# transformer core explanation required

#### Abdullah15

Joined Jul 6, 2021
8
hello everyone, this is a stupid question but if anyone can answer this, i'd be so grateful. the question is about the transformer core. take a simple EI laminated silicone steel core for example. we know primary winding magnetises and secondary demagnetises. and at a some point an equilibrium is achieved. now tell me what happens if, assume our windings can support any amount of current, short the secondary winding so that theoretically infinite current can flow. according to my understanding, primary winding will draw more current to achieve that equilibrium. so here it is, till what point will primary keep on increasing current until the equilibrium is achieved. and secondly, what does the size of the core have to do here? secondary is demagnetising and primary is magnetising, so as per my understanding, core should never saturate because secondary doesn't let it magnetise at such levels. plz plz help. i can't find answers anywhere

#### ronsimpson

Joined Oct 7, 2019
2,747
Primary and Secondary are actually more than just turns. There is wire and the wire has resistance.

Look at a transformer with a perfect primary and a real world secondary; 12V secondary, 1 ohm of wire resistance, no current we get 12V, but short out the secondary, 0V, 12A and all 12 volts are across the 1 ohm of resistance.

Look at a real primary and a "text book" secondary; primary has resistance. 220Vac primary, 10 ohms, If a perfect secondary shorts out the primary it leaves the 10 ohms across the power line.

Also "coupling" is how well the P & S are tied together. So a secondary can't 100% short out the primary.

Because a real transformer is not "perfect"; Shorting out the secondary does not short out the power line. But the transformer will try its best to move the short from Secondary to Primary.

#### Abdullah15

Joined Jul 6, 2021
8
Primary and Secondary are actually more than just turns. There is wire and the wire has resistance.

Look at a transformer with a perfect primary and a real world secondary; 12V secondary, 1 ohm of wire resistance, no current we get 12V, but short out the secondary, 0V, 12A and all 12 volts are across the 1 ohm of resistance.

Look at a real primary and a "text book" secondary; primary has resistance. 220Vac primary, 10 ohms, If a perfect secondary shorts out the primary it leaves the 10 ohms across the power line.

Also "coupling" is how well the P & S are tied together. So a secondary can't 100% short out the primary.

Because a real transformer is not "perfect"; Shorting out the secondary does not short out the power line. But the transformer will try its best to move the short from Secondary to Primary.
let me put it this way, primary provides flux and secondary uses the flux. there has to be a factor related to the core that puts a stop to how much flux the core can provide. i know we use bigger cores for larger current demands, but im confused that if secondary demagnetises the core why do we use bigger cores? im not sure if im wording it correctly, plz try to understand that I'm trying to visualise the power transition from primary to secondary and also the limitations. plz elaborate more. thanks

#### nsaspook

Joined Aug 27, 2009
11,744
let me put it this way, primary provides flux and secondary uses the flux. there has to be a factor related to the core that puts a stop to how much flux the core can provide. i know we use bigger cores for larger current demands, but im confused that if secondary demagnetises the core why do we use bigger cores? im not sure if im wording it correctly, plz try to understand that I'm trying to visualise the power transition from primary to secondary and also the limitations. plz elaborate more. thanks
Visualize the transformer as a machine like a car transmission. The flux is the equivalent to the connecting shafts between reduction gears. It takes a small amount of power to turn the shaft (flux) but there are limits to the amount of stress (saturate) each shaft can take transmitting power between gears. (turns) Low power, small shafts (core flux) are needed, high power, large shafts.

It's only a visual analogy, not an explanation.

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#### Abdullah15

Joined Jul 6, 2021
8
Visualize the transformer as a machine like a car transmission. The flux is the equivalent to the connecting shafts between reduction gears. It takes a small amount of power to turn the shaft (flux) but there are limits to the amount stress each shaft can take transmitting power between gears. (turns) Low power, small shafts (core flux) are needed, high power, large shafts.

It's only a visual analogy, not an explanation.
you understood my question. i know its hard to explain, if you can't explain, can you guide me from where i can read this?

#### crutschow

Joined Mar 14, 2008
32,847
It took me awhile to understand transformers, but here's the basics:

(I believe nsaspook's analogy is indeed bad and not correct.)

The fundamental is that the transformer core not saturate at the peak input voltage, so there has to be enough primary turns to create enough inductance so that the primary magnetizing current always stays below this core saturation point.
There is thus a trade between the number of required primary turns and the core size.
A smaller core will require more turns for a given primary voltage.
This is not related to load current.

Any secondary load current will reduce core flux which reduces the primary inductive impedance.
This causes more primary current to flow to balance that reduction and keep the core flux essentially constant.
The flux level in the core is thus basically independent of the primary and secondary current.
The means the core flux is unrelated to the maximum allowed current of an ideal transformer (with negligible leakage inductance) since net flux is not significantly changed by load current.

So the maximum allowed transformer current is determined by the transformer heating from the wire resistance of the primary and secondary.
Thus a higher current transformer requires larger wire, which requires more core area for the wire, giving a larger transformer (which also gives more area for heat dissipation).
This means a transformer with superconducting wire could be much smaller, limited only by the critical current limit of the wire.

That all make sense?

