motor calc help

[furbis]

I am taking the colorado journeymans this week and in my studying came across a ? I am unsure of.

"Determine the current of a 10hp motor,208volt, 1 phase, operating at 90% efficiency, with an 85% power factor."

A.46.88 B.50 C. 55 D. 68.75

I am clear on 430.248 for the FLC and 430.22 for 125% but not sure how to figure efficiency and power factor.

 

[jdengr]

Re: motor calc help

First, you need to know that 1 hp = 746 watts. Then, you can convert the hp of the motor to watts:

10*746 = 7460 Watts

But that is output power. To get input power, you need to factor in efficiency:

Pin = Pout/Eff, so
Pin = 7460/.9 = 8289 Watts.

To get current, use the single phase power formula:

P = I*V*Pf

Then solve for current:

I = P/(V*Pf), so
I = 8289/(208*.85) = 46.88 Amps, or answer (A).

 

[furbis]

Re: motor calc help

thanks I will try and fit that into my brain with all the other stuff I have been studying.