# Example question

Discussion in 'Homework Help' started by jaymo3141, Feb 21, 2014.

1. ### jaymo3141 Thread Starter New Member

Feb 16, 2014
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Can someone explain why i1 doesn't lead i3 by 345 degrees?

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2. ### MrChips Moderator

Oct 2, 2009
12,633
3,453
Try drawing the three currents as vectors on a polar plot.

3. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
First, get all three into the same form, namely

i_n(t) = I_N cos(ωt + θ_n)

Then determining how much one leads another becomes trivial.

4. ### screen1988 Member

Mar 7, 2013
310
3
In i_n(t) = I_N sin(ωt + θ_n) what is the range of θ_n?

-180 degrees to 180 degrees or 0 to 360 degrees?
I got two different results with the question i1(t) leads i3(t).
If θ_n is from -180 to 180 degrees then i1(t) leads i3(t) by 165 degrees.
If θ_n is from 0 to 360 degrees then i1(t) leads i3(t) by -195 degrees.

Last edited: Feb 21, 2014
5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
It shouldn't matter what range you choose. Either way, you may need to normalize the answer, but the answers should be the same modulo 360°.

screen1988 likes this.
6. ### screen1988 Member

Mar 7, 2013
310
3
I realise I made a mistake. The first case is 165 degrees and the second is -195 degrees.
And as you said both are correct. There is an infinite number of the correct answers here?

7. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Yes, though common practice is to reduce it to less than one complete revolution in either direction and to normally restrict even that to a total range of just one revolution overall.