Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] Transformer Primary & Secondary Current on load and off load

Status
Not open for further replies.
rjkrocks

rjkrocks

Member level 3
Joined
Sep 12, 2011
Messages
66
Helped
9
Reputation
18
Reaction score
9
Trophy points
1,288
Location
Kerala, India
Activity points
1,844
My question is simple,

Will the the primary current of transformer vary when load is connected to the secondary (from no load state or vary the secondary load)?
or Will the primary current vary when no load is in the secondary (from the load state)?

Please give me the correct answer with the reason (weather yes or no).

I would like you to consider this fact while answering : Transformer will provide isolation.

(because of this fact i am little bit confused)

Thanks for taking time....
 

if you increase the load on secondary i.e. try to draw more current from secondary .....the current on the primary will also increases.... they are mostly lineraly proportional as VA is contant .... Aslo at no load on secondary the primary will draw some current to over come magnetic and hystersis losses...

And you are right transform provides electro-magnetic isolation .....

Good Luck
 

sir,

But the emf in the secondary is created because of the flux linkage from primary (the cutting of the flux) by the secondary coil, right?
Weather if there is a load in secondary or no load in secondary, the flux linkage will be same...... right?
then how is the current in primary is affected?
Means, How the primary knows if there is load in secondary or not, since the only connection between them is through the flux linkage,which is a constant?
(please correct me if I am wrong).
 

Flux linkeges changes with load current .....NI = phi*S ....basic magnetic relation....where N is no of turn I is current and phi is magnetic flux and S is reluctance of circuit.... now if your N*phi is a flux linkage.... you will get phi as N*I/S from above basic relation.... as N and S are constant.... so Phi is directly proportional to I ....so u will get Flux linkage as N*phi = N*N*I/S ..... So as load current increases flux linkage increases....it is not constant....Now to give you an analogy....see if you are getting more energy ( load) on secondary then you need to put more energy also to primary....so how this enegry will get transfer from primary to secondary by means of flux linkage ....thus as load increases .....you need to have more flux linkage too.....

I think this answers your question.....

Good Luck
 
Thank you sir,

It is almost cleared my doubt, but still something bugging me. how the primary will know the load is connected to the secondary.. is there some thin to do with the constant VA rating you mentioned in post #2....

Please explain sir.I also like to know about the VA rating you mentioned in #2. Is it relatedto primary or secondary.........

One more question
consider for a 12-0-12V transformer if VA rating is 12VA, that means 12V 500mA. right?--> this is for secodary right/
How can we determine the maximum primary current that can safley flow? Is it 500mA

Thanks again..
 

Now let take the example that you are given only let we have transformer of 230/ 12-0-12 with 500ma on senconday thus as u said the there iti 12-0-12 transformer thus effectively is is 0-24 v transformer then VA on secondary is 24*500 ma = 12VA......not this is maximum rating of transformer so when you have 500 ma rating the current on primary will be 12VA/230v = 0.0522 amps equal to 52 ma max......now you are load on seconday is only 100ma then your VA = 24* 100ma = 2.4VA now your primary current = VA/230 = 2.4/230 = 10.43 ma only....

Generally manufacture if transformer give maximum current rating on seconday side i.e. 500 ma....

I hope this answers your question.....

Good Luck
 

rjkrocks said:
how the primary will know the load is connected to the secondary
Click to expand...

Some simple explanations about ''Transformers/Magnetic flux/mutual inductance/Magnetomotive force'' we can find here:

Mutual inductance and basic operation : TRANSFORMERS

No current will exist in the secondary coil, since it is open-circuited. However, if we connect a load resistor to it, an alternating current will go through the coil, in-phase with the induced voltage (because the voltage across a resistor and the current through it are always in-phase with each other).
At first, one might expect this secondary coil current to cause additional magnetic flux in the core. In fact, it does not. If more flux were induced in the core, it would cause more voltage to be induced voltage in the primary coil (remember that e = dΦ/dt). This cannot happen, because the primary coil's induced voltage must remain at the same magnitude and phase in order to balance with the applied voltage, in accordance with Kirchhoff's voltage law. Consequently, the magnetic flux in the core cannot be affected by secondary coil current. However, what does change is the amount of mmf in the magnetic circuit.
Magnetomotive force is produced any time electrons move through a wire. Usually, this mmf is accompanied by magnetic flux, in accordance with the mmf=ΦR “magnetic Ohm's Law” equation. In this case, though, additional flux is not permitted, so the only way the secondary coil's mmf may exist is if a counteracting mmf is generated by the primary coil, of equal magnitude and opposite phase.………………

So when an AC voltage is applied to the primary coil, it creates a magnetic flux in the core, which induces AC voltage in the secondary coil in-phase with the source voltage. Any current drawn through the secondary coil to power a load induces a corresponding current in the primary coil, drawing current from the source.
Click to expand...
 
So when an AC voltage is applied to the primary coil, it creates a magnetic flux in the core, which induces AC voltage in the secondary coil in-phase with the source voltage. Any current drawn through the secondary coil to power a load induces a corresponding current in the primary coil, drawing current from the source.
Click to expand...
Thank you very much mister_rf.....................this cleared by doubt.
Thanks Milind Kulkarny for all help..............
 

rjkrocks said:
Weather if there is a load in secondary or no load in secondary, the flux linkage will be same...... right?
Click to expand...

This is what I had learnt in power systems class as well.

From the no-load to full-load there should be no change in flux.

Although, I think this only holds for an ideal transformer.

Of course as you increase the load on the secondary, the current flow in the primary will increase as well, and since the leakage flux is proportional to the winding current, we should see more flux lost.

If you were to look at the waveform for the current primary from no load to full load conditions, it would start off looking like a very small and non sinusoidal trace, but as you move closer to the full load condition it will begin to transform into a cleaner sine wave.
 

jegues said:
If you were to look at the waveform for the current primary from no load to full load conditions, it would start off looking like a very small and non sinusoidal trace, but as you move closer to the full load condition it will begin to transform into a cleaner sine wave.
Click to expand...

Hi jegues,

This is a new knowledge.Thanks, but have my doubts.Please clear me........ As for transformer, to maintain the loss due to hysteresis a small amount of current is needed and this has to get from the supply. So are you saying this current will be non-sinusoidal in nature? (if so what will be the reason?)
or because of the small value, the sinusoidal current drawing from the power supply to the transformers primary appear as non-sinusoidal?
What you think?
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top