3-Phase Motor Formula

[LarryFine]

... in my experience most exam questions for electricians are based on basic ohms law and will ignore PF, impedance, capacitance and efficiency.

I agree, except maybe for PF, which is mentioned in the question for a reason when it is stated. If your calculated answer doesn't match one without PF, see if it does with it.

I would have aced my Master's test if I had remembered that PF is never greater than 1. :roll:

 

[Besoeker]

Exactly.

These test are not trying to see how much you know or to what level your knowledge goes, like many school test will do, all they are design to do is to make sure you have a basic understanding of electrical theory and the NEC, with a few other things added depending upon the local of the test and type, while a few of the test around here are local written, and can have some of the dumbest questions I have ever seen, they all are wrote around the basic needs of doing electrical work not engineering.

Basic electrical work would include running the correctly rated cabling to a motor, wouldn't it? Ignore motor power factor and you could come up with an inadequately rated cable size.

 

[iwire]

Basic electrical work would include running the correctly rated cabling to a motor, wouldn't it? Ignore motor power factor and you could come up with an inadequately rated cable size.

But he is not sizing a cable, he is taking a test.

Also when an electrician sizes a cable for a motor under the NEC they use the motors labeled HP and the tables in the NEC.

 

[hurk27]

Basic electrical work would include running the correctly rated cabling to a motor, wouldn't it? Ignore motor power factor and you could come up with an inadequately rated cable size.

Remember the NEC allows an engineer to do things that it don't allow an electrician to do, such as size conductors to a load that can be smaller then if we size them.
You see the words "under the supervision of an engineer" in the NEC in many places.

This is one of the reasons these test don't or shouldn't have these type of questions.

 

[Besoeker]

But he is not sizing a cable, he is taking a test.

Also when an electrician sizes a cable for a motor under the NEC they use the motors labeled HP and the tables in the NEC.

So, it's asking the person taking the test to do a calculation that he will never need to in real life. And then not providing all the required information* he needs to do the calculation anyway. Brilliant, that is.

*That is, if the opening post gave all the information contained in the question.

 

[hurk27]

You have to remember most times when we size conductors for a motor circuit we don't have much more then the HP and voltage to go by, when we are wiring these buildings, the motor is not on site, an engineer might have these spec's but we go by the table in the NEC and run the size conductors it requirees, which in most cases will be more then large enough.

 

[gar]

110302-0944 EST

Thread Title:

3-Phase Motor Formula

This seems to imply the desire for a formula.


Following is the pertinent information provided by jkpyke:

Post 1:

I can't seem to find a formula for a practice question that I have. I am supplied the wattage, the H.P. and the voltage for the 3-phase motor, and it is asking me what the current is. Can anyone let me know what the formula would be for this?

At this point there is no information that the practice question is a multiple N-choice question. Since the request for a formula is the question of this post the only answer is that there is no formula. There can not not be a specific solution to the practice question.

Post 7:

Thanks for the help guys. Sorry Gar, there is an answer for this, it was actually on my State test and that was all the info given in the question and I did not know how to answer it/calculate it.

This has now changed to a State Test and not a practice question. Also now the problem is "I did not know how to answer it". Good reason, there is no solution.

Post 14:

--- Anyways, the question was "A 3-phase motor drawing XXXX watts, that is XX H.P. and XXX volts, calculate the current." Then it said "Hint not FLC" The question was multiple choice. Sorry I did not want to come out and say that it was from a state test from the beginning and I don't remember the exact numbers of the questions, since I took the test 2 weeks ago. This question has been stuck in my head since the test. It is not the fact of whether or not that I got the question right or wrong, but on how to calculate it correctly that has been bothering me. Gar is correct in his response, and since I am from the same state that Gar is from, he should know how the Journeyman test is. I can't really argue with the people from the State giving me my Journeyman test. Either way it has made me really think.

Now we learn this is a multiple choice type of question. A whole different set of conditions. Now it is a guessing game. I have no idea what is the purpose of the hint, but the numbers are not available. Maybe the HP given was the HP rating of the motor, and by the power input it is clear that the motor is running at partial load.

Suppose an entire test is made up of questions with four possible answers provided, and the requirement is that you must choose one of the given possible answers. With no knowledge of the subject whatsoever, and you randomly pick answers your score on average will be 25%. If the desired answers were randomly positioned, then you could pick the first position for all answers and on average get a 25% score.

Now continue with a person with no knowledge of the subject, but a good understanding of the language, English in this case, and just by the structure of the question and the answer choices and intelligent guessing can probably do much better than 25%.

