Hi:

Can someone tell me how I would set the specs for an LTSpice transformer model.
The transformer should have a 120 volt primary and a 12 volt center-tapped secondary
at 1 amp.

Thanks,

M

 

hi maker,
Check this .
Note LTS Vsource expects the Vpeak voltage.
E


or

 

hi maker,
Check this .
Note LTS Vsource expects the Vpeak voltage.
E
View attachment 330186

or

Hi E:

Thank you for your help as always. How do you determine what values of inductance
are required to get the desired secondary voltage? Also, what does ".tran .2" do? And
finally, does AC = 120 need to be specified?

Thanks,

M

 

Hi maker,
Look at this link.
E
https://www.analog.com/en/resources...-basic-steps-for-simulating-transformers.html

And
https://web.cecs.pdx.edu/~tymerski/ece222/How to create a transformer using LTSpice.pdf


https://www.google.com/search?client=firefox-b-d&q=modelling+transformers+in+ltspice

 

Hi maker,
Look at this link.
E
https://www.analog.com/en/resources...-basic-steps-for-simulating-transformers.html

And
https://web.cecs.pdx.edu/~tymerski/ece222/How to create a transformer using LTSpice.pdf


https://www.google.com/search?client=firefox-b-d&q=modelling+transformers+in+ltspice

Thanks.

 

Connect your transformer to the mains and measure the primary current. Or, if you have a datasheet, use the value for the off-load primary current.
From that, calculate its impedance.
Measure the primary resistance.
Calculate Xpri = √(Zpri^2-Rpri^2)
From the mains frequency, work out the inductance.
This is the inductance to use for the primary.
You can specify the winding resistance as the resistance of the inductor, or you can add a separate resistor in series.
(I use the resistor in series, as the inductor symbol doesn't show its series resistance on the schematic)
Then calculate the secondary inductance as Lpri/(turnsratio)^2
Include a SPICE command that sets K=1
If you have the transformer to hand, and an inductance meter, short out the secondary and measure the primary inductance.
This is the leakage inductance. Add an inductor of this value in series with the primary. I find it much more satisfactory that calculating a value of K in the 0.99 region.

You could simply measure the primary inductance with the inductance meter, but the value is usually to high for the inductance meter, and also varies with current.

 

Connect your transformer to the mains and measure the primary current. Or, if you have a datasheet, use the value for the off-load primary current.
From that, calculate its impedance.
Measure the primary resistance.
Calculate Xpri = √(Zpri^2-Rpri^2)
From the mains frequency, work out the inductance.
This is the inductance to use for the primary.
You can specify the winding resistance as the resistance of the inductor, or you can add a separate resistor in series.
(I use the resistor in series, as the inductor symbol doesn't show its series resistance on the schematic)
Then calculate the secondary inductance as Lpri/(turnsratio)^2
Include a SPICE command that sets K=1
If you have the transformer to hand, and an inductance meter, short out the secondary and measure the primary inductance.
This is the leakage inductance. Add an inductor of this value in series with the primary. I find it much more satisfactory that calculating a value of K in the 0.99 region.

You could simply measure the primary inductance with the inductance meter, but the value is usually to high for the inductance meter, and also varies with current.

Thanks IanO

 

Thanks IanO

You'll get an inductance that a lot, lot bigger than 100uH on the examples!

 

You'll get an inductance that a lot, lot bigger than 100uH on the examples!

Hi,
Check the 2nd simulation file primary value in post #2. 1Henry

E

 

Hi,
Check the 2nd simulation file primary value in post #2. 1Henry

E

Thanks. I thought the second picture was just the thumbnail of the first.
1 H is a sizeable transformer, about 2kVA.
(just measured a 1100VA EI type at 1.77H)
toroids will be higher

 

hi maker,
Watch this video, it shows how to measure the actual values of a transformer that you may have on the bench and then transform those values into an LTSpice asc file.

About 7 minutes in from the start, it shows the method.

Also, it covers the general design of transformers
E

 

hi maker,
Watch this video, it shows how to measure the actual values of a transformer that you may have on the bench and then transform those values into an LTSpice asc file.

About 7 minutes in from the start, it shows the method.

Also, it covers the general design of transformers
E

I watched part of it and its kinda sketchy...I don't recommend.

 

I watched part of it and its kinda sketchy...I don't recommend.

Hi,
I do not agree, I recommend it.
That guy 'Fesz' knows what he is talking about, I have used that method a number of times, and it works just fine.

' its kinda sketchy', what sort of professional analysis is that supposed to be.?

Instead of dismissing someone other person's work, let's see what you have to offer that will help the TS.

E

 

hi maker,
Watch this video, it shows how to measure the actual values of a transformer that you may have on the bench and then transform those values into an LTSpice asc file.

About 7 minutes in from the start, it shows the method.

Also, it covers the general design of transformers
E

Excellent video! Thanks E.

One last question. If I hover over the schematic, none of the voltages or currents are displayed.
Why is that?

M

 

Measuring the inductance with an inductance meter doesn’t give anywhere near the right value. For instance, the 1100VA transformer which had a magnetisation current of 400mA at 240V had a measured inductance about a factor of 10 too low.
I was taught to measure leakage inductance by shorting out the secondary and adjusting the primary voltage with a variable so that the normal working current flows. At that point the leakage inductance can be derived from the primary voltage.

Putting a power analyser on the transformer primary with the secondary open circuit will give a power factor of almost but not quite zero (φ ≈ 90°) If you obtain the exact phase of the current, you can also calculate the resistive component of the magnetisation current, which is due to the core loss, and add that as a resistance in parallel to the magnetisation inductance.