How does back EMF contribute to the operation of an autotransformer?

[Merlin3189]

When the two currents are closest and they nearly cancel then the load would have minimal current (Is=Ip+ I' below).. why do you say it's more efficient?

Is= Ip + I' , so when Is nearly equals Ip, then I' is small. That's what makes it so efficient.
The large common winding carries little current I', so the I²R loss is small in this large winding.

But don't think this is something that can be arranged for any transformer ratio. It's just that autotransformers are very efficient when making small changes in voltage, because then the current in the bulk of the winding is small. As the step up/down ratio increases, the autotransformer advantage decreases.

 

[kiki_danc]

Is= Ip + I' , so when Is nearly equals Ip, then I' is small. That's what makes it so efficient.
The large common winding carries little current I', so the I²R loss is small in this large winding.

But don't think this is something that can be arranged for any transformer ratio. It's just that autotransformers are very efficient when making small changes in voltage, because then the current in the bulk of the winding is small. As the step up/down ratio increases, the autotransformer advantage decreases.

How about for typical 240v to 120v step down transformers? How efficient is it? When you mentioned efficient.. you were just referring to the winding being bigger or smaller to support bigger or smaller current demands (smaller wire for smaller current demand).. isn't it? You were not describing the operating losses? I know Isolation Transformer has more core loss (in terms of hysteresis and eddy current losses), so were you describing resistance loses in the efficiency of the winding or just the material during construction or both?

Why is current and magnetic field connected? What higher laws of physics require them to be binded? It seems strange that it is only an accident of nature. Sometimes I wonder if they are made that way so transformers and ac power is possible so humans can use them.

 

[Merlin3189]

Yes, I was using efficiency in a rather vague way, thinking of I²R losses in the windings. As you say, real losses include eddy and hysteresis. And autotransformer design may well say, use thinner wire and have similar I²R losses with a smaller, cheaper transformer.

For your example, both the series and common windings carry the same current as the primary of a similar dual wound transformer, but then you have no secondary winding at all. In the DWT the secondary would be half the turns of the primary, carrying double the current. So the AT saves that amount of copper and all the secondary I²R loss.
In a DWT if primary and secondary were the same size wire (usually the secondary would be thicker) the AT would have 1/3 the total I²R losses of the DWT. If the secondary used wire with double the area and half the resistance per unit length, the AT still has 1/2 the I²R losses and only uses half the copper of the DWT.

I know Isolation Transformer has more core loss (in terms of hysteresis and eddy current losses),

Than what? Core losses depend on core properties and winding losses on winding properties. Winding losses can be reduced by using more copper to have thicker wires. The AT always scores some reduction in winding losses against a DWT for a given amount of copper.

Why current and magnetic field are connected, is a bit beyond me. I only got as far as, they are connected and some of the rules for that.

 

[kiki_danc]

Yes, I was using efficiency in a rather vague way, thinking of I²R losses in the windings. As you say, real losses include eddy and hysteresis. And autotransformer design may well say, use thinner wire and have similar I²R losses with a smaller, cheaper transformer.

For your example, both the series and common windings carry the same current as the primary of a similar dual wound transformer, but then you have no secondary winding at all. In the DWT the secondary would be half the turns of the primary, carrying double the current. So the AT saves that amount of copper and all the secondary I²R loss.
In a DWT if primary and secondary were the same size wire (usually the secondary would be thicker) the AT would have 1/3 the total I²R losses of the DWT. If the secondary used wire with double the area and half the resistance per unit length, the AT still has 1/2 the I²R losses and only uses half the copper of the DWT.

For a similar rated 500VA 240v-120v step down isolation transformer vs 500VA 240v-120v step down autotransformer. Are their primary windings same sizes? Or could either one be larger and why?

Than what? Core losses depend on core properties and winding losses on winding properties. Winding losses can be reduced by using more copper to have thicker wires. The AT always scores some reduction in winding losses against a DWT for a given amount of copper.

