Why the efficiency of air core transformer changes (up and down)

[UtsavRaj]

An explanation on magnetising current and it's losses please. I cannot find anything about it for some reason.

 

[jim hardy]

This tells us whether it behaves like a solenoid or not?

Tells us how closely it approximates the ideal properties of inductance

recall definition of inductance, L = $\frac{NΦ}{I}$

 

[jim hardy]

try http://www.rfcafe.com/references/po...nsformer-october-1960-popular-electronics.htm

from an old magazine article he thankfully saved

Core Losses

Since the iron core itself, as well as the coils, is cut by the expanding and contracting magnetic field, a current is induced here, too. As this eddy current flows in the core, it steals energy from the primary circuit and dissipates it as useless heat. The eddy current flows at right angles to the magnetic flux. It can be reduced by substituting several thin layers of iron for the solid core. These thin layers - laminations - are separated by layers of glue which electrically insulate the laminations from each other. In practice, a small eddy current is set up separately in each lamination, but the total loss is much less than for a solid-core transformer.

Still another core loss is caused by the alternating current itself. Since this current reverses its direction 120 times a second, the iron core - in effect, an electromagnet - must continually reverse its polarity. And since the minute magnetic elements in the core tend to resist this change, power must be expended to realign them. This is called hysteresis loss. Engineers reduce it by building transformer cores of steels which change magnetic polarity with comparative ease, so that less power is consumed in making the switch.

http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1537&context=gradschool_theses
https://www.bing.com/search?q=transformer+core+loss+scholarly&pc=MOZI&form=MOZTSB

 

[The Electrician]

"One part of the current is due to the secondary current being "reflected" to the primary."

I do not get this.

This is the essence of what a transformer does.

Google for the search phrase "How transformers work.". You'll find links to several YouTube videos.

In an ideal transformer where there is no loss in the core (and the core has infinite permeability) or in the resistance of the wire making up the primary and secondary, there would be no current drawn by the primary when the secondary is unloaded. If a load were then connected to the secondary, current would be drawn by the primary and that would be supplying power to the secondary load. We say that the secondary load is "reflected" to the primary. The transformer with a load on the secondary would behave as though a load was in parallel with the primary terminals. The magnetic coupling of primary to secondary allows power to be transferred from primary to secondary even though there is no direct electrical connection between the copper wire of the primary and the copper wire of the secondary--the connection is only by means of the magnetic flux that links both windings. This "reflected" load current would be the only current in the primary.

BUT, in a real transformer some current is drawn by the primary even when there is no load on the secondary. This current supplies the losses (in the iron core and the resistance of the copper wire of the primary) and the reactive current drawn by the finite inductance of the primary (the permeability of a real iron core is not infinite as was the case with the ideal transformer). This current is called the "exciting current", and it's still part of the primary current even when there is a load on the secondary. The primary current drawn when there is a load on the secondary consists of two parts--the exciting current and the reflected load current from the secondary load.

 

[Baluncore]

Before I comment I need to:
1. See a definitive data set with the decimal points in the correct places.
2. Know what colour (temperature) the filament lamp glows at the highest currents.
3. Know the primary exciting current when there is no filament of other load present.
Without that there will be confusion.

 

[Baluncore]

An explanation on magnetising current and it's losses please. I cannot find anything about it for some reason.

Magnetising current is reactive current. It is not real power therefore it is not a real loss, apart from primary series resistance which is very small.

 

[UtsavRaj]

these graphs are all for the Iron core transformer.
As I thought the effency-voltage graph looked like an inverse-inverse relationship (1/x =1/y) (The last graph with no title)
But shouldn't it go through origin?

Before I comment I need to:
1. See a definitive data set with the decimal points in the correct places.
2. Know what colour (temperature) the filament lamp glows at the highest currents.
3. Know the primary exciting current when there is no filament of other load present.
Without that there will be confusion.

2. yellow
3. Cannot be checked due to the school being closed.
Data for Air core

Data for Iron core

from an old magazine article he thankfully saved

thank you

 

[UtsavRaj]

Also, I was looking at what variables i had to control.
I know that the number of coil is controlled by me along with the temperature by using therostat and also the small breaks i took after the wire got too heated.
But when i thought that i controlled the impendance by using 50Hz AC Current and the same wire for all experiment; I remember that my graph says different impendance so i thought someone may help me clarify this.
Anything else that i controlled by using my method?The method went like this:

• Powerpack attached to rheostat
• rheostat attached to digital ammeter
• Digital ammeter attached to Transformer coil (primary winding)
• Transformer coil attached back to powerpack.
• The digital voltmeter attached in parallel to the primary coil.
• The secondary winding was attached to another Digital ammeter.
• The digital ammeter attached to a bulb of 2 ohms.
• Bulb attached back to the secondary winding.
• Another digital voltmeter attached in parallel to secondary winding.
• All wires were copper wires.
• All the voltage and current readings were taken from the respective devices and recorded.

Same equipments was used for alll trials and experiment.

 

[UtsavRaj]

On side note:
I had some medical emergency in my family and that's why i had not been active.

I am sorry.

 

[jim hardy]

..

As I thought the effency-voltage graph looked like an inverse-inverse relationship (1/x =1/y) (The last graph with no title)
But shouldn't it go through origin?

i'm really confused.

Graph 1 says it's efficiency vs voltage but it doesn't look much like the last graph(untitled)
did you swap axes or something ?

How can you get to origin plotting Pout/Pin ? Origin for that ratio has zero in denominator, and you know one has to be wary of any ratio with a denominator that's infinitesimal. One arranges his experiments to avoid them.

old jim

 

[jim hardy]

By the way -

nice job with this one.

Gives one confidence when a result comes out just what physics says it should, doesn't it ?