Understanding Transformer Dinamics

[Mike Filipovic]

I have some problem with understanding transformer dinamics at the moment when load is applied to the secondary.
I understand that flux in the core stays unchanged in order to maintain counter-emf to balace applied voltage. Additional mmf (i.e. Current) will balance mmf created in secondary.
How this is possible considering that voltage in the primary circuit is unchanged and leakage and mutual inductivites should not be changed (magnetization current and core flux are unchanged)?
Also, if during saturation mutual inductance drops, to my understanding magnetic energy could't disappear i.e. leakage flux will increase. How then current goes high? What I'm missing?

 

[rattus]

Mike,
Magnetizing current flows only in the primary and is ideally constant.

Load currents flow in both primary and secondary and the fluxes so generated cancel each other.

 

[Mike Filipovic]

Understanding Transformer Dinamics

Hi rattus,
Thank you for your reply.
Your statements are completely true but they are not answers on my questions.
For the question No.1, related to normal (unsaturated) operation, applying of the mesh current method for two loops, primary and secondary, could give the answer?
Best regards,
Mike

 

[rattus]

Hi rattus,
Thank you for your reply.
Your statements are completely true but they are not answers on my questions.
For the question No.1, related to normal (unsaturated) operation, applying of the mesh current method for two loops, primary and secondary, could give the answer?
Best regards,
Mike

Sorry, I misunderstood your question. The answer is the the primary inductance goes down which causes the magnetizing current to rise.

 

[Mike Filipovic]

Sorry, I misunderstood your question. The answer is the the primary inductance goes down which causes the magnetizing current to rise.

Hi rattus,
Now, I'm a bit confused.
In my original post I made two qestions:
The first one was related to increase of the primary current at the moment when secondary load is applied.
The second one is related to increase of the primary current in case of saturation.

I guess your answer is related to the second question.
When you say "primary inductance" do you mean leakage inductance of primary coil?

Best regards,
Mike Filipovic

 

[rattus]

Hi rattus,
Now, I'm a bit confused.
In my original post I made two qestions:
The first one was related to increase of the primary current at the moment when secondary load is applied.
The second one is related to increase of the primary current in case of saturation.

I guess your answer is related to the second question.
When you say "primary inductance" do you mean leakage inductance of primary coil?

Best regards,
Mike Filipovic

I will try again.

Leakage inductances are in series with both primary and secondary. They arise as a result of incomplete coupling between primary and secondary.

Primary inductance, Lp, is simply the inductance of the primary coil--unsaturated core of course. Magnetizing current flows through this inductance.

As the iron saturates, Lp goes down which allows more and more magnetizing current to flow.

The "exact" transformer model comprises a 'tee" network as described below:

Primary resistance--series
Primary leakage inductance--series

Primary inductance--shunt
Equivalent core loss resistance--shunt

Reflected secondary resistance--series
Reflected secondary leakage inductance--series

Reflected secondary load impedance--shunt