# Transformer and alternator

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#### nix85

##### Member level 3
I got one simple question which i'm sure you will find easy to answer.

Namely, if we feed transformer with pulsed DC, output is AC as demonstrated here:

and here

What's confusing me is this...

How do we explain this in comparison with alternator..

Lets say north pole of rotor magnet approaches the coil and voltage reaches the peak as it comes closest to the coil. As it starts moving away voltage drops and reaches 0. This is first half-cycle.

Then south pole starts approaching and voltage rises in opossite direction, then falls to 0 and cycle repeats.

Now... Same thing happens in transformer, only solid state.

Feeding pulsed dc to the primary is the same as if alternator's rotor poles are both north or both south, each time they pass near the coil they induce voltage in same direction.

So if this action would produce pulsing DC in a generator, why does it produce AC in a transformer?

I think question is clear, but if it's not i will gladly clearify.

#### kripacharya

##### Banned
Re: How to correctly calculate coil's field strength?

I got one simple question which i'm sure you will find easy to answer.

Namely, if we feed transformer with pulsed DC, output is AC as demonstrated here:

and here

What's confusing me is this...

How do we explain this in comparison with alternator..

View attachment 154126

Lets say north pole of rotor magnet approaches the coil and voltage reaches the peak as it comes closest to the coil. As it starts moving away voltage drops and reaches 0. This is first half-cycle.

Then south pole starts approaching and voltage rises in opossite direction, then falls to 0 and cycle repeats.

Now... Same thing happens in transformer, only solid state.

Feeding pulsed dc to the primary is the same as if alternator's rotor poles are both north or both south, each time they pass near the coil they induce voltage in same direction.

So if this action would produce pulsing DC in a generator, why does it produce AC in a transformer?

I think question is clear, but if it's not i will gladly clearify.
This is electromagnetics 101, and an entirely different thread.

Any varying magnetic field will create a corresponding varying current in a conductor.

This explanation should be enough to explain all of your queries.

#### nix85

##### Member level 3
Re: How to correctly calculate coil's field strength?

This is electromagnetics 101, and an entirely different thread.

Any varying magnetic field will create a corresponding varying current in a conductor.

This explanation should be enough to explain all of your queries.
You reply is useless as usual. Read the question again.

- - - Updated - - -

I think the answer lies in the fact that transformer action - primary field building up and collapsing is akin to sliding a magnet into and out of the solenoid, while in the alternator case magnetic field is passing horizontally across the coil, not into and out of it. This difference in relative motion results in different output signal.

#### kripacharya

##### Banned
Once again .. A varying magnetic field will create a corresponding varying current in a conductor. This could be a relative variation or motion. A steady magnetic field does not create any current. A well understood application of this fact should be enough to answer all of your queries.

Conversely a steady current through a conductor will create a steady magnetic field, while a varying current (i.e. accelerating electrons) will create a varying magnetic field.

These are the general principles. It just needs diligent and careful application in specific cases.

#### summitville

##### Member level 3
Feeding pulsed dc to the primary is the same as if alternator's rotor poles are both north or both south, each time they pass near the coil they induce voltage in same direction.
Since there appears to be no answer, I will try ...

I will start with - I disagree with your above assumption.

AC Alternator ...
When the North Pole of the rotor passes by the coil,
the motion is always in one direction - the rotation of the rotor.
Causing the "+" half cycle voltage in the coil to increase to max voltage, then falls back to zero volts.
Then the passing of the South Pole, which causes the opposite max "-" half cycle voltage, then fall back to zero volts.

Pulsed DC Transformer ...
When the primary coil is "charged" to max current ...
the magnetic field expands and then stops expanding, causing the secondary voltage to "+" max voltage and then falls back to zero.
When the primary coil is "discharged" to zero amps ...
the collapsing magnetic field reverses direction and returns to zero flux, causing the secondary voltage to "-" max voltage and then falls back to zero.

The rotor moving the North Pole past the coil is equivalent to increasing the amps in the primary of the transformer = "+" half cycle
The rotor moving the South Pole past the coil is equivalent to decreasing the amps in the primary of the transformer = "-" half cycle

It is this reversal of direction of the magnetic flux in the transformer that is different than an AC Alternator spinning in one direction.

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#### nix85

##### Member level 3
Since there appears to be no answer, I will try ...

I will start with - I disagree with your above assumption.

AC Alternator ...
When the North Pole of the rotor passes by the coil,
the motion is always in one direction - the rotation of the rotor.
Causing the "+" half cycle voltage in the coil to increase to max voltage, then falls back to zero volts.
Then the passing of the South Pole, which causes the opposite max "-" half cycle voltage, then fall back to zero volts.

Pulsed DC Transformer ...
When the primary coil is "charged" to max current ...
the magnetic field expands and then stops expanding, causing the secondary voltage to "+" max voltage and then falls back to zero.
When the primary coil is "discharged" to zero amps ...
the collapsing magnetic field reverses direction and returns to zero flux, causing the secondary voltage to "-" max voltage and then falls back to zero.

The rotor moving the North Pole past the coil is equivalent to increasing the amps in the primary of the transformer = "+" half cycle
The rotor moving the South Pole past the coil is equivalent to decreasing the amps in the primary of the transformer = "-" half cycle

It is this reversal of direction of the magnetic flux in the transformer that is different than an AC Alternator spinning in one direction.
You start with disagreeing and then you back that up with false claim. I thought that was the case too until i saw the video below. What happens is the exact opposite.

