# Subtracting Phasors



## cruzy (Oct 26, 2010)

You cannot just subtract phasor quantities by their magnitude, correct? In the Chelapati book, page 3-54, they subtract 1036 angle -6.9 kVA from 1135.5 kVA to get 117.4 kVA. That's an error, correct?

Does anyone know if there is an errate available from the Chelapati books anywhere? I'm sure this is an error and there might be some more.


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## vinnius (Oct 26, 2010)

cruzy said:


> You cannot just subtract phasor quantities by their magnitude, correct? In the Chelapati book, page 3-54, they subtract 1036 angle -6.9 kVA from 1135.5 kVA to get 117.4 kVA. That's an error, correct?
> Does anyone know if there is an errate available from the Chelapati books anywhere? I'm sure this is an error and there might be some more.


no you cannot. the best way is to convert back into rectangular form and do the addition/subtraction there. Only multiplication/division can be done in the polar form.

or.....use the fx casio 115 from what i hear.


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## nmh0408 (Oct 26, 2010)

vinnius said:


> cruzy said:
> 
> 
> > You cannot just subtract phasor quantities by their magnitude, correct? In the Chelapati book, page 3-54, they subtract 1036 angle -6.9 kVA from 1135.5 kVA to get 117.4 kVA. That's an error, correct?
> ...



You can add and substract only if the two phasors are in phase (equal angels).


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## cableguy (Oct 26, 2010)

I think their "out" is the phrase they use just before the answer - "at the same power factor". Their power factor is cos(-6.9), so they are just leaving that part out. Basically, they're saying 1153.5&lt;(-6.9) - 1036.4&lt;(-6.9) = 117.4&lt;(-6.9) kVA

I've found a couple of typo type errors in the Chelapati book, but nothing as far as theory or formulas. Nothing on the level of the Kaplan sample exam.


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## nmh0408 (Oct 26, 2010)

cableguy said:


> I think their "out" is the phrase they use just before the answer - "at the same power factor". Their power factor is cos(-6.9), so they are just leaving that part out. Basically, they're saying 1153.5&lt;(-6.9) - 1036.4&lt;(-6.9) = 117.4&lt;(-6.9) kVA
> I've found a couple of typo type errors in the Chelapati book, but nothing as far as theory or formulas. Nothing on the level of the Kaplan sample exam.



What is teh PF associated with 1036.4 KVA?


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## cableguy (Oct 26, 2010)

cos(-6.9) = .9927... darn near 1.


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