208Y buck-boost

[GoldDigger]

Yes, but the angles are not all the same. The coils (green lines) are equally spaced 120deg from each other. 3*120=360, but because one spoke length is different (a shorter coil) the lengths between phases are not all the same.

:thumbsup:
But is this seen as an imbalance by the motor?

 

[jim dungar]

Yes indeed. but if the ph-ph voltage is not consistent wont a 3ph motor "buck"

.

A 3-phase open-delta configuration does involve all three voltages.
The neutral connection at the 208Y/120 source is irrelevant if there is no neutral connection at the load.
A buck-boost wye-wye cannot be used when a 120V load line-neutral connection is required.

The only reason to use 3 transformers in a wye, is to meet loading requirements that exceed what is available from an open-delta.

 

[gar]

190423-1937 EDT

If you have a triangle with two equal sides and an angle between those two sides of 60 degrees, then the third side must be the same length as each the first two sides. And in turn each of the other two angles is 60 degrees. As a result an equilateral triangle.

.

 

[FionaZuppa]

.

A 3-phase open-delta configuration does involve all three voltages.
The neutral connection at the 208Y/120 source is irrelevant if there is no neutral connection at the load.
A buck-boost wye-wye cannot be used when a 120V load line-neutral connection is required.

The only reason to use 3 transformers in a wye, is to meet loading requirements that exceed what is available from an open-delta.

To boost phase from 120 to 138 the N is needed by the boost xfrmer, so it is relevant. Surely the N does not carry to the 3wire 3ph motor.

Are you saying if you use the setup as shown by Schneider (the pics I posted in #35), you can then no longer use any of the source wye phases for 120v bc's?

 

[LarryFine]

Helpful pics:

 

[LarryFine]

How does connecting some coils in series (with taps) across the lines of 208Y (208v) cause a voltage increase in the chain of coils? I assume this pic (1=3)>2 when ref'd to the 208Y N/gnd?

Click on these puppies:


https://acmefaq.wordpress.com/2012/...former-differ-from-an-insulating-transformer/

 

[gar]

190423-2118 EDT

FionaZuppa:

In your post #40 you are confused. The spokes of the wye power source are of no concern when the boost circuit of post #1 is used to generate a delta output. The angles of the newly created voltages are still basically derived from from the original power source, and thus are 120 apart.

.

 

[LarryFine]

Agreed. That's why a utility open delta works.

 

[oldsparky52]

190423-2118 EDT

FionaZuppa:

In your post #40 you are confused. The spokes of the wye power source are of no concern when the boost circuit of post #1 is used to generate a delta output. The angles of the newly created voltages are still basically derived from from the original power source, and thus are 120 apart.

.

Help me if you can please.

In the 2 transformer boost drawing, A and C load phase are run through the transformer while B phase is not. My guess would be that B phase would still be 120V to ground. Assuming a 24V boost with a 208V feed, what would A and C phases voltage to ground be after the transformer?

What would AB, BC, CA voltages be after the transformer?

Thanks.

 

[LarryFine]

In the 2 transformer boost drawing, A and C load phase are run through the transformer while B phase is not.

Think if it as A-B and B-C run through the transformers. With an open-sided equilateral triangle, when you extend the two sides, the open gap increases by the same amount. The line-to-line-to-line voltages theoretically always match.

My guess would be that B phase would still be 120V to ground. Assuming a 24V boost with a 208V feed, what would A and C phases voltage to ground be after the transformer?

That would be my guess, too.

I'd have to calculate those numbers.

What would AB, BC, CA voltages be after the transformer?

See above. Remember, all of this math is theoretical.

We first need to know if we're feeding 208v to a primary rated for 240v. If so, the ratio gives us 0.8667, so the secondary output will be 0.8667 x the 12 or 24 volts, or 0.8667 x the 16 or 32 volts, then add that derived voltage to the incoming line-to-line voltage.

So, let's say we have 208v L-L, and we want the highest voltage we can get. We feed a 120/240v-16/32v BB with 240v primary with 208v, giving us a secondary of (0.8667 x 32) 27.7344v, which added to 208v gives is 235.7v. That should be line-to-line-to-line.

In theory, theory and practice are the same. In practice, theory and practice are very different.

 

[gar]

190423-2239 EDT

oldsparky52:

In post #1 there are two boost transformers. The primary of one transformer is connected from A to B, and its boost voltage is added in phase to A to produce at A' 208+24 = 232 V from A' to B. The same is true for C and B, C' to B is also 232. A' to B' will be 232 also because the angles don't change.

The voltage from A' to neutral is
(120^2-60^2)^0.5 = 103.92
( (208-60)^2 + 103.92^2 )^0.5 = 180.84

In my earlier post where boost was to the full 240 my result was 207.9 V.

The line to neutral voltage won't ratio the same as the line to line ratio because the angle is changing from line to neutral. But check both my calculations and see if there are any mistakes.

Note the 103.92 remains the same because it derives from 120 V and a 30 or 60 degree angle. 30 or 60 depends upon how you want to view the triangle.

240/232 = 1.0345 and
207.9/180.84 = 1.1496

.

 

[MTW]

3Φ 3W 208 to 240V Boost Transformer Setup

3Φ 3W 208 to 240V Boost Transformer Setup

You could use the "three-phase open delta" on the Scneider Calculator rather than the "three-phase wye" option.

