Scott transformer connection - Page 1
Forum section: electronics / general

author: nForce
author_rank: Frequent Contributor
timestamp: August 08, 2018, 07:38:05 pm
content: I have read quite a lot web articles about this subject, but I still don't understand how the scott transformer connection works.And I don't understand those indian guys on youtube who are trying to explain this concept.Can someone explain to me how it works?I do understand that we transform from 3 phase to 2 phase or vice versa, and symetrical.

author: jmelson
author_rank: Super Contributor
timestamp: August 08, 2018, 08:02:02 pm
content: The "two phases" are not like residential service in the US, which is actually a "split phase", ie. a single phase with a center tap.A Scott Tee converts between 90 degree two phase and 120 degree three phase. If you take a drawing of a 3 phase system, (it is an Isoceles triangle) and draw in a vertical line, you can see you have two lines that are now at 90 degrees. So, the 3 phase system has a bottom line that is the base of the triangle, and the vertical line extends from the center of the base to the point at the top. The center of the base can be created with a simple center-tapped transformer. Now, you have the full base line-to-line voltage, let's say that is 230 V. Each side of the triangle is also 230 V. But, the vertical line from the center of the base to the peak is265 V (just simple trig solves this). So, 265 / 230 is 1.15 or the reciprocal of 0.866 (how convenient, the sine of 60 degrees).So, to convert between that vertical line, you need a transformer with a ratio of 86.6 : 1.So, a Scott Tee is actually TWO transformers, one is a simple center-tapped autotransformer, the other has an 86.6 : 1 turns ratio.Jon

author: nForce
author_rank: Frequent Contributor
timestamp: August 09, 2018, 10:38:07 am
content: Quote from: jmelson on August 08, 2018, 08:02:02 pmThanks, but I need more detailed. Maybe you meant equilateral triangle and not isosceles triangle? Because each side (line-to-line voltage) is 230 V. This means that the vertical line from the base to the top is less then each side, because in this case the side is a hypotenuse.

author: jmelson
author_rank: Super Contributor
timestamp: August 09, 2018, 07:34:53 pm
content: Quote from: nForce on August 09, 2018, 10:38:07 amOh, yes, you are absolutely RIGHT! Equilateral, for sure!QuoteWell, cut the equilateral in half, and you have a 30-60-90 triangle. 230 on the hypotenuse, 120 on the bottom, and must be 208 on the vertical. Maybe I've got it right this time??Jon

author: nForce
author_rank: Frequent Contributor
timestamp: August 09, 2018, 08:14:00 pm
content: Quote from: jmelson on August 09, 2018, 07:34:53 pmHmm, not quite. If we have 230 V line-to-line voltage, half of 230 V is 115 V. And then we use Pythogerean theorem and we get that the vertical is approx. 200V.So how do we get then that 0.866?

author: nForce
author_rank: Frequent Contributor
timestamp: August 11, 2018, 10:56:24 am
content: So no one else understands what am I saying?Jon?

author: Edison
author_rank: Frequent Contributor
timestamp: August 11, 2018, 01:41:47 pm
content: Just employ Google uncle and aunt Wiki

author: station240
author_rank: Unknown
timestamp: August 11, 2018, 02:46:16 pm
content: An example install with 2 phase,Fran Blanche - PhiladelphiaQuote from: jmelson on August 09, 2018, 07:34:53 pmExcept it's a tranformer ratio, 86.6% to 100%, or 208 (\$\sqrt3 /2 * 240\$) to 240 (1*240)

author: nForce
author_rank: Frequent Contributor
timestamp: August 12, 2018, 04:31:53 pm
content: Yes, but how do we get from 3-phase system which has 120° to a 2-phase which has 90°? I know from phasor diagram we can see this, but it blows my mind if I am trying to see this from the actual connections.

author: jmelson
author_rank: Super Contributor
timestamp: August 13, 2018, 10:31:14 pm
content: Quote from: nForce on August 12, 2018, 04:31:53 pmIt is really IN THERE!Look at the equilateral triangle. Now, a center-tapped transformer at the bottom creates a midpoint there.So, now you have a point that is exactly below the top point of the triangle. So, we call the base of the triangle zero degrees. The line from the center tap of the base to the top point is 90 degrees from the 2 base points. So, right THERE is your 90 degree phase shift. it was actually there all the time in the 3-phase waveform.Now, the only remaining issue is the vertical line is not equal to the base (or any other side) so you need the 86.67 : 100 transformer to fix that difference in the voltage.Jon