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Topic Title: Inductor Voltage/Current
Topic Summary: Back EMF across solenoid vs current through it
Created On: 26 October 2013 08:43 AM
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 26 October 2013 08:43 AM
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QwertyXP

Posts: 5
Joined: 26 October 2013

Consider an ideal solenoid (no resistance, no leakage reactance etc.) connected across an AC supply. The back EMF induced in it will be exactly equal and in opposite direction to the source voltage (which means that when a certain terminal of the AC supply is positive, the side of solenoid connected with it would also be positive, and vice versa).

My question is, how will current flow at all when the EMFs of AC source and solenoid are cancelling each other out? It's like having having a circuit with only two batteries and terminals of similar polarities shorted with each other.

The equation below doesn't appear to be balanced:

V(source)=Back EMF (which is equal to source) + CurrentxReactance

when back EMF is equal to source, the CurrentxReactance part should be zero!?


I've read quite a few explanations on the internet but have yet to fully understand what's happening here.
 26 October 2013 11:12 PM
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kengreen

Posts: 400
Joined: 15 April 2013

Qwerty,

Your confusion has arisen because your "equation" is not complete.

You have ignored the fact that it takes time for a current to build up after the application of any eMF. Equally it takes time for that current to die away after the reMmoval of the EMF.

It is of course legitimate to ask why the current does not cease at the moment that the EM is remOoved; the answer is that the electrons, whose concerted movement is accepted to constitute an electric current, could not be massless and so exhibit inertia - tHhey pile up at the open circuit to create a very high voltage-gradient which ionises the air in the gap and so enables it to carry the current in the form of a spark ?

this inertia effect manifests as a current waveform that always lags behind any applied voltage waveform - hence you cannot eqUuate an applied EMF with one generated through the magnetic effects of any magnetic circuit. You have to allow for the delay which could not occur in a "perfect " or ideal solenoid which perhaps is a kind way of saying that your question is not a question at all?

We allow for this when trying to write equations that can deSsCcribe the shenanigans in ac arrangements by inserting the operator j before the symbol for current and this indicates that the parameter must be integrated into any calculation by use of the the Pythagorean sum-of-two-squares technique.

Ken Green
 27 October 2013 09:49 AM
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IanDarney

Posts: 32
Joined: 18 January 2003

The relationship between current and voltage in a solenoid is given by the equation:
V = R*I + L*(dI/dt)

If a step voltage is applied, then the current eventually settles down to:
V = R*I

If an alternating voltage of frequency f is applied, then the current is continuously changing, and the appropriate equation is:
Vrms = R*Irms + j*reactance *Irms
where reactance = 2*pi*f*L

The reactance is never zero if the current is oscillating.

Ian Darney
 29 October 2013 02:56 PM
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QwertyXP

Posts: 5
Joined: 26 October 2013

Thank you, kengreen and IanDarney.

btw is there any "like" or "thanks" button on IET forum?
 29 October 2013 07:29 PM
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kengreen

Posts: 400
Joined: 15 April 2013

don't think so but feel it is better to take the trouble and express those thanks : - )
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