You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old August 25th, 2008, 04:26 PM   #1
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
Exclamation why the two out of phases is called single phase ??

at US , why the two out of phases is called single phase ??

is there phase shift betwen these two phases ??


then , when do we call these are two phases and when do we call these are three phases ??


see figure C blow : the last figure



thanks

Last edited by magdyfayad; August 25th, 2008 at 04:34 PM.
  Reply With Quote
Old August 25th, 2008, 05:41 PM   #2
keithkyll
Member
United States

keithkyll is offline
 
Join Date: Jul 2005
Location: Heath, TX
Posts: 1,968
Quote:
Originally Posted by magdyfayad
at US, why the two out of phases is called single phase ??
is there phase shift between these two phases ??
No. 120/240 house current is single phase.
Quote:
Originally Posted by magdyfayad
then, when do we call these are two phases and when do we call these are three phases ??


see figure C below : the last figure



thanks
Never 2 phase. A, B, and C are all 3 phase.
  Reply With Quote
Old August 25th, 2008, 06:11 PM   #3
Jeev
Member
Australia

Jeev is offline
 
Jeev's Avatar
 
Join Date: May 2008
Location: Brisbane
Posts: 430
As best I understand your question, all configurations in your figures are 3-phase. I think you may be getting the terminology slightly confused as it appears you're talking about phase-to-phase and phase-to-neutral.

For example, from your figure in (A) if you had:
o A-N (1 phase, phase-to-neutral)
o A-B (2 phases, phase-to-phase)
o A-B-N (Still 2 phases, but you can have either phase-to-phase or phase-to-neutral)

As for differences between the phases, they are usually 120 degrees out of phase with each other.
  Reply With Quote
Old August 25th, 2008, 08:05 PM   #4
keithkyll
Member
United States

keithkyll is offline
 
Join Date: Jul 2005
Location: Heath, TX
Posts: 1,968
I understand the question now. B-C is one phase. B-N and N-C are part of the same phase. The voltage between B-C is 240 volts. When you add a Neutral, you center-tap the transformer. Now you can have 120V from B-N or N-C, or 240V from B-C. All of these are still in phase, because they are all still part of the same winding - a single phase - B-C.
  Reply With Quote
Old August 26th, 2008, 12:08 AM   #5
leitmotif
Member
United States

leitmotif is offline
 
Join Date: Nov 2004
Location: Seattle Wa. USA
Posts: 3,680
Quote:
Originally Posted by magdyfayad
at US , why the two out of phases is called single phase ??

is there phase shift betwen these two phases ??


then , when do we call these are two phases and when do we call these are three phases ??


see figure C blow : the last figure



thanks
Because when you take any two and put em on a scope you will see a sine wave. Remember it is the difference between the two conductors. With all 3 phases showing on the scope the difference between A and B will be the net difference between each with them at 120 degrees apart.

With only A and B on the scope you will see only the net sum of the two sine waves (still 120 apart back in the three phase) but on the scope you will see just the difference between them ie a single sine wave.

Same for any phase to neutral

Look at it mathematically do a sine function for 0 to 360 degrees in 10 degree increments and label it A
Do another stagger it by 120 and label B

Do another stagger by another 120 and label C
Now subtract the difference betweeen A and B.
Do again for B and C and for A and C

NOw draw graphs for all calculations. Kinda hard to visualize it until you do the math and do the graphs. That is what I had to do to really understand what is going on.

Dan Bentler
  Reply With Quote
Old August 26th, 2008, 02:52 AM   #6
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
first thanks leitmotif , keithkyll , and Jeev

second i am not electerical power engineer , so this forum may be simple to many , but i need to understand this as i need to understand this concept for a forum which is :

http://www.plctalk.net/qanda/showthr...4&page=2&pp=15


i still do not understand well why the two phases of delta connection acts as a single phase ??

how then we can obtain two phases or three phase for delta connection ?? as if we take any two phases for delta connection they are called a single phase - not two phases - ??

if we take two phases for star connection - not delta connection - and connected them to the scope , what do we show at the scope for these two phases for star connection ??

