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14th October 2010, 05:52 AM  #1 
diyAudio Member
Join Date: Jul 2010

Instantaneous VoltageMath Question!
Could someone help me out with some Instantaneous Voltage calculations please.
I need to calculate the value of the resultant waveform at 45deg 120deg & 240deg that includes a third harmonic content of 20v. Vmax is 200V, third harmonic amplitude is 20v I know that to find the instantaneous voltage the calculation is: e = Vmax sin0 So e = 200 x sin 45 = 141.4V e = 200 x sin 120 = 173.2V e = 200 x sin 240 = 173.2V Do I just ADD the 20v to each result to include the third harmonic amplitude or is there more to it? Cheers 
14th October 2010, 06:25 AM  #2 
diyAudio Member
Join Date: Mar 2010

There is more to it. First, you need to know the phase relationship. The 20 V (peak?) third harmonic has to be evaluated at the instant in question. It too is sinusoidal and is defined by peak voltage times sin (3f) Plus the shift due to phase difference.

14th October 2010, 06:56 AM  #3 
diyAudio Member
Join Date: Jul 2010

bob91343,
Thanks for the reply, I think I get what your saying, I'll try to work it out & see how I go. Cheers. 
14th October 2010, 03:32 PM  #4 
diyAudio Member
Join Date: Oct 2006
Location: Charlotte, NC

Try this out.
Just an Excel 97 spreadsheet that plots out what the waveform would look like with a number of harmonics and phase shifts. 
14th October 2010, 04:30 PM  #5 
Banned

Its:
e=200*sin(45)+20*sin(45*3) e=200*sin(120)+20*sin(120*3) e=200*sin(240)+20*sin(240*3) Here's a spreadsheet showing the graph of the actual values you're interested in: 10percent_3rd_harm.zip w I've taken the phase relationship as 0 degrees. Last edited by wakibaki; 14th October 2010 at 04:32 PM. 
14th October 2010, 10:27 PM  #6 
diyAudio Member
Join Date: Jul 2010

Thanks for the replies guy's & also the Excel spreadsheets,
I see how this is calculated, much appreciated. If I could ask one more question. How does the phase relationship factor into the equation, could you possibly give me a quick example, basically for future reference. Thanks Guy's Cheers 
14th October 2010, 10:54 PM  #7 
diyAudio Member
Join Date: Mar 2010

The phase relationship causes the voltages to add in different ways. As the waveform changes you will get different results, although the rms will be unchanged.
Just pick a phase and plot it with graph paper, adding the voltages at each instant of time. The wave could be flattened or peaked but will be symmetrical with only odd harmonics. 
14th October 2010, 11:33 PM  #9 
diyAudio Member
Join Date: Jul 2010

Thanks very much, I understand.
The posts have been very useful. Thanks again Cheers 
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