Transformer tries to maintain flux a constant, then why does it still 'Saturate'? 1

[Soloten]

My understanding is that at no load, a transformer does have a certain no-load flux in the core, due to the magnetizing current in the branch. When it is loaded, the secondary draws more current, create an opposing flux (Lenz's law) that opposes the primary flux - and this inturn reduces the self induced emf drawing more current from primary. So overall, in this process, the secondary flux and primary flux cancel out, and the flux still remains close to its open-circuit flux.

So my question is: What causes the transformer to saturate then? If primary voltage is increased, the load in the secondary will draw more current and should cancel out the primary flux and keep the flux in the core a constant right?

 

[prc]

Soloten, Let me try to clear your doubt. Let us consider a 1:1 (voltage wise) transformer. When such a transformer is loaded , the secondary current will be less than primary current as a part of current will be required for magnetizing the core. This component used to be very high (as high as 50 %) in the initial days when iron sheets were used as core. Nowadays it is <0.5 %. This loss in current is required for transferring voltage from primary to secondary circuit. But when the core gets saturated, it will require more primary current to create the extra flux demanded by higher V/f to generate higher voltage at secondary terminals. When you take V/f say 30 % more than rated condition, more flux will be generated and the flux will over flow from core in to air ( permeability 1 instead of 6000 in CRGO) and huge current will be required due to poor permeability. This extra current will not appear in secondary as it is consumed for generating the extra flux out side core to keep up higher v/F in secondary, in line with primary.

Similarly at no load, secondary voltage will be same as primary voltage. But when you start drawing current from secondary, secondary voltage will dip and the loss in voltage is what you pay for transferring current from primary to secondary.

 

[HamburgerHelper]

Transformers
and power
quality – Part I

https://hrcak.srce.hr/file/246841

"It should be noted here that contemporary
designers design transformers so that
they operate with a magnetic flux density
close to saturation point for the maximum
use of the ferromagnetic core material. In
general, the more the transformer core is
saturated, the larger the harmonic components
will become.

Consequently, even when supplied with
sinusoidal voltage, their no-load current
will have a pure non-sinusoidal shape,
i.e. transformers will generate high harmonics
which penetrate into the grid and
reduce the quality of electricity"

"This article investigates the generation
of higher harmonics in small singlephase
transformers supplying power to
household, office and industrial equipment.
The results of the finite element analysis
of the magnetic field have been used to
obtain the waveform of the current in the
primary side of the transformer operating
at no load. Its shape is different from sinusoidal
wave, i.e. the current contains higher
harmonics, which occur due to non-linearity
in the magnetic characteristics of the
magnetic materials used in transformers,
and due to saturation effect. To obtain
magnitudes of these harmonics, harmonic
analysis was performed by the FFT."

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If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

So my question is: What causes the transformer to saturate then? If primary voltage is increased, the load in the secondary will draw more current and should cancel out the primary flux and keep the flux in the core a constant right?

The voltage increase causes saturation independently of the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

HH, you can't let it go?
This quote from your link is pure rubbish: "their no-load current
will have a pure non-sinusoidal shape"

What the heck is a "pure non-sinusoidal shape"? A dirac pulse, perhaps?




Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[HamburgerHelper]

Skogsgurra,

Saturation due to the transformers entering into the non-linear region during no-load. They show pictures if you want to look at them.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[Skogsgurra]

I do this for a living. And has done so even Before FFT analyzers were easy to operate. Graphical DFT at those times.

So, I know more than most guys about harmonics IRL.

It is the term "pure non-sinusoidal" that I am wondering about. Did you even understand what I asked? I Think no. As I said, we don't want a new "frequency discussion". It leads nowhere.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[stevenal]

Probably a typo. Try shifting it to "non-pure sinusoidal."

 

[waross]

Saturation due to the transformers entering into the non-linear region during no-load.???

There are two non-linear regions in the flux curve.
The non-linear region that causes harmonics at no load (or at all loads for that matter) is not the non-linear region which is related to saturation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

stevenal, it can hardly be a typo.

A typo is poor orthography or finger-trouble.

The phrase "pure non-pure sinusoidal" is neither. It is poor thinking - aka brain-trouble.

Anyhow, it has no bearing on the OP's question, which I think you should read again.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[HamburgerHelper]

"pure non-pure sinusoidal"

Is that a typo?

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[prc]

H&H I do not think harmonics in transformer excitation current is going to pollute the grid etc. It is true % harmonics in no-load current has gone up with better grade of transformer steels. When hysteresis loss is brought down keeping the same rated flux density, the loop shape becomes leaner and the area of loop( measure of loss ) comes down, but due to sharp bend at saturation point, harmonics goes up. But absolute harmonic current is reducing drastically as the excitation current as % of load current is very much low for new grade of steels.

 

[Skogsgurra]

HH, listen to prc. He has massive knowledge in this field. But remember that the thread is not about that subject.

I cannot help wondering about your "If you can't explain it to a six year old, you don't understand it yourself"
There are quite a few things in technology that go far beyond what a six-year (or even a 15 year) child has the theoretical underpinning or experience to understand. We try to stay at a somewhat higher level of understanding in EngTips. It is for professional engineers after all.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

VERY educational thread about transformers; thank you to all posters & contributors.

There is however also lots of "blacksmithing" going on...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]