# 3-Phase of an Electrical Machine

#### Management

Joined Sep 18, 2007
306
Hi,

I would like clarification on nomenclature when it comes to report out specs of a generator that goes to a 3-phase system.

If the specs of a generator is reported to have an voltage of 10 kV and a PF of 0.9, is that voltage the line-line voltage or the per phase voltage? I'm thinking phase voltage but not sure.

Also, it is reported to have an output power of 6 MW. Since it says megawatts I am going to assume that this is real power and in RMS, correct? Is this power into the load?

So if I wanted to find the current per phase it would be the power into the load divided by the product of the PF and voltage per phase, correct? Or should I divide the power by 3 first?

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#### Management

Joined Sep 18, 2007
306
No one?

I read that if they don't state what it is that I should assume it to be line-line voltage, Vab.

So if this is the case then the phase voltage is:
Vph = Vab / (sqrt(3)*ang(30º)

Is this correct. I need to know the phase voltage and the phase current. If someone can confirm this or correct me it would be great appreciated.

Also, if the power is 6 MegaWatts, then the phase current would be (6 MW / 3) / Vph or is it 6 MW / Vph.

Is this correct?

#### rjenkins

Joined Nov 6, 2005
1,013
Without seeing the actual machine specs, it's a guess that the voltage is phase-to-phase.

I can't remember the 'official' calcs for phase current, but if you measure all phases you are measuring the same current twice. Think of a single phase load drawing 100A, if you measure both connections they read 100A each, it's still drawing 100A not drawing 200A.
(100 x 2) / 2 (feed + return) = 100A

With three phase it's something like
(100 x 3) / 2 (feed plus return) = 150A equivalent on single phase.

6MW @ 10KV = 600A single phase or 400A three phase??
(Please allow for power factor & me being half asleep...)

#### Management

Joined Sep 18, 2007
306
So if they give you a power spec should I assume that it is 3 phase and not the power per phase? Does it make a difference? Was reading this site's 3-phase power section: http://eece.ksu.edu/~starret/581/3phase.html#8 It says to assume it's 3 phase. and it's equations says that it is the sum of each "phase power".

This website said: Power (single phase): P = Vp×Ip×cos φ

So should I divide that 6 MW by 3 before trying to find that current?

Also, I found that I can just consider the voltage value and get Vp = Vab/sqrt(3). And then get Ip from the above equation where P is 6 MW / 3.

Do you agree with this approach?

#### t_n_k

Joined Mar 6, 2009
5,455
I would think ...

The 10kV is probably line-to-line [phase-to-phase]. Probably quoted 6MW is real power otherwise it would 6MVA - but it seems strange one would quote MW rather than MVA...?

Then

Vph=10,000/√3=5774V
Power per phase=2MW

At unity pf, Iphase=2e^6/5774=346Amps
At 0.9pf, Iphase=346/0.9=385Amps

#### Management

Joined Sep 18, 2007
306
I would think ...

The 10kV is probably line-to-line [phase-to-phase]. Probably quoted 6MW is real power otherwise it would 6MVA - but it seems strange one would quote MW rather than MVA...?

Then

Vph=10,000/√3=5774V
Power per phase=2MW

At unity pf, Iphase=2e^6/5774=346Amps
At 0.9pf, Iphase=346/0.9=385Amps
Great! You confirmed what I said above. Thanks a lot.