3-Phase Motor Formula

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jkpyke

Member
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?
 

xformer

Senior Member
Location
Dallas, Tx
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?
If I remember right, KW = I x E x 1.732 / 1000 and KW = HP x 746 / 1000 with a little algebra one can come up with a formula form there.
 

hurk27

Senior Member
Wattage/voltage/1.732= current if nothing else is required such as efficiency?

a 5 HP motor using the 746 watts per HP would be 5x746=3730/480volts/1.732=4.5 amps.

The first mistake many make is when given a KW rating and not inputing the whole figure like in the above 3.7kw and using 3.7 instead of 3700 in the above equation.

keep in mind that since you have been given the wattage, you don't need the HP figure, other then to check if the wattage figure is correct, most likely put into the question to see if it throws you off.

heres a link to the many free stuff on here at Mike Holt, including formulas, just go down the list and select what you need.

http://www.mikeholt.com/freestuff.php?id=freegeneral <<click here
 
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gar

Senior Member
110301-0626 EST

jkpyke:

There is no answer for your question.

You have provided insufficient, and incomplete information. From the information provided you could make some estimates (guesses).

There is no information on the loading of the motor, power factor at whatever load, and the efficiency at that load.

To apply "some formula" you need to know the theory of the system and how that formula applies to that system. You can get all sorts of incorrect answers from a highly accurate, correctly working, calculator by putting in wrong data or using the wrong instructions to process that data.

.
 

kwired

Electron manager
Location
NE Nebraska
110301-0626 EST

jkpyke:

There is no answer for your question.

You have provided insufficient, and incomplete information. From the information provided you could make some estimates (guesses).

There is no information on the loading of the motor, power factor at whatever load, and the efficiency at that load.

To apply "some formula" you need to know the theory of the system and how that formula applies to that system. You can get all sorts of incorrect answers from a highly accurate, correctly working, calculator by putting in wrong data or using the wrong instructions to process that data.

.
Another no so well thought out test question. What type of calculation(s) they wanted the person taking the test to be able to use could have still been done with a resistance load. With a motor there are many other things that come into play such as actual load on the motor, power factor, efficiency.
 

jkpyke

Member
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.
 

kwired

Electron manager
Location
NE Nebraska
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.
That is the problem I was trying to get at. There is not enough information in the question to come up with only one correct answer. Person writing or selecting the question for use on this test is who is wrong.
 

gar

Senior Member
110301-0830 EST


Following are the first 5 definitions for "answer" from dictionary.com :
http://dictionary.reference.com/browse/answer

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.

Number 5 most closely approximates your situation. There is no solution to the stated problem.

Probably a reasonably correct answer to the question would be --- there is no solution to the question with the supplied information.

Your state exam may have provided some number as an answer to the question, but there is no valid answer, and there is no equation to provide you an answer.

The reason you did not know how to answer the question is because there is no way without additional information. Thus, your answer probably should have been to tell them what was wrong with the question, or make assumptions that would allow you to provide an answer and include those assumptions as part of your answer.

I had a prof, he was also head of the department, that if you had a question about a test question he would simply tell you to explain what you thought was wrong with the question as your answer to the test question. Also, I might point out that most of our tests were open book and we operated on the honor system where the instructor was not in the room during the test.

.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Or maybe the answer is "Not enough information is provided to give an accurate answer." Sometimes a test is not about a value you get to but whether or not you are thorough in your knowledge.

I = P (in kW) / (E x pf x eff. x 1.732)
The fact that they give you kW and HP is a red herring, kW and HP are both just expressions of Power; 0.746kW = 1HP

If it s a multiple choice question, I agree the tester is a nincompoop. If the answer is a write-in sentence, I would put in the above, followed by "But if we assume a .8 power factor and 95% efficiency at full load, and assume the motor is fully loaded, the answer would be XXXA."
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
110301-0830 EST


Following are the first 5 definitions for "answer" from dictionary.com :
http://dictionary.reference.com/browse/answer

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.

Number 5 most closely approximates your situation. There is no solution to the stated problem.

Probably a reasonably correct answer to the question would be --- there is no solution to the question with the supplied information.

Your state exam may have provided some number as an answer to the question, but there is no valid answer, and there is no equation to provide you an answer.

The reason you did not know how to answer the question is because there is no way without additional information. Thus, your answer probably should have been to tell them what was wrong with the question, or make assumptions that would allow you to provide an answer and include those assumptions as part of your answer.

