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