Three Phase Power Question

[Eng1001]

Hello guys,
It's been a while since I last dealt with generators and 3-phase - so need a refresher (and an electrical engineer that I've been trying to hire!!!).

I have current demands for each phase of a load on the generator (say, a motor). If I were to work-out the individual phase power, it is = V(line) * I (line).
If I am confirming the size of the 3-phase generator for the load, how would I calculate the 3-phase equivalent power?
Is it
a) P = sqrt(3) * V(line) * I(line); or
b) P = sqrt(3) * V(line-line) * I(line)

line voltage is 220, and line-line voltage is 440V. I(line) would be the calculated maximum demand any one of the lines.

Sorry, such a high-school question, but as they say, don't use it, you loose it.

 

[LSpark]

The 3 phase equivalent power is:
b)P = sqrt(3) * V(line-line) * I(line)

a) is wrong and should be:
P = 3 * V(line) * I(line)

V(line-line) = sqrt(3) * V(line)
So V(Line-line) would be more like 381V (not 440V) if V(line) is 220V.

 

[Skogsgurra]

If your load isn't lamps or heaters, you also need to consider power factor, cos(phi) or PF.

P = sqrt(3)*U*I*PF. U is line-line voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

I think there is confusion about power vs VA. You have asked about the VA rating, not the power rating. You hinted this is a generator sizing question. So at the most basic level, the alternator needs to support the required VA, which is sqrt(3)*V*A. The prime mover needs to support the required power which is sqrt(3)*V*A*pf.

Since you also hinted you'll be starting a motor, you also have to consider the generator overloading capability vs the motor starting method so you can purchase the most practical (read cheapest) generator that will still do the job required.

 

[waross]

With 220:440 Volts three phase, is this a four wire delta connection?
If your power factor at full load is greater than 80% you must calculate both KVA and KW. The KVA of the generator must be equal to or greater than the KVA of the load and the KW of the generator must be equal to or greater than the KW of the load.
BUT, if you have motor loads, you should consider the largest motor at 300%. When the generator is sized for any large motor loads the KW will generally be adequate regardless of the power factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Eng1001]

Ah,
it's slowly coming back.
Sorry, VL-N is 230 not 220, and VL is 400 not 440.

Going back to sizing, the demand data I have seems to assume resistive (or near) loads as the pf is indicated around 1.

The generator capacity is specified in KVA (100) and KW (80), indicating rated power factor: 0.80

Therefore, if I had 90 KVA (or KW) of resistive loads, would this generator be undersized?

 

[electricpete]

Your prime mover could be undersized if it is matched to the 80kw rating.

=====================================
(2B)+(2B)' ?

 

[rbulsara]

Both kW and kVA limits need to be observed. You cannot exceed 80kW nor can you exceed 90 kVA. So 90 kW is over the rating.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

If you have a diversity factor and your load only exceeds 80 kW for one hour or less out of twelve, you may get by with a prime rated set. An 80 kW prime rated set is allowed to produce 88 kW for one hour out of twelve. You could probably fudge the extra 2 kW. I would have to see the actual loads and conditions before giving a firm answer when it's that close. An 80 kW prime rated set may be a good idea in one case and a really bad idea in another case. It depends. A standby rated gen-set would be undersized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Remember, an overloaded transformer runs hotter. You may often live with this.
An overloaded generator runs slower and the voltage drops almost proportionally to the frequency. You often CAN NOT live with this.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[erputra0615]

thanks alot frend for explanation...