I determine this voltage drop polarity due to the mutual inductance in general ??

Best Answer

The drop due to the mutual inductance is negative because the magnetic field generated by the current flowing in the mutually coupled inductor is in the opposite direction. Pay careful attention to the direction the wire wraps around the core. Trace the path of the current around that wire and notice how it is travelling in a direction that will impart a magnetic field in the opposite direction in the core.

If the mutual inductance was equal to the self inductance (perfect coupling) then the two effects would cancel and you would have zero magnetic field. Zero magnetic field means zero inductance.