How to get experience with three phase power

5 posts in this topic

I got denied on a job interview, there might have been a few things but I think not knowing NEC and lack of experience with three phase hurt me.

The NEC stuff is for another thread but how do you get experience with three phase? Unless you're in a shop already 3ph can be tricky to find. We talked about it at school in multiple classes but never anything hands on. Yet the descriptions for these entry level jobs all expect you to have a few years in of working with 3ph.

It looks like I can make my own three phase from 120VAC with a small VFD. Maybe find some small 3ph motor at the scrap yard, get a new contactor and practice taking measurements/hooking stuff up. I've no problem building a trainer if I get to study up on 3ph and NEC at the same time.

Crazy? Any advice?

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I did this, by the way. Or at least got a decent start.

Wired up a little VFD, fuses, load bulb, etc and ran a 1/3 HP 3ph motor off of single. The fun part was that the motor is too big and I had to tweak dozens of variables in the VFD just to get it to start/run without overloading, even then it would overload if too above/below %25 load. But there is so much to learn just from the book it came with, I look forward to finding a tiny 3ph motor and doing some more stuff with it.

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I can't tell you where to actually get hands on experience with 3 Ø but here is a short list of what I believe is important to know:

Read up on and understand transformer connections: wye, delta, open delta, grounded leg, etc.

Understand where & how to identify the different phases (legs) in a panel and MCC.

Be able to properly test L1-L2, L1-L3, L2-L3, etc.

Know how to hook up a 9-lead motor on hi-voltage and low-voltage.

Know the various wire color codes.  NEC rules are different from UL, NFPA-79, residential, and commercial color codes.

Hope this helps.

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i would expect that candidates have strong theoretical understanding and can explain what goes on and can prove to me they are quick learners. experience is nice but i prefer good attitude and no experience - specially over bad experience.

not experienced person can quickly gain experience with a bit of coaching in the workplace (and this gives me a chance to make sure he/she does it the proper way). wrong kind of experience is still experience but... it is bad and dangerous... correcting this requires a lot of un-learning... and i don't have time for that. 

actually using VFD is not a perfect substitute for real 3-phases - VFD output is a chopped DC, not a sinewave. This also means you need to at least use TrueRMS multi-meter and know what to expect. anything less is waste of time.

if you can get your hands on it, try ganging two motors, one as motor and other as a 3-phase generator. this can also be very small or low power for those willing to experiment.


things that bug me is candidates that waste my time. that includes people that:

- don't understand social effects of (not using) shower, laundry and personal hygiene (you would not believe how many people show up with BO, greasy hair etc.). this is what makes meetings VERY short...

- cannot read and write. knowing alphabet and ability to communicate verbally is only the most rudimentary skill set. i still need to see handwriting, basic algebra, basic computer skills, critical thinking etc. (yes, give them small word problem and see how they do, my 15 year old son is doing that in school and he is good at it, adult should be able to at least match it). if i cannot EASILY read their notes, i cannot have them complete service reports, note down reasons why equipment is locked down etc. if someone failed to learn how to read and write in 12+ years of schooling, they came to a wrong place to ask for another chance, i am not that good of a mentor.

- send resumes without proofreading them. when they type it, word processor tells if there is a spelling mistake. demonstrate that you know how to use it before you email that resume. and i specially love it when they state great communication skills but use inappropriate language, terminology or mess up resume.

- don't understand electric power but want job that requires knowing it. those who cannot explain difference between P=V*I, P=V*I*cos(phi) and P=V*I*cos(phi)*1.73 need not apply...

- don't understand Ohms or Kirchhoff's Laws or how multi-meter behaves in a circuit. easily tested on paper by drawing mixed circuit of various meters and a power source. i may also throw in resistor just to give them some hope. 

- don't understand operation of transformer and voltage divider


That would be just a set of "no nonsense" checks to weed out 95% of "candidates". real interview starts after this...



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3-phase power, as an engineer or technician, is a very broad subject.  You are aware of the different voltage levels (240V, 480V, etc.)?  You are aware of the impact of 50Hz and 60Hz?  Panic is correct...a VFD creates a simulated 3-phase circuit but is not the real thing.

There is a wealth of knowledge on the web on this subject.  My suggestion is that you start to write your own Primer on the subject.  50% of my technical skill set has NOT been built in a classroom, and that I can demonstrate with my own library of Primers.  

The NEC dovetails with this subject, as it is the framework of guidelines for design of 1-phase and 3-phase networks.

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