#### nsaspook

Joined Aug 27, 2009
11,744
you understood my question. i know its hard to explain, if you can't explain, can you guide me from where i can read this?
I didn't try to explain transformer operation. You said "visualize the power transition from primary to secondary and also the limitations" so I explicitly tried to help with what only. @crutschow has a good explanation of WHY to the transformer core sizing question.. The overall core size is set by both magnetic and wire winding requirements acting together.

https://www.engr.colostate.edu/ECE562/98lectures/l27.pdf
In summary choice of optimum dimensions for magnetic cores employed involve several apparently disparate factors: · Value of required magnetizing inductance · Cost of core material at core geometry specified · Cost of making Cu Windings · Required thermal dissipation of heat generated in cores and surrounding wires as set by the core geometry via conductive, convective and radiative means · Desired values of leakage inductance · Desired reduction of proximity effects in wire resistance
1. Power rating
2. Voltage levels (primary and secondary)
3. Currents on both sides
4. Primary and secondary coils wire diameter/size
5. Iron Core area
6. Numbers of turns (primary and secondary)
https://engineerexperiences.com/design-calculations.html

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Joined Jul 18, 2013
27,601
Some transformers are designed to run at saturation, SOLA regulator versions for e.g.

#### crutschow

Joined Mar 14, 2008
32,847
Some transformers are designed to run at saturation, SOLA regulator versions for e.g.
True.
Some inverter transformer circuits also use saturation to control the inverter oscillation.

My explanation was, of course, for common linear transformer operation, such as a main's type.

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#### Abdullah15

Joined Jul 6, 2021
8
thanks everyone for helping me clear my concepts. i think i have narrrowed down my real question now. lets just assume heating issue resistance window area etc are not our problem. and we want to assume this because we want to narrow down our problem. suppose there is a see saw. well yes if you put 100kg at one end and 100kg on the other end, both ends will remain at same level. but if you keep on increasing weight you will break the see saw. now come back to the core. im magnetising primary and demagnetising secondary. is there any limit to it? or i will be safe as long as both currents are somewhat equal and i will not saturate

Joined Jul 18, 2013
27,601
True.
Some inverter transformer circuits also use saturation to control the inverter oscillation.

My explanation was, of course, for common linear transformer operation, such as a main's type.
That is exactly what SOLA is, it uses a common power transformer and capacitor circuit to saturate the output, and any load decreases the saturation circuit current in proportion.

#### crutschow

Joined Mar 14, 2008
32,847
now come back to the core. im magnetising primary and demagnetising secondary. is there any limit to it? or i will be safe as long as both currents are somewhat equal and i will not saturate
Yes, there is no theoretical limit to the current due to core flux in an ideal transformer.
The primary current will always follow the secondary current to keep the core flux at a constant level.

#### Abdullah15

Joined Jul 6, 2021
8
Yes, there is no theoretical limit to the current due to core flux in an ideal transformer.
The primary current will always follow the secondary current to keep the core flux at a constant level.
thankyou. confusion cleared

#### crutschow

Joined Mar 14, 2008
32,847
That is exactly what SOLA is, it uses a common power transformer and capacitor circuit to saturate the output, and any load decreases the saturation circuit current in proportion.
The transformer is apparently not completely common.
It has some extra windings (below).

Joined Jul 18, 2013
27,601
Agreed, but not really an invertor transformer
Also run exceedingly hot in the process, of course.

#### Abdullah15

Joined Jul 6, 2021
8
thankyou. confusion cleared
another question, im sorry im so annoying. when we short out the secondary of a transformer, which i have done many times. and im talking about 220v on primary and lets say 12v output 50hz EI core. after shorting out the secondary, primary draws a certain amount of current and secondary also has a certain amount of current flowing. i want to know what factors are limiting the current in this scenario. it cant be copper loss alone. please please explain

#### crutschow

Joined Mar 14, 2008
32,847
i want to know what factors are limiting the current in this scenario. it cant be copper loss alone. please please explain
It is mostly the resistance of the primary and secondary, plus the effect of any leakage inductance.

If you measure the primary and secondary winding resistance and transform that resistance to one equivalent winding using the square of the turns ratio, you will see what the maximum current would be from just winding resistances.

Be careful in shorting the secondary of a transformer.
I accidentally burned one out doing that.

#### Abdullah15

Joined Jul 6, 2021
8
so if i want to design a transformer that can withstand output short circuit, i should aim to have some leakage inductance right?

#### nsaspook

Joined Aug 27, 2009
11,744
another question, im sorry im so annoying. when we short out the secondary of a transformer, which i have done many times. and im talking about 220v on primary and lets say 12v output 50hz EI core. after shorting out the secondary, primary draws a certain amount of current and secondary also has a certain amount of current flowing. i want to know what factors are limiting the current in this scenario. it cant be copper loss alone. please please explain
With zero impedance reflected back to the primary the load will be essentially resistive copper losses with leakage flux effects.

To help in the calculation of fault condition currents larger transformers are rated for secondary shorts .

https://studyelectrical.com/2018/05/percentage-impedance-of-transformer-and.html
The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.

It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions.
The percentage impedance is the magnitude of the per-unit equivalent series impedance. Higher equivalent series impedance, lower primary fault current. You can design with the series impedance to make something like a door bell transformer that's fairly resistant to shorted secondary faults.

https://studyelectrical.com/wp-cont...rcentage-impedance-of-transformer-testing.jpg

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