Transformer Primary Impedance
- Thread starter sdilucca
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Re: Transformer Primary Impedance
Let's talk step-down, just for the sake of clarity.
The primary side sees a voltage that is higher than that seen by the secondary component, by a factor of the "turns ratio" of transformer.
The primary side sees a current that is lower than that seen by the secondary component, by a factor of the "inverse of the turns ratio" of transformer.
The resistance (as seen from the primary) is equal to the voltage (as seen from the primary) divided by the current (as seen from the primary). Therefore, the primary side sees a resistance that is higher than that seen by the secondary component, by a factor of the "square of the turns ratio" of transformer.
Let's talk step-down, just for the sake of clarity.
The primary side sees a voltage that is higher than that seen by the secondary component, by a factor of the "turns ratio" of transformer.
The primary side sees a current that is lower than that seen by the secondary component, by a factor of the "inverse of the turns ratio" of transformer.
The resistance (as seen from the primary) is equal to the voltage (as seen from the primary) divided by the current (as seen from the primary). Therefore, the primary side sees a resistance that is higher than that seen by the secondary component, by a factor of the "square of the turns ratio" of transformer.
rcwilson
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- Redmond, WA
Re: Transformer Primary Impedance
A transformer transforms power from a high voltage to a lower voltage. Power is the same on both sides of the transformer (neglecting the minor losses of the transformer). So the kVA on the high voltage side is the same as the kVA on the low voltage side.
KVA = Volts x amps x 1000. Since KVA is constant, but the voltage is different on the two sides of the transformer, the current has to be different. With different voltages and currents Ohms law says there will be different impedances. (Impedance = Volts/Amps)
To make it simple, assume a single-phase 100 kVA transformer stepping voltage down from 1000 V to 100 V feeding a 10 kVA load.
Primary voltage = 1000 V.
Primary amps (for 10 kVA load) = (10 kVA x 1000) / 1000V = 10 amps.
Secondary voltage = 100V,
Secondary amps (for 10 kVA load) = (10 kVA x 1000) / 100V = 100 amps.
The impedance of the 10 kVA load is given by ohms law: Z= V/I.
10 kVA impedance on the primary side = Zp = 1000V/ 10 A = 100 ohms.
10 kVA impedance on the secondary side = Zs = 100V/ 100 A = 1 ohm.
Note that the turns ratio on this transformer is 10:1. Voltage ratio is 1000/100 = 10:1. Current ratio is 10/100 = 1:10. Impedance ratio = 100/1 = 100:1 = (10:1) squared
I hope this helps you see that transformers transmit power and that power is the same from the generator through the transmission lines and transformers and panels to the load. The actual values of voltage, amps and impedances depend on which voltage level we are looking at. That?s why we have to change the impedance value when we go through a transformer. (That is also why engineers use the per unit method in calculations involving several voltage levels, but that?s another thread.)
A transformer transforms power from a high voltage to a lower voltage. Power is the same on both sides of the transformer (neglecting the minor losses of the transformer). So the kVA on the high voltage side is the same as the kVA on the low voltage side.
KVA = Volts x amps x 1000. Since KVA is constant, but the voltage is different on the two sides of the transformer, the current has to be different. With different voltages and currents Ohms law says there will be different impedances. (Impedance = Volts/Amps)
To make it simple, assume a single-phase 100 kVA transformer stepping voltage down from 1000 V to 100 V feeding a 10 kVA load.
Primary voltage = 1000 V.
Primary amps (for 10 kVA load) = (10 kVA x 1000) / 1000V = 10 amps.
Secondary voltage = 100V,
Secondary amps (for 10 kVA load) = (10 kVA x 1000) / 100V = 100 amps.
The impedance of the 10 kVA load is given by ohms law: Z= V/I.
10 kVA impedance on the primary side = Zp = 1000V/ 10 A = 100 ohms.
10 kVA impedance on the secondary side = Zs = 100V/ 100 A = 1 ohm.
Note that the turns ratio on this transformer is 10:1. Voltage ratio is 1000/100 = 10:1. Current ratio is 10/100 = 1:10. Impedance ratio = 100/1 = 100:1 = (10:1) squared
I hope this helps you see that transformers transmit power and that power is the same from the generator through the transmission lines and transformers and panels to the load. The actual values of voltage, amps and impedances depend on which voltage level we are looking at. That?s why we have to change the impedance value when we go through a transformer. (That is also why engineers use the per unit method in calculations involving several voltage levels, but that?s another thread.)
