Hello,

It's my first post, so, greetings to all the folks around and thank you in advance for all the responses on my threads! Unfortunately, I'm not properly educated in this area to fulfill all my ambitions, therefore, don't mind some very basics questions!

To get started ... I am wondering, hot to wire a simple transformer (say 220V to 12V) to a single phase current. I've taken one apart and found that it has no resistors inside, or anything that will limit the current. I suppose that, if we plug a simple wire into the wall plug, one end into each hole, it would short circuit and the breaker will hopefully prevent further connection. Why doesn't it happen here? Have I missed some resistors somewhere?

 

Hi and Welcome , At the top of of this page in the headings is a section labled "education" Have a look through the sub menues to answer your "basic questions" and give you a clear understanding of how it all works

 

The transformer windings have inductance which has an impedance (roughly equivalent to resistance) which is what limits the current.
With no load on the transformer secondary, the primary will take only a small current known as the magnetising current. As the load on the secondary is increased so the current in the primary will also increase.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/ac-inductor-circuits/

 

Hello,

It's my first post, so, greetings to all the folks around and thank you in advance for all the responses on my threads! Unfortunately, I'm not properly educated in this area to fulfill all my ambitions, therefore, don't mind some very basics questions!

To get started ... I am wondering, hot to wire a simple transformer (say 220V to 12V) to a single phase current. I've taken one apart and found that it has no resistors inside, or anything that will limit the current. I suppose that, if we plug a simple wire into the wall plug, one end into each hole, it would short circuit and the breaker will hopefully prevent further connection. Why doesn't it happen here? Have I missed some resistors somewhere?

Welcome to AAC!
I guarantee that you will find this site very useful in advancing your knowledge in everything electrical and electronic.

Your assumption is correct. If you connect a simple wire across a wall plug you will get some beautiful sparks, a blown fuse or tripped circuit breaker. (Don't try this at home or any place else.)

There are no hidden resistors in a transformer, except for the inherent resistance in the wires of the transformer.

You may have noticed that a transformer is many turns of wire wrapped around a ferro-magnetic core. This makes it an inductor. An inductor opposes the flow of changing current. If you connect the leads of the primary windings across a high capacity battery you will get the same effect as with the wire across AC mains, i.e. sparks and a melted wire.

The transformer across AC mains is a different case. Since the AC mains voltage is oscillating at 50 or 60 times a second, very little current will flow in the primary windings when there is no load connected to the secondary windings.

When a resistive load is connected across the secondary windings, things start to get interesting. Current is induced from the primary winding to the secondary winding and power is delivered to the load. Transformers are very efficient. If the transformer is 100% efficient, the power taken from the AC mains on the primary winding will be equal to the power drawn by the load on the secondary winding.

As an example, suppose the load resistor is 12Ω. The load will draw 1A @ 12VAC = 12W from the secondary winding.
On the primary winding, the current draw will be 12W / 220V = 0.055A or 55mA

 

I just found that education sector, you should put an add besides it, it'll certainly get it seen in first 3 seconds marvelous, all on one place!

Thank you all for answers. Having read the link provided above, it seems simple for ac now. However for dc, how easily can a winding be fried, when, for example, a rotor inside of a DC brush motor gets jammed, or if it starts directly from a battery?

 

I don't know much about motors, but frying a wire takes a certain amount of current which depends on the type and size of wire. If your DC source can supply this current in a short circuit situation, then indeed your wire will fry. If the output impedance of the DC source limits the current to within the useable limit of the wire, it should remain intact.

 

for dc, how easily can a winding be fried, when, for example, a rotor inside of a DC brush motor gets jammed,

First, there is no such thing as a DC transformer. If you try to run them on DC there won't be any useful output and the time to melting depends on simple wire resistance ohms and voltage applied. A DC rotor? I have personally seen the smoke coming out of a DC motor with the rotor stuck. Smoke is the normal result so, don't do that! You have a matter of seconds before damage becomes significant.