Let's talk about motors...

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celtic said:
Size what?
CB..fuse..conductors???
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yes to all, I agree with the notion of using what the motor nameplate tells you and design circuit accordingly. But that's my opinion.
 
Cody K said:
yes to all, I agree with the notion of using what the motor nameplate tells you and design circuit accordingly. But that's my opinion.
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Your opinion is wrong.

Re-read the entire thread.
 
I'm not the sharpest knife in the drawer when it comes to motors. However, I seem to remember that NEMA has classifications of motor design, being namely Design A, B, C and D. Design B motors are relatively ubiquitous, or at least more common. Is it possible that the specification for OC/OP could be based on the NEMA design of the motor?
 
Celtic, if keeping score on help, we owe you plenty.

NEC Digest published a brief procedure for Motor Circuits below, which simplifies most of the critical code considerations.

Key Elements For Designing A Reliable Motor Circuit
http://www.nfpa.org/assets/files//PDF/necdigest/CodeIssues083005.pdf

I'm still looking for a good NEMA reference for motor-circuit, design considerations, but for phase-balance tolerances, related to warranty violations, here is a simple 1-page reference.
http://www1.eere.energy.gov/industr...minate_voltage_unbalanced_motor_systemts7.pdf
 
Celtic,
There were two posts in a row. So my vote is to third his sentiment. I was fortunate to work for awhile with a very senior iaei inspector. When I asked why we didn't use the nameplate values for conductor sizing, he gave me the face and tone but not the wagging finger. We use the HP and tables, because a motor manufacturer might make claims for a super efficient motor, was his answer. We do not design based on that.

In addition to what Roger has posted, A-B has some excellent articles. There's a very clear simplistic overview in chapter 19 of the eaton site,
http://www.eatonelectrical.com/unsecure/html/101basics/101basics.html

Then, there are some excellent motor fundamentals pages. These are so cool you might end up liking something. They each have a different slant, whatever works for you is best. The Yaskawa file is a large download which sets up an introduction to motors, modern controllers, and simple control theory.

http://www.freescale.com/webapp/sps/site/homepage.jsp?nodeId=02nQXG&tid=tChl

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125&param=en026674

http://www.yaskawa.com/site/dmdrive.nsf/SearchV/86256EC30069E432862571710078C4AC?OpenDocument&Source=SearchResultPage
 
Awesome!

I don't have time today to look at those links....but I will tom'row....THANKS!
 
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celtic said:
Awesome!

I don't have time today to look at those links....but I will tom'row....THANKS!
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on company time :)

(messin with you celtic)
 
The main reason behind using the tables and not actual motor nameplate is compatability and replacement. The tables "should" actually be based on the upper limits of NEMA standards. Thus when the motor is replaced with another motor of the same Hp the circuit is adequate - only the OL needs adjusted - the SC protection and ampacity of the circuit would be adequate. Non-standard motor applications may be put in using nameplate (cycloconverters, wound-rotor, High E)
 
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