Page 2 of 2 FirstFirst 12
Results 11 to 16 of 16

Thread: Utility specs large transformers for motors

  1. #11
    Join Date
    Apr 2009
    Location
    Williamsburg, VA
    Posts
    5,307
    Quote Originally Posted by petersonra View Post
    I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

    These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

    My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.
    Very simple... If more people use power at off-peak times, the power company does not have to upgrade its infrastructure. Virginia power used to pay us an exorbitant amount to run our generators at peak times so they would not have to bring additional equipment online ( or go with rolling brownouts). years later, after they had to upgrade anyway, they tried to get out of that contract. Tried... and failed. they did make it so that it was practically a National Emergency if the generator was not on and loaded by 2 p.m.
    Electricians do it until it Hertz!

  2. #12
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,237
    Quote Originally Posted by JFletcher View Post
    Wouldn't you want a higher transformer impedance than what is listed for a lower fault current?
    Yeah oops, that's what I meant
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #13
    Join Date
    Apr 2009
    Location
    Williamsburg, VA
    Posts
    5,307
    Quote Originally Posted by electrofelon View Post
    Yeah oops, that's what I meant
    Just making sure everyone is reading your post, right?

    The surest way to get an answer on the Internet is to post something incorrect.
    Electricians do it until it Hertz!

  4. #14
    Join Date
    Mar 2013
    Location
    Rutland, VT, USA
    Posts
    320
    Quote Originally Posted by electrofelon View Post
    I guess they want a stiff supply to keep flicker and VD within target during motor starting. This might mess up my fault current calcs. IT was a catch 22 when I set this up: the utility asked me a few questions and then said just said build it and get it inspected, and after I did that they sent it to engineering I assumed a full 200 amps for transformer sizing as absolute worst case. Its going to be close. Hopefully the transformer impedance is less than what the book says, probably is.
    Instead of using the infinite bus fault current for your sizing calcs, if you are close, you may want to use the utility actual available fault current.

  5. #15
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    36,333
    Quote Originally Posted by petersonra View Post
    I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

    These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

    My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.
    They have plenty of 240 volt transformers available - they are the same ones they are using on 120/240 single phase services. They apparently aren't interested in stocking 480 volt transformers, but will still supply them - with additional cost. To build a 480/277 system they are 277 volt secondary. Rural POCO's around here do supply 240/480 single phase, most of the time it is in remote areas where there is only single phase lines running and customer will need to supply phase conversion equipment if three phase is needed. Usually for limited KVA loads - like irrigation machines. If three phase primary is available they will build an open delta system for such loads - then customer at least doesn't need phase conversion equipment. About 75 kVA and up they prefer to build 480/277 systems but still may see occasions with open delta when all three phases aren't readily available in the area.
    I live for today, I'm just a day behind.

  6. #16
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,237
    This is a very populated area, therefore I find it unlikely that they don't stock 25's 50's and 75's.


    Wbdvt, yes not assuming infinite primary would help, but getting that information may be a challenge.


    It's just kinda funny, usually we are like, "ooo did they make a mistake by installing such a small transformer? " this time it's the other way around. Never thought I'd see the day!
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •