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brianafischer

Hardware Design Training

11 posts in this topic

I was wondering how everyone here has learned hardware design. I am currently the only electrical engineer employed by my company, and need to refresh my memory of converting plant voltages (US and Canada) to 208/230, and 120V systems. Also, I have some questions regarding overall design principles such as when line filtering is necessary, when diodes are necessary, and other misc questions that don't seem to be readily available in most manuals. Any books/reference material/training courses that might help? Thanks!

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what are you converting from, what power and what for?

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Some panels require >15A of 120V, which would lead me to using a 230V drop. Some motor drives require 230V also. However, most PLC's, power supplies, and other misc devices require 120V. Thus the need for multiple voltage levels in a single control panel.

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normally i use single 3-phase power drop and match all 3-phase components to that voltage. if 3-phase components are not supporting available 3-phase power, i simply use properly sized transformer. sometimes this is done (use of 3-phase transformer) if the service is 600V, because motors and drives for lower voltage are more common or cheaper for example (prices of transformers are usually insignificant). transformers are sometimes required by specs of the equipment (like to provide isolation, meet required interrupting capacity, or provide proper ground reference if the feed happens to be one of those rare deltas etc.) for 120V, separate transformer, single output if load is up to some 20A, if it's more, use split secondary ("center tap") to get 2x120V. this has nice feature that max. current on any of three conductors is half of what would be required if this was just single 120VAC output (neutral only carries imbalance). smaller conductors are easier to work with (talk to you electrician or panel building shop). diodes on ac circuit are used primarily for rectifying, on dc circuit for suppression (accross inductive loads) or indication (LEDs). on PCB level they can have many uses but those are rarely something we get to worry about when working on panel wiring. line filters are used to deal with noise. this is something to be expected specialy with big nonlinear and variable loads like vfd or servo drives for example. good infos come from number of sources, i cannot name one particular book as a reference. as far as i can tell, people tipically learn basics in school (el. laws explained) but to get somthing done in field, one has to also follow local code etc. local electrical code is normally offered just about everywhere (evening classes on college). take it, don't just buy book. it is a good idea to see some customer's or goverment specs or design guides. this will at least specify which standards are to be followed but some of them will go to great detail (like Toyota's NAAMS for example). some companies like to keep track who has access to their documentation but there is lots on the net too, specially for goverment. for example here is what google suggested: http://www.codehandbook.com/ http://www.amazon.com/gp/product/007022005...ce&n=283155 http://www.amazon.com/gp/product/084938574...ce&n=283155 http://www.eh.doe.gov/techstds/Standard/hd...dbk10922004.pdf http://www.electricityforum.com/bookstore/...r-handbook.html and don't forget to talk to your field inspector, he will have quite few things to say... also this forum is great way of sharing information. i'm sure others will be glad to help too

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Thanks for all the info. I definately understand the THEORY behind why everything is used. However, I am having a difficult time determining what to use in the "practical" world and not in the "ideal" world. Every panel could have it's own 120V isolation xfmr (even if the feed was 120V), line filter, only single-wire/terminals, but what standards are followed. Thanks for the help! Edited by brianafischer

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You're flying the US flag, so I assume you are in the States, if so try this website: http://www.mikeholt.com/index.php?id=homegeneral It is a NEC code website, check out the forum.

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I don't have all the answers so I'll just address the one's I'm sure about... I like to separate AC vs DC either left to right or top to bottom depending on where the signals are going out of the main panel. It's a simple thing to do in the design phase that makes an electrician's life easier later. If I'm changing from AC to DC, an isolation transformer is needed. If I'm just changing AC voltage levels with high power requirements (more than 5kVA), I tend to go with an autotransformer with taps for the different voltages needed. The RFI filter depends on whether the plant power is clean or not or whether there are VFDs or induction coils nearby, etc. As for codes, I like the NFPA website's free document access: http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp You can look inside any of the codes for the specific information you need, but you can't print from there. NFPA 79 is for Industrial Machinery. NFPA 70 is for general wiring, but also has sections on Industrial Machines. I bought both of them in paper copies, but it's so much easier to surf them than to thumb thru when I'm just looking for one or two things. My favorite practical book is a very beat-up copy of Wiring Diagrams from Square D: http://ecatalog.squared.com/search97cgi/s9...ryText=sm304r10 I got this book back in 1986 as a EE co-op student & I think it's better than the textbooks I have kept from that period. HTH!

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Any critique of this circuit? I know that the neutral of an isolated xfmr is grounded. What about the plant neutral? Should that be connected to the chassis ground?

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Now I'm no electrical engineer, but I'm pretty sure you never connect a Neutral feed to Chassis ground !!

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Correct the neutral is connected to ground by the supplier this may be at the meters if you have a PME supply or further down the line at the substation but you should NEVER connect to ground. I have seen some examples of bad practice where a panel has needed a neutral for some mods and this has just been connected to ground as there is no neutral at the panel, in these circumstances you should either pull in a neutral or I sometimes fit an extra transformer to get my 240V

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Unfortunately, I have inherited some flawed designs such as the one above.I am a degreed Electrical Enginer/EIT, so I am getting up to speed on the scope of the NEC/NFPA documentation. It seems there was a lack of understanding the difference between a neutral and GND in the designs I inherited. My preference is to include a 208V to 120V step-down transformer for both voltages in the system. The "testing" area that we work in has a 4-wire L1/L2/L3/N NEMA-rated power cord and plug for machines. This concerns me considering the system is ungrounded. Any suggestions on how to ground the above system while testing? Edited by brianafischer

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