Basic transformer question

[mark32]

I should know this but can't seem to figure it out. If one were to look at a schematic of a transformer's primary you would see that only the windings are what lies between the ungrounded conductors. Is there that much resistance in the windings that would keep the current down in order to keep the ocp from opening as soon as one is energized? Is it like in a motor where the cemf keeps the current from going through the roof during operation? That can't be though as there are no moving parts in a transformer. What's wrong with my thinking?

 

[iwire]

A transformer is not much more than a stationary motor so the answer to your question IMO is yes.

 

[charlie b]

Agreed. A transformer's windings have very little resistance, so it would seem reasonable to expect very high currents. However, owing to the simple fact that each winding consists of a wire wrapped around itself many times, the windings have a high value of inductive reactance. That reduces the current, in the same way that resistance would have done. You don't need motion in the windings (as in a motor) in order to get the inductive reactance. All you need is a wire wound in circles.

 

[augie47]

It is something to keep in mind in sizing your primary OCP/wiring.
On occasion, one will use an oversized from their surplus stock and find OCPs based on load will trip.
(example: using a 45kva transformer and wiring designed for a 15 kva)

 

[gar]

101231-1253 EST

mark32:

Under steady-state conditions, voltage has been applied for a while, there is a small AC current flowing in the primary with the secondary open. This current creates a changing flux in the core. In turn the flux induces an opposite voltage in the primary. The magnitude of the current is determined by the difference between the applied voltage minus the induced voltage and the winding resistance. This current component is often referred to as the "magnetizing current", and may be on the order of 1% of full load current.

When you first apply voltage to a transformer primary with the secondary open there may or may not be a very large initial inrush current. This transient current is determined by the residual flux state of the transformer core from the last turn off relative to the turn on point of the AC waveform.

See photos P6 thru p8 at my web site
http://beta-a2.com/EE-photos.html

Essentially when the high inrush current occurs the input inductance of the transformer primary is low because the core is driven into saturation. Low inductance relative to the frequency means low impedance and therefore high current.

.

 

[glene77is]

101231-1253 EST

Gar,
If your responses were not so good,
then the thread discussion would be a little longer.

 

[Kartracer087]

Any inductor will produce back emf once the fields have set up. Before that, you have a nice little short circuit, essentially. The back emf or back voltage means that the voltage the inductor sees is actually less than at the terminals due to that back emf. Its like negative voltage.

 

[gar]

110105-0854 EST

Kartracer087:

Your description for an inductor is incorrect. The current in an inductor can not be changed instantaneously. If the initial current is zero in the inductor and a voltage is applied to the inductor, then at that first instant after the voltage is applied the current is still zero. Thus, the inductor looks like an infinite resistance at that instant. R= V/I = V/0. Note if I know how I got to 0, then I can divide by 0 and get a meaningful result. The basis of differential calculus.

From where did you get the information that resulted in your statement about an inductor at t=0+?

Write the differential equation for a series RL circuit and study the result.

Your description of a short circuit at t=0+ does apply to a capacitor.

.

 

[mikeames]

I always remember.

I always remember.

ELI the ICE man !

 

[mark32]

Thanks all for the replies, especially Charlie's simplified explanation and Gar's technical one. Gar, thanks for the link to your page, when I get some more time I'd like to read your article on grounding.