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.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?
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.110301-0626 EST
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.
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.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.
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!110301-0830 EST
Following are the first 5 definitions for "answer" from dictionary.com :
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.
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.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.
[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]
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.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.
Completely agree.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."
Exactly.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.