motor related questions

[tdotboys]

Que 1 - 120 volt 5hp motor draws 35 amps, the efficiency is:

a)93
b)104
c)83.9
d)62

Que 2 - the speed of a 2 pole motor connected to a 60-cycle source with a 5 -- slip is: (quesion paper is kinda blurry, i dont know what is written between 5--- slip) sorry

a)3456 rpm
b)3600 rpm
c) 7200 rpm
d) 3420 rpm


Help will be appreciated with these question.Thanks all. Great forum and helping thread for dumbo like me.:D

[Ohm1]

Please post how you worked the problem, and we will go from there.


Thank you!

[junkcollector]

Que 2 - the speed of a 2 pole motor connected to a 60-cycle source with a 5 -- slip is: (quesion paper is kinda blurry, i dont know what is written between 5--- slip) sorry


it's % (percent slip)

[suemarkp]

I'm not so sure you can answer question 1. I don't believe that efficiency is truely equal to theoritical amps / actual amps because of motor power factor. What really matters is output power / input power and they didn't give you the input power.

A 2-pole motor makes 1 complete revolution per Hz. So 60 Hz = 60 rotations/second = 3600 rotations/min. A 5% slip means it is turning 95% percent of that, so .95 * 3600 = 3420.

[Ohm1]

I'm not so sure you can answer question 1. I don't believe that efficiency is truely equal to theoritical amps / actual amps because of motor power factor. What really matters is output power / input power and they didn't give you the input power.

A 2-pole motor makes 1 complete revolution per Hz. So 60 Hz = 60 rotations/second = 3600 rotations/min. A 5% slip means it is turning 95% percent of that, so .95 * 3600 = 3420.
Not to be mean, but goodness, give the OP time to post how they worked the problem (s)!!! We don't need to be superman for every question on the site. Give it time so everyone can learn!!

[Ohm1]

I guess I should add a smiley---seeing that I respect, and admire Suemark. So here it is:

:) :D

[junkcollector]

Hey guys, did you get the first one to come out? I couldn't get it to come out to any of the given answers. Just curious.

We will assume that it is a theoretical question, because that is all the test writers give us...

[dznewz]

For what it's worth, here are my calculations. They seem right to me, which isn't saying much.

35 Amps at 120 V = 4200 watts
4200 watts / 745.7 watts per HP = 5.6 HP

Assuming that the 5HP motor produces 5 HP,
5 HP / 5.6 HP = about 89 % efficient.

[junkcollector]

about 89 % efficient.

Exactly what I got. .88

[Ohm1]

Que 1 - 120 volt 5hp motor draws 35 amps, the efficiency is:

a)93
b)104
c)83.9
d)62

I'm on board with Suemark for this one
Que 2 - the speed of a 2 pole motor connected to a 60-cycle source with a 5 -- slip is: (quesion paper is kinda blurry, i dont know what is written between 5--- slip) sorry

a)3456 rpm
b)3600 rpm
c) 7200 rpm
d) 3420 rpm

60cps(120)/2 pole= 7200/2=3600rpm's
Then: 3600(.05 slip)=180
So: 3600rpm's-180=3420
I guess my way is much longer!