VA & Watt Confusion

[yjwong]

I have a motor with 22kW (I presume this is the power of the motor), I measured the current actually goes through it with a clamp meter and get a reading 17A. The voltage which sourced from a generator is 415V. So I get the Apparent Power reading 17x415=7055VA (7.05kVA). By definition Power Factor (PF) = kW/KVA then I get 22/7.05=3.12! What is going on? I have a list of all motors in the plant and they all give PF greater than 1. I think the way I did my calculation has problems but I can't figure out what's wrong and I haven't got any electrical background so I would really appreciate if somebody can explain this in a simple way to me. Thank you very much indeed.

 

[electricpete]

You assumed the motor is putting out nameplate power - it may in fact be putting out much less... motor genearlly matches the load power requirement.

Also if it is a three-phase machine, there is a sqrt(3) in the real and reactive power calculations.

VA=sqrt(3)*Irms*Vrms where Vrms is line-line voltage

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[yjwong]

First what do you mean by "nameplate power"? and what are Irms and Vrms and how can I get those readings? Afterall where can I get the real power reading? Thank you very much.

 

[yjwong]

sorry but when you said sqrt(3) do you mean cubis root as sqrt(3)*8=2? Many thanks.

 

[supraskate13]

yjwong,

I know you don't have an electrical background so here is a tool we use. It is called the power triangle. A power triangle is a right triangle that shows real power (watts), reactive power (vars) and and apparent power (VA)

Watts is the horizontal X-Axis, Vars is the verticle Y-axis and VA is the hypotonose (sp is definitley wrong!) as you can see, your apparent power is smaller than the real power leg of the triangle. Geometrically speaking.....this is impossible. The motor will also only put out as much power as it has too. If it is not loaded down, it will not be putting out close to 22kw.

 

[TheBlacksmith]

Sqrt(3) as in equals ~ 1.73

Blacksmith

 

[yjwong]

so is this "Sqrt(3)" kind of constant like "e" or "pie"? if so what is it actually called? Cheers.... and by the way even I multiply my VA with 1.73 it's still smaller than 22kW, so what else can I do? Thank you.

 

[rbulsara]

yjwong:

How much effort did you put in to learn some basic electrical engineering material and 3 phase power formulas before asking this forum to teach it to you?

Please look up some 3 phase formulas for electrical , there are plenty on the web as well. If you have a friend who is electrical engineer may be sit down for 1 hr with him/her.

By the way, name plate means rated power, but machine does not use rated power all the time. Like if you have a car whose engine is rated for 250HP, that is maximum it can put out. You do not use it all the time, unless you are doing 140mph with 6 people in it all the time.