Now to the specific question here. We have no numbers with which to work. So it is a bigger guessing game for us.

Suppose that, as some have suggested, that watts was the power input, and HP was the power output. I will pick two values from the motor performance table I presented in an earlier post. 17,000 watts and 19.5 HP. So convert HP to watts and the result is 14,547. This seem reasonable because it is 86% of the input power. We do not know the motor rating, but we guess at 90% for the power factor assuming the motor is operating near its rating. With input power, assumed power factor, known voltage (440 here), and that the motor is a balanced load we can estimate line current. Theoretical line to neutral voltage is 440/1.732 = 254 V. Input VA per phase is 17,000/(3*0.9) = 6296 VA. Thus, I = 6296/254 = 24.8 A. The actual value from the table is 25 A which I had ask the reader to determine.

Now is 24.8 A sufficiently close to any one of the multiple answers provided. At this point you make a judgement. Suppose you are midway between two answers, then you need to rethink your assumptions and make a judgement of which way to go for your answer choice.

Tests with multiple choice answers make it easy for grading the tests, but may not do a good job of teaching and determining a student's real understanding of the subject.

To answer the question you are really dealing with the need for basic knowledge of motors, some equations, some practical data, and common sense to answer this type of question. How ridiculous the non-desired answers are determines how easy it is to answer the question.

Suppose some answers for my 35 HP motor were 19.1, 22.3, 25.0, 34,3, 43.5 A what would you pick given 35 HP, 17 KW, and 440 V delta were the input values of the question?

.

 

[Besoeker]

110302-0944 EST
Suppose that, as some have suggested, that watts was the power input, and HP was the power output.

I (and maybe others) offered such a suggestion. On reflection having read post 14 again, I think maybe the HP is the rating of the motor of the motor which is not necessarily the same as the power output at the operating duty and kW is the input power at that operating point. You can't calculate the current from that which is what the OP asked for.

But, if it's a multiple choice question, you could rule out any obviously wrong/impossible answers. Maybe there would then be one remaining choice. Who knows........

 

[iwire]

This has now changed to a State Test and not a practice question. Also now the problem is "I did not know how to answer it". Good reason, there is no solution.

Again, there is a solution to it.

Just because you do not agree with the solution does not not mean there is not.

Post 14:Now we learn this is a multiple choice type of question. A whole different set of conditions. Now it is a guessing game. I have no idea what is the purpose of the hint, but the numbers are not available. Maybe the HP given was the HP rating of the motor, and by the power input it is clear that the motor is running at partial load.

No guessing, all they want is basic ohms law.

How many of the EEs here have taken American Journeymen electricians courses?

How many of the EEs here have taken an American Journeymen license tests?

I have taken 3 of those tests in three different states and passed them all without issue using just basic ohms law.

Now we can go on and on about how we are being taught incorrectly, and I will not dispute that.

But I am telling you that without a doubt to pass the test you need to ignore many factors and just use the info given.

Notice how the other electricians that have passed these tests see it the same as I do.

 

[iwire]

But, if it's a multiple choice question, you could rule out any obviously wrong/impossible answers. Maybe there would then be one remaining choice. Who knows........

Who knows?

I know and so do other electricians that have taken these tests successfully.

 

[iwire]

I can't seem to find a formula for a practice question that I have. I am supplied the wattage, the H.P. and the voltage for the 3-phase motor, and it is asking me what the current is. Can anyone let me know what the formula would be for this?

For what it is worth, as far as the NEC is concerned this can be correctly answered with only the voltage and HP. No other info is needed other than having the NEC with you. (Most places test 'open book')

The wattage is not needed and likely put in the question to through you off.

 

[bob]

It was my thinking that you used tables 430 -148, -149 and 150 to obtain the amperage for your motor. You of course need to specify the voltage to
choose the correct motor amperage. If you do not have your book, you can look here http://electricmotorshop.net/index.php?option=com_content&view=article&id=687&Itemid=249

 

[jim dungar]

Wow, 32 posts to defend positions based on a poorly worded OP question.

In post # 14 we get everything we need to know in order to solve the test question.

... the question was "A 3-phase motor drawing XXXX watts, that is XX H.P. and XXX volts, calculate the current." Then it said "Hint not FLC" The question was multiple choice.

The question clearly stated what the motor was drawing in W. And the hint pointed out that the rating of the motor was immaterial.