Re: Transformer Primary Impedance
You probably know all this already.This will be a very elementary answer since I am not a engineer or Physicist.
I could be wrong but I don't know if anyone can answer the question you ask.I will leave that up to others.
A transformer is equal to a AC. generator or Alternator.
The difference is that a alternator uses a rotating field as a magnet to make a moving magnetic field.
The transformer uses a moving magnetic field made by the alternating voltage of the AC. voltage applied to its primary coil or winding.
The only connection between the primary and the secondary is magnetic.
In real use we do have a connection through the ground but has nothing to do with the way it generates electrical power.
What we do know about this is that the primary coil makes Xs amount of magnetic lines of force or magnetism.
And the added load or lowered impedance or ac resistance requires more magnetic lines of force or magnetism to make enough current to drive the applied load. And the transformer without protection will destroy itself if not protected trying to satisfy the secondary load.
Is there any Physicist or Physis out there that can tell exactly what happens to the magnetic forces between the primary and secondary please help?
What about it Sam?
You probably know all this already.This will be a very elementary answer since I am not a engineer or Physicist.
I could be wrong but I don't know if anyone can answer the question you ask.I will leave that up to others.
A transformer is equal to a AC. generator or Alternator.
The difference is that a alternator uses a rotating field as a magnet to make a moving magnetic field.
The transformer uses a moving magnetic field made by the alternating voltage of the AC. voltage applied to its primary coil or winding.
The only connection between the primary and the secondary is magnetic.
In real use we do have a connection through the ground but has nothing to do with the way it generates electrical power.
What we do know about this is that the primary coil makes Xs amount of magnetic lines of force or magnetism.
And the added load or lowered impedance or ac resistance requires more magnetic lines of force or magnetism to make enough current to drive the applied load. And the transformer without protection will destroy itself if not protected trying to satisfy the secondary load.
Is there any Physicist or Physis out there that can tell exactly what happens to the magnetic forces between the primary and secondary please help?
What about it Sam?
al hildenbrand
Senior Member
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- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
Thinking in terms of an analogy, sdilucca, did you ever turn a bicycle upside down and turn the pedals with one hand and slow down the wheel with your other hand?
The gearing and chain is the transformer function.
The drag of a hand on the wheel is reflected to the hand on the pedal. Changes in the drag telegraph immediately.
Thinking in terms of an analogy, sdilucca, did you ever turn a bicycle upside down and turn the pedals with one hand and slow down the wheel with your other hand?
The gearing and chain is the transformer function.
The drag of a hand on the wheel is reflected to the hand on the pedal. Changes in the drag telegraph immediately.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Transformer Primary Impedance
Here's a few rules about how electromagnetism works:
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Finally, let's loop the wire around a chunk of metal. Make it a square shape, but hollow in the middle, like a square donut. Wrap the windings around the left side. The magnetic field created inside the loops will be felt by the metal. Presuming that we picked a metal with good magnetic properties, the magnetic field will be present throughout the square shape of the metal. That brings us to,
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But you can get a "moving magnetic field," even with no moving parts. If the source of the magnetic field is an alternating current, then the magnetic field that it creates will rise and fall with the rise and fall of the current. This is equivalent to a "moving magnetic field," in that it is the same as taking a bar magnet and bringing it closer, then pulling it away, then bringing it closer, and continuing in this pattern until your hand gets tired.
So let's wrap the right hand side of that square, hollow, metal thing with more wires. Since there is a varying (i.e., "moving") magnetic field, having been created by the wires on the left side, that field will be felt by the electrons in the wires on the right hand side. They will therefore feel a push, because of Rule #3. That push will set them in motion (i.e., it creates a current in the wires on the right-hand side). The direction of motion will be along the wires. You have just created the secondary of the transformer.
I'll give it a try. Let me know if this helps.