Why current and magnetic field are connected, is a bit beyond me. I only got as far as, they are connected and some of the rules for that.

 

[kiki_danc]

Yes, I was using efficiency in a rather vague way, thinking of I²R losses in the windings. As you say, real losses include eddy and hysteresis. And autotransformer design may well say, use thinner wire and have similar I²R losses with a smaller, cheaper transformer.

For your example, both the series and common windings carry the same current as the primary of a similar dual wound transformer, but then you have no secondary winding at all. In the DWT the secondary would be half the turns of the primary, carrying double the current. So the AT saves that amount of copper and all the secondary I²R loss.
In a DWT if primary and secondary were the same size wire (usually the secondary would be thicker) the AT would have 1/3 the total I²R losses of the DWT. If the secondary used wire with double the area and half the resistance per unit length, the AT still has 1/2 the I²R losses and only uses half the copper of the DWT.

Than what? Core losses depend on core properties and winding losses on winding properties. Winding losses can be reduced by using more copper to have thicker wires. The AT always scores some reduction in winding losses against a DWT for a given amount of copper.

Why current and magnetic field are connected, is a bit beyond me. I only got as far as, they are connected and some of the rules for that.

Oh before it forgot. I want to understand more this phenomenon of floating secondary output of isolated transformer. Say a 240v-120v step down isolation transformer produced 120v output. And one of its leads is not connected to ground. What are the situations when the voltage can rise above 120v? Like 1000V. Is it not the output will always be 120v? But if you let the neutral or one of the leads touch say 1000v.. the output can become 1120v? how? by series? How does this work when output is supposed to be 120v but the actual is 1120v? how is the 1120v measured? between the load (but it should be 120v).. hence the confusion.

 

[gneill]

Oh before it forgot. I want to understand more this phenomenon of floating secondary output of isolated transformer. Say a 240v-120v step down isolation transformer produced 120v output. And one of its leads is not connected to ground. What are the situations when the voltage can rise above 120v? Like 1000V. Is it not the output will always be 120v? But if you let the neutral or one of the leads touch say 1000v.. the output can become 1120v? how? by series? How does this work when output is supposed to be 120v but the actual is 1120v? how is the 1120v measured? between the load (but it should be 120v).. hence the confusion.

Electric potential difference is measured between two points. That's why it's called a "difference". An isolated transformer secondary has an effectively undefined potential with respect to anything else. What is defined is the potential difference between its leads.

When you connect a lead of the secondary to something with an established potential (with respect to what? Ground?), then you elevate the whole secondary by that potential. Note that there will still be 120 V between the two leads. That doesn't change.

 

[kiki_danc]

Electric potential difference is measured between two points. That's why it's called a "difference". An isolated transformer secondary has an effectively undefined potential with respect to anything else. What is defined is the potential difference between its leads.

When you connect a lead of the secondary to something with an established potential (with respect to what? Ground?), then you elevate the whole secondary by that potential. Note that there will still be 120 V between the two leads. That doesn't change.

Can you give an example of what is this something with established potential (except ground)? What is the consequence if your elevate the whole secondary by that potential? What would be the adverse effect on the equipment or the surrounding?

 

[gneill]

Can you give an example of what is this something with established potential (except ground)? What is the consequence if your elevate the whole secondary by that potential? What would be the adverse effect on the equipment or the surrounding?

You can choose anything you want as a reference. Then when you quote potentials it is understood that they are potential differences with respect to the chosen reference point. In isolated equipment such as a laptop computer or your cell phone, they will have a their own reference potentials entirely internal to themselves, usually associated with one of their battery terminals.

If you raise the potential of just one winding on a transformer and the other winding happens to be tied to ground (say by being connected to AC mains where the neutral is at ground potential, then you'll have that potential difference between the windings on the transformer. If the transformer isn't designed to handle that potential difference then it'll fail, probably due to insulation breakdown.

