# AC with INDUCTOR IMP DOUBT

1. Nov 15, 2009

### sudar_dhoni

What i know the most fundamental thing is that the emf supplies energy to the charges and these charges have to reach the other end(say for electrons to reach the + end). IN order to do that somehow or the other they have to drop their voltage or energy for this to happen .
AC with inductor alone
In inductor the thing which drops the voltage is the induced electric field acting to oppose the electrons to flow.OK all this fine i understand.
But in the figure below when the inductor voltage or when the induced electric field(which is generated by the changing magnetic field) becomes 0 or when the current becomes quite constant then no induced electric field as a result there wont be a voltage drop there then how can current be maximum when there is no voltage drop there since the current to reach the other end (+). what i expected that when this induced electric field is zero there wont be any voltage drops as a result current should not flow but it is maximum at that time.
PLZ any 1 if i can understand this most of my problems are over
PLZ dont explain with water analogy.It works for water analogy even i know that but here it is not working.pLZ explain in a more theoritical way than mathematical

2. Nov 16, 2009

### Staff: Mentor

Are you familiar with the differential equation that relates the voltage drop across an inductor to the change in the current through the inductor?

$$V(t) = L \frac{dI(t)}{dt}$$

3. Nov 17, 2009

### sudar_dhoni

yes i know that
but where is the answer to my doubt

4. Nov 17, 2009

### Staff: Mentor

You mention "in the figure below" in your original post (OP). Did you intend to upload an attachment?

5. Nov 18, 2009

### sudar_dhoni

What i know the most fundamental thing is that the emf supplies energy to the charges and these charges have to reach the other end(say for electrons to reach the + end). IN order to do that somehow or the other they have to drop their voltage or energy for this to happen .
AC with inductor alone
In inductor the thing which drops the voltage is the induced electric field acting to oppose the electrons to flow.OK all this fine i understand.
But in the figure below when the inductor voltage or when the induced electric field(which is generated by the changing magnetic field) becomes 0 or when the current becomes quite constant then no induced electric field as a result there wont be a voltage drop there then how can current be maximum when there is no voltage drop there since the current to reach the other end (+). what i expected that when this induced electric field is zero there wont be any voltage drops as a result current should not flow but it is maximum at that time.
PLZ any 1 if i can understand this most of my problems are over
PLZ dont explain with water analogy.It works for water analogy even i know that but here it is not working.pLZ explain in a more theoritical way than mathematical

k i have posted the image
atleast now give the answer because i need it desperately

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6. Nov 18, 2009

### b.shahvir

I did reply to your post which most probably would have quelled your doubts. Unfortunately, my good friend Berkeman deleted it claiming that my post posed a threat to the sanctity of the forum... still beats me!!

7. Nov 18, 2009

### sudar_dhoni

hey plz post it

8. Nov 18, 2009

### b.shahvir

Dear Sudar,

Here's the edited post;

Your doubt is not uncommon. The problem is, nature acts in more mysterious ways than what is obviously evident. Collapsing of a magnetic field and building up of emf or electrical charges is a gradual process and is governed by the laws of nature. A magnetic field cannot suddenly collapse or build up at time t = 0. Any physical reaction consumes some finite amount of time although it would be infinitesimally small and could be neglected in theory.

One more important thing to note is that the voltage vs. current waveforms normally depicted are not relevant in actual practice. For example as soon as you throw the switch on a pure inductor, the voltage and current will never become 90 Deg out of phase immediately, i.e., at t = 0. To do that, it will take some finite amount of time, which is normally not depicted in waveforms (such as the one appended by yourself) for reasons of simplicity.
However, if you refer to higher grade articles/literature on electrical machine design, this aspect is sometimes depicted in form of accurate waveforms as machine design is a practical subject .

You can also examine the article in the link below. The waveforms depict behaviour of electrical parameters in practical magnetic circuits of transformers.

Also given below are my postings on similiar posts from another forum;

A) To visualize the concept of inductance is pretty difficult. I have been dabbling with it for quite sometime now! In order to explain you the working of an ‘ideal’ inductor, I will have to make certain assumptions;

1) the coil of inductor should possess zero resistance

2) The inductor should have a magnetic material core (iron, ferrite, etc…. but not air-core). In an air-core inductor, the voltage and current waveforms will be practically in-phase with each other thus defeating the purpose.

3) The magnetic core should be ‘Lossless’

4) The input voltage to the ideal inductor is purely sinusoidal.

The voltage and current waveforms do not immediately phase shift by 90 deg as depicted by textbook diagrams. Consider positive half cycle of ‘voltage versus current’ waveform for an ideal inductor. At ‘t (time)’ = 0, as the voltage wave just rises above zero, the current wave rises in-phase with it. However as the voltage wave further progresses at ‘t (time)’ > 0 towards it’s peak value, the rate of change of magnetic flux becomes more pronounced ad the current wave now starts going out of phase w.r.t the voltage wave. Eventually as the cycle progresses, the current wave now completely goes 90 deg out of phase w.r.t the voltage wave (assuming an ideal inductor).

If you concentrate on the negative half cycle of the voltage vs. current waveform, you will find that the phase shifting is due to the ‘discharge’ of electrical energy by the inductor back to the power source as the magnetic field collapses (or rises, whatever may be the case), which precisely occurs when the input voltage wave has already reversed itself. This charge/discharge action of an inductor occurs due to the finite time taken my the magnetic field to rise or collapse……as force fields cannot change it’s states abruptly at ‘t = 0’ as per the laws of Physics or Mother Nature (if it would, the consequences would be disastrous). This in turn causes a delay in the flow of current thru the inductor and results in the phase shift between the voltage and current waveform. This delaying property of an inductance is also sometimes known as ‘Magnetic Inertia’.

Although the query deals with magnetic core design pertaining to transformer, I have attempted to intuitively explain the concept of inductance and the term ‘Magnetic Inertia’. I hope my explanation helps you visualize inductors better!

B) Hi, the phenomenon you are after is somewhat abstract in nature and hence a bit difficult to visualize. I myself have been dabbling with it for quite sometime now! A strong ‘H’ in the magnetic core tends to saturate it, as a result a finite reluctance in the form of an air-gap is introduced in the core circuit so that now a much higher ‘H’ is required to saturate the core. It is somewhat akin to a magnetic de-rating process.

Coming to your query on storage of ‘H’; It is helpful to remember that electrons in motion are infinitesimally small mechanical particles. Having said that, the mechanical particles undergo moment of inertia. When the voltage wave is at off cycle, the electrons still remain in motion as per the laws of particle mechanics since they are unable to cope with the speed at which the voltage wave passes thru zero.

However, it is important to note that this inertia is mainly caused by the changing magnetic flux associated with the electrons in motion (that vary their flow rate and direction of motion after each half cycle). This changing magnetizing force (‘H’) in turn ‘opposes’ or ‘assists’ the action of the electrical current, i.e., the changing magnetic field delays the fall or rise of the electric current thru the coil…..as each time the magnitude of coil current changes, the strength of ‘H’ also changes. As a result a self-induced alternating voltage appears in the coil (due to Faraday’s laws of electromagnetic induction) which ‘opposes’ or ‘sustains’ the flow of electrons at each minute step in the primary coil. This phenomenon is sometimes known as Magnetic Inertia.

Please note, my explanation may not be scientifically accurate (as the physics of charges in motion is quite complex), but this is how I have come to understand this abstract concept….awaiting a better and scientifically accurate reply myself !

Hope this helps!

Kind Regards,
Shahvir

Last edited: Nov 18, 2009