# Engineering exam

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#### Bryan Joles

I am studying for the NYS PE exam and have 2 questions.
1. Are the terms "boost" & "Buck" used only for referencing auto-transformers, and "Step-up" & "Step-down" used for ideal transformers?
2. assuming I was given a 3 phase source with a phase sequence of a,b,c and I can choose Vca to be the reference, what is the correct angle for the two other voltages? I have solved it as
Vca angle of 0
Vab angle of 120
Vbc angle of -120
Although I would assume it to be correct a past exam had conjugate angles for the latter two.
What is correct?

#### PhilCorso

Responding to Brian Joles', Wed, 30 Aug, 10:15 am, questions... speaking of single-phase:

A1) No. Boost & Buck refers an arrangement of transformer windings arranged whereby the output voltage is higher (primary & secondary windings additive) or lower (primary & secondary windings subtractive) than the input voltage.

Auto-transformers are considered as having two fixed, interconnected windings with three terminals. On the other hand, B&B transformers have four independent, isolated, windings each with two terminals. They can be interconnected to provide an AT function.

In general, B&Bs are applied where small changes, as well as small kVA rating, in output voltage are required. An example would be changing 115 Vac to 127 Vac (or 120 Vac), at 20-100 kVA. ATs are used where higher voltages and kVA ratings are necessary. An example would be: changing 6.6 kV to 6.9 kV (or 6.0 kV), at 4,000 kVA).

A2) Your vector tags and angles are correct for the counter clock-wise phase sequence abc. Note that -120 degrees equals +240 degrees.

Regards,
Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

A

#### Al Pawlowski

1) No. Boost, and buck, refer to adding, or subtracting multiple outputs to obtain a desired value (usually by various connections of separate windings in transformes), while step-up and step-down refers to whether a device just increases or decreases an input (usually this means the turns ratio between a primary and secondary in a transformer).

2) A conventional, 3 phase source has equal 120 degree phase difference between its outputs. These are normally labelled a, b and c with angle increasing from a to b to c in a clockwise rotation when drawn as vectors i.e. if angle Va = 0, then Vb's angle is 120 and Vc's is 240. However, your terminology is confusing. In my
experience, Vac means Va with respect to c. In other words, the second subscript specifies the reference. If you say Vab and Vbc, you have specified two different references; not one (Vca) as stated. As you have stated it, angle Vab and Vbc would both be 120 (or -120 for reverse measurement rotation). The only way I see to get answers like I think you are trying for is to make Vb the reference. Then Vab = -120 with Vbc = 120 for clockwise rotation and vice versa (your answers) for counter-clockwise rotation. In other words, I think you got your rotation reversed (from normal) and the problem is stated badly.

Al Pawlowski, PE
[email protected]
dba ALMONT Engineering
Baton Rouge, LA USA

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#### Roger Irwin

> I am studying for the NYS PE exam and have 2 questions.
> 1. Are the terms "boost" & "Buck" used only for referencing auto-
> transformers, and "Step-up" & "Step-down" used for ideal
> transformers?

Boost and buck are generally used to describe switched mode regulators, which commonly use inductors rather than transormers, they are not comparable to autotransformers (they are based on flyback techniques). Maybe these terms are also used for autotransformers, but I have never heard of this.

> 2. assuming I was given a 3 phase source with a phase sequence
> of a,b,c and I can choose Vca to be the reference, what is the
> correct angle for the two other voltages? I have solved it as
> Vca angle of 0
> Vab angle of 120
> Vbc angle of -120
> Although I would assume it to be correct a past exam had
> conjugate angles for the latter two.

I could express 120 in complex form

> What is correct?

Both, but I suppose theoretical study of 3 phase circuits would suggest use of conjugate form rather than degrees, just as theoretical study of dynamics would suggest the use of angular velocity in radians rather than "revs per minute".

#### PhilCorso

Ec the info for Brian Joles in my Wed, 30 Aug, 2:05 pm:

Using USA convention, a line-to-line voltage vector that is labeled Vca, is a vector directed from line c to line a. Following is the derivation
of vector Vca from the line-to-neutral voltages Van and Vcn. If line voltage between phases is 480 Volts, hence, line-to-neutral voltage is 277 Volts, then:

1) Vca = Vcn - Van. eg, 480 V, 0 deg.

2) Vab = Van - Vbn, eg, 480 V, 120 deg.

3) Vbc = Vbn - Vcn, eg, 400 V, 240 deg.

Phase sequence is still a-b-c. However, in this case their respective line-to neutral vectors, eg Vna, will lead the line-to-line voltage vector, Vca, by 30 degrees. And of course, Vca is displaced from Vac, by 180 degrees.

Any AC Circuits text (perhaps SCHAUM's Outline) should clear up the alphabet soup.

Regards,
Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

M

#### Mark Erdle

The Buck-Boost transformer is an insulating transformer that can be used for a variety of applications by wiring the transformer as an autotransformer. A common application is in a three phase 208 / 120 volt wye distribution system to boost the 208 volt to 230 volt. The advantage of this approach is that you only need to rate the transformer for the KVA of the Buck or Boost. In this example a 10.5% boost is required. If your load was 10 KVA, your transformer would only need to be rated at 1 KVA. Step Up and Step Down are refers to a non-autotransfer configuration.

#### PhilCorso

Responding to off-list queries about... phasor sequence convention.

First, let me state that there is no conventional way to label phasors. The method one uses depends on the text he or she used in school. One can choose any direction for the reference phasor.
But once selected, then convention dictates what the others should be. I use the line-to-neutral vectors, not the line-to-line vectors. However, I suggest the following:

Draw and orient an equilateral triangle so that one leg points vertically. Label the top of this leg 'a', and its bottom 'c'. Place the arrow head at 'a'. (This is the Vca vector referenced by Brian Joles.) Of course, its magnitude is its length, and its direction is "chosen" as 0 deg.

Now label the remaining point 'b'. Draw the arrow heads for the ab and bc vectors at ends 'b' and 'c', respectively. I use the "from-to" method. Note, that 'b' will appear to the right of 'a', or to its left. If at any other location, then I suggest that you freshen up your drafting
skills.

If point 'b' is to the right of 'a', then this is the sequence a-b-c. That is, if you rotate the triangle about its equidistant center point, first 'a', then 'b', and finally 'c', will pass any reference point, in that sequence. Conversely, if point 'b' is to the left of 'