# Tic...Toc...Tic...Toc...clock counts down



## Wolverine (Oct 10, 2006)

Just venting...

I've completed 75% of my planned study modules, with a 75% success ratio on working the problems and two weeks left to go, putting me in the good solid "F" category. Not feeling very optimistic today, but then maybe some space aliens will come down in the next few days and implant a microchip in my head containing all the electrical knowledge in the universe, so there's still hope.

A balanced three phase motor of efficiency 80% requires a line current of 10 amperes (effective value) and operates at rated line to line voltage of 220volts. Determine the horsepower out of the motor assuming a power factor of .9 lagging.


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## Frontier05 (Oct 10, 2006)

What book is this from? I got 3.68 hp - is it close?

There is no reference to the 10 amp "line" current so I assumed it was "line to load per each line" --- (phase current) and not an overall load current. The voltage was specifically referenced as line to line voltage so that is clear.


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## Art (Oct 10, 2006)

sqrt(3) x 220 x 10 x 0.9 x 0.8 = 2743.6 W

1 HP = 745.7 W

2743.6 W/745.7 W/HP = 3.68 HP

line current ~ phase current

'effective' means RMS as opposed to sinusoidal peak

RMS=Peak/sqrt(2)


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## Wolverine (Oct 11, 2006)

3.68HP is the correct answer.

This problem is from an old Ga Tech review course.

One of my many worries is that there will be ambiguous problem wording as I've found in some examples, and no one to argue with over the intended meaning. In my business I'm used to "line" meaning "three-phase-line-to-line" and "phase" meaning "phase-to-ground" but that's not necessarily true in the problems where line and phase are used interchangeably and only clarified by the connections (and sometimes not). Hope this generates enough confusion to further elevate my own relative score.


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## Art (Oct 11, 2006)

> 3.68HP is the correct answer.
> This problem is from an old Ga Tech review course.
> 
> One of my many worries is that there will be ambiguous problem wording as I've found in some examples, and no one to argue with over the intended meaning. In my business I'm used to "line" meaning "three-phase-line-to-line" and "phase" meaning "phase-to-ground" but that's not necessarily true in the problems where line and phase are used interchangeably and only clarified by the connections (and sometimes not). Hope this generates enough confusion to further elevate my own relative score.


the problem said 'balanced' meaning all 3 phases are equal, and the neutral current must be 0...

line = line for a delta or wye circuit...

sqrt 3 x load equal line current for a delta

and for a wye the line equals the neg of the sum of the other 2 phases lines

line to line current is a misnomer, there is only line...

there really is no confusion...


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