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Author Topic: question concerning self-induction in a coil  (Read 272 times)

Group: Tinkerer
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question concerning self-induction in a coil:

What effect does a DC bias have on the induced EMF in a coil when the coil receives a pulse?

Example:
You apply a 1kv pulse, 100v counter emf is produced by the coil to resist the current.
You bias the coil with 200vdc, same polarity as your pulse, and then apply the pulse over the bias.
Does the bias negate/block the 100v counter emf? or does the counter emf lower the bias by 100v while active?

   
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Posts: 14
Back EMF has to do with a change in current and so not strictly an effect from back emf. There could potentially be some kind of effect due to how the bias effects core saturation, cant say for sure though.
   
Group: Experimentalist
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question concerning self-induction in a coil:

What effect does a DC bias have on the induced EMF in a coil when the coil receives a pulse?

Example:
You apply a 1kv pulse, 100v counter emf is produced by the coil to resist the current.
You bias the coil with 200vdc, same polarity as your pulse, and then apply the pulse over the bias.
Does the bias negate/block the 100v counter emf? or does the counter emf lower the bias by 100v while active?

So we have a coil in series with a DC generator ?
I agree with Polyrhythm.
The induced pulse signal produces an EMF in the coil, which is in series with the voltage supplied by the generator.
The variable signal is therefore the same as if the coil was short-circuited, but shifted by the DC value, assuming that the DC generator does not modify the inductance or parasitic capacitances, and that the permeability of the coil core, if it has any, is independent of the magnetic field.
The variable signal being the same, the counter EMF is the same.



---------------------------
"Open your mind, but not like a trash bin"
   

Group: Professor
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Posts: 1739
question concerning self-induction in a coil:

What effect does a DC bias have on the induced EMF in a coil when the coil receives a pulse?

Example:
You apply a 1kv pulse, 100v counter emf is produced by the coil to resist the current.
You bias the coil with 200vdc, same polarity as your pulse, and then apply the pulse over the bias.
Does the bias negate/block the 100v counter emf? or does the counter emf lower the bias by 100v while active?
When you apply a voltage step V to an inductor L the current rises from zero initially at a rate of V/L amps/second.  The initial cemf holding off the current is therefore V, the applied voltage.  That cemf reduces exponentially allowing the current to rise exponentially until eventually the current reaches its maximum value when the cemf is then zero.  So your 100V cemf against a 1KV pulse will only occur at an instant in time after the step.  Your perception of a constant cemf being produced is wrong.  Your 200V DC will bias the core at a DC current of 200/R where R is the coil resistance and your 1KV pulse sitting on top will cause an increase in that current at a rate of 1000/L A/s with the cemf initially being 1000, then depending on the pulse width the cemf decreases while the current increases.  It is wrong to think of the coil being biased by voltage, it is the current that is important.  The current increases during the pulse, and if you consider current as a bias then the bias increases. 

Smudge
   

Group: Tinkerer
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When you apply a voltage step V to an inductor L the current rises from zero initially at a rate of V/L amps/second.  The initial cemf holding off the current is therefore V, the applied voltage.  That cemf reduces exponentially allowing the current to rise exponentially until eventually the current reaches its maximum value when the cemf is then zero.  So your 100V cemf against a 1KV pulse will only occur at an instant in time after the step.  Your perception of a constant cemf being produced is wrong.  Your 200V DC will bias the core at a DC current of 200/R where R is the coil resistance and your 1KV pulse sitting on top will cause an increase in that current at a rate of 1000/L A/s with the cemf initially being 1000, then depending on the pulse width the cemf decreases while the current increases.  It is wrong to think of the coil being biased by voltage, it is the current that is important.  The current increases during the pulse, and if you consider current as a bias then the bias increases. 

Smudge

Hello Smudge,

I agree with what you say.  I'm asking these questions because I am applying a pulse to a coil with a requirement that the pulse train NEVER goes below zero.  This is difficult because of ringing associated with the inductor and switching elements.  Even with the best snubber I can manage, there is a dip between pulses of less than -1v, but still negative.

The instructions I have for the pulse requirements allow two options:
1. bias the coil with 50v to 500v DC (pick a stable value) and switch the pulse on top of the bias, bias current not mentioned, presume 2ma to 5ma as reasonable
2. wrap a second coil around the first and apply DC voltage of 15v to 24v, at a current of up to 1A

Note that the inner coil has about 2000 turns (DC bias), the outer coil only about 200 turns (15v to 24v)

As you can see the two options are not exactly equivalent with one applying higher voltage lower current to a much longer coil of wire.  The other applying much lower voltage, much higher current to a much shorter coil.

I'm trying to determine if I should use option 1 or 2 to meet the NEVER go negative requirement.
   
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Posts: 14
I will try to describe what we are doing and I hope that it answers some of your dilemma.
 When we fire our coils, and I will take an example from one coil, which is wounded in a clockwise direction. It automatically creates its reaction in CC coil. Now they are working equally as one. When the coils are turned off, the electric field collapses and creates an opposite polarity within the core. At just about the same time, the CC coil is fired, in the opposite direction, which now aligns with the direction of the core without creating any opposite effects.
In this way there is no resistance to disturb the flow.

We can visualize this in a different way. We throw a ball into the air to fulfill action within the positive half of the wave. Now we can cross over the equator to the negative side where reaction is happening. The ball is falling towards us, returning the power that was received in the first half of the cycle.

We always knew that generation and radiation creates one wave cycle. They are 2 opposite effects. Somehow, we squeezed these 2 effects into the first half of the cycle.  This is very unnatural – it creates a lot of friction and resistance, heat, unwanted spikes and different unnatural effects, which are not present when we operate them in the normal cycle.

Abnormal action will always create abnormal reaction.

Best Regards
   

Group: Tinkerer
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Posts: 3905
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I'm proceeding with the idea that the field I am trying to build-up cannot have a reversal, so either method works, and both shouldn't hurt either.
   
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