transformer undergoing saturation

[Disha Karnataki]

if at all i have a transfromer with the ferromagnetic core it's always stated that we need to connect the primary of the transfromer to ac source than we get at secondary either stepped up/stepped down ac.
now,suppose if i apply dc at the primary the the core is said to be saturating and will soon damage itself if at all kept in the same condition for some time i really don't follow this. So what if the core saturates?? only thing is if at all the core saturates then it will be like the primary has no more opposition for it's back emf from secondary or the core's flux which used to be produced at the primary when ac used to flow That's it & hence the turns at primary will now be like wire or may be short circuit.
if i am not right then explain with your points

 

[FvM]

When connecting DC to a transformer, you don't need think much about saturation (yes, it does saturate, but it's really not important).

For now, it's sufficient to know that a transformer can't handle DC voltage. After a short transient response, the current will be only determined by the windings resistance. It's up to you to calculate, if the transformer can widthstand a specific DC current or not.

You can connect an air coil to DC, it does not saturate, but the current will be also determined by the windings resistance (and burn the coil if it's too high). For DC neither the transformer's inductance nor the saturation curve matters.

 

[FANT]

If you allow to saturate the core, there will not be back EMF and the current will be limited only by the DC resistance of the winding ( for the part of the sine that saturates the core ).
In this case the power is given by R*I2 and will increase greately, bringing to burning of the transformer.
For this reason it is mandatory not to saturate the core.

Mandi

 

[Disha Karnataki]

i will first keep my point of view:
according to me passing ac(whether core saturates or no) is more dangerous to the x'mer.
explaination: Consider the x'mer where ac is passed in the primary so firstly as ac means always fluctuating so consider ac in it's positive 1/2 cycle. now,as slowly current increases then there will be a back emf generated by the coil to oppose the original ac current this induced emf produces its own emf(as per lenz law) and, the flux now linked with this emf is linked with the whole core so, as this flux further passes to the secondary, again due to lenz law there will be an emf produced such that inorder to reduce the induced emf in the primary. till here is it fine?? next,as there will be a try given by secondary of the x'mer to reduce the voltage the inductor at the primary will try to increase the current flowing through it inorder to compensate for the opposition given by the secondary so like this it will happen for the full positive 1/2 cycle & eventually there will be excess current flowing into the primary(as explained b4) so the magnitude of flux linkage also increases and so there will be every possiblity that flux linkage may go to the saturation point & may increase cause the heating up of the core.
so this is my explaination for the ac current while flowing in the primary of the x'mer. i no it's contradictory because dc causes lot of heating but this is what i have imagined

- - - Updated - - -

i will first keep my point of view:
according to me passing ac(whether core saturates or no) is more dangerous to the x'mer.
explaination: Consider the x'mer where ac is passed in the primary so firstly as ac means always fluctuating so consider ac in it's positive 1/2 cycle. now,as slowly current increases then there will be a back emf generated by the coil to oppose the original ac current this induced emf produces its own emf(as per lenz law) and, the flux now linked with this emf is linked with the whole core so, as this flux further passes to the secondary, again due to lenz law there will be an emf produced such that inorder to reduce the induced emf in the primary. till here is it fine?? next,as there will be a try given by secondary of the x'mer to reduce the voltage the inductor at the primary will try to increase the current flowing through it inorder to compensate for the opposition given by the secondary so like this it will happen for the full positive 1/2 cycle & eventually there will be excess current flowing into the primary(as explained b4) so the magnitude of flux linkage also increases and so there will be every possiblity that flux linkage may go to the saturation point & may increase cause the heating up of the core.
so this is my explaination for the ac current while flowing in the primary of the x'mer. i no it's contradictory because dc causes lot of heating but this is what i have imagined

 

[Venkadesh_M]

Hi

If there is a change in flux in any conductor It will have induced EMF...
in a Turns of wires, if you apply a DC voltage It produces a steady magnetic flux from zero. So that induces voltage in the same turns and according to lenz law that opposes the applied voltage. So current cant pass instantly It will take its time to reach the level.... This is called inductance and that is what happening in a DC applied transformer..

When applying AC (I am not talking about +ve, _ve cycles) If the input is changing (It is called AC not to call as fluctuation) The current will also changing so It induces voltage in both primary and secondary windings... The induced voltage in the primary opposes the Applied voltage and makes ideally zero current (practically some current due to losses and magnetization current)... If you load the secondary The current in secondary produces some current and produces some flux which opposes the primary flux and the makes increasing the primary current....

 

[Disha Karnataki]

yes this is the same i have actually thought of.
Now, my question is: Is it dangerous passing dc to the primary of the x'mer as i have read many posts & it's specified that dc should never be passed(mostly avoided) to the x'mer why?? because as u mentioned if dc voltage is applied, It produces a steady magnetic flux So current cant pass instantly It will take its time to reach the level.... yes agree to this.
So there should not be a problem applying dc to the primary of the x'mer because now primary becomes as if a short(not instantly but takes some time) due to dc being applied.
and hence applying dc should as if give rise to an electromagnet placed in series with the supply that's it....Why are in many posts it's mentioned that passing dc to the x'mer is dangerous is my question..

 

[Venkadesh_M]

yeah it takes time to reach but it is typically may be 5ms after that ?? The resistace of the primary of the transformer is about very low for example 10 ohms if ypu apply a 100V DC it could make 10A of current but in ac it will be designed foe 500mA.. Now thats what will happen when you apply DC....