 

[kiki_danc]

You can choose anything you want as a reference. Then when you quote potentials it is understood that they are potential differences with respect to the chosen reference point. In isolated equipment such as a laptop computer or your cell phone, they will have a their own reference potentials entirely internal to themselves, usually associated with one of their battery terminals.

If you raise the potential of just one winding on a transformer and the other winding happens to be tied to ground (say by being connected to AC mains where the neutral is at ground potential, then you'll have that potential difference between the windings on the transformer. If the transformer isn't designed to handle that potential difference then it'll fail, probably due to insulation breakdown.

Can you give example of anything used as reference (besides ground) because I don't have idea. Or let's take the example of 1000V output of a power supply in the lab. You mean if one of the leads of the secondary output of the 500VA isolation transformer touches one of the leads of the output of 1000v, then even though the output voltage between the transformer leads is still 120v.. the potential becomes 1120v? But potential between what? And how do you measure it? This is a bit confusing. Thanks.

 

[gneill]

Can you give example of anything used as reference (besides ground) because I don't have idea.

Airplanes have electrical systems so they need their own reference. Cars, too, as they're insulated from contact with the ground by rubber tires.

Or let's take the example of 1000V output of a power supply in the lab.

First, what do you mean by a "1000V output of a power supply in the lab". With respect to what reference is it 1000 V? It may be that the 1000V is between two terminals on that supply, and not referenced to ground. Potential is ALWAYS with respect to some reference.

You mean if one of the leads of the secondary output of the 500VA isolation transformer touches one of the leads of the output of 1000v, then even though the output voltage between the transformer leads is still 120v.. the potential becomes 1120v? But potential between what? And how do you measure it? This is a bit confusing.

It's confusing for you I think because you haven't properly grasped the idea of potential difference. Potentials are ALWAYS measured as a difference between two points. That's why I asked you about that 1000V supply. You need to know what it's reference is before you can talk meaningfully about what the potential of the transformer secondary will be after being connected to the 1000V supply.

"1000V" stated alone is meaningless without knowing "with respect to what".

 

[Merlin3189]

AFAIK the point about an isolation transformer is that there is no other connection of the secondary circuit to ground. If the user touches the electric supply (by fault or by error) it does not make a circuit through which current can flow and do him harm.

The secondary of the transformer is at an undefined potential wrt the ground and, when he touches part of that circuit, that point becomes grounded through him. A small transient current could flow as contact is made, but that would be to discharge any static potential that had arisen on the small capacitance of the wires. Once this is discharged, conduction through him maintains that point at ground potential. No significant current can flow, because there is no low resistance path to ground from anywhere else in the secondary circuit.

To get a dangerous shock he would need to touch two parts of the circuit, say either side of the tool, which were at 120 V pd.

Can you give example of anything used as reference (besides ground)

here I measure the anode voltage (= 402.7 V wrt chassis 0 V ) by measuring the pd wrt to the +500 V line. I may do this because the anode resistor is simply a convenient place to attach my meter, or maybe my 100 V range (or other low range) is easier to read accurately than the 500 V range.
(BTW I can't think of any reason this circuit would be biased like this. Its just for the sake of argument ! Though perhaps I could make it into a class B/C amplifier with a few mods?)
=========
Edit: scrub the decimal of the voltages. Even on the 100 V range I couldn't read 0.3 V ! That was just me trying to show an accurate reading. I'd probably be able to estimate to 1 or 2 V on the 100 V range, but only to 5 or 10 V on the 500 V range (or the 600 V range which I would need to use on my meter.)

 

[kiki_danc]

Airplanes have electrical systems so they need their own reference. Cars, too, as they're insulated from contact with the ground by rubber tires.

First, what do you mean by a "1000V output of a power supply in the lab". With respect to what reference is it 1000 V? It may be that the 1000V is between two terminals on that supply, and not referenced to ground. Potential is ALWAYS with respect to some reference.

