Hi, all.

I have read the material given on this site regarding inductors and its working.
But, I am still finding it difficult to understand the concept of Inductor working.
I have listed the questions which confuses me.
I am not clear about the AC response of the Inductor.

My doubts :

1. How is the BACK EMF created when the current increases in the inductor?
2. What is the polarity of the Back EMF in the inductor with respect to the circuit?
3. In a simple circuit consisting of an AC source of 10KHz and an inductor of some value ( for explanation purpose only ), what would the inductor voltage across the inductor and what would be its polarity ? Does the polarity of the inductor change during positive and negative half cycle or does it remain constant during both the half cycles?

The Inductor is just a coil of wire. So, when an AC passes through the inductor, why should there be some voltage drop across the inductor ( as its just a simple piece of wire ) ?
I also get the fact that inductor also has some value of wire resistance (and DCR). But why is a voltage drop across the inductor and what would be its magnitude across the inductor ?
And my final doubt :

Is the, Inductor voltage = (source voltage - BACK EMF) ?
What would be their values for a simple circuit example (please explain this with a simple example circuit of your own)?
Please help me understand.
Thanks.

 

If you don't already have it, I suggest you get hold of LTspice (free circuit simulator software downloadable from Linear Technology) and try simulating simple circuits involving an inductor with that. It will help to answer your questions.

 

When you apply a voltage across an inductor the current will start to increase as determined by the inductance and applied voltage.
The change in current is equal to the applied voltage times the time divided by the inductance (di = V dt / L) where di is the change in current and dt is the time the voltage is applied).
This inductance is caused by the magnetic field generated in the inductor by the current.
This magnetic field stores energy so it provides an impedance to the buildup of the current as the energy is stored.
The total stored energy is equal to ½LI².

When the voltage is removed, this stored energy of the magnetic field will keep the current flowing until the energy is dissipated.
The voltage generated by this will be such as to keep the current flowing in the same direction (thus the generated voltage is opposite of the previously applied voltage).
If the source voltage is removed by opening the circuit, then a large negative voltage (spike) will be generated that will spark across the switch gap (or worst-case will cause the insulation breakdown of the inductor wiring) until the inductor energy is dissipated.
That's why a diode (or other transient suppression circuit) is often placed across an inductor (such as a relay coil) to absorb that energy and prevent a damaging voltage spike.

Note that even a straight piece of wire has a small inductance, but it's usually not enough to cause a problem except in very high frequency or fast-switching circuits.

 

In my head, I think of an inductor as a flywheel. The speed that it's spinning is the current, and the force you exert on it is the voltage. If it's sitting still, there is no current. To get it spinning quickly (get current flowing quickly) you have to give it a hard push (high voltage). Once it is spinning a steady speed, this is the equivalent of DC; it takes very little force to keep the speed steady (the current steady). Now to stop it suddenly or switch direction suddenly takes a very large force (very high voltage), but once the speed (current) is steady there is very little force (voltage) across it. With a flywheel the energy is stored by the inertia of the mass, with an inductor the energy is stored by the magnetic field.

Does this make sense?

 

1. A changing magnetic field creates an electric field. This is one of Maxwell's equations.
2. The back EMF is called "back" because it opposes the flow of current. It is the opposite polarity of the applied voltage.
3. If the coil is connected to an AC source, the voltage across the coil must be same same as the AC voltage. If there is resistance in series, the voltage across the coil will be shifted in phase compared to the AC source. The larger the resistor, the large the shift, up to 90 degrees.

Bob