Primary vs secondary turns

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mbrooke

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I constantly hear education text books talking about winding ratios, ie a 480 to 120 volt transformer will have 4 times more primary turns than secondary turns with graphics showing the same size wire. However come real I don't think that always to be the case. I have seen many actual transformers where the primary is of much thinner wire while the secondary is of thicker wire.


Does size also play a role in voltage/amp ratios? Ie if I had 20,000 primary turns of 24 gauge wire and 20,000 turns of 14 gauge secondary wire would my secondary voltage be lower with more available current?
 

ron

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The wire size is related to the voltage. The higher voltage carries less current so the wire can be thinner.

The turns ratio must be a reality or the voltage relationship (primary / secondary) is out the window.
 

Carultch

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I constantly hear education text books talking about winding ratios, ie a 480 to 120 volt transformer will have 4 times more primary turns than secondary turns with graphics showing the same size wire. However come real I don't think that always to be the case. I have seen many actual transformers where the primary is of much thinner wire while the secondary is of thicker wire.


Does size also play a role in voltage/amp ratios? Ie if I had 20,000 primary turns of 24 gauge wire and 20,000 turns of 14 gauge secondary wire would my secondary voltage be lower with more available current?

The primary windings carry a lot less current than the secondary windings. Therefore, they can be a lot thinner.

The thickness of the winding, whether it be a wire or a sheet of metal, is not relevant to the "TURNS RATIO". What matters is the number of complete closed paths the current in that winding has to make around the magnetic core.

The governing physical laws of transformers are Ampere's law of magnetic fields due to currents and Faraday's law of induction. The more turns in the winding on the primary side, the more its current contributes to the magnetic field in the core material. The more turns on the secondary side, the more voltage is induced from the time-varying magnetic field in the core material.


A mechanical analogy of a transformer is a gear assembly. The ratio of the number of teeth sets the ratio of the rotational speeds, since it rigidly links the speeds of the teeth where they mesh. Since the gear assembly cannot create energy, power is at most the same on the output shaft. The larger gear that spins slow will experience a low torque, and the smaller gear that spins fast will experience a high torque.
 

mbrooke

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The primary windings carry a lot less current than the secondary windings. Therefore, they can be a lot thinner.

The thickness of the winding, whether it be a wire or a sheet of metal, is not relevant to the "TURNS RATIO". What matters is the number of complete closed paths the current in that winding has to make around the magnetic core.

The governing physical laws of transformers are Ampere's law of magnetic fields due to currents and Faraday's law of induction. The more turns in the winding on the primary side, the more its current contributes to the magnetic field in the core material. The more turns on the secondary side, the more voltage is induced from the time-varying magnetic field in the core material.

But wouldn't winding size also play a role in the amount of induction? Larger primary wire means thicker lines of flux? Or am I off (probably am:lol:)
 

Carultch

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But wouldn't winding size also play a role in the amount of induction? Larger primary wire means thicker lines of flux? Or am I off (probably am:lol:)

Not true. Take a closer look at Maxwell's equations again.

Larger windings mean the current is more distributed, but it still is the same current.
 

mbrooke

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Not true. Take a closer look at Maxwell's equations again.

Larger windings mean the current is more distributed, but it still is the same current.


So turns ratio always applies then? A 46000 volt 100kva transformer will always have 384 primary turns to one secondary turn even though the winding sizes will differ (2.1amps to 833 amps)

I ask because I have seen pole transformers where secondary windings look like only a few turns of large sheet metal while the primary is a spool of thin wire. So 4 turns giving 240 volts will always equal 220 turns of primary coils at 13200 volts?
 

jim dungar

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So turns ratio always applies then? A 46000 volt 100kva transformer will always have 384 primary turns to one secondary turn even though the winding sizes will differ (2.1amps to 833 amps)

I ask because I have seen pole transformers where secondary windings look like only a few turns of large sheet metal while the primary is a spool of thin wire. So 4 turns giving 240 volts will always equal 220 turns of primary coils at 13200 volts?

It is possible to use paralleled small conductors when creating a coil, which gives the impression of more windings.
 
To expand on the discussion, i have two questions about turns and wire size:

1) two transformers, one with twice the KVA capacity, all other things equal. What is the difference?
2) two transformers, one designed for twice the frequency, all other things equal. What is the difference?
 

Carultch

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Location
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To expand on the discussion, i have two questions about turns and wire size:

1) two transformers, one with twice the KVA capacity, all other things equal. What is the difference?
2) two transformers, one designed for twice the frequency, all other things equal. What is the difference?

I'm not an expert in transformer design, so I cannot answer quantitatively. Don't expect a linear relation of transformer design geometry to electrical characteristics. Here is my conceptual explanation nonetheless.

1) The transformer with the higher KVA capacity needs to have thicker windings for both the primary and secondary, as well as larger termination assemblies. The ratio of number of windings from primary to secondary is still the same. It is the thickness of the windings that changes. As well as all other components of the heat management system (dielectric fluid volume, fin design, housing size, etc). Parallel winding assemblies might also be used in order to achieve higher KVA. As well as wound sheets of metal, instead of coils of wire.

2) Transformers work best at high frequencies. In order to make it work for lower frequencies, the windings need more inductance. You can use a 50 Hz transformer for a 60 Hz application, and it will work just the same. But you cannot necessarily use a 60 Hz transformer for a 50 Hz application, and expect the same performance. To achieve more inductance, this can mean more turns, longer winding "wires", a larger radius of the core and winding turns, a different composition of iron in the core, and numerous other geometric differences in the design.
 

Carultch

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Location
Massachusetts
I would also like to add another topic to this discussion. Efficiency considerations.

I often think of a transformer as analogous to a gear system. Gears trade torque for speed (or vice versa), while nearly preserving power from input to output. Similarly, transformers trade voltage for current (or vice versa), while nearly preserving power from input to output.

In the case of gears, the speed ratio is rigidly constrained by the geometry of the gears. The teeth, the pitch diameters, etc. The torque is what gets "sacrificed" due to inefficiencies within the transmission system. For a 2-to-1 reduction system, the speed on the output has to be 1/2 the speed on the input. But the torque on the output is not necessarily twice the torque on the input. In the limit of perfection it is, but in practice it is slightly less.

What about transformers? How do they handle inefficiency? Is the rigid constraint on the voltage ratio matching the turns ratios, while the current will be slightly different than what you'd expect? Or the other way around?

Or perhaps, are the inefficiency properties of a transformer a little more amorphous, such that neither property is rigidly constrained to the turns ratio? If this is the case, you might want to expect a slightly different turns ratio in practice, than that which theory predicts. It might be larger on the secondary, in order to compensate for internal voltage drops. And it can make a difference which way the transformer operates, such that a step-up application should have a slightly different turns ratio, than the equivalent step-down transformer.
 
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