It's confusing for you I think because you haven't properly grasped the idea of potential difference. Potentials are ALWAYS measured as a difference between two points. That's why I asked you about that 1000V supply. You need to know what it's reference is before you can talk meaningfully about what the potential of the transformer secondary will be after being connected to the 1000V supply.

"1000V" stated alone is meaningless without knowing "with respect to what".

By potential you mean "Electric Potential".. like this? https://en.wikipedia.org/wiki/Electric_potential

My room TV now is connected to a Toroid isolation step down transformer with 120v output. It's not connected to ground. It's floating. So can you give any example of how the potential value may rise? And how it may damage the TV? Since it's isolation transformer, even if you measure one of the leads to ground.. it's not connected so it won't show any value as the above illustration shows. I was familiar with this. But not the potential thing where it can raise too. Please give example how it can affect the TV or me.

 

[kiki_danc]

AFAIK the point about an isolation transformer is that there is no other connection of the secondary circuit to ground. If the user touches the electric supply (by fault or by error) it does not make a circuit through which current can flow and do him harm.
View attachment 234102
The secondary of the transformer is at an undefined potential wrt the ground and, when he touches part of that circuit, that point becomes grounded through him. A small transient current could flow as contact is made, but that would be to discharge any static potential that had arisen on the small capacitance of the wires. Once this is discharged, conduction through him maintains that point at ground potential. No significant current can flow, because there is no low resistance path to ground from anywhere else in the secondary circuit.

To get a dangerous shock he would need to touch two parts of the circuit, say either side of the tool, which were at 120 V pd.View attachment 234103 here I measure the anode voltage (= 402.7 V wrt chassis 0 V ) by measuring the pd wrt to the +500 V line. I may do this because the anode resistor is simply a convenient place to attach my meter, or maybe my 100 V range (or other low range) is easier to read accurately than the 500 V range.
(BTW I can't think of any reason this circuit would be biased like this. Its just for the sake of argument ! Though perhaps I could make it into a class B/C amplifier with a few mods?)
=========
Edit: scrub the decimal of the voltages. Even on the 100 V range I couldn't read 0.3 V ! That was just me trying to show an accurate reading. I'd probably be able to estimate to 1 or 2 V on the 100 V range, but only to 5 or 10 V on the 500 V range (or the 600 V range which I would need to use on my meter.)

This is my room Toroid 240v-120v step down isolation transformer (secondary not grounded) connected to my 120v LG OLED TV. Can you give example how the potential can rise (by giving any example you can think of) and how it can damage the tv or affect me)?

 

[gneill]

By potential you mean "Electric Potential".. like this? https://en.wikipedia.org/wiki/Electric_potential

Yes.

My room TV now is connected to a Toroid isolation step down transformer with 120v output. It's not connected to ground. It's floating. So can you give any example of how the potential value may rise? And how it may damage the TV? Since it's isolation transformer, even if you measure one of the leads to ground.. it's not connected so it won't show any value as the above illustration shows. I was familiar with this. But not the potential thing where it can raise too. Please give example how it can affect the TV or me.

Unless the transformer fails (say, the primary shorts to the secondary) or you connect the secondary or your TV to some other voltage source it won't do anything.

 

[kiki_danc]

Yes.

Unless the transformer fails (say, the primary shorts to the secondary) or you connect the secondary or your TV to some other voltage source it won't do anything.

Can the secondary voltage be connected to another voltage source? Can ac source be connected in series to another ac source? I haven't heard of this done. Can you share link or illustration showing any circuit where ac power is added?

And if the secondary is not connected to any voltage source. you said the potential can still be anything.. Like it can really be 5000v or become 1 million volt much? But where does this come from? From fluctuation in the quantum vacuum? Is it related?