Here's a few rules about how electromagnetism works:
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- <font size="2" face="Verdana, Helvetica, sans-serif">Rule #1: The presence of a charge will cause an electric field to be created.</font>
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Rule #2: A charge that is in motion will create around itself a magnetic field.</font>
Finally, let's loop the wire around a chunk of metal. Make it a square shape, but hollow in the middle, like a square donut. Wrap the windings around the left side. The magnetic field created inside the loops will be felt by the metal. Presuming that we picked a metal with good magnetic properties, the magnetic field will be present throughout the square shape of the metal. That brings us to,
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Rule #3. A charge that is in the presence of a moving magnetic field will feel a force.</font>
But you can get a "moving magnetic field," even with no moving parts. If the source of the magnetic field is an alternating current, then the magnetic field that it creates will rise and fall with the rise and fall of the current. This is equivalent to a "moving magnetic field," in that it is the same as taking a bar magnet and bringing it closer, then pulling it away, then bringing it closer, and continuing in this pattern until your hand gets tired.
So let's wrap the right hand side of that square, hollow, metal thing with more wires. Since there is a varying (i.e., "moving") magnetic field, having been created by the wires on the left side, that field will be felt by the electrons in the wires on the right hand side. They will therefore feel a push, because of Rule #3. That push will set them in motion (i.e., it creates a current in the wires on the right-hand side). The direction of motion will be along the wires. You have just created the secondary of the transformer.
Re: Transformer Primary Impedance
i am also trying to understand the behavior of the magnetic field in in a transformer. Here is a good question:
since a magnetic field originates from a specified point in space, and it is always closed loop... can it be said that the magnetic loop between the primary and secondary is always stretching back and forth (across the hollow section of the core) between primary and secodary? The best way to picture it is making believe the primary blowing a soap bubble till it reaches the core of the secondary, and then deinflactes till it's back at the primary, then cycle repeats. Hope you get the picture. Does this sound correct?
i am also trying to understand the behavior of the magnetic field in in a transformer. Here is a good question:
since a magnetic field originates from a specified point in space, and it is always closed loop... can it be said that the magnetic loop between the primary and secondary is always stretching back and forth (across the hollow section of the core) between primary and secodary? The best way to picture it is making believe the primary blowing a soap bubble till it reaches the core of the secondary, and then deinflactes till it's back at the primary, then cycle repeats. Hope you get the picture. Does this sound correct?
Re: Transformer Primary Impedance
Charlie if you where in front of me I would have to give you as cigar for the effort you took to try to explain that,you are probably right and a lot of people probably understand it but explaining it is another matter.I know enough on the magnetic coupling to be dangerous.
Charlie if you where in front of me I would have to give you as cigar for the effort you took to try to explain that,you are probably right and a lot of people probably understand it but explaining it is another matter.I know enough on the magnetic coupling to be dangerous.
al hildenbrand
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- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
The instant when the field in a transformer core is zero is not all that helpfull in the basic understanding.
Rather, it is that the field in the core is changing in strength. The change is what couples the windings.
The shape of the field is determined by the physical shape of the core.
I don't think of it as a monopole.
The instant when the field in a transformer core is zero is not all that helpfull in the basic understanding.
Rather, it is that the field in the core is changing in strength. The change is what couples the windings.
The shape of the field is determined by the physical shape of the core.
Re: Transformer Primary Impedance
how is it possible for the magnetic field to induce current in the secondary wires, if the magnetic field is confined to the core? Doesn't a magnetic field have to physically intersect the wire to induce current? Someone please shed some light.
how is it possible for the magnetic field to induce current in the secondary wires, if the magnetic field is confined to the core? Doesn't a magnetic field have to physically intersect the wire to induce current? Someone please shed some light.
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Re: Transformer Primary Impedance
But I am not sure we are both using the same meaning of the phrase "hollow space." I meant the "donut hole." I am presuming that the iron core is made of solid iron, and has no hollow space internal to the iron itself. Magnetic fields are more easily "stretched" from pole to pole through iron than through air. The field will pass through the hollow space, but not easily. It will prefer to pass through the iron core; that is why we use iron cores. What is happening is that the primary current is causing the iron that its loops are encircling to become magnetized. That area of (now magnetized) iron causes the iron just outside the loops to become magnetized. That causes the remainder of the iron core to become magnetized. You might say that the magnetic field "flows" through the iron, in the same sense that water flows through a hose, and current flows through a wire. The hollow space, the air, plays only a minor role in the operation of a transformer.
