Electronics > Beginners

Isolation transformer and electrons

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Zero999:

--- Quote from: nForce on December 11, 2018, 04:24:58 pm ---Hmm, but I want to know from electrons perspective. Maybe IanB gave a better answer.

So if I understand correctly: When we make a short circuit on the grid, those electrons flow from a transformer through wire into ground. Now the transformer has electron "holes" and the earth (our planet) has negative charge. Because of that the transformer tap, which is grounded suck electrons from the ground, to fill the "holes". Now everything is again balanced the earth and the transformer. Altough our planet is very large, so excess of electrons would be negligible.

Sorry but my question is pure physics, and not engineering.

--- End quote ---
In the distribution transformer scenario, the earth simply acts as a return conductor for the current path. If one side of the transformer isn't connected to earth, then there will be no return current path, so no current will flow, as is the case with the isolation transformer, when the other side isn't earthed.

Brumby:
YES!  I've been wanting to express something like this - but haven't had the time to put the words together.  Thanks to wbeaty!!

I would make one change, which I have highlighted...


--- Quote from: wbeaty on December 12, 2018, 08:06:28 am ---Why do electrons only flow in loops?   It's because if we wanted them to flow in a one-way direction, we'd need kilovolts, megavolts, to apply enough force.
--- End quote ---
eg Lightning.


--- Quote ---Electrons are actively held inside of wires by their enormous attraction to the positive charges of atom nuclei.  The electrons of the metal can flow in a loop, but can't leave the metal circuit.

If electrons in a circuit are like a flywheel, then it's very easy to spin the flywheel, but very hard to pull chunks out of it.    That's why electrons stay inside the isolated circuit: an immense voltage-force would be needed if you wanted to push any of them out of the loop and into the Earth.   

Why?  It's because metals are half electrons, half protons, all in balance and strongly attracting.  If we make electrons flow in a circle, then they always stay near some protons.  But if we try to make them flow in one direction from point A to point B and not in a loop, then we're fighting against an immense-but-unnoticed attraction-force.  We're pulling them away from the enormous positive charge inside the copper.

--- End quote ---

Brumby:
The first thing you need to do is set aside electrostatic examples such as lightning.  That deals with single discharge events on not a continuous current flow.

The other thing you need to understand - and there has been a LOT of effort in this thread to try and make this point - is that there is no such thing as an absolute voltage.  It is ALWAYS a voltage at one point with respect to another point.  Whether you realise this or not, you prove this point every time you make a voltage measurement.  If you connect one probe from your multimeter to a voltage source (say a 9V battery) and don't connect the other probe, then you aren't going to get a voltage reading.

Further, if these two points are not electrically connected in some way, then you cannot measure the voltage between them - because there is no difference in their potential.  To prove this, connect one probe from your multimeter to the positive terminal of one 9V battery and connect the other probe to the negative of a second 9V battery - with no connection between the batteries.  Again, you aren't going to get a meaningful voltage reading.

However, once you make a connection between these two batteries, you have defined voltage relationships and you will get a voltage reading that is meaningful.  Note that if you make exactly one connection between the two batteries, you will not create any short circuits, but depending on which of the four possible connections you could make (with the probes connected as above), then you will get a voltage reading of 0V, +9V or +18V.  Swap the multimeter probes and the sign of the voltages will change.

In all these cases, the multimeter completes a circuit when meaningful measurements are given.


The primary and the secondary of a standard transformer are two separate electrical circuits.  The fact that they are coupled magnetically has NO impact on their behaviour as separate electrical circuits (other than the transformer action).

IanMacdonald:
Interesting point is that an electric current is rather like a hydraulic circuit, in that the 'fluid' (electrons) doesn't have to go through the whole length of the pipe (cable) to do work. Electrons going in one end, jostle the rest along, so some come out the other end. Not the same ones that went in, though. In fact, the to-and-fro movement of the electrons in a 50Hz AC circuit is quite short. So it's more like a vibration than a flow.

https://en.wikipedia.org/wiki/Drift_velocity#Numerical_example

ArthurDent:
Hero999 – “In the distribution transformer scenario, the earth simply acts as a return conductor for the current path. If one side of the transformer isn't connected to earth, then there will be no return current path, so no current will flow, as is the case with the isolation transformer, when the other side isn't earthed.”

That is absolute incorrect. If you had read my post #27 and post #30 you would know that the earth grounding of the neutral wire (that is the actual return conductor for the current path) is a safety feature and the earth isn’t relied on for carrying current.

The reason for this is simple, the earth’s resitivity is a variable that varies wildly from fairly low in some areas to near infinity in rocky areas. Power distribution has to be known and controlled and utilities can’t have one customer’s lights dim or flickering while the neighbor’s lights are very bright. Two wires are used and ground is only a safety connection.

This confusion is possibly caused by the misunderstanding of what the proper definition of ‘ground’ is. Here is a good description to help clear the confusion many have and is titled “An Introduction to Ground: Earth Ground, Common Ground, Analog Ground, and Digital Ground”.
https://www.allaboutcircuits.com/technical-articles/an-introduction-to-ground/

More specifically, in the case of power distribution being discussed, here is a document that describes a power distribution system and the ground connections used for safety.
http://www.samlexamerica.com/support/documents/13007-0612_GroundedElectricalPowerDistribution.pdf

1.0 CONDUCTORS FOR ELECTRICAL POWER DISTRIBUTION
For single-phase transmission of AC power or DC power, two conductors are required that will be carrying the current. These are called the “current-carrying” conductors. A third conductor is used for grounding to prevent the build up of voltages that may result in undue hazards to the connected equipment or persons. This is called the “non current-carrying” conductor (will carry current only under ground fault conditions).
2.0 GROUNDING TERMINOLOGY
In electronics, Ground is considered to be a common point, a point of zero potential and an infinite sink of electrons. In reality, most points we consider “grounded” are far from this perfect state. For purposes of electrical power transmission and distribution, the term “Grounded” indicates that one or more parts of the electrical system are connected to Earth, which is considered to have zero voltage or potential. In some areas, the term “earthing” is used instead of grounding. Connection to Earth may be made using Earth Electrodes like ground rods, buried wires, metallic pipes and other conductors in contact with the Earth.
Earth Ground is described in the context of lightning protection, safety and operational performance and is used for the following functions:
• Dissipate lightning strike energy in a manner that protects the surrounding area. Please see details under the Section 6 titled “Grounding System and Lightning / Ground Fault Protection”
• Provide a low impedance connection back to the AC power mains Ground or Neutral to reliably clear fault conditions by blowing a fuse or by tripping a circuit breaker. A “fault condition” occurs when a live, ungrounded current carrying conductor comes in contact with the exposed metal parts of electrical equipment
• Reduce the step potential gradient to safe levels
• Form a natural sink for atmospheric and radiated noise
• Provide an electrical antenna counterpoise.

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