So to follow on that point, for a demonstration.

From the chosen Square [D]etroit calculator

Using the correct voltage and bank type, gives a different answer.

Instructions:
Buck Boost Electronic Calculator, Calculates proper Square D Transformers to use when doing small changes in voltage: 5%, 6.6%, 10%, 13.3%, 20%, and 26.6%

• Step One: Single Phase or Three Phase Application
  IF Three Phase:
  • Three wire source: Must use Open Delta
  • Four-wire source:
    • Use Open Delta if only need three wires for load
    • Use Wye configuration for all four-wire applications.
      Note: Wye configurations offer expanded kVA Capacity - so try if Open Delta does not meet your requirment.
• Step Two: Input Source Voltage
• Step Three: Start changing the voltage change percentage until desired voltage appears in either the buck or boost windows (Exact voltage might not appear, please pick closes voltage: 240 required from 208, 236 is closest voltage)
• Step Four: Choose the voltage that will be used. (Buck or Boost)
• Step Five: Input Load Amps
  Calculate Load Amps by inputting Load kVA
• Step Six: If source is 50HZ click yes for required

Calculator will then calculate Proper Square D Buck Boost Transformer and wiring diagram.

OUTPUT from Tool:

• Part number
• Units required
• kVA capacity using the diagram
• Wiring Diagram
• Options for printing and e-mailing

1 Search Result | Format to Print | Email a Friend	Click image for larger version
Transformer: 750SV46F TRFMR DRY 1PH .75KVA 120X240V-16/32V Actual KVA: 9.58 THREE PHASE VOLTAGE KVA Rating 3PH 208V 3PH 240V 3PH 480V 3PH 600V AMPS 9.58 26.59 23.05 11.52 9.22 Frequency: 60HZ Number Required: 2.00

 

[MTW]

[D] calculator

[D] calculator

Here is the [D] calculator screenshot, the forum character limit, clipped it from the last post.

 

[LarryFine]

The primary of one transformer is connected from A to B, and its boost voltage is added in phase to A to produce at A' 208+24 = 232 V from A' to B.

Remember, supplying a 240v primary with 208v will not produce 24v on a 24v secondary; that's why they make the 16/32v version, too.

 

[MTW]

For protection of the transformer bank input. 26.59A @208V, 26.59 x 1.25= 33.24A Lowest over current protection device, for full input capacity.

So you would need a 35 or 40A fuse, on the input. A 40A would be better to prevent nuisance trips on energizing. 26.59A x 1.5= 39.88A

The output current is rated at 3Φ 3W 240V @ 23.05A, so you could protect it at 23.05A x 1.25= 28.81A or 30A for the full output capacity.

Two small .75KVA transformers , results in 9.58KVA 240V 3Φ 3W, worth of capacity. Seems a bit untrue, when compared to an isolation style transformer. But the rub is, only the secondary winding, (the amount of CHANGE in voltage) is really doing most of the work. The amount of current on the secondary windings, is the limiting factor of KVA rating.

With an open delta, you should divorce yourself from the idea of a neutral reference voltage. It's a delta, there is no true neutral.
Neither does it have a center tap, for a neutral.

Its an autotransformer bank, so the input is common with the output. No isolation, directly connected. That means that the equipment grounding conductor, is common to both sides.

Larry was doing a fine job with presenting the math, so I'll let him continue onward.

Remember, all of this math is theoretical.

We first need to know if we're feeding 208v to a primary rated for 240v. If so, the ratio gives us 0.8667, so the secondary output will be 0.8667 x the 12 or 24 volts, or 0.8667 x the 16 or 32 volts, then add that derived voltage to the incoming line-to-line voltage.

So, let's say we have 208v L-L, and we want the highest voltage we can get. We feed a 120/240v-16/32v BB with 240v primary with 208v, giving us a secondary of (0.8667 x 32) 27.7344v, which added to 208v gives is 235.7v. That should be line-to-line-to-line.

In theory, theory and practice are the same. In practice, theory and practice are very different.

MTW Ω

 

[LarryFine]

Keep in mind here that this represents two separate transformers, left and right, not one. They should have been drawn in the delta position, or with the cores represented, to make that more obvious to the neophyte.

 

[MTW]

Well, yes. I just used the [D] calculator, that he picked, to keep it same same.

I did include one in post #27

I liked the old Acme catalog, for lessons on buck boost setups, but didn't find it when browsing Hubbell briefly. I could upload one to a site for downloading if he's interested. Or someone could find and post a link.

MTW Ω

 

[Besoeker3]

Good grief!
How did this get so convoluted?
Surely it just needs a 3-phase 208/240V autotransformer.

 

[oldsparky52]

Good grief!
How did this get so convoluted?

Because some old guy is asking questions above his education level.

:dunce:

 

[oldsparky52]

I drew 3 lines (each 120 units) from a common point, one at 0 degrees, one at 120 degrees and one at 240 degrees that in my mind represent 208Y/120.

Then I extended lines A and C (without changing the angle) so that they are each 240 units from B. (This was my mistake and the source of my confusion).

This morning I realized I needed to change the angle of the boost to follow a line from B to (A or C). When I extended line (boost) on this angle instead of the original angle, everything worked out. (I think).

A pretty ignorant mistake and my apologizes to everyone for the questions that came from it.