 

[Disha Karnataki]

so nothing harm in applying dc to the primary of the x'mer is it?? do u agree??

 

[FvM]

so nothing harm in applying dc to the primary of the x'mer

If Vdc is quite small so that Vdc/Rdc doesn't exceed the rated transformer current. Typically less than 5 percent of the regular transformer voltage.

 

[Disha Karnataki]

ok if i apply same value of once ac voltage to the primary of the x'mer and other time dc voltage to the primary. Which do u thing will generate more amount of heat in the x'mer & why???

 

[permute]

What do you intend to accomplish by applying DC to a transformer?

Keep in mind that transformers are used in DC-DC conversion, but all methods use some mechanism to either convert it to AC, or otherwise reset the core. Flyback and singled-ended forward use the flyback effect to reset the core (the flyback transformer is designed to store energy as well). Push-pull uses two primary windings in an alternating fashion. Half-bridge and full bridge alternate the polarity of DC applied to the single primary.

For many applications, the source is a voltage source with low output impedance. This allows very high currents when the core saturates. In current-source applications, the current is limited and DC isn't a huge issue from a safety perspective.

There are also devices like saturable reactors and mag-amps that are designed to saturate or have DC/LF applied to a winding. This behavior can be an issue at higher frequencies though, as there is also a power loss that scales with the "flux excursion" of the core.

 

[Disha Karnataki]

Push-pull uses two primary windings in an alternating fashion. Half-bridge and full bridge alternate the polarity of DC applied to the single primary.

do u mean as centre tap x'mer??
see this link they have said how dangerous it is if we apply dc to the x'mer:
https://wiki.answers.com/Q/What_wil...input_to_the_primary_of_a_transformers#slide1
but if i place x'mer in series with some high impedance circuit like that of the ce based transformer coupeled amplifier circuit where dc is applied at the primary of the x'mer then applying dc should not at all be a problem as they have explained in the slides.

 

[Venkadesh_M]

Ok I think you need Theories right take a primary of a Transformer as inductor and the Inductive reactance is 2*PI*f*L.. for AC main the frequency is 50 Its OK but for DC the frequncy is zero so no impedance So high current blowing the winding I say the same in before thread but you got it wrong....

The transistor circuit applies the pulses in the primary but not a pure DC, The pulsating component for different cycles applies in output as a single sine stream...

 

[Disha Karnataki]

no i got what u said i.e if dc is passed the flux will not be produced in whole core due to it's transient response right??
then i told u my reason why i think passing dc is not dangerous because initially no matter that current will not pass instantly but after some time it will gain constant value.
so isn't it that after some time it will behave like an electromagnet???
am i right in this point of view??

 

[Venkadesh_M]

no i got what u said i.e if dc is passed the flux will not be produced in whole core due to it's transient response right??

No the flux will be produced whatever the current flow but not change in flux in DC.........

so isn't it that after some time it will behave like an electromagnet???

Yes but No, In an electromagnet the poles will be open (No way for flux) thats why it attracts magnetic materials but a Transformer is acting electromagnet but it cant attract anything because its poles are closed already tightly.......

 

[Disha Karnataki]

sorry i meant that there will not be instant current at the start it will take time even for the dc to build current flow in the primary.

why i told about the electromagnetic concept is that to highlight the thing that the similarities(not in the properties whether attracting/repelling) in the formation of electromagnet.
we give dc supply when we want to have an electromagnet as per faradays experiment. And there we never spoke about giving dc supply would harm the windings or may be due to low impedance the windings would be burnt or something like that.
then why do we bother in x'mer??? That is why i had compared an electromagnet formation for the x'mer when dc is supplied & after constant primary current has developed

 

[Venkadesh_M]

we give dc supply when we want to have an electromagnet as per faradays experiment. And there we never spoke about giving dc supply would harm the windings or may be due to low impedance the windings would be burnt or something like that.
then why do we bother in x'mer???

Because Transformer has low resistance windings To provide a low voltage regulation......

 

[FvM]

I think, you are making too much words about a simple thing. As previously said, you get Idc = Vdc/Rdc, that's valid for a transformer winding, an electromagnet, an air coil or piece of wire. Maximum Idc is determined by wire cross section, thermal resistance to ambient, temperature resisting of insulation. So you can calculate the maximum Vdc that can be applied to the winding/electromagnet/coil/wire without damage.

 

[muhammad11]

if at all i have a transfromer with the ferromagnetic core it's always stated that we need to connect the primary of the transfromer to ac source than we get at secondary either stepped up/stepped down ac.
now,suppose if i apply dc at the primary the the core is said to be saturating and will soon damage itself if at all kept in the same condition for some time i really don't follow this. So what if the core saturates?? only thing is if at all the core saturates then it will be like the primary has no more opposition for it's back emf from secondary or the core's flux which used to be produced at the primary when ac used to flow That's it & hence the turns at primary will now be like wire or may be short circuit.
if i am not right then explain with your points

I am going to explan in simple way.: If Ac is passed then there will be a large amount of current in primary due to its low resistance.It will produce much heat in primary such that it will burn the primary winding

 

[Disha Karnataki]

I am going to explan in simple way.: If Ac is passed then there will be a large amount of current in primary due to its low resistance.It will produce much heat in primary such that it will burn the primary winding

if ac is passed???
this is contradictory to what we usually observe when we pass ac.
i don't think so we burn our x'mers when ac is passed is it??