Resonance Simulation error . Beginner Finding!!!!

[snouto override]

Hello Guys in this great forum , i would like to ask something .

i am testing resonance Circuit on multisim , to discover the resonant effect for the first time on multisim simulation 10 .

i placed the following components : note all components are in SERIES order .

1 - AC Power supply at 300 Vrms 1000 KHZ.
2- Resistor 10 Ohm
3- Capacitor 106pf.
4 - Coil 239 uH . MicroHenry


i would like to know the total current in this circuit and each voltage drops for across all components .

i calculated my results but multisim shows me totally different answers , whereas i used default mathematical equations but multisim gives me totally different values .

Multisim values are :
Total current I = 7.45 A.
Vr = 74.282 v.
Vc = 11 Kv.
Vl  = 11.393 kv .

ARe these values wrong or right please ????????????

[lacy]

I am not sure how you arrived at these numbers ,but I get different values. I am simulating with all default settings.

Vl=300.242 uV

Vc=300V

Vr=1.999 Mv

Total current: 200.5 uA It actually fluctuates from 199 uA up to 201 Ua

I am measuring these values directly across the component. If you are measuring a different way, let me know

Message Edited by lacy on 10-15-2007 01:44 PM

[lacy]

My apologies. I mis-read your voltage frequency. Once I changed that I did get the same results. I am not sure if these are correct, I would have to do brush up on my resonant circuit math before I could say that.

From what I do know that in a AC resonant circuits (if I rermember this correctly) is that you cannot directly measure the drops across the capacitor and inductor because of phase differences and large EMF voltages generated by the inductor/ capacitor combination and that these have to be calculated using vectors. I would have to refresh my memory on this since it has been a while since I have done any vectior calculations. (I usually dont deal with resonant circuits much)

[lacy]

I found a little tib-bit on the web that may explain the reason behind the voltages you are seeing. This come from the NEETS Engineering Manuals.

"Q Relationships in Series Circuits

Q can be used to determine the "gain" of series-resonant circuits. Gain refers to the fact that at resonance, the voltage drop across the reactances are greater than the applied voltage."

This is about the best explaination of the very high voltages you are witnessing across the inductor and capacitor.

If you would like to read this whole text on this subject you can Google "NEETS" and you should find it. These are in-depth manuals that are free to view on the web. Look for "Electronics" on the sites navigation bar on the left.

I hope this helps some. If any of my assumptions are wrong I apologize. It has been a while for me when it comes to resonant circuits.

Message Edited by lacy on 10-15-2007 02:30 PM

[snouto override]

Thank You Lacy , for your reply my friend , i would like to tell you that i did my calculations as the book says , but i found that my multisim simulation environment gives me totally different values for that resonant circuit that is why i am telling you my friend  that these values may be wrong , but in my calculations which i repeated more than a t ime now that the values given by the multisim are totally different from these ones i retrieved by mathematical equations.

i just wonder , i want to know who did the wrong calculations "multisim" or me???? but i did like the book told using the same equations more than once .

[lacy]

Your calculations are probably correct. Do these calculations take in consideration the voltage gain of the series resonant circuit at resonance? How about the singal phase relationships of the reactive components?  Does your calculations take into comsideration perfect inductors and perfect capacitors and their effect on a resonant circuit?

One thing I will point out here that may be causing these voltage reading to be higher in MS than they may be in the real world is the fact that the models used for the capacitor and the inductor are perfect models with no losses. Most real world components have losses that is represented by a series resistor in line with the component. This is called ESR (equivalent series resistance). Since Multisim does not model this ESR, then the Q of a circuit is going to be quite high at resonance. This is the only way to explain these voltages and why they differ from you calculations. I could be wrong and if someone has a different take on this I would be more than willing to hear what you have to say.

I would like to see your formulas that you are using and do the math myself, then maybe I will have a better clue as to what is happening. If you could post this for me, I will take them and analyze them in conjunction with this circuit. Maybe then we can get to the actual heart of what Multisim is doing as compared to what the calcualtions are showing. (I am am not questioning your math, but I need a basis for comparison in order to do deductive reasoning on this)

[haklesup]

Without even firing up the S/W or taking out a pen, I can see that the sum of the voltages across the loads is much much more than the voltage of the source thus violating Kirchhoff's Voltage Law (KVL).  Even if you scale kV to V it still sums to only 100V too low.  Clearly the results from the program are wrong.  Really 11kV, this is not a voltage multiplier, you should have no load with a voltage greater than the source.

At resonance I would expect the sum of the voltages across the cap and inductor to be minimized and across the resistor to be maximized (since current will be at its max during resonance) but they should still add up to the source voltatge.  Its hard to see the effect of resonance while looking at only one frequency.

Try simulating using the function generator instead of the AC source.  With the Agilent function generator, you can sweep the frequency and use the spectrum analyzer or bode plotter to see resonance effects nicely.  If Kirchhoff and Ohms laws can't make sense of the result, it is garbage.

If you get a correct result with the function generator but not the AC source, I suggest you save both circuits in the same file and post it here for the experts to see side by side. 

[lacy]

Hi haklesup,

I tried the agilent Function generator and was unable to set it up to 300Vrms the most I was able to get was 20Vpp (I may not have this set up right). I then tried the other funtion generator and still had the same results as before.

What we have not considered in this circuit is that he has 300Vrms. This is not the toal voltage in the circuit. The peak amplitude is 420V AC and the Peak to Peak is 840V AC. Now if Multisim is baseing its calculation on one of these other votages and not the RMS value, then that could be why it is throwing off the manual calcualtions and the results don't match.

I have a question for you about what I have been saying about the voltage gain in a series resonant circuit. The information I quoted was verbatim from the Naval Electrical Engineering Training manual from T-Pub. It says you can have a voltage gain above the applied voltage. Now is this information incorrect? You can check this out for yourself, but this is where I got my information from.

[lacy]

I am going to post the link to my information for you to read. This way you can analyze what it says better than I can explain it. You can draw your own conclusions from it.

Here's the link: http://www.tpub.com/content/neets/14181/css/14181_37.htm

[digitalguy]

I just briefly looked at your Multisim results and lacking time calculate the results myself, you can check the Multisim answer by calculating

v

=

((vL - vC)² + vR²)½

Where v is the applied voltage from the Multisim analysis and the other voltages are self explanatory (also from the Multisim analysis).