# Ltspice transformer power mismatch

#### 00ai

Joined Jul 11, 2021
7
Hello,

I simulated the transformer using ltspice but couldn't figure out why don't the power value calculated with Vrms and Irms values match with the power value calculated by ltspice. Thank you for your time.

#### ericgibbs

Joined Jan 29, 2010
18,646
hi 00.
Welcome to AAC.

What values do you calculate using the waveforms.?

E

#### 00ai

Joined Jul 11, 2021
7
Thank you for the response, Eric. I found 56.36 mA using the rms values of the waveforms. Ltspice calculates average power as 30.3 mW. Theory states that Irms should be 17.1 mA (in case of no power loss), however, it is seen as 32 mA in ltspice.

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#### ericgibbs

Joined Jan 29, 2010
18,646
hi,
A quick check shows Pin = Pout [ across R1] are equal.

Will check the currents next.
E

Update:
Could you post your calculations figures.?

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#### ericgibbs

Joined Jan 29, 2010
18,646
hi 00,
My Volts * Current , Watts value is close the the 30mW given by LTS

Post how you calculated the values you calculated.

E

#### 00ai

Joined Jul 11, 2021
7
Hi Eric,

Using the rms values of waveform I and waveform V from the first ss that I uploaded, the power generated by the primary circuit is 1.75 V * 32.115 mA = 56.36 mW. I mistakenly wrote it as 56.36 mA above; sorry for that.

#### Papabravo

Joined Feb 24, 2006
20,993
Hi Eric,

Using the rms values of waveform I and waveform V from the first ss that I uploaded, the power generated by the primary circuit is 1.75 V * 32.115 mA = 56.36 mW. I mistakenly wrote it as 56.36 mA above; sorry for that.
You might want to repeat that calculation over an interval where both waveforms are in a steady state. the contribution from the transient part of the waveform may be skewing your results. You can do this by rerunning the simulation, but don't start collecting data until steady state has been reached.

#### ericgibbs

Joined Jan 29, 2010
18,646
hi 00,
Does this IV plot of the primary current give you a clue ref the Power ?
E

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#### 00ai

Joined Jul 11, 2021
7
You might want to repeat that calculation over an interval where both waveforms are in a steady state. the contribution from the transient part of the waveform may be skewing your results. You can do this by rerunning the simulation, but don't start collecting data until steady state has been reached.
Thank you for the response. I took some data by increasing stop time, changing relative response values, and solver. However, the current passing through the primary circuit is almost the same which is approximately 32mA.

#### Papabravo

Joined Feb 24, 2006
20,993
PFC -- Power Factor Correction. In an inductive load the voltage and the current are not in phase as they would be with a resistive load.

#### ericgibbs

Joined Jan 29, 2010
18,646
hi,
Note the I to V phasing, last posted.#8
E

#### 00ai

Joined Jul 11, 2021
7
hi 00,
Does this IV plot of the primary current give you a clue ref the Power ?
E
Hi Eric,

The rms of current passing through the primary circuit is still around 32mA here, which yields the same result (56 mW) as I calculated before. Am I missing something?

#### 00ai

Joined Jul 11, 2021
7
Thank you very much for your time. This is clear now.

#### ericgibbs

Joined Jan 29, 2010
18,646
hi 00.
So please show your phase correction calculation for the P = I *V * ? = .................

E

#### eetech00

Joined Jun 8, 2013
3,820
try Lp=73mh Ls=7.3H

Suggestion:
.param Vrms=1
.param Vpk=Vrms*1.414

in V1:

SINE(0 {Vpk} 1k)

will generate Vpk-pk = Vrms

#### 00ai

Joined Jul 11, 2021
7
hi 00.
So please show your phase correction calculation for the P = I *V * ? = .................

E
Hi Eric,

The time difference between the wave crests is around 0.163ms. The frequency of our source is 1kHz, so the phase difference is 0.163ms * 2π / 1ms ≅ 1.023rad. Vrms*Irms*cos(1.023) ≅ 30 mW.

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