I had a prof, he was also head of the department, that if you had a question about a test question he would simply tell you to explain what you thought was wrong with the question as your answer to the test question. Also, I might point out that most of our tests were open book and we operated on the honor system where the instructor was not in the room during the test.

.
For whatever reason, my browser didn't show this response until after I posted. So I apologize for being redundant to gar. We were however thinking the same thing!
 
I crabbed about questions like this during my apprenticeship. I had one teacher that put it into perspective. He told us that when dealing with multiple choice questions, the answer they are looking for may not be the exact answer, they are looking for the best answer.

I never liked that strategy, but I used it and finished at the top of my class by doing so.
 

jkpyke

Member
Sorry I am slow to respond, my 7mo old has me hopping around the house this morning. 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.
 

hurk27

Senior Member
Sorry I am slow to respond, my 7mo old has me hopping around the house this morning. 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.
The statement "not FLC" is the key, the HP would have given you the FLC of the motor, but not what the motor is actually loaded to, the wattage being drawn is the actual load on the motor, so with this info I can see that they wanted to see if you knew the difference, I have taken many test here as we don't have a state wide test, so in each and every locality you have to test, and many questions are written just like this.

I think they like to play with our minds to see how good our comprehension is.
 

gar

Senior Member
110301-1310 EST

Jraef:

No problem. You provided a different presentation and that is good. Many times I do not provide a direct answer, but instead try to provide some broad information and try to force the questioner to think a little.


jkpyke:

Here is some old textbook data for a 35 HP Induction-Motor. From Bailey and Gault, p 211.

Code:
[FONT="Courier New"]
Rating 35 hp, 1800 rpm, 3-phase, 60 Hz, 440 V

Approximate load          0.25     0.5      0.75      1.00     1.25

Input power   - watts     9850    17000    23900     31200    38500  
Output power  - watts     7710    14561    20921     27484    33814
Output power  - HP        10.3     19.5     28.0      36.8     45.3  
Efficiency               0.782    0.856    0.875     0.880    0.879
Power Factor             0.758    0.891    0.924     0.942    0.938
Line current                      you do the calculation
  [/FONT]
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Also, I might point out that most of our tests were open book and we operated on the honor system where the instructor was not in the room during the test.
I want to spend a few more years in the industry before going for grad school as I think I will get the most out of it with that approach. I have, however, been taking some graduate level courses here and there to address some intersets I have. In the most recent course, as with many engineering courses, the exam was open book. The instructor also left the room during the exam, and I had not experienced that in previous courses. Sadly, I must report that the honor system failed miserably! With the level of "group work" that was going on I was surprised that I did quite well since I worked on the exams individually, considering the number of meets I missed since I work full time and deadlines, etc. came up.
 

Besoeker

Senior Member
Location
UK
Or maybe the answer is "Not enough information is provided to give an accurate answer." Sometimes a test is not about a value you get to but whether or not you are thorough in your knowledge.

I = P (in kW) / (E x pf x eff. x 1.732)
The fact that they give you kW and HP is a red herring, kW and HP are both just expressions of Power; 0.746kW = 1HP

If it s a multiple choice question, I agree the tester is a nincompoop. If the answer is a write-in sentence, I would put in the above, followed by "But if we assume a .8 power factor and 95% efficiency at full load, and assume the motor is fully loaded, the answer would be XXXA."
Completely agree.
Maybe the question, if it was presented as stated, offered kW as input power and HP as output power so that would get you efficiency.
It still doesn't tell you power factor.
 

iwire

Moderator
Staff member
Location
Massachusetts
Never ask an engineer about an NEC based exam.:grin:

All they will do is confuse you and make you get the answer wrong.:roll:

What the EEs do not know is what is expected of us as electricians.

As much as the EEs here will say there is no answer, you can bet that to the exam administrator there is a correct answer on the exam and part of the test is the electricians ability to know what the test is looking for.

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

It may not be 'right' but it is what it is and if you want to pass these tests you must accept that.
 
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hurk27

Senior Member
you can bet that to the exam administrator there is a correct answer on the exam and part of the test is the electricians ability to know what the test is looking for.

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

It may not be 'right' but it is what it is and if you want to pass these tests you must accept that.
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. (except one I took that was written by an engineer):mad:
 
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