 

[gneill]

Can the secondary voltage be connected to another voltage source? Can ac source be connected in series to another ac source? I haven't heard of this done. Can you share link or illustration showing any circuit where ac power is added?

A simple example is where a transformer has multiple secondaries and each is isolated from the others. One could connect them in series to obtain different total voltages.

And if the secondary is not connected to any voltage source. you said the potential can still be anything.. Like it can really be 5000v or become 1 million volt much? But where does this come from? From fluctuation in the quantum vacuum? Is it related?

The most common source of potential differences arising between isolated things is static electricity. The quantum vacuum has nothing to do with any of this.

 

[kiki_danc]

A simple example is where a transformer has multiple secondaries and each is isolated from the others. One could connect them in series to obtain different total voltages.The most common source of potential differences arising between isolated things is static electricity. The quantum vacuum has nothing to do with any of this.

What is the worst source of static electricity? So do you recommend I ground the secondary of my Hammond toroid isolation transformer or not? What is usually done in home isolation step down transformer?

 

[gneill]

What is the worst source of static electricity?

In the home? Probably a carpet on a dry winter's day Shuffle across the carpet in your slippers and then touch a doorknob. Zap!

So do you recommend I ground the secondary of my Hammond toroid isolation transformer or not? What is usually done in home isolation step down transformer?

Note that I am not a certified electrician, so I cannot legally recommend you do anything with respect to your electric wiring. I can only give you my layman's opinion. That said...

I'd not recommend that you do anything. So long as you're not connecting your TV to other mains-powered equipment, it'll be quite happy sitting in isolation. If you do connect to other mains-powered equipment, your TV will then likely have its chassis ground tied to the house wiring Earth ground (if it exists in the wiring system where you live. Different countries do neutrals and grounds differently) through the connection.

 

[kiki_danc]

In the home? Probably a carpet on a dry winter's day Shuffle across the carpet in your slippers and then touch a doorknob. Zap!

Note that I am not a certified electrician, so I cannot legally recommend you do anything with respect to your electric wiring. I can only give you my layman's opinion. That said...

I'd not recommend that you do anything. So long as you're not connecting your TV to other mains-powered equipment, it'll be quite happy sitting in isolation. If you do connect to other mains-powered equipment, your TV will then likely have its chassis ground tied to the house wiring Earth ground (if it exists in the wiring system where you live. Different countries do neutrals and grounds differently) through the connection.

The TV is flat panel LG OLED TV, all plastic enclosure. Not those old CRTs.

This is my Toroid model: M117 https://www.hammfg.com/files/products/1182/1182-insert.pdf
It's connected as 234v-117v isolated transformer.

So there is no way to measure electric potential by itself. So without grounding it. It's even possible for my Toroid to be in 1 million volts potential? Won't I feel a thing even if it's 1 million volt?​

 

[gneill]

So there is no way to measure electric potential by itself. So without grounding it. It's even possible for my Toroid to be in 1 million volts potential? Won't I feel a thing even if it's 1 million volt?

Assuming that potential is due to a static electric charge then it'll be the same as touching a doorknob on a dry winter's day.

 

[kiki_danc]

Assuming that potential is due to a static electric charge then it'll be the same as touching a doorknob on a dry winter's day.

You mean touching a doorknob on a dry winter's day can produce 1 million volts potential? any reference so I can just read about this huge value. Thanks.

 

[gneill]

A million volts was your suggestion. I thought you were exaggerating for effect. No, typical static electric buildups are automatically limited to be less than about 35 kV since after that coronal discharge will dissipate the charge into the air. You wouldn't get anywhere near that high in a typical household environment where the air has some humidity. More typically you can expect less than 12 kV.

Wikipedia has an entry on static electricity which covers the matter.

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

 

[Merlin3189]

So there is no way to measure electric potential by itself.

