Batteries and Door Bell Transformer

[Dennis Alwon]

Thanks all. In my drawing I forgot to reverse the dots but I did realize that that would happen if the winding was reversed. Well, I wasn't sure but I thought that was what was being said.

I still don't see why changing the turn of the wire changes the polarity. I understand that it does but WHY? How does the core know which way the wire is wound

 

[romex jockey]

View attachment 21935

grand visual.....~RJ~

 

[romex jockey]

Thanks all. In my drawing I forgot to reverse the dots but I did realize that that would happen if the winding was reversed. Well, I wasn't sure but I thought that was what was being said.

I still don't see why changing the turn of the wire changes the polarity. I understand that it does but WHY? How does the core know which way the wire is wound

Kinda like the whole PV interconnection, electrons passing some go south, some go north

all of which reeks of atomic level theory , none of which i 'get' either.....:weeping:~RJ~

 

[SG-1]

Here is what the core is seeing that we cannot:

The link is to an interactive tutorial from the Nation High Magnetic Field Laboratory: Magnetic Field Around a Wire

https://nationalmaglab.org/educatio...ay/interactive/magnetic-field-around-a-wire-i


Here is a link to the full list of interactive tutorials:

https://nationalmaglab.org/education/magnet-academy/watch-play/interactive

 

[LarryFine]

I still don't see why changing the turn of the wire changes the polarity. I understand that it does but WHY? How does the core know which way the wire is wound

For a given half-cycle, the current of the primary magnetizes the core in one direction (polarity) and that magnetic flux (change in flux, actually) induces a voltage in the secondary in one direction.

If you reverse any one parameter, such as the primary connections, the primary winding direction, the secondary winding direction, or the secondary connections, the output voltage polarity reverses.

 

[Dennis Alwon]

For a given half-cycle, the current of the primary magnetizes the core in one direction (polarity) and that magnetic flux (change in flux, actually) induces a voltage in the secondary in one direction.

If you reverse any one parameter, such as the primary connections, the primary winding direction, the secondary winding direction, or the secondary connections, the output voltage polarity reverses.

Thank you.

 

[Besoeker]

I still don't see why changing the turn of the wire changes the polarity. I understand that it does but WHY? How does the core know which way the wire is wound

The core doesn't care. It's the relationship between the windings on that same core that matters.

If you have two windings that are in phase the both go positive at the same time. Reverse how one of the windings is would and the two are in antiphase. When one goes positive, the other goes negative.

Whether that matters is another matter.

 

[Dennis Alwon]

Here is what the core is seeing that we cannot:

The link is to an interactive tutorial from the Nation High Magnetic Field Laboratory: Magnetic Field Around a Wire

https://nationalmaglab.org/educatio...ay/interactive/magnetic-field-around-a-wire-i


Here is a link to the full list of interactive tutorials:

https://nationalmaglab.org/education/magnet-academy/watch-play/interactive

Thank you-- First link explains it well. I guess I lost tract of this being ac and the power cycling at 60 times per sec

 

[mivey]

I still don't see why changing the turn of the wire changes the polarity. I understand that it does but WHY? How does the core know which way the wire is wound

Because it doesn't like change. It wants to oppose the force trying to change the flux through the coil.

If the wire is wound in one direction, the current moves in a direction to create enough flux to oppose the acting force. If you wind in the opposite direction and the acting force hasn't changed direction, the current then has to act in the opposite direction to create the same opposing flux.

This is based on Faraday's Law and Lenz's Law that are based in Maxwell's equations.

See these links:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c2

 

[Dennis Alwon]

Thanks mivey.. This is a different question. What happens if the core of the transformer was a magnet instead of an iron core?

 

[LarryFine]

What happens if the core of the transformer was a magnet instead of an iron core?

DC offset? Asymmetrical sine wave?

 

[K8MHZ]

Thanks mivey.. This is a different question. What happens if the core of the transformer was a magnet instead of an iron core?

Read all about it here:

https://en.wikipedia.org/wiki/Magnetic_core

 

[gar]

181231-1149 EST

Dennis Alwon:

A magnet is a steady state DC unchanging magnetic field. No field change no induced voltage. The induced voltage is e = N*df/dt where df/dt is the rate of change of the field. However, with a magnet you can move either the magnet or the coil relative to each other to get a changing field.

To your fixed magnet add a primary coil that you excite, and now you have a biased varying magnetic field, and there is an induced voltage in the secondary, but only from the varying part of the field.

.

 

[winnie]

Two things:

Imagine that the secondary was a _single_ turn of wire, rather than a coil. At any given instant, one lead of that single turn will be + and the other -, and this changes over time.

Now imagine a stack of these single turns. If you connect the 'left' of each single turn to the 'right' of each turn above it, then you have a coil going one direction. If you connect the 'right' of each single turn to the 'left' of each turn above it, then you have a coil going in the opposite direction. By thinking of a coil as a stack of single turns in series, it is pretty easy to see that the 'direction' of the coil will change the polarity of the output.

Second:
If the core of the transformer is somehow made with a permanent magnet which you _rotate_, then instead of a transformer you have a generator

-Jon

 

[Dennis Alwon]

Second:
If the core of the transformer is somehow made with a permanent magnet which you _rotate_, then instead of a transformer you have a generator

-Jon

Thanks Jon

I was thinking that if I used a circular magnet as the core and then wrapped a wire around opposite sides- One the primary and the other the secondary-- what would happen. Would I get nothing?

 

[mivey]

Thanks mivey.. This is a different question. What happens if the core of the transformer was a magnet instead of an iron core?

Depends on the magnet.

The AC signal replication depends on the ability to reverse the magnetic field. We tend to do this along a path of hysteresis that is fairly linear. We don't drive the core to saturation. Even so, we have to overcome some residual magnetization at each reversal and that causes some distortion and energy loss.

If we could reverse the magnet, it would require a lot more energy to reverse it out of saturation. See the article here on coercivity and the reverse field needed:

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/magperm.html#c1

So, we could possibly get the magnet reversed and start alternating in a linear region but probably not because a "permanent" magnet material is not as good for an alternating field as transformer material.

We might could get a signal but it would be distorted at the least.

 

[Sahib]

I was thinking that if I used a circular magnet as the core and then wrapped a wire around opposite sides- One the primary and the other the secondary-- what would happen. Would I get nothing?

It depends on the permeability of the magnet. If the magnet is as permeable as the iron core, considerable voltage would be induced on the secondary. On the other hand, if the permeability of the magnet is as low as air, the voltage induced on the secondary would be negligible.

 

[mivey]

Dennis,

In the following article in the "Variations in Hysteresis Curves" section at the bottom is an illustration on the difference in the hysteresis curve for a magnet vs a transformer core:

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/hyst.html#c4

 

[Dennis Alwon]

You all are amazing. I actually got most of it. Thank you

Another question... How does one determine the number of turns of wire needed to get the needed transformed voltage. Can I assume the number of turns on the primary and secondary are different?

 

[LarryFine]

Another question... How does one determine the number of turns of wire needed to get the needed transformed voltage. Can I assume the number of turns on the primary and secondary are different?

Yes, quite. I know the actual turns numbers are based on desired capacity, from which they select conductor sizes and desired voltages.

Made-up example: If you supply a primary with 240v, and it has 480 turns, that means every turn receives 1/2 of a volt. That same number will be created by every other winding on the same core; if you need 120v out, you wind the secondary with 240 turns.

The actual turns ratio is wound slightly higher than the theoretical I just described, to compensate for losses. That's why a reverse-fed transformer puts out a slightly lower voltage than its rating; it's "de-compensating" because of the backwards ratio adjustment.