do we show single sine wave ?? or do we show two sine waves ?? and why for either answer ??

my questions may be simple to many , but i still need to understand the answres of it well !!

i know very little informatiom for star and delta connection as i know the voltage between phase to phase for star connection is the voltage between phase to nuetral multiplied by the root of three , and the phase current is the line current and vice versa for delta connection , but i need to understand this concept well as i said i need it for the above forum , as i want to know when we consider they are two phases and when we consider it acts as a single phase !!


the explain by leitmotif is good as it opened to me some things , but i still need more !!


another additional question : what is the advantage of use less operated voltage at USA which is 120 VAC while at Egypt and Europe the operated voltage is more as it is 220 VAC , as if the electerical power of any device is same , so the current pass through the device which operated by 120 VAC is the double of the current pass through the device which operated by 220 VAC , as then we need cable with big cross section area and the cost increases , so i want to know - as an additinal question to my orignal question - what is the advantage of use less voltage at USA which is 120 VAC while at Egypt and Europe is more voltage as it is 220 VAC ??

thanks

Last edited by magdyfayad; August 26th, 2008 at 03:46 AM.
  Reply With Quote
Old August 26th, 2008, 08:16 AM   #7
boardmaker
Member
United States

boardmaker is offline
 
Join Date: Mar 2007
Location: missouri
Posts: 66
As for your last question, I cannot explain the exact reason we chose one voltage over another, but I can tell you that using obviously using a lower voltage is much safer than using a higher voltage. Take for instance a motor control circuit. Say you have to take some measurements with the power on. Would you feel safer with 120v versus 240v. or better yet 24v. I've had some do it yourselfers who decide they can fix their dryer (but really shouldn't) tell me they got shocked by 240v. So I ask did you did you touch both lines. Of course, no. Line to ground-120v. Don't fix your dryer any more or you waterheater, stove, furnace, anything.
  Reply With Quote
Old August 26th, 2008, 08:28 AM   #8
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
thanks , but i still need more information about the additional last question


for the orignal question , i want to ask a question which may be the key of the answer which is :

the two hot wires - which come to home at USA and which are still in phase not are two phases - are the output of a tranformer , i.e : these two hot wires are the secondary of this transformer , are the primary of this transformer also two hot wires or the primary of this transformer are only one hot wire and the another is a nuteral ?? or the primary may be the both , i.e. : anything or any connetion ??

i think the answer of this question is the key for the answer for more questions !!

thanks

Last edited by magdyfayad; August 26th, 2008 at 08:33 AM.
  Reply With Quote
Old August 26th, 2008, 08:39 AM   #9
keithkyll
Member
United States

keithkyll is offline
 
Join Date: Jul 2005
Location: Heath, TX
Posts: 1,968
The power from the power company is 3 phases at very high voltage. 40,000 volts I believe. It is stepped down at power stations. Final stepdown is 3 phase for industrial, and single phase for homes.
Neutral is supplied from a copper rod driven deep into the Earth.
Cross section of wire is larger for 120 verses 240, but that's only the final run from the pole to the house. Higher power devices (water heater, stove, etc.) are 240 volt.

Study generators to see how 3 phases are developed. Look at how a sine wave is made using a magnet and 1 coil.

Last edited by keithkyll; August 26th, 2008 at 08:43 AM.
  Reply With Quote
Old August 26th, 2008, 08:59 AM   #10
PeterW
Member
Canada

PeterW is offline
 
Join Date: Jun 2006
Location: Edmonton
Posts: 2,515
3 phase shows a potential difference between 3 phase wires and neutral/ground (hence 3 phase).

Single phase only has one wire with a potential difference to neutral/ground (hence single phase).

In the UK the potential difference between phases and neutral (after substation transformation) are 240V AC, between phases it is 415V AC, therefore the household supply is simply a single phase supply and the neutral which gives 240V AC supply to households.

Industrial control circuits used to be 110V AC, this was achieved by transformers in the panels, 110V AC was seen to be safer for electricians to work with.

For this reason as well it is common in industry and construction for 110V power tools to be used, trailing extension leads cannot exceed 110V.

