NCEES #110 & 121

Can someone explain to me why the power factors were not used in these two problems to find the currents?

Thanks!

Apparent power (S) is determined by: S=VI*.

You only start caring about the power factor if you are concerned about real and reactive components such as what happens when you start examining loads with non-like power factors.

Since all they are looking for is the magnitude of the current, the angle doesn't matter.

Flyer_PE said:

Apparent power (S) is determined by: S=VI*.
You only start caring about the power factor if you are concerned about real and reactive components such as what happens when you start examining loads with non-like power factors.

Since all they are looking for is the magnitude of the current, the angle doesn't matter.

Click to expand...

But in the problems they did not mention what kind of power so how can i tell they are apparent, real or reactive powers if I see any similar problem like this?

Anyway, thanks for your time.

truong said:

Flyer_PE said:

Apparent power (S) is determined by: S=VI*.
You only start caring about the power factor if you are concerned about real and reactive components such as what happens when you start examining loads with non-like power factors.

Since all they are looking for is the magnitude of the current, the angle doesn't matter.

Click to expand...

But in the problems they did not mention what kind of power so how can i tell they are apparent, real or reactive powers if I see any similar problem like this?

Anyway, thanks for your time.

Click to expand...

They did mention the type of power. It was KVA, therefore ILine = S/(sqrt3)(ELine) for 3 phase. No need to use power factor. Power factor was just a distraction.

55 Days togo

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Why does the answer for # 110 not require the sqrt 3, but # 121 requires using the sqrt 3 ? Is it because in #110 the load is only from phase B to C, and in 121, the output KVA is for all of 3 Phase?

Problem 110 is a single phase problem. Even though they are connecting to a three-phase system, they are only connecting a single phase load and the voltage between the phases is given.

Problem 121 is a three phase problem.

Flyer_PE said:

Problem 110 is a single phase problem. Even though they are connecting to a three-phase system, they are only connecting a single phase load and the voltage between the phases is given.
Problem 121 is a three phase problem.

Click to expand...

I understand how this was calculated, I'm just confused as to if this is the standard way they state the specifications of a generator. How do I know that the 2.4 kV stated is phase voltage and not line voltage?

FatDirk said:

Flyer_PE said:

Problem 110 is a single phase problem. Even though they are connecting to a three-phase system, they are only connecting a single phase load and the voltage between the phases is given.
Problem 121 is a three phase problem.

Click to expand...

I understand how this was calculated, I'm just confused as to if this is the standard way they state the specifications of a generator. How do I know that the 2.4 kV stated is phase voltage and not line voltage?

Click to expand...

I've never seen a 3-phase generator rated in anything other than VLine. If the information is given as a phase voltage, it should explicitly state that it as such.

For Clarity purposes:

1. If I had a Delta-Connected 3-Phase Load with Vline = 2.4KV, and S = 1325 KVA. What is Line Current?

2. If the above is WYE-Connected 3-Phase Load with same values, Do I get same Line Current?

My Answer is the same for 1 and 2: Iline = Stotal/(sqrt3)(Vline) = 1325K /(1.73)(2400) = 319 Amps

I have seen some solutions to this type of problem not include the sqrt3 for 3-phase Delta Loads. I want to make sure this logic is wrong. Does anyone not agree with my answer for 1&2?

^Regardless of whether you are connected delta or wye the line current for a balanced 3-phase system will be:

ILine= STotal/(sqrt3*VL-L)