The point about static charges (in everyday life) is that although the potential can be thousands of volts wrt Earth (how else would you get a spark when you touch it ?) the capacitance of objects is very small and the total charge stored on them is very small.
That makes it difficult to measure, because all measurements draw some charge from the object and therefore change its potential. The usual rough and ready way to measure static potential is with an electroscope (gold leaf), but even this accepts some of the charge and reduces the potential a bit. Valve and insulated gate fet voltmeters can have ultra high resistance inputs with very small capacitance. These sorts of things could give you a ball park measurement (or maybe better.)

In practice I think it unlikely that you would ever build up very big static potentials in the sort of situation you're thinking about. We have to try very hard with our insulation in static electric experiments. In your transformer, wire and TV the insulation has only to prevent significant current flowing with 250 V emf. I'm not sure what the test criterion is (I'd guess in domestic low voltage work, you can have μA currents), but to preserve a static charge you need much better insulation than that. A leakage current of 1 μA would discharge a 1 nF capacitance charged to 1000 V in one second.

To get charged in the first place it would need something to be charging it faster than it was losing charge. When you walk across the carpet and touch the TV maybe you'd charge it up instantly. Then you sit and watch for a few hundred seconds while that charge leaks away, before touching it again to recharge it. The charge is never likely to build up.

While you're questions have raised some interesting comments, I can't help feeling that your fundamental worry is misplaced. You have bought an isolating transformer, whose main purpose is to prevent your getting electric shocks and your equipment getting damaged by line faults perhaps. So then you ask how that transformer can fail in its primary objective. It is very unlikely that it will fail. It is the solution of choice for this application, so much so that it is a legal requirement in many workplace situations. That wouldn't be the case if this were just some unreliable gimmick. So you're now asking us to come up with some outlandish extreme circumstance which might circumvent or prevent this transformer from doing its job. What is the point?

 

[kiki_danc]

The point about static charges (in everyday life) is that although the potential can be thousands of volts wrt Earth (how else would you get a spark when you touch it ?) the capacitance of objects is very small and the total charge stored on them is very small.
That makes it difficult to measure, because all measurements draw some charge from the object and therefore change its potential. The usual rough and ready way to measure static potential is with an electroscope (gold leaf), but even this accepts some of the charge and reduces the potential a bit. Valve and insulated gate fet voltmeters can have ultra high resistance inputs with very small capacitance. These sorts of things could give you a ball park measurement (or maybe better.)

In practice I think it unlikely that you would ever build up very big static potentials in the sort of situation you're thinking about. We have to try very hard with our insulation in static electric experiments. In your transformer, wire and TV the insulation has only to prevent significant current flowing with 250 V emf. I'm not sure what the test criterion is (I'd guess in domestic low voltage work, you can have μA currents), but to preserve a static charge you need much better insulation than that. A leakage current of 1 μA would discharge a 1 nF capacitance charged to 1000 V in one second.

To get charged in the first place it would need something to be charging it faster than it was losing charge. When you walk across the carpet and touch the TV maybe you'd charge it up instantly. Then you sit and watch for a few hundred seconds while that charge leaks away, before touching it again to recharge it. The charge is never likely to build up.

While you're questions have raised some interesting comments, I can't help feeling that your fundamental worry is misplaced. You have bought an isolating transformer, whose main purpose is to prevent your getting electric shocks and your equipment getting damaged by line faults perhaps. So then you ask how that transformer can fail in its primary objective. It is very unlikely that it will fail. It is the solution of choice for this application, so much so that it is a legal requirement in many workplace situations. That wouldn't be the case if this were just some unreliable gimmick. So you're now asking us to come up with some outlandish extreme circumstance which might circumvent or prevent this transformer from doing its job. What is the point?

Last part of questions. You discussed a lot about static charge.. but what has it got to do with the transformer secondary floating to any potential. I mean how does this static charge related to the isolation transformer secondary floating and getting any potential value? When you touch the door knob. The transformer is elsewhere, unless you mean the transformer secondary can take up static charge and become millions of volts in potential? Does the transformer absorb static charge via it's two leads or the windings? How does it able to absorb it by itself since it is not moving or can touch any knobs.

