BACK-EMF?!

[Vis99]

If there is an electromagnet and a magnet system
Where the electromagnet is fixed and the magnet can move freely with low friction.
The gap between the two is 0. The orientations set-up in a way, that they repel each other,
Now I understood that EMF is generated when there is a "change" in the magnetic field, or simply put an interaction between to fields.
In my case, the interaction is from the electromagnet and the magnet, now here's my question.
When the electromagnet repel's the magnet is back EMF generated BEFORE or AFTER that interaction? I guess its after? Since then the change will occur.

 

[Skogsgurra]

Simultameously. E=-d(phi)/dt

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

ditto: not before, not after, only during a change or during motion.

 

[Vis99]

Thank you.

 

[waross]

Change causes an EMF to be generated. When the voltage is applied to the magnet coil, an EMF is generated as the current increases from zero to the steady state value. When the power is interrupted the collapsing field around the magnet coil causes an EMF to be generated.
If the magnets are moved closer together or further apart an EMF is generated. Some of these situations generate an EMF that opposes the flow of current, some will be in the forward direction, tending to increase the current or to slow the decay of the current.
Change causes an EMF and the EMF is generated only while some other value is changing.
Not all EMFs are back EMFs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Are you thinking seconds or picoseconds?

Not that it matters. But if you are thinking seconds, you may be missing the finer details.

Simultaneously it is.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

We may be on the verge of a breakthrough in electromagnetic seduction here Gunnar. Possibly the inter-reducted fields of a permanent magnet intersecting with an electromagnetic field may actually generate a useful energy. Self induction will soon reach a climax and will the influence of the earth's magnetic field cause the back EMF to become a leading flow of electrons in the southern hemisphere.
GIGO I may if GIGO I must.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Overunity warning?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Well if the power factor of the electromagnet is improved to unity and the permanent magnet is installed vertically above it, Could happen. Interesting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Bill, you know it can't.

Careful now so the oil companies don't shut down the whole Eng-Tips

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

Bill and Gunnar are respected members, but sometimes they get carried away and need to be reminded this is a professional forum.

Now in this system, where the electromagnet is turned on, EMF is applied from a power source to allow current to flow to create a magnetic field to repel the magnet, ONLY when the magnet moves back EMF is generated, in that case its AFTER when the force of repulsion/attraction is applied.

I'm not sure I get the point of your question either. But let's define it better.

You had steady state before t=0, and then something changed at t=0

Was it scenario A:
prior to release you have dc voltage applied with steady dc current and you are mechanically restraining the magnet. Then at t=0 you release the magnet.

or scenario B:
prior to release you have no voltage applied and magnet sitting on top of coil. Then at t=0 you apply voltage.


=====================================
(2B)+(2B)' ?

 

[LionelHutz]

My first thought was that repel was a poor choice of words.

When the electromagnet repel's the magnet is back EMF generated BEFORE or AFTER that interaction?

To me, the electromagnet repelling the magnet might mean it moves the magnet but also might mean it's just pushing on the magnet without moving it. The electromagnet can push against the magnet without moving it and no back-emf will be generated until movement begins.

I'm really not sure where you're going with your further questions or statements. That the magnet won't move until some time after voltage is applied to the electromagnet. The electromagnet has to build up enough of an opposing magnetic field to begin moving the magnet. It may be a very short time but there will be a time delay. However, once the magnet begins to move, back EMF is generated simultaneously with the movement, not after the movement.

 

[waross]

When the coil is energized, there is a back EMF opposing the build up of the current. When the magnet moves, there will be an EMF generated by the movement.
CHANGE. EMF is generated by a change in the magnetic flux. This may be a change in position or a change in current.
CHANGE. DURING a CHANGE.
I did put a GIGO disclaimer, PETE. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Thanks for putting us non-serious almost non-engineers back on track, Pete.We got carried away. Will never happen again - until next April 1st.

BTW, did you read about the sad mishap in Switzerland?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Vis999]

Sorry of not clarifying this! But what I meant is that back EMF is generated after the force is applied.
And only then the magnet moves and with that movement back EMF is generated.
I was interested about the force. And would back EMF happens before or after that force is applied.
Now I know back EMF only occurs when the magnet moves.


Thanks everyone!
Much appreciated.

 

[Vis999]

Thank you.