# THD, (not to do with marijuana!)

Discussion in 'Homework Help' started by ninjaman, Dec 15, 2013.

1. ### ninjaman Thread Starter Member

May 18, 2013
307
1
hello
I have a question that im not sure about. I have a fundamental of 70vrms 50Hz, a 3rd harmonic of 23vrms and 5th harmonic of 14vrms. the frequencies of the harmonics are 150Hz and 250Hz respectively.
I have this question,

Part 3:
If the 5th harmonic has a lag of 70º
a) Produce graphs of individual waves and resultant complex waveform.(I have done this)
b) Produce a multisim simulation to support your graph. (I have done this also)
Part 4: Find the voltage after 20ms from graphical and mathematical methods. (I have done this)

• Derive the harmonic distortion between the 2 results.
this is where it went pear shaped!
I have found what I think is the right formula
(taken from wiki)
putting in my values of:
V2 ^2 = 23V^2
V3^2 = 14V^2
over V1 = 70v
these are all rms values, is that correct?
I get 3.21825, which I rounded to 3.22, which I think is a percentage?

so I get that the harmonics are a ratio to the fundamental which is why there is a percentage. though I have seen dB as well. I don't like dB personally, it muddles me up!!!

CHRISTMAS SOON!!!! TIME TO SIT THERE AND DO NOTHING WHILE EATING LOTS OF RICH FOOD, GAIN WEIGHT AND WATCH CRAP ON THE TELLY!!!

all the best
simon

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,675
1,536
You take the square root of the sum of the squared numbers, then divide.

That result is the decimal from which you convert to a percentage.

What you did was sum the squares, then divide, then took the square root of that result and did not convert that to a percentage.

On edit ...

You might want to show us your illustrations as well ...

Last edited: Dec 15, 2013
3. ### ninjaman Thread Starter Member

May 18, 2013
307
1
hello

so i did the above again the way jester said, i got 0.384.

he then said about convert to a percentage, does this mean *100
if so this gives me 38.5%(rounded)???

im not sure what you want to see jester?

4. ### Metalmann Active Member

Dec 8, 2012
700
224
I think the OP means "THC", instead of THD.

One of Nature's greatest cure-alls.

5. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,675
1,536
Using just the numbers you gave does have 38.4% distortion. That is why I asked you to show the other work you said you had completed.

And yes, converting the decimal to percentage typically means multiply by 100 ... and adding the percent symbol.

If you did that to your first example you would be over 300 percent distortion and you would have realized that answer was "incorrect". You weren't doing it the way I said to arrive at the 38.5% figure, you were doing it the way the formula that you posted stated it should be done.

All I did was do the calculation, then troubleshot how you arrived at your figure ... where you missed a crucial calculation.