For inductrial control purposes, 110V AC is not as common anymore, as 24VDC is now the most common voltage for inductrial control.
  Reply With Quote
Old August 26th, 2008, 09:01 AM   #11
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
yes , the power from the power company is 3 phases at very high voltage , as it is stepped down at power stations. Final stepdown is 3 phase for industrial and single phase for homes.


but the first transformer after the generator staion at USA or the last transformer before the home at USA , its primary are also two hot wires or only one hot wire and the another is the nuteral ??

i asked this question as the all know the three phases voltage are two types : star and delta ,

as the many said the two hot wires which come to home at USA are still in phase , so the primary of this transformer must be also in phase - for my think - , i.e : i want to say primary of this transformer must not be two hot wires ( delta connection ) as it must be only one hot wire and the another is the nuteral !!

if my think is wrong and the primary of this transformer can be also two hot wires ( delta connection ) , i.e : the primary of this transformer as in phase as the secondary , how this comptable by naming three phase delta connection !!

thanks
  Reply With Quote
Old August 26th, 2008, 09:28 AM   #12
Jim Dungar
Member
United States

Jim Dungar is offline
 
Join Date: Apr 2002
Location: WI
Posts: 1,152
The primary can be two phase (hot) conductors which is a single voltage, or it can be one phase (hot) conductor and the neutral. I am an advocate of describing the number of phases in a system by the number of different Line to Line voltages that are present.

If you are only using two conductors it does not matter if the come from a delta or a star connection.

Most times when I see confusion about single phase and three phase it is because people forget that the word phase is usually context sensitive. For example, to measure a phase voltage you need two points, but many people call a single conductor a "phase" when they mean a "phase conductor"
  Reply With Quote
Old August 26th, 2008, 09:38 AM   #13
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
the problem if the primary can also be two hot wires which are delta connection , how this comtable by naming three phase delta connection , as we called the two hot wires which come to home at USA are still in phase , which mean the primary is also in phase , it is not compatable
or i have miss understand !!

as i want to say the primary mustnot be two hot wires ( delta connection ) as it must be only one hot wire and the another is the nuteral , which then the primary is in phase, and as the result of the primary is in phase , the secondary must be also is in phase although it contains two hot wires !!

i think my question is very clear now


wait for your answers

thanks all
  Reply With Quote
Old August 26th, 2008, 09:44 AM   #14
skipfast
Member
United States

skipfast is offline
 
skipfast's Avatar
 
Join Date: Jan 2004
Location: missouri
Posts: 5
I have attached a drawing of a single phase, residential distribution transformer. I hope this helps.
Attached Images
File Type: bmp HV-LV.bmp (84.7 KB, 583 views)
  Reply With Quote
Old August 26th, 2008, 09:52 AM   #15
magdyfayad
Member
Egypt

magdyfayad is offline
 
Join Date: Aug 2007
Location: cairo
Posts: 245
yes , thanks the primary of this example as i said contain only one hot wire , i.e : the primary is in phase so the secondry is also in phase although it contain two hot wires

then can the primary of the transformer may be also two hot wires ??
i.e : after the generator station which produce three phase delta connection can we put two hot wires from the output of this delta generator which are two phases and obtain two hot wires at the secondry
- which come to home at USA - but not two phases as it called in phase i.e : single phase ??

thanks

Last edited by magdyfayad; August 26th, 2008 at 09:57 AM.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
choosing capacitor when translate 3 phase AC motor into single phase alezi LIVE PLC Questions And Answers 12 June 5th, 2008 01:36 PM
Single Phase motor breakers - 1 or two poles? TimD LIVE PLC Questions And Answers 11 May 2nd, 2008 08:22 AM
motor 3phase to single phase billyboot LIVE PLC Questions And Answers 22 July 6th, 2005 07:14 AM
Automation or not ? Pierre LIVE PLC Questions And Answers 23 May 26th, 2004 06:06 AM
Mechanical/Material Handling and ISA 88 rlmunoz LIVE PLC Questions And Answers 6 December 25th, 2002 02:49 AM


All times are GMT -5. The time now is 05:49 AM.


.