Complex Imaginary Test 4 Problem 29

[Wildsoldier PE]

I think complex imaginary have this answer wrong.

At first they have P=(SQ3) (460) (96) (.95) = 72.7 KW

Then an errata goes out on this problem saying.

P = (3) (460) (96) (.95) = 125.9 KW

Now this is how I resolve this problem i assume this motor is a 3-phase motor:

Let me know what you all think!!

 

[DK PE]

I think the CI team asked another question with no answer as you have no clue what real power is produced by the motor since it states nothing on efficiency. You could make an engineering assumption (this is a ~100HP motor so efficiency = ~ 90%) but that is probably beyond the scope of the class. All the input real power could be dissipated as heat and answer = 0

That said, your terminology confuses me a bit but I agree with real power in = Pin = √3 * 460 * (96*√3) * 0.7 since the problem statement seems to clearly state a phase current in a delta winding. As long I'm picking on the CI team, motors don't "produce" current (at least not normally), they consume current.

 

[vdubEE]

Since it is delta-connected, the voltage given is VLL and/or Vphase. Since the problem statement gives you Iphase and you already have Vphase, the easiest thing to do is use P=3*Vphase*Iphase*PF. The way you worked it out works fine as well since you calculated ILine and then used P=squareroot(3)*VLL*IL*PF.

However, that is where the problem seems to take a turn for the worse. Since all the examples in the NCEES practice exam point to the fact that P is not affected by the capacitor bank installation. It seems the Complex Imaginary problem use the wrong power factor number. I think they got lost in exactly what they were trying to accomplish with this problem. The problem has a pretty good trick to it if you don't realize that they asked for the P, gave you too much information, and you use the new power factor to calculate P. Seems they forgot that the capacitor bank reduces Q not increases P of the motor.

Hopefully we can get some more feedback to see if others agree. Nevermind it seems like someone posted while I was typing my response.

 

[Wildsoldier PE]

I think i am not going to post any more complex imaginary problems i think they need to get their things straight. Pay some good professors with knowledge in this field to revice their stuff. I hope they get every thing corrected because their test have pretty good questions but the errors makes every one wonder who is wrong, the book or us.

 

[BamaBino]

Wildsoldier, I agree with your solution and answer.

 

[Insaf]

I think complex imaginary have this answer wrong.

At first they have P=(SQ3) (460) (96) (.95) = 72.7 KW

Then an errata goes out on this problem saying.

P = (3) (460) (96) (.95) = 125.9 KW

Now this is how I resolve this problem i assume this motor is a 3-phase motor:

Let me know what you all think!!

---------------------------

Motor produces power !! (not consumimg ? seems its a generator)