Transformer simulation

Hi, I am having a hard time understanding the simulation results of this stepdown T/F. I am giving the primary an input voltage RMS of 8600V, T/F turns ratio 25:1 with R_prim=0.1ohm and L_prim=0.1H. The R_sec = 0.1ohm and R_load is 100ohm. When I calculate the current through the primary it should be 8600/((sqrt(R_prim^2+L_prim^2)+(R_load*(turnsratio^2)) ,that's 3.439 but it appears that the secondary current that is in the simulation says RMS 3.44A and primary current as RMS 382.6A which I don't understand why. Even when I divide 3.44/382.6 is not equal to the turns ratio in the equation. The secondary voltage in the simulation is however correct which is RMS 344V (8600/25). Am I missing anything here. Pl advice.

Circuit below

by archsada
July 03, 2018

huh...when i simulate that exact circuit i get zero volts everywhere...

by Demoniak
July 23, 2018

I am having similar issues. Every time I insert a step-up transformer in my circuit, my output node dbV reading goes negative

by Hussyh
September 08, 2020

This note is in response to the most recent reply, from @Hussyh.

You haven't provided a circuit, so I will refer to the origin of the thread, about 2 years ago now. That transformer is a step-down, that is, NODE2 (SEC) is at a lower voltage (-dBV) with respect to NODE1 (PRI). If you want the SEC voltage in that configuration to be higher than the PRI (step-up), then you will need a turns ratio of less than 1.

It is unfortunate that the original question was not answered - there is so much learning to be had from a close look at the circuit! If your "similar issues" cover the original circuit too, have a look at these observations and suggestions.

1] Look closely at the first cycle of the Primary current I(XFMR1.nPRI_A) in the plot/graph. You will see that it starts at 0, goes to a positive peak, then back to 0. A +ve DC pulse in an AC circuit!

2] Now look at the final cycle of Primary current: the lower part of the waveform is just negative. (The +ve DC component is decaying with a time constant of Lpri/Rpri = 0.1/0.1 = 1Sec.)

3] The formula given for the Pri current: 8600/((sqrt(R_prim^2+L_prim^2)+(R_load*(turnsratio^2)), that is, Volts/sqrt(Impedance^2). The term L_prim^2 needs a factor of omega^2 to make it an impedance^2.

Things to try:

i) Add 5 (whole) seconds to both the start and the stop times of the T-D simulation. After a longer wait for the simulation results, you should find that the Pri current is almost symmetrical +/-.

ii) In the V1 parameters, set Phase to -180. You should find that the first cycle of Pri current is now -ve. (Does this give you a clue about where the DC pulse comes from?)

iii) In the XFMR1 parameters, set R_PRI to a higher value. You should see that the DC decay time constant is lower.

by EF82
September 09, 2020

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