As magnetic pole approaches the coil voltage is induced in one direction - coil's field repelling the magnet, and as it starts to receed from the coil now voltage is induced in opposite direction and coil attracts the magnet, in both cases resisting rotation, as shown (Lenz). Much like when magnet is shoved into and out of the coil.

https://youtu.be/bht9AJ1eNYc?t=2735

If we look at a single conductor moving through a magnetic field, however, then as the conductor approaches the magnet and starts to cut more flux lines, higher voltage is induced until it gets closest to the magnet, then as it receeds we get decreasing voltage in same direction, as shown.

#### summitville

##### Member level 3
That You-Tube video is of a DC Generator, not an AC Alternator with magnets on the rotor.
In an AC Alternator ... as the North Pole is leaving the stator coil, the following South Pole is simultaneously approaching the coil.
A N-S pair passes the coil, then a S-N pair passes the coil, then a N-S pair passes the coil ...

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#### nix85

##### Member level 3
That You-Tube video is of a DC Generator, not an AC Alternator with magnets on the rotor.
In an AC Alternator ... as the North Pole is leaving the stator coil, the following South Pole simultaneously approaching the coil.
No, it is AC generator as it clearly states.

South pole approaching produces the same voltage
as north pole leaving and that is irrelevant in the context.

#### summitville

##### Member level 3
OK, I see now that it is not a commutator but two slip rings = AC generator.
Initially, the video does not show the alternating current - which is wrong and very confusing!
Finally at time 46:43, the video does show that the current flips to the opposite direction, once every 180° = once per N-S pair = 1/2 cycle of voltage
Do you see the Voltage reversing in your latest You-Tube video?
As the armature spins through one N-S pair, the current reverses in the next S-N pair.

The magnets are in N+S, S+N, N+S pairs on an AC rotor, like you showed in message #1, and also N-S Pairs in the stator like your latest video

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#### nix85

##### Member level 3
OK, I see now that it is not a commutator but two slip rings = AC generator.
Initially, the video does not show the alternating current - which is wrong and very confusing!
Finally at time 46:43, the video does show that the current flips to the opposite direction, once every 180° = once per N-S pair = 1/2 cycle of voltage
Do you see the Voltage reversing in your latest You-Tube video?
As the armature spins through one N-S pair, the current reverses in the next S-N pair.

The magnets are in N+S, S+N, N+S pairs on an AC rotor, like you showed in message #1, and also N-S Pairs in the stator like your latest video
It does show alternating current from the start. Look at the arrows representing the current flow.

Unlike what you said and what i initially thought, polarity of the current flips as magnet starts to receed from the coil.

#### kripacharya

##### Banned
The video is inexact. Polarity does not flip when rotor passes the magnetic pole, but rather at the midpoint when it is at 90°.

If it changed direction when passing the pole then we would have a discontinuity at that point - i. e. Going from max in one direction to max in other direction instantaneously.

- - - Updated - - -

Your post #6 makes incorrect use of Lenz's law, and hence results in incorrect conclusion. Please revisit.

#### summitville

##### Member level 3
From time 45:33 until 46:08 in your You-Tube video, the armature is spinning but the Arrows are not changing direction.
That is very wrong & very misleading.
Finally, at time 46:09, the video shows an actual Voltage reversal.

Feeding pulsed dc to the primary is the same as if alternator's rotor poles are both north or both south, each time they pass near the coil they induce voltage in same direction.

I was not discussing WHEN the Voltage flip occurred, only that the voltage does flip with each N-S pair.

#### kripacharya

##### Banned
Also worth investigating is the affect of Special Relativity on the relative motion of conductor/ magnets, and especially related to Lorentz effect and Maxwell equations vis-a-vis Maxwell-Faraday equations.

#### nix85

##### Member level 3
From time 45:33 until 46:08 in your You-Tube video, the armature is spinning but the Arrows are not changing direction.
That is very wrong & very misleading.
Finally, at time 46:09, the video shows an actual Voltage reversal.

Feeding pulsed dc to the primary is the same as if alternator's rotor poles are both north or both south, each time they pass near the coil they induce voltage in same direction.

I was not discussing WHEN the Voltage flip occurred, only that the voltage does flip with each N-S pair.
Now that i looked into it better, we were right afterall and video is wrong.

As for my comparison of alternator with same poles and tranformer fed pulsed dc, you are right, it is not the same. I knew this but confused it. Clearly as voltage on the primary rises to the peak voltage on the secondary rises to the peak and drops to zero, as voltage on the primary starts collapsing, secondary voltage is now in other direction, unlike alternator with same poles where flux is in same direction all the time.

#### nix85

##### Member level 3
Video is NOT incorrect afterall.

Video is right if we consider field first cuts the left side of the coil producing one polarity and as it crosses the center and starts to move away cuts another side of the coil producing opposite polarity.

Just like shown in simulation of 3 phase axial alternator below.

#### kripacharya

##### Banned
Blades of magnetic flux?
Twelve magnets?
Zeropoitnflux?
What's a 'poitn'?
Where's the North/South poles? Or is it all monopoles?

Not really a 'simulation', it's more a graphical representation, and seems a poor & incorrect one too.

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##### Super Moderator
Deleeted an inflammatory comment, but had to also remove a couple of comments that followed. If you had posted something that you still feel is useful for the thread then please post it again. Just avoid inflammatory and/or personal attacks on other members.

#### nix85

##### Member level 3
You first elaborate what is incorrect in the video.

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#### kripacharya

##### Banned
You first elaborate what is incorrect in the video.
I already did. See my post #16

#### nix85

##### Member level 3
I already did. See my post #16
You did not. You only made unclear references, you did not explain what you consider incorrect.

If you are asking about polarity black and yellow squares are alternating poles. This is assumed.

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