Power factor question

[DT1878]

revising for my 301 on Thursday and Ive come stuck on a question, any ideas?



A 30KW 3 phase delta connected motor has a power factor of 0.86 lagging. Calculate the line current.

answer is 50.35a but i dont know how to get there





thanks in advance

 

[Guest123]

Ok

30000/(400x1.732x0.86) = 50.35A.

 

[pmac10]

rearrange the equation
power=volts*amps*SQ RT 3*power factor
to make amps the subject
30000=400*50.35*root3*0.86

 

[pmac10]

that was quick lenny

 

[DT1878]

thanks lads thats great

 

[Guest123]

that was quick lenny

Story of my life....

 

[DT1878]

erm another quick problem.......


A medium sized factory has a three phase load of 50kVA and 30kVAr. Calculate the power factor?

a. 0.8



again im struggling with the formula, i must be arranging it in the wrong order, sorry to be a pain but anybody help us out?

 

[pmac10]

post the formulae, what is 30kvAr, should that read 30kvA, is 50kvA apparent power and 30kvA true power?

 

[DT1878]

well it will be this wont it?

The power factor for a three-phase electric motor can be expressed as:
PF = P / [(3)1/2 U I] (2)
where
PF = power factor
P = power applied (W, watts)
U = voltage (V)
I = current (A, amps)

 

[pmac10]

where did the motor come from???
can you type the question as it reads??

 

[DT1878]

sorry i must be using the wrong forumula then, this is exactly how the question reads:


A medium sized factory has a three phase load of 50 kVA and 30 kVAr. Calculate the power factor

 

[pmac10]

to me, and im pretty sure im right(99.9%), you have a factory with apparent power 50kvA and true power of 30kvA, power factor=true power/apparent power, 30kvA/50kvA=0.6 which means your answer is wrong. text books with incorrect answers are not uncommon. does this sound plausible?? also i'm not sure what kvAr is?

 

[DT1878]

to me, and im pretty sure im right(99.9%), you have a factory with apparent power 50kvA and true power of 30kvA, power factor=true power/apparent power, 30kvA/50kvA=0.6 which means your answer is wrong. text books with incorrect answers are not uncommon. does this sound plausible?? also i'm not sure what kvAr is?

kvar is Reactive Power so i think the question is right im just not getting it.

or you could be right and the paper is wrong lol

 

[pmac10]

you are right, i didnt think you would that at level 3, i cant cope with these questions after a few beers so ill get back to you tomorrow nite if thats ok

 

[DT1878]

haha ok pmac no probs, cheers anyway