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

[gneill]

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.

If you're worried about what can happen outside of your service entrance, then there's nothing to give advice about other than to say that your wiring should meet local code specifications. This would involve engaging a local electrician/inspector. If secondaries remain isolated, nothing that happens to the primary side (other than voltage surges or dropouts) can affect it.

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?

Nope, not me. You might want to visit the Electrical Engineering forum pinned thread,

Beauty of old electrical and measuring things, etc.

and engage some of the members who hang out there.

 

[kiki_danc]

If you're worried about what can happen outside of your service entrance, then there's nothing to give advice about other than to say that your wiring should meet local code specifications. This would involve engaging a local electrician/inspector. If secondaries remain isolated, nothing that happens to the primary side (other than voltage surges or dropouts) can affect it.

I'm not worried.. Just pointing out the analogies. In power utility poles, they also use isolation transformers.. and they ground the neutral. One very important reason is to get a grounded reference.. so equipment metal chasses with shorted live can trip the breakers. But I'm more interested in the other reasons.. why or how the neutral able to take on non-zero voltages. Here exactly the same analogy with small home 500va step down transformers (with difference the power lines can take lightning strike while the small transformer beside your tv may not). This is the reason I asked what potentials the leads can take. Because I read that in power lines, the neutral can get non-zero values. So next time in other forums. I'll simply ask about the reasons for grounding utility neutrals and use the same reasoning for small isolated transformer besides tv.

Nope, not me. You might want to visit the Electrical Engineering forum pinned thread,

Beauty of old electrical and measuring things, etc.

and engage some of the members who hang out there.

Thanks for all your help. 5 stars to you :)

 

[kiki_danc]

I'm not worried.. Just pointing out the analogies. In power utility poles, they also use isolation transformers.. and they ground the neutral. One very important reason is to get a grounded reference.. so equipment metal chasses with shorted live can trip the breakers. But I'm more interested in the other reasons.. why or how the neutral able to take on non-zero voltages. Here exactly the same analogy with small home 500va step down transformers (with difference the power lines can take lightning strike while the small transformer beside your tv may not). This is the reason I asked what potentials the leads can take. Because I read that in power lines, the neutral can get non-zero values. So next time in other forums. I'll simply ask about the reasons for grounding utility neutrals and use the same reasoning for small isolated transformer besides tv.

What I was saying above was that if the neutral centertap in the secondary output of isolation transformation is not grounded. The Neutral can shift to Line 1 or Line 2, right. So without grounding.. say a resident user put the Line 1 to the ground, then it can become the neutral. And neutral would become hot. The scenario I mentioned above may not work, that is, neutral connecting to other live and it becomes hot. This is because if the neutral connects to hot, it is short circuit and trip the breaker or fry the primaries.

Before I was not so clear about floating secondaries potential. So far this thread makes it clear that

1. Sources of secondary potential floating is from static charges which is little and not permanent.. I thought it was like HUP that the potential can take on any value even the Electroweak potential. And since you can't measure the potential. I wondered yesterday why if the potential take on the electroweak potential, we don't feel any plasma in the room.

2. Even if static charges cause potential changes, the output leads would still be 120v. I though the output lead voltage would vary.

3. If one of the leads touches other hot wires from other transformer, the two leads would still be 120v and it is the potential of the other lead with respect to ground that can have potential differences with respect to the Primary neutral (not the secondary which doesn't have it assuming it's not grounded).

4. And above... only if one of the leads are referenced to ground that it becomes the neutral. So without grounding the centertap, the neutral can shift to the side (L1 and L2 above). Please comment if the above scenario is correct.

At least I don't have to worry about my flat screen TV exploding from potential increase to millions of volts or even the GeV. And don't worry about me trying to change the utility grounding or terminals (of course I won't do it.. I'm afraid of getting shock). I just want theoretical understanding of it. Thanks a lot for those who helped.

 

[kiki_danc]

View attachment 234152

What I was saying above was that if the neutral centertap in the secondary output of isolation transformation is not grounded. The Neutral can shift to Line 1 or Line 2, right. So without grounding.. say a resident user put the Line 1 to the ground, then it can become the neutral. And neutral would become hot. The scenario I mentioned above may not work, that is, neutral connecting to other live and it becomes hot. This is because if the neutral connects to hot, it is short circuit and trip the breaker or fry the primaries.

Before I was not so clear about floating secondaries potential. So far this thread makes it clear that

1. Sources of secondary potential floating is from static charges which is little and not permanent.. I thought it was like HUP that the potential can take on any value even the Electroweak potential. And since you can't measure the potential. I wondered yesterday why if the potential take on the electroweak potential, we don't feel any plasma in the room.

2. Even if static charges cause potential changes, the output leads would still be 120v. I though the output lead voltage would vary.

3. If one of the leads touches other hot wires from other transformer, the two leads would still be 120v and it is the potential of the other lead with respect to ground that can have potential differences with respect to the Primary neutral (not the secondary which doesn't have it assuming it's not grounded).

4. And above... only if one of the leads are referenced to ground that it becomes the neutral. So without grounding the centertap, the neutral can shift to the side (L1 and L2 above). Please comment if the above scenario is correct.

At least I don't have to worry about my flat screen TV exploding from potential increase to millions of volts or even the GeV. And don't worry about me trying to change the utility grounding or terminals (of course I won't do it.. I'm afraid of getting shock). I just want theoretical understanding of it. Thanks a lot for those who helped.

Let me verify if I understand the concepts with regards to floating potential very well.

When you first turn on the transformer and its building the magnetic field.. and you plug a load to the secondary side after a minute. And there is no stray static field. The potential would start from zero, right? Or is it undefined (related to quantum field potential) and it can start at any value? (I mean is it based on classical ignorance or inherently unknown similar to quantum randomness? don't blame me from asking this because the electric field is also quantum in essence)? If you still insist it can start at any random value. Then it's not far from HUP and can still reach random potential even millions of volts? Or is it undefined because you can't measure it but assuming the potential is zero. So imagine the graph starts from zero potential and fluctuating up and down perhaps due to the capacitive coupling of the human body, etc. passing by it or inherent quantum nature of fields (like electric field)?

 

[anorlunda]

When we get to HUP in a discussion about basic electricity, the thread has gone on too long.

Thread closed.