That means it was time to ignore one of the most misused and mis-taught conversions in the electrical field 1HP=746W, which is the conversion for the output power of a motor (traditionally expressed in HP in the US and KW in most of the world). It is useless for determining input KW without knowing or making assumptions about efficiency and power factor. If you are going to guess 2 of the 4 factors in the conversion formula, you might as well just guess at the final answer.

So the answer was simply I=KW/(V*1.732).

 

[Besoeker]

For what it is worth, as far as the NEC is concerned this can be correctly answered with only the voltage and HP. No other info is needed other than having the NEC with you. (Most places test 'open book')

The wattage is not needed and likely put in the question to through you off.

Doesn't the NEC give you rated current at full load? From post #14 we learn that it isn't full load.

 

[hurk27]

the question was "A 3-phase motor drawing XXXX watts, that is XX H.P. and XXX volts, calculate the current." Then it said "Hint not FLC" The question was multiple choice.

After taking so many of these test, the above is so very clear to me what they want?

they clearly do not want full load current of a 5 HP motor via the statement in red, they want the current the motor is "Drawing" converted from the wattage as stated in green.

Why are we trying to complicate this?

Edited to add "guess I should have read Jim's post better Whoops

 

[iwire]

Doesn't the NEC give you rated current at full load? From post #14 we learn that it isn't full load.

You are right there, I had not seen the OPs 14.

 

[gar]

110302-1854 EST

Some definitions from dictionary.com:

Solution ---
?noun
1. the act of solving a problem, question, etc.: The situation is approaching solution.
2. the state of being solved: a problem capable of solution.
3. a particular instance or method of solving; an explanation or answer: The solution is as good as any other.
4. Mathematics .
a. the process of determining the answer to a problem.
b. the answer itself.


Answer ---
?noun
1. a spoken or written reply or response to a question, request, letter, etc.: He sent an answer to my letter promptly.
2. a correct response to a question asked to test one's knowledge.
3. an equivalent or approximation: a singing group that tried to be the french answer to the Beatles.
4. an action serving as a reply or response: The answer was a volley of fire.
5. a solution to a problem, especially in mathematics.


In my post at #27 I tried to go through the sequence of the changes in the statement of the original post.

By saying there is "no solution" to the original question it meant there was no singular solution because there was no s attached to solution. There are an infinite number of solutions, but we have no way to pick any one because there is insufficient information. Also note: that the word solution tends to imply a reasonably valid result.

The word "answer" in the broad sense, as generally used, does not imply a correct or even useful result. Answer is basically a response to an input.


By the time we got into post #14 there was probably enough information to provide guidance on how to make an estimate for an answer to the question. But we did not have the specific input values to determine what was probably the desired answer among the multiple possibilities.

The likely answer was probably (Input watts / RMS Line to neutral voltage) or this value modified by multiplying by ( 1 / an assumed power factor ). If the given values for answers had enough difference between them, then a couple quick calculations would probably identify what to pick for an answer.

Ohm's law does not apply to this problem. Power = Voltage * Current is not Ohm's law. This power equation is really a combination of Ohm's and Joule's laws. See http://en.wikipedia.org/wiki/Joule's_laws .

.

 

[Besoeker]

Wow, 32 posts to defend positions based on a poorly worded OP question.

In post # 14 we get everything we need to know in order to solve the test question.

The question clearly stated what the motor was drawing in W. And the hint pointed out that the rating of the motor was immaterial.

That means it was time to ignore one of the most misused and mis-taught conversions in the electrical field 1HP=746W, which is the conversion for the output power of a motor (traditionally expressed in HP in the US and KW in most of the world). It is useless for determining input KW without knowing or making assumptions about efficiency and power factor. If you are going to guess 2 of the 4 factors in the conversion formula, you might as well just guess at the final answer.

So the answer was simply I=KW/(V*1.732).

But it isn't.
It's I=W/(V*sqrt(3)*cosφ))
You can't ignore cosφ and get the right answer.
On a lightly loaded motor, the power factor could be 0.5 or worse in which case the current would be double what you would by ignoring pf.

 

[mivey]

But it isn't.
It's I=W/(V*sqrt(3)*cosφ))
You can't ignore cosφ and get the right answer.
On a lightly loaded motor, the power factor could be 0.5 or worse in which case the current would be double what you would by ignoring pf.

So what is the most misused part of the formula: k or cosφ ? :grin:

 

[Besoeker]

So what is the most misused part of the formula: k or cosφ ? :grin:

The extra bracket at the end.............