Inductive Kickback

[SeaMonkey]

Well, I got as far as page six in this very interesting discussion.

Has it yet been concluded that both voltage and current which
are induced within the inductor during charge and discharge
do indeed change direction?

When pulsing an inductor with DC pulses and monitoring the
voltage across the inductor the polarity change cannot be
seen directly but its effect will be observed.

As current from a source pulse increases from zero to its
maximum the induced Counter ElectroMotive Force opposes
the source voltage and current.  The result is the charging
time constant.  The source current through the inductor
is stronger than the induced Counter ElectroMotive Force
current which opposes it or pushes back against it with the
result that the charging current increases from zero to
maximum (at inductor saturation) in the characteristic time
constant curve;  gradually increasing as it overcomes the
induced Counter ElectroMotive Force.

Then when the source current is discontinued and the
charged magnetic field collapses the induced voltage
and current will reverse as the inductor itself becomes
a source of current which will sustain current in the
same direction as the current from the source which
charged it, thereby effectively aiding that source.

While charging; the inductor will oppose the source.

While discharging; the inductor will aid the source.

Yes, the induced voltage within the inductor does
change polarity as well as the induced current flow.

A parallel winding on the same core as the inductor
under test will demonstrate the polarity reversals as
the inductor is charged and discharged.  There the
reversals will be clearly seen as it is separated from
the overpowering influence of the source current in
the driven winding.

This is how Inductors were taught in Naval Electronics
Technician "A" School, Treasure Island, California in the
'60s.  At that time Navy Electronics Schools were the
absolute best in the whole nation.

[citfta]

Hi SeaMonkey,

Yes I think we pretty well established with some good scope shots that while the voltage reverses when the power is removed from an inductor the current continues to flow in the same direction until the magnetic field has collapsed.  One of our senior members explained that paradox this way.  When the applied voltage is removed the inductor which was the load becomes a source instead of a load.  Since it is now a source of current the polarity has to be the opposite of the direction of current flow.

Another way to look at it is which way does the current flow inside a DC generator?  If the electron flow (conventional way of thinking) flows from negative to positive outside the generator then which way must it flow inside the generator?  Obviously it has to flow from positive to negative which of course is because of the generating forces being applied internally.  And that is exactly what happens as the magnetic field collapses in an inductor when the source voltage is removed.

So it appears what you were taught is still valid.

Regards,
Carroll

[allcanadian]

@cifta

Yes I think we pretty well established with some good scope shots that while the voltage reverses when the power is removed from an inductor the current continues to flow in the same direction until the magnetic field has collapsed.  One of our senior members explained that paradox this way.  When the applied voltage is removed the inductor which was the load becomes a source instead of a load.  Since it is now a source of current the polarity has to be the opposite of the direction of current flow.

In a generator or transformer the external magnetic field change which may expand into or contract from the coil region determines the induced Electro-Motive Force direction thus the direction of current flow. When the field change is expanding the current flows in one direction and when it contracts the current reverses flowing in the other direction, an Alternating Current.


In an inductor the applied Emf from let's say a battery "produces" a current flow which then produces an expanding magnetic field which then "induces" a Cemf against the applied Emf from the battery. The Cemf is "induced" due to the expanding magnetic field and has the same direction as if an external magnetic field had induced it as in a generator or transformer. However the battery Emf maintains the current flow forward against the Cemf.


Then when we remove the battery Emf/current the contracting magnetic field induces an Emf thus a current in the same direction as the battery Emf/current. The inductor acts just like a coil in a generator or transformer and the magnetic field change induces an Emf in the same direction. However the source battery applied Emf/current dominates over the the induced Cemf from the expanding magnetic field.


Let's apply this thought, when a magnet passes a coil the expanding magnetic field into the coil induces an Emf and current in one direction and as the magnetic field contracts from the coil it induces an Emf and current in the opposite direction... an Alternating Current. However if we applied an external Emf from let's say a battery or another coil to exactly "Counter" the induced Emf from the magnets expanding magnetic field then no current would flow. The only output would be from the contracting magnetic field as the magnet leaves the coil inducing a current in one direction only... a Direct Current. We could say the external applied Emf opposing the induced Emf has rectified the output current. Now you know how to produce DC from an AC generator without rectifying the current with diodes. 


In fact we can have any number of applied or induced Emf's in a circuit acting in different directions however the sum of the Emf's will determine the direction of the current flow.


AC

[Farmhand]

I propose a new way of defining the voltage apparent reversal which I think will make things less confusing. I'll explain.

The way I see it is very much the same as Citfa, MileHigh and others except that rather than just stating that the voltage reverses which is vague.

I propose that when the coil is switched off the applied voltage can be forgotten and a new voltage is caused due to the energy in the magnetic field, the current and the resistance if faces.

It is  the new voltage which is produced that has an opposite polarity across the coil when compared to the initial applied voltage. The initial applied voltage from the initial supply remains the same. The opposite voltage is a different voltage, emf.

It is the voltage produced at the coil when the coil becomes the separate supply that is opposite to the initial voltage applied from the supply "battery for example".

To put it another way.

The voltage applied to the coil from the initial supply does not reverse at all, the new voltage that the coil applies to the load  resistance is opposite polarity to the initial voltage applied to the coil.

The discharge current can be thought of as new as well, the new discharging current is in the same direction as the charging current.

The one thing that is not new is the energy. The same energy in both, some energy is lost as heat and radiation ect. so the total output power is always less than the total input power per cycle on a whole system basis.

..

Quote from: allcanadian on July 26, 2016, 05:12:08 PM

In fact we can have any number of applied or induced Emf's in a circuit acting in different directions however the sum of the Emf's will determine the direction of the current flow.


AC

As clearly stated by AC and probably others, the above would appear to be true of course. And that is very relevant.

Generally when thinking of this kind of situation the coil is switched off, the initial applied voltage is removed until the coil is discharged fully. Sometimes a new current path is enabled by whichever means to discharge back to the supply battery, sometimes to another load, so the voltage simply has to be a new and different voltage, this is reinforced by the fact that the discharge voltage can be very high if the coil discharge is through a high resistance.

..   

[SeaMonkey]

I can well remember at the age of 11 onward beginning
my education in electricity/electronics.

My dad was a railroad switchman who had a supply of
six volt lantern batteries.  I had a good supply of discarded
radios which yielded several very useful transformers.

I discovered that connecting the low voltage winding of a
filament transformer to the six volt battery briefly caused
a large, flamelike spark to be produced.  It seemed almost
magical at the time.

Once while creating those large flaming sparks I held the
bare ends of the wires to get a firm grip while sparking
the connection and got a tremendous jolt.  It was a
strong and painful jolt of high voltage electricity.

In those days the only way to find answers to questions
such as "What causes the large flamelike spark?" or
"Why did I get shocked from the six volt battery and
filament transformer combination?" was to go to a
library and find books on electricity.  Fortunately, most
libraries then had a good selection of books on electricity
for experimenters which contained instructions for lots of
projects.  Another very useful resource was the encyclopedia.

The answers didn't come quickly but they were eventually
found.  Of course, playing with inductors at an early age
assures that one is "bitten by the bug" which causes the
life-long addiction to the study of all things electrical and
electronic.

Then a bit later Navy Electronics Schools played a very big
role in my ongoing education.  The types of equipments that
the Navy Electronics Technician is able to maintain are incredible.
The subject matter of the schools where technicians are trained
was fascinating and diverse.  And of course life aboard ship which
plies the seas to numerous foreign destinations was 'icing on the cake'
of Adventure.  It was a good life!