Calculating Kw

[nitro]

I'm an electrical engineer in the US and need to clarify a dispute with a co-worker. We are working on a project in Russia where voltage and hertz are different from the US. We are sizing Generator Sets and have to calculate kVA to kW @ 50Hz.

What is the formula for converting kVA to kW for 50Hz? I know it is not he same as 60Hz (kVA * pf). I say kW is higher than kVA at 50Hz and kW is lower or equal to kVA at 60Hz depending on Power Factor (pf). Any feedback would be appreciated.

Thank You,
Nitro

 

[infomaniac]

nitro,

Sorry to say but you will have to buy the drink for your co-worker. The laws of electrical engineering do not change just because the fundamental frequency is different. KW is always equal to KVA times power factor. This can get more detailed if we start talking about displacement power factor versus true power factor so for arguments sake just assume no harmonics exist.

 

[infomaniac]

nitro, just one more thing. KW can never be larger than KVA. Again, the fundamental definitions of electrical engineering are still maintained.

KVA = sqrt(KW^2 + KVAR^2)

If you play around with this in a spreadsheet, you will see that KW can never exceed KVA. The best you can get is KW=KVA and that occurs when the power factor is 100%.

 

[nitro]

Thank Infomanic, but we still have some old-timers here think there's a difference depending on hertz, I'm not one of them. Ant published works on this? Thanks again.

Nitro

 

[TheBlacksmith]

Since there is no mathematical variable for frquency in the equation KVA = sqrt(KW^2 + KVAR^2) then it should be easy to assume that the frequency has no impact. Basic math, not some obscure reference should suffice.

Blacksmith

 

[infomaniac]

Thanks Blacksmith. I did not want to have to respond to that question.

 

[nitro]

Thanks Blacksmith & Informanic, now if I can just convince the old-timers here. You would think this would be simple.

 

[Bung]

Sounds like you and the old-timers are talking at cross-puroses. If you take a motor designed for 60Hz operation and run it at 50Hz, its kW/kVA characterisitcs will be different, because of the design of the core / windings being optimised for one particular frequency.

However, at any particular frequency the relationships between the kW and kVA are calculated identically. Just the numbers going in (current and power factor, assuming V is same for both) will be different for a particular motor depending on whrther you try to energise it at 50 or 60Hz.

So, you are both right and shold buy each other a drink

Bung
Life is non-linear...

 

[towkay238]

I like Bung's comment that everybody is right but I think Infomaniacs answer is correct to the question of kW vs kVA. The value does not change with the frequency - the speed of the machine is different and as mentioned the characteristics of the generator may be different for the different speeds.