Transformers

[uzair_ha91]

When the primary of a transformer is connected to A.C. mains, the current in it...
a) is very small if the secondary circuit is open, but
b) increases when the secondary circuit is closed.
PLEASE EXPLAIN THESE FACTS.

[negitron]

What are your thoughts on this?

[jhooper3581]

This link (http://www.animations.physics.unsw.edu.au/jw/AC.html) gives a good insight to AC circuits. Also, try this link (http://en.wikipedia.org/wiki/Alternating_current).

[uzair_ha91]

My thoughts:
When the circuit is closed, the load placed in the secondary circuit draws current...Is that right?

[vk6kro]

So, where does this power come from?

[uzair_ha91]

So, where does this power come from?
The power comes from the A.C. generator. (P=IV)

[negitron]

Right. And if the voltage in the primary is 3 times the voltage of the secondary, then the current through the secondary will be three times the current in the primary; and power in = power out (neglecting losses).

[J_Cervini]

When the secondary is open there is no current flow thus it consumes no power, power = voltage*current (there is also an additional term 'power factor' that we can neglect for this example).
When the secondary is attached to a load, the primary coil feeds the secondary power through induction. A voltage is induced onto the secondary coil in the amount equal to the ratio of secondary windings/primary windings. The power required by the secondary comes from this voltage.

[Pumblechook]

A real transformer does consume power with no load.

The output voltage is not the ratio of the turns x the input voltage for a real transformer... certainly not with with more than just a very light load on it.

[Mr.Green]

As you said when the secondary is open the primary current is very small; when the secondary is closed current flows in it and the primary current increases.

To understand what is going on you should have some knowledge of electromagnetic fields and Lenz's law.

When curren is flowing through any conductor a magnetic field is produced. Now, when a curren is flowing through the primary winding of a transformer the produced magnetic field is flowing in the iron core of the transformer. In this case the magnetic field isrepresented by lines known as flux lines. This lines intersect the secondary winding and induce a voltage across it.

http://www.clayeng.net/Training/myweb6/Module04/Output/images/t-f3.gif

A) Secondary is open:
No current flows through the secondary and consequently there is no magnetic field produces by the secondary. The only field in the core is that due to the primary current (which is small).

B) Secondary is closed:
Current is flowing through the secondary and a magnetic field is produced (so, now there are two magnetic fields: from Primary and Secondary). The secondary magnetic field opposes the primary magnetic field according to Lenz's law. So, the total magnetic field in the core which is now the resultant of primary and secondary magnetic fields is reduced. Since, EMF = N dф/dt , by reduction of flux the induced emf in the primary winding is reduced causing more current to flow in the primary winding. This continues untill a balance is achieved.

Firstly, it seems to be complicated... but it isn't.

[uzair_ha91]

You said that when the secondary circuit is closed, a current flows through the circuit which produces its own magnetic field. From where does this current come from? I thought that it is the induced current that flows through the secondary circuit. (Induced when the primary current being AC fluctuates between positive and negative peak emfs and causes change of flux to occur through the secondary coil)

[Mr.Green]

From where does this current come from?

The current flows because there are an induced voltage across the secondary winding.

This lines intersect the secondary winding and induce a voltage across it.

There are voltage...by closing the circuit there will be current.

I thought that it is the induced current that flows through the secondary circuit.

What do you mean by Induced Current?
If you mean the current flowing due to the induced voltage, you are right.
Induced voltage causes the induced current to flow.