This is not true. There can never be a point source of magnetic field. No matter how many pieces you try to break a bar magnet into, and no matter how small they get, each and every piece will always have a North pole and a South pole.
A magnetic field can be said to stretch from a North Pole to a South pole. In a transformer, the magnetic field is called into being by the current in the primary. That creates a magnetic field both inside the loop (where it is felt by the iron core) and outside the loop (where it can be felt by the magnetic stripe of your credit cards). The North pole will exist at the beginning of the first loop of the primary windings. The South pole will exist at the end of the last loop of the primary windings. On the other half-cycle (i.e., when the primary current is negative), the North and South poles are reversed. I might use the phrase "stretches back and forth" in the sense that the AC source flips back and forth between positive current and negative current, causing the North and South poles to shift back and forth.
But I am not sure we are both using the same meaning of the phrase "hollow space." I meant the "donut hole." I am presuming that the iron core is made of solid iron, and has no hollow space internal to the iron itself. Magnetic fields are more easily "stretched" from pole to pole through iron than through air. The field will pass through the hollow space, but not easily. It will prefer to pass through the iron core; that is why we use iron cores. What is happening is that the primary current is causing the iron that its loops are encircling to become magnetized. That area of (now magnetized) iron causes the iron just outside the loops to become magnetized. That causes the remainder of the iron core to become magnetized. You might say that the magnetic field "flows" through the iron, in the same sense that water flows through a hose, and current flows through a wire. The hollow space, the air, plays only a minor role in the operation of a transformer.
al hildenbrand
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- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
One conductor is the primary. As the current in this conductor increases, the field strength around the conductor increases.
The other conductor, the secondary, reflects the increasing magnetic field with an induced current.
Here's an important point. If the magnetic field is constant, that is, the current in the Primary is DC, no current is induced in the secondary.
Part of what tricks my mind is that I have two basic conductors inside of one concentric magnetic field.
One conductor is the primary. As the current in this conductor increases, the field strength around the conductor increases.
The other conductor, the secondary, reflects the increasing magnetic field with an induced current.
Here's an important point. If the magnetic field is constant, that is, the current in the Primary is DC, no current is induced in the secondary.
al hildenbrand
Senior Member
- Location
- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
The magnetic field is concentrated in the core.
The core is a material that is highly permeable by the magnetic field. The field finds it much easier to be in the core than in free air.
In fact, there is field in the copper windings, the air between the windings and the core, and the entire volume of the universe (no kidding) outside of the windings. But that field strength is not large, compared with the field strength inside the core.
The magnetic field is concentrated in the core.
The core is a material that is highly permeable by the magnetic field. The field finds it much easier to be in the core than in free air.
In fact, there is field in the copper windings, the air between the windings and the core, and the entire volume of the universe (no kidding) outside of the windings. But that field strength is not large, compared with the field strength inside the core.
al hildenbrand
Senior Member
- Location
- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
One last thought. All the field that is not in the core is small enough, that, for purposes of trying to understand the basic notion, it is simply ignored.
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Re: Transformer Primary Impedance
I like Al's comments. Let me add another tidbit.
The current in the primary causes a field to exist. See my earlier "Rule #2." That field is concentrated within the center of the loops. That is why we put the iron core within the loops, so it can pick up the greater part of the magnetic field.
The magnetic field then "flows" (if you will allow that term) through the iron core to the other side.
The secondary windings now have a magnetic field that is rising and falling within the space of their loops of wire. The wire will sense that field, and a current is created in the secondary windings. See my earlier "Rule #3."
Yes it does. You are right about this. But any magnet will have surrounding it a magnetic field. The iron core has become a magnet, and the secondary windings are within the range of the magnetic field that surrounds the iron core.
I like Al's comments. Let me add another tidbit.
The current in the primary causes a field to exist. See my earlier "Rule #2." That field is concentrated within the center of the loops. That is why we put the iron core within the loops, so it can pick up the greater part of the magnetic field.
The magnetic field then "flows" (if you will allow that term) through the iron core to the other side.
The secondary windings now have a magnetic field that is rising and falling within the space of their loops of wire. The wire will sense that field, and a current is created in the secondary windings. See my earlier "Rule #3."
al hildenbrand
Senior Member
- Location
- Minnesota
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- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: Transformer Primary Impedance
LOL
LOL
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