3 Phase Machine Line Current problem

  • Thread starter AdamBG10
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  • #1
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Hi guys,

I would really appreciate some help with this problem(probably quite easy for anybody with Electrical Engineering experience).

Thanks in advance

Homework Statement



A 415V, 50Hz, 3-phase supply is connected to a 3 phase load of 10kw with a power factor of 0.8 lagging. Find the magnitude of the line currents.


Homework Equations



P = VICos(power factor)

Total Power = 3 V(p)I(p)cos(power factor)


The Attempt at a Solution



Attempted the question using the equations above, except I obtained the wrong answer.

Correct answer = 17.39Amps
 

Answers and Replies

  • #2
394
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Assuming the 415V is line to line rms and that the 10kW is the total wattage for all three phases:

[tex]V_{\phi}=\frac{V_{LL}}{\sqrt{3}}[/tex]
with [tex]V_{\phi}[/tex] being a per phase voltage and [tex]V_{LL}[/tex] being the line-to-line voltage.

Next, we know that:
[tex]P_{3\phi} = 3V_{\phi}I_{\phi}PF[/tex]
with PF = powerfactor. And if this is star connected...
[tex]I_{\phi}=I_{LL}[/tex]

so
[tex]P_{3\phi}=3\frac{V_{LL}}{\sqrt{3}}I_{LL}PF=\sqrt{3}V_{LL}I_{LL}PF[/tex]
or
[tex]I_{LL}=\frac{P_{3\phi}}{V_{LL}\sqrt{3}PF}[/tex]

You seem to be confusing something. cos(angle) = PF. You don't say cos(PF). As far as I know, cos(PF) means nothing of importance.
 
  • #3
4
0
Assuming the 415V is line to line rms and that the 10kW is the total wattage for all three phases:

[tex]V_{\phi}=\frac{V_{LL}}{\sqrt{3}}[/tex]
with [tex]V_{\phi}[/tex] being a per phase voltage and [tex]V_{LL}[/tex] being the line-to-line voltage.

Next, we know that:
[tex]P_{3\phi} = 3V_{\phi}I_{\phi}PF[/tex]
with PF = powerfactor. And if this is star connected...
[tex]I_{\phi}=I_{LL}[/tex]

so
[tex]P_{3\phi}=3\frac{V_{LL}}{\sqrt{3}}I_{LL}PF=\sqrt{3}V_{LL}I_{LL}PF[/tex]
or
[tex]I_{LL}=\frac{P_{3\phi}}{V_{LL}\sqrt{3}PF}[/tex]

You seem to be confusing something. cos(angle) = PF. You don't say cos(PF). As far as I know, cos(PF) means nothing of importance.

Thanks very much!
 

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