And you missed this question. For a similar rated 500VA 240v-120v step down isolation transformer vs 500VA 240v-120v step down autotransformer. Are their primary windings same sizes? Or could either one be larger and why?

Thanks a lot!

 

[gneill]

A grounded device won't develop a static charge with respect to ground. The ground connection provides a path for that charge to escape. An isolated circuit has no path to ground, so static charge has a possibility to build up (but that requires that some effort like rubbing the device with silk or wool, or other ways and means of depositing a charge on it). The total amount of charge will be very small despite the potential, due to the tiny capacitance of isolated bodies. Also, millions of volts is out of the question. Maybe 12 kV or so in a typical household environment with relatively dry air.

Step-down autotransformers typically use less copper in their construction, so they may be less expensive for a given application, but they do not provide isolation (from the mains); There's a physical connection between the mains and the load.

You will need to tell us about your local electrical codes with regard to ground and neutral in order for us to give you any ideas about doing something to mitigate static charge buildup on isolated devices.

**Note that we are not licensed to give electrical wiring advice for any jurisdiction!**, so we cannot give you (code certified) instruction on what to do for your particular case. We can only point you to information that you may find helpful in your own researches. Do be sure to read the Wikipedia article on static electricity and in particular the section titled "Removal and prevention".

 

[kiki_danc]

A grounded device won't develop a static charge with respect to ground. The ground connection provides a path for that charge to escape. An isolated circuit has no path to ground, so static charge has a possibility to build up (but that requires that some effort like rubbing the device with silk or wool, or other ways and means of depositing a charge on it). The total amount of charge will be very small despite the potential, due to the tiny capacitance of isolated bodies. Also, millions of volts is out of the question. Maybe 12 kV or so in a typical household environment with relatively dry air.

Step-down autotransformers typically use less copper in their construction, so they may be less expensive for a given application, but they do not provide isolation (from the mains); There's a physical connection between the mains and the load.

You will need to tell us about your local electrical codes with regard to ground and neutral in order for us to give you any ideas about doing something to mitigate static charge buildup on isolated devices.

**Note that we are not licensed to give electrical wiring advice for any jurisdiction!**, so we cannot give you (code certified) instruction on what to do for your particular case. We can only point you to information that you may find helpful in your own researches. Do be sure to read the Wikipedia article on static electricity and in particular the section titled "Removal and prevention".

My electricity is identical to USA and I'm very familiar about the ground wire needing to be bonded to neutral only once at service entrance. I can easily connect the ground to the secondary output of the isolation transformer, but the isolation would be lost. Hence concerned how the potential could rise by its own. I was concerned the potential could even rise to the vev (vacuum expectation value) of the electroweak plasma potential and I don't want this happening near my room, but you commented it won't happen and the potential rise could occur by rubbing the transformer in the door knob and the amount of charge will be small.

But then, if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?

 

[gneill]

My electricity is identical to USA and I'm very familiar about the ground wire needing to be bonded to neutral only once at service entrance. I can easily connect the ground to the secondary output of the isolation transformer, but the isolation would be lost. Hence concerned how the potential could rise by its own. I was concerned the potential could even rise to the vev (vacuum expectation value) of the electroweak plasma potential and I don't want this happening near my room, but you commented it won't happen and the potential rise could occur by rubbing the transformer in the door knob and the amount of charge will be small.

No, an isolated body can gain or lose charge by rubbing with various materials (silk, wool, nylon, cat fur... the list is extensive) or by encountering another already charged body (yourself shuffling across a carpet). Nothing to do with the electroweak plasma potential is relevant here. At all.

But then, if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?

Why would this be a concern if the transformer and its mounting is properly constructed?

