Motor calc, what on earth did i do wrong?

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yanert

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What the heck?

I am going to take my masters electrical test in a bit.
My state uses Experior testing to provide the exam.
In the information they provide with the candidate information bulletin, they give nine practice questions that are supposed to represent a sample of what is going to be on the test.

This is number 4.
Given: A 25 horsepower synchronous, code letter A motor is to be installed with 460 volt, 3-phase alternation current.
The largest nontime delay fuse to provide short circuit and ground fault protection for the motor is how many amperes?
A 30
B 40
C 50
D 60
My figures:
25 HP, Synch, 3 phase, 460
T 430.150 = 26 amps
Nontime delay fuse., Synch 300%
78 amps
240.6 = 80 amp fuse
Also, to get more technical,
430.52C1 exception 2a , go as far as 400% FLC
104 amps = so you could use a 100 amp fuse.
What did I do wrong? I think my figures are right, but heck it?s one of only nine example questions from the company that is giving the test, and this is on the actual test information bulletin so I have to be wrong, right??
 
Re: Motor calc, what on earth did i do wrong?

Look at note 3 in 430.52. It would seem they would give you more information in the question though.

Roger

[ February 06, 2005, 12:04 PM: Message edited by: roger ]
 
Re: Motor calc, what on earth did i do wrong?

why would I need to sit down with somebody? I have done tons of these calcs. This is the first one that has been wrong in my studies. It just really makes me wonder, since this is in the study guide for the test. What I have quoted in my text is the exact wording and format for the question on the study guide. Thanks!
 
Re: Motor calc, what on earth did i do wrong?

Also, make sure your're good on ohm's law, voltage drop, power factor and general electrical knowledge, their probably going to ask about the wiring of control circuits, and transformer and dual voltage motor connections.
Then they will be asking somethings your just going to have to know.
 
Re: Motor calc, what on earth did i do wrong?

Yanert
Your calculation looks correct to me. Suspiciously, if you ignore synchronous and just calculate a squirrel cage motor, you get 60.
Jim T
 
Re: Motor calc, what on earth did i do wrong?

I think your answer of 80 amps is right. If I saw this question on the exam, and since the right answer doesn't seem to be listed, I would have to pick the closest one of 60 amps.

Steve
 
Re: Motor calc, what on earth did i do wrong?

experior lists the answer as 40 amps
Click to expand...
Motors aren't my strong point, but I believe that 40 amps is wrong. If they can't figure out the correct answers for their practice questions, I wonder how well they do on the actual exam. Is it possible that contractors have been denied a license because experior doesn't know the right answer to their own questions?


For the PE exam, one can complain about questions they feel are unfair, or incorrect. I don't supose experior has anything like that?

Steve
 
Re: Motor calc, what on earth did i do wrong?

They do give you a sheet to file with them on questions you have issues with. Also I noticed while taking thier test that they also provide the wrong answer as one of the choices, that is if you were to forget a step, that number would be there.
 
Re: Motor calc, what on earth did i do wrong?

Originally posted by ken987:
experior lists the answer as 40 amps
Click to expand...
This is what I come up with:
430.3 Part-Winding Motors (synchronous)
Top right of page 70-285/2002 nec
Each motor-winding connection shall have branch circuit and ground fault protection rated "not at more than one-half" that specified by 430.52

*no power factor given.

Table 430.152
25hp-460v-------26 amperes
430.7(b) code letter A,locked rotor---3.14


26---- winding two-table 430.150
+ 3.14--- winding one-table 430.7(b)
----------
29.14
table 250.52 "Question denotes Max.size"

29.14
x 300%
---------
87.42 divided by 2= 43.71
430.3: "not more than half" =40amp NTD Fuse
Note 3 on table 430.52 no need in sizing more than 200%,so 300% of table is max size..

REAL good question ;)
 
Re: Motor calc, what on earth did i do wrong?

I used my Square D cheat card, the sliding motor data card. It says 50Amp.
T430.150 26Amps for synchronous motor 25HP 460 Volts
Table 430.52 Type of motor synchronous maximum Time delay fuse 175%
1.75 * 26 Amps = 45.5 next higher 50Amps

Hope this helps.
 
Re: Motor calc, what on earth did i do wrong?

Yikes! Good thing I have never sized a 25 horsepower synchronous, code letter A motor!. I would have gone right out of the book, and would have been completely wrong. Man, I hope the entire test is not like that. Total sleeper question that is really not a sleeper at at all!
Thanks for all the imput folks.
 
Re: Motor calc, what on earth did i do wrong?

Originally posted by dillon3c:
Originally posted by ken987:


26---- winding two-table 430.150
+ 3.14--- winding one-table 430.7(b)
----------
29.14
Click to expand...
I don't get what you are doing in this part of your calculation? Why are you adding 3.14 to the FLC? Table 430.7(B) is based on kVA per horsepower, meaning there could be as much as 3.14 kVA of locked-rotor current during the srart up of a Code Letter A motor.

25 hp, 3?, 460V, code A motor =

25 x 3.14 = 78.5 x 1000 / 460 x 1.732 = 98.5-amperes of locked-rotor current.
Click to expand...
 
Re: Motor calc, what on earth did i do wrong?

dillon3c - Where did you come up with part-winding?
The question, as stated, does not say part-winding. It only says synchronous.
If Experior, or anyone else, thinks that "synchronous" automatically means "part-winding" - then they are mistaken.
 
Re: Motor calc, what on earth did i do wrong?

If you look at the defination in 430.3 it spells it out. Key would be standard part-winding start induction motor.
 
Re: Motor calc, what on earth did i do wrong?

That looks good but wouldn't the calc look like this.

26 FLA * 300% =78
78/2=39
This would work for your, part winding start, however, the question does not say it is a part winding start motor. Simply knowing the answer and making the numbers work doesn't cut it for me.
 
Re: Motor calc, what on earth did i do wrong?

Originally posted by ken987:
That looks good but wouldn't the calc look like this.

26 FLA * 300% =78
78/2=39
Simply knowing the answer and making the numbers work doesn't cut it for me.
Click to expand...
->And table 430.7(b)where does it fit?<-
Well heck Ken,
That's exactly what I've been trying to do.Make the numbers fit by digging all around in article 430.I would asumme the only book these guys have in option to use as a tool in a time related testing.I have NEVER had this in a test question, on any of my test,or test questionnaire books I have purchased.It is in fact, an excellent test question with that answer that yourself provided Ken, via Experior Site/test book.Nor ken/bryan have I walk on water and divide fish for the multitudes.Nowhere in my logg-in name,Says Mr. Definitive Electrician,,Sirs. I don't, as of yet know the answer to this question,fitting this answer.Seems that I remeber someone posting, that Experior has a 99% fail rate for "first timers".I HAVE copied this question down,and will pose it to my Electrical Inspection Instructor either Friday night/Saturday morning after class for his opinion.If my silly a-- equation offends any of my electrical form patrons in trying to find a answer what would somehow fit this logic and I might add, what doesn't seem to be posted fitting this question/answer yet.In all due respect sirs.Y'all have a Good day..

[ February 08, 2005, 12:39 PM: Message edited by: dillon3c ]
 
Re: Motor calc, what on earth did i do wrong?

I must confess that I just don?t see how they arrived at the answer they gave. The only reason for specifying Code Letter A is to make sure that 430.52(C)(1) Ex 2 doesn?t apply. (by restricting the LRC they also restricted the starting current)

The rest of your analysis is correct.
 
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