NCEES 2001 Problem 116

[SK82 P.E.]

I don't know how many have the NCEES 2001 accessible, but I need help on problem 108.

I attached pics of the problem and the solution.  I used:

IAmp = IAmp = ((sf*Power)/phase)/(V*pf*〖Eff〗_motor ) = ((1*24.4kW*1000W)/(3 phase*kW))/(460*0.9*0.93)

I’m not sure how they got 1.73 in the denominator.  When I move the 3 to the denominator I get 2.79.

 

[tbova]

This comes from the power being three-phase. The square root of 3 = 1.73

I = P / ( V * pf * SQRT(3))

Note the mechanical efficiency is not used.  It's superfluous information since they give you the motor load.  You would need that if they gave you the work done by the fan and needed to work backwards to get the motor load.

 

[Audi Driver P.E.]

See 19-6 in the 13th Edition of the MERM for a handy table.

 

[Audi Driver P.E.]

And while it looks like you have this part handled, note that the Power Factor (given as a percent), in some references is required as Cos(theta).  They are equivalent.

 

[SK82 P.E.]

Thanks tbova and Audi driver,P.E.!

 

[AAGR2013]

the answer 34A is not counted 93% efficiency, why? it will be 36.6A. I did a few same problems, efficiency are all counted.

 

[Audi Driver P.E.]

the answer 34A is not counted 93% efficiency, why? it will be 36.6A. I did a few same problems, efficiency are all counted.

Because the mechanical efficiency has nothing to do with current draw in this problem.  It is extraneous information.  You're given the full load wattage and are asked for the full load amperage, which is calculated without respect to mechanical efficiency.