I cannot provide any advice beyond what what I would surmise might happen in a given situation. I am not licensed to dispense instruction to guarantee conformance with your local electrical codes, and will not accept the potential ramifications of doing so. If this is a Do-It-Yourself (DIY) project where you are building your own step-down / isolation interface to the mains, I'd suggest consulting a local licensed electrician to approve your build.

 

[kiki_danc]

No, an isolated body can gain or lose charge by rubbing with various materials (silk, wool, nylon, cat fur... the list is extensive) or by encountering another already charged body (yourself shuffling across a carpet). Nothing to do with the electroweak plasma potential is relevant here. At all.

Why would this be a concern if the transformer and its mounting is properly constructed?

I cannot provide any advice beyond what what I would surmise might happen in a given situation. I am not licensed to dispense instruction to guarantee conformance with your local electrical codes, and will not accept the potential ramifications of doing so. If this is a Do-It-Yourself (DIY) project where you are building your own step-down / isolation interface to the mains, I'd suggest consulting a local licensed electrician to approve your build.

Actually. I buy isolation transformer just to understand the concept of magnetic flux. In my country. We don't have any isolation transformer. We only have autotransformer. But as I learned in this thread, autotransformer also has magnetic flux and same principle.

At least please share the hypothetical situation that if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v? The logic is that if you don't ground the secondary, and the neutral is bitten by rats and it touches a 240v source, the neutral can take that value... and secondary voltage may change, true? so if the secondary is 120v.. and one lead touches a 240v source, would the total become 360v or 240v? Of course I won't try it.

 

[gneill]

At least please share the hypothetical situation that if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?

The potential difference variation between the transformer leads would remain 120 V. Your load device would remain happy. The potential of the "shorted" lead and the other lead with respect to your mains wiring neutral or ground would vary according to mains voltage (240 V). This would not affect the connected load, since it only "sees" the transformer leads. You would definitely have a problem with exposed wiring/chassis on your load device, since it'll share the potential of a "iive" wire. Not a happy circumstance.

The logic is that if you don't ground the secondary, and the neutral is bitten by rats and it touches a 240v source, the neutral can take that value... and secondary voltage may change, true?

No. The secondary is isolated. The neutral is on the mains side of the circuit. The rat will die, the GFCI will note the ground current and trip the breaker. The neutral is bonded to ground as you've already stated in a previous post, so it won't change potential w.r.t. ground (ignoring any small resistance in the bonding wiring). Your load is safe in this situation (in my unlicensed opinion). The rat is not safe of course .

 

[kiki_danc]

The potential difference variation between the transformer leads would remain 120 V. Your load device would remain happy. The potential of the "shorted" lead and the other lead with respect to your mains wiring neutral or ground would vary according to mains voltage (240 V). This would not affect the connected load, since it only "sees" the transformer leads. You would definitely have a problem with exposed wiring/chassis on your load device, since it'll share the potential of a "iive" wire. Not a happy circumstance.No. The secondary is isolated. The neutral is on the mains side of the circuit. The rat will die, the GFCI will note the ground current and trip the breaker. The neutral is bonded to ground as you've already stated in a previous post, so it won't change potential w.r.t. ground (ignoring any small resistance in the bonding wiring). Your load is safe in this situation (in my unlicensed opinion). The rat is not safe of course .

Oh I read that if the USA utility pole isolated transformer is not grounded. Then the neutral can take positive values as it touches live wire.. meaning it can reverse.. so the house neutral can become hot.. this is why they ground it at the pole and house service entrance. Similarly. If you have 3 leads in small 500Va isolated transformer and assume the middle is neutral but didn't ground it.. then the neutral can take hot value. So I guess this is importance of grounding the secondary of isolation transformer if the load would become complicated.. and also to suppress common mode surges and capacitive coupling.

I want to collect rare transformers just for collections. Most in the market now are the shell type. Know any commercially available small 500VA transformer that still uses the core type?