Induction coil question

[R_W_B]

Hello I am trying to learn the deeper theory behind automotive and motorcycle electronics. I've read that current is induced when a conductor is moved through a coil OR the coil is moved around the conductor. I get the jest that movement is key.

More specifically I'm trying to see exactly what's happening when the spark jumps the spark plug. I'ver read that for this to occur the positive side connection to the primary coil is cut, thereby creating a voltage spike in the secondary coil resulting in the spark jumping the gap.

My confusion is I don't readily see what this disconnection does to cause the voltage spike. I understand that the secondary windings are 10 times the amount of primary windings and would have 10 times more volts (or around 120 volts with a 12 volt battery).

But why does collapsing the field in the primary cause the secondary to soar even higher creating enough voltage to jump the plug gap. Is this disconnection another facet of the "movement" semantics ?

Appreciate any info on this.

 

[JimW]

It looks like you have most of it, just a few comments:

This is all about magnetic fields.
- Current through a wire creates a magnetic field around the wire.
- A wire passing through a magnetic field causes current to flow in a wire.
Movement of wires next to each other does nothing unless there is a magnetic field

An ignition coil has a an AC signal on the primary side, like the 12Volts beign turned on and off. This causes the magnetic field from the primary to be created and then removed. This is similar to a moving magnetic field. It builds, then it collapses. A stationary wire in this field would view this the same as moving through a stationary magnetic field.

The secondary side of the ignition coil is affected by this "moving" magnetic field. And the secondary winding has many more turns than the primary side winding. So the current that is caused by the moving magnetic field across the secondary windings results in a much larger voltage than the 12 volts switching on the primary. The more the windings, the more the wire being "moved" through a magnetic field. This results in a large secondary voltage, large enough to create a spark across the spark coil gap.

Jim

 

[R_W_B]

Thanks for the reply. Ok so when the magetic field collapses it effects the same thing as movement. That makes sense as far as being somewhat equivalent in semantics.

I guess what's puzzling me is how the movement causes such a dramatic increase of voltage on the secondary side, i.e. much more than the 10 times number of windings ratio. I obviously have not fully gotten under what happens in the field as the collapse occurs.

I.e. would a conductor moved thru the same coils create this same voltage amount ?

 

[MrAl]

Hi,

It's not *only* that it collapses, it's *also* how fast it collapses (or how fast it changes).

v=K*d(Phi)/dt

In the above, d(Phi)/dt is the rate of change of flux. Obviously the larger this is the larger the voltage will be with any constant K, and K could be the number of turns of the coil.
The division d(Phi) divided by dt could be looked at as an increment in the flux divided by the increment in time that it took to see that said increment in flux. If we see a larger change in flux for a given amount of time, that means the rate is higher, and so multiplying by the constant K gives a larger value of voltage v.
The value K multiplies the rate, so 50 turns would produce 5 times the voltage as 10 turns would.

So there are two ways to get a higher voltage:
1. Increase the number of turns.
2. Increase the rate of change of the flux.

Sometimes you can not change either of these things because they are already set by the design of the coil and external parts.

 

[R_W_B]

Hi, It's not *only* that it collapses, it's *also* how fast it collapses (or how fast it changes).

Ok this is helping me to see some more to this. (And I also surmise the ratio I'm quoting is merely one that was used for an example and probably is a larger ratio in reality)

But you are saying the speed of the collaspe (which would be great) is effecting the large voltage spike. I.e. then I would assume that this equates also that the faster one could turn an alternator the voltage would rise higher per revolution respectively ? Or is that a different scenario ?

 

[MrAl]

Hi,

That sounds about right because the coil would sweep past the magnets faster so the rate of change of flux would be higher.

With a coil that is disconnected (like an ignition coil) however the rate of change would be limited by the external circuit, such as a small capacitor or the resistance or even the nonlinear response of the spark plug.

 

[audioguru]

Take a little 12V relay coil and hold it in one hand so its terminals touch the skin. Then apply a little 9V battery to both contacts then remove the battery. You will feel a SHOCK!
When you remove the battery then the magnetic field quickly collapses which steps-up the 9V to hundreds of volts.
Don't hold the relay contacts in two hands because the shock might stop your heart.

 

[R_W_B]

Thanks again I believe I can see this now thanks to you guys.