Sample Test question (need help)

[elecrookie]

Hello to Everyone
i found this question and i was totally confused , maybe i just don't understand the question and looking for HELP question below thanks




1. A balanced 3 phase, 60Hz load consists of machinery of 500 kva at 0.5 PF lagging, and a 3 phase resistive load of
300 kW. The two loads are supplied by 4160 volts feeder.

a. What is the magnitude of the feeder current?

b. Specify the number and ratings of the capacitors required if it is desired to improve the feeder power factor to
0.8. Assume caps will be connected in Y.

 

[Smart $]

a. With both a reactive load and a resistive load, you have to use vector addition to determine the magnitude of combined currents. To do so start by determining A phase current and angle for each load. You only have to use one phase because it is assumed the 3? current is balanced. Angle (theta) for reactive load is cos (theta) = pf. I assume you know, but state anyway: lagging pf means current angle lags voltage angle.

b. You can improve power factor using capacitors. Plenty of info' on 'net. Search KVAR power factor correction...

 

[iceworm]

Hello to Everyone
i found this question and i was totally confused , maybe i just don't understand the question and looking for HELP question below thanks. ...

Following $'s comment:
You didn't pick a tough problem, but it does require an understanding of Complex Power. We need to know where you are in your math skills and what level you are in any power classes.

How is your trig?
vectors:

vector addition
vector multiplication​

define power factor:

as $ said: cos of the angle between the phase voltage and phase current.
also pf = (real power)/(apparent power) = kw/kva​

are you familiar with the power triangle - KW, KVA, KVAR?

I'm not trying to overwhelm you. These are the concepts you will need to understand the question.

I'm pretty sure you are not up on all of these. No sin there - that's where we all started. So tell us where you are at - maybe we can help with the rest.

edit to add: It will be an interactive process - you will have to participate.

ice

 

[Smart $]

...edit to add: It will be an interactive process - you will have to participate.

:rotflmao:

 

[Tony S]

Hello to Everyone
i found this question and i was totally confused , maybe i just don't understand the question and looking for HELP question below thanks




1. A balanced 3 phase, 60Hz load consists of machinery of 500 kva at 0.5 PF lagging, and a 3 phase resistive load of
300 kW. The two loads are supplied by 4160 volts feeder.

a. What is the magnitude of the feeder current?

b. Specify the number and ratings of the capacitors required if it is desired to improve the feeder power factor to
0.8. Assume caps will be connected in Y.

I?d put the figures in to the spreadsheet I wrote many years ago.

But then I?m just plain and simply lazy.

 

[elecrookie]

Following $'s comment:
You didn't pick a tough problem, but it does require an understanding of Complex Power. We need to know where you are in your math skills and what level you are in any power classes.

How is your trig?
vectors:

vector addition
vector multiplication​

define power factor:

as $ said: cos of the angle between the phase voltage and phase current.
also pf = (real power)/(apparent power) = kw/kva​

are you familiar with the power triangle - KW, KVA, KVAR?

I'm not trying to overwhelm you. These are the concepts you will need to understand the question.

I'm pretty sure you are not up on all of these. No sin there - that's where we all started. So tell us where you are at - maybe we can help with the rest.

edit to add: It will be an interactive process - you will have to participate.

ice

i been working for little on it and this is what i have come up with see attach

 

[Smart $]

i been working for little on it and this is what i have come up with see attach

Omit theta in your second pf equation.

pf <> theta <> W/VA

pf = cos (theta) = W/VA

Hint for solving second question: overlay a second triangle (first with 0.5 pf, second with 0.8 pf).

 

[iceworm]

i been working for little on it and this is what i have come up with see attach

That looks great. You have a pretty good handle on complex power and power factor.
First some 3 phase booking keeping issues - probably stuff you already know:

S = E x I X sqrt(3)

The 4160V given in the problem statement is line to line. Most of the calculations will be for one phase so we will use Line to neutral voltage. V(L-N) = V(L-L)/sqrt(3). It's okay there is no neutral. Call it a virtual neutral. Fits the math model, makes the calculations easier​

Reactive load:
First calculate the current magnitude. I = S/(E x sqrt(3)). But it is a vector. The current is not in phase with the voltage. And the phase anlge is the same theta from the power factor.

Translate to vectors:

I(reactive load) = |I| >(theta) I (vector) = magnitude (I) at phase angle (theta) (Using ">" as "phase angle"

Next calculate the resistive load current. Note the phase angle is zero.

Add the two current vectors together and you get the magnitude and phase angle of the line current.

This might help:
S = P + jQ
So for the reactive load, 500 kva at .5pf
P =
Q =

If this makes any sense, fill in the blanks and we will go on.

ice