PPI Camera Power Sample Exam #1 - Problem #8 - Unbalanced Loads

[afacemire]

Hi Everyone,

I haven't seen this question asked about on here yet.

8. An electrical distribution system has the following:

three-phase, four wire

wye connection, neutral groud

480V line voltage

A complex unbalanced load of 500 + j300 VA is connected across phase A and phase C. What is most nearly the neutral current?

~~~~~~~~~~~~~

Solution:

Vphase = 480 / sqrt(3) = 277

-In = Ia + Ic (no Ib b/c it is equal to zero)

= (500+j300)/(277 angle0) + (500+j300)/(277 angle-240)

= 2.1A

~~~~~~~~~~~~~

The issue I have with this problem is that that I think the current formula should be using the live voltage instead of the phase voltage.

If you had the (500+j300) connected between Aphase and Neutral AND (500+j300) connected between Cphase and neutral, I think the solution would be correct.

However, from the problem statement, I am assuming that there is only one load and it is connected between Aphase and Cphase.

Thoughts? Thanks in advance!

 

[luckyboy886]

I think you are correct to assume that there are two loads for Phases A and C. The problem statement seems to be unclear.

If there is only one phase between Phases A and C, there will be no flow in the neutral.

 

[afacemire]

Thanks for the response. I think I might submit to the errata to change the question to:

“A complex unbalanced load of 500 + j300 VA is connected across phase A and neutral and phase C and neutral.”