Discussion in 'Homework Help' started by JimmyB, Jun 19, 2010.

Jun 1, 2010
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Hi is anyone able to help me with this question??

draw to scale 3 phasors representing the following voltages, taking e1 as reference phasor

e1:= 80 sin ωt volts

e2= 60 cos ωt volts

e3 = 100 sin (ωt-∏/3) volts

I think i understand this part, is it 1 sin wave and 1 cosine wave and 1 lagging sin wave on one 2∏ cycle chart,

the part im not sure is it then asks to sum these 3 voltages and express it in the form Em sin (ωt ± θ), i think i can sum the 2 sin waves though i'm unsure, it is the cosine i'm unsure with...

is anyone able to help please

jim

Last edited: Jun 20, 2010
2. t_n_k AAC Fanatic!

Mar 6, 2009
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782
Consider the phasor e2 cosine as leading the reference e1 sine phasor by 90° [∏/2 radians]. You've typed e2 twice but I assume the 3rd phasor is e3. Phasor e3 lags e1 by ∏/3 [60°].

Use phasor addition to find the sum e1+e2+e3

$e_{tot}=e_{1}+e_{2}+e_{3}=80\angle 0^{o}+60\angle 90^{o}+100\angle -60^{o}$

Then convert the resultant phasor back to the time-domain equivalent.

Last edited: Jun 19, 2010

Jun 1, 2010
38
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sorry it was ment to be e3.. edited now...

Thanks tnk, does that mean convert into rectangular add the votages then back into polar form..

Am I right about the drawing?? all 3 waves are drawn on one 2∏ cycle..

thanks

jim

4. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
If you are lucky enough to have a calculator that will do everything (including addition) in polar form then no - you don't have to convert. Otherwise you will probably have to go to rectangular form to do the addition and then back to polar.

Are you sure you know what a phasor diagram is? I'm not sure of your meaning in - "all 3 waves on a 2∏ cycle". You may be thinking of the time based waveform diagram - which is not a phasor diagram.

Jun 1, 2010
38
0
are Phasor diagrams straight lines?

or can I draw 2 sin waves and 1 cosine wave superimposed on one graph..

If they are staright lines do i have e1 horizontal mag 80, e2 same origin but vertical mag 60, e3 same origin mag 100 -60° from e1.. is that what is ment by phasor diagram.. or should e2 start from the end of e1, and e3 start from the end of e2.. I know that is what you should do to add the phasors..

thanks

jim

Last edited: Jun 20, 2010
6. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Something like this .....

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