Transformer high side

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wirenut1980

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Hello, a line foreman asked me today about transformer windings and how they actually worked. Some apprentices were confused on why, under normal operation, is there not a short between phase and neutral on the high side on a, let's say a single phase wye connected transformer. I really did not know how to explain it in the way he wanted (as he put it, "in layman's terms). I know that the technical explanation is that under no load, there is a counter emf generated in the primary winding, due to the inductance of the coil, that is equal to and opposes the potential between line and neutral and limits current from flowing. This emf is also induced on the secondary windings. Once a load is applied on the secondary, the counter emf is decreased due to Lenz's Law, yada yada yada (Seinfeld reference, Georgestoltz), and this results in more current flowing on the primary winding to re-establish the flux in the core.

How can I explain this better?
 
It?s too early in the morning for me to try to figure out what you said.

But in laymen?s terms, if you have a voltage source, and if you connect a wire to it (nothing else, just a wire), you have a ?short circuit.? The resistance of the wire is so low that, according to Ohm?s Law (I = E/R), the current (I) will be very high.

If, however, before you connect the wire, you wrap it in loops, a bunch of loops, that by itself will not change the ?resistance? of the wire. It will, though, create a new factor: ?reactance.? The net total values of ?resistance? plus ?reactance? add up to a value called ?impedance.? You don?t add the two in a simple manner, however. Resistance and reactance are added as though they were two sides of a right triangle, the hypotenuse of which is impedance. So they add up using the A^2 + B^2 = C^2 formula.

Anyone who has taken a loop or two of wire around a nail and connected it to a battery will remember that this creates a magnetic field. Well in the primary of the transformer you have lots of loops of the same wire, and thus have a large magnetic field. It is the current flowing in the loops that call into existence the magnetic field. But it (like all else in nature and in life) comes at a price. There is energy lost in the loops of wire. That is because the wire has a significant value of ?impedance,? and that happens because the wire is wrapped in loops.
 
In addition to Charlie's brilliant explanation, there is an additional interaction. If the primary loops are wrapped with the secondary loops, particularly around an iron core, they are interconnected magnetically.

At that point the power in the primary loops must also flow in the secondary loops. That is why there is no short on the pirmary side, unless you also short the secondary side.

If there is little or no magnetic interconnection, then you can indeed have a short on the primary side. The current will be limited by the impedance of the coil only, not also by the transformer principle.

Jim T
 
Building on and adding to Charlie's and Jim's equally excellent explanations:
wirenut1980 said:
...transformer windings and how they actually worked.

...(why) is there not a short between phase and neutral on the high side on a, let's say a single phase wye connected transformer.

...(as he put it, "in layman's terms). ...
Try this:
When you wrap a coil of wire around an iron core and put an AC voltage on it, the coil has to build and collapse a magnetic field in the core. The magnetic field follows the voltage. Building and collapsing the magnetic field impedes the current flow. The property is called Inductance and the holding back of the current flow is called Impedance. So, the DC resistance of the primary winding is really small compared to the AC impedance of the primary winding. The rest of this discussion ignores the DC resistance - it really doesn't affect anything. (For the physics twits - which includes me - add "for this discussion"):smile:

When you wind two coils on the same iron core, they share the magnetic field in the core you could say the two coils are coupled by the magnetic field. So when the primary builds and collapses a magnetic field in the core, that same magnetic field is seen by the secondary. Two important things happen when two coils share the same magnetic field:

1. The voltage on the primary winding is seen on the secondary winding. The ration of V(primary) to V(secondary) = N(turns ratio) or
Vp/N = Vs

2. In order for secondary current to flow, there has to be a cooresponding current on the primary. The ratio of I(primary) to I(secondary) = 1/N or:
Ip*N = Is

So, if the secondary is open circuit, then I s = 0, so Ip = 0. This is pretty true. The only primary current is the magnetizing current - the current required to build and collapse the magnetic field - and it is really small compared to the transformer FLA

So:
1. To DC, the primary is virtually a dead short - but we don't use transformers on DC - so this is a "so what".

2. To AC, the Impedance limits the current a lot more than the resistance. The resistance has very little effect.

3. Take a look at the two formulas - don't forget the piece in red.
a. With the secondary open circuit there is very little current flow in the primary (magnetizing current only).
b. With load on the secondary, current flows in the primary in ratio to the secondary current.
c. A short circuit on the secondary pretty much looks like a short circuit on the primary. Not exactly true, but close enough for this discussion

Additional notes:
Why did they call it Impedance instead of Resistance? Well, cause it is different, this works on changing currents(AC only), not on steady currents (DC)

Why doesn't this Inductance work on DC? Well, cause on DC, the magnetic field builds, and stays still, no energy transfer to build and collapse the magnetic field. No energy transfer, no Impedance.

Hope this helps. Others are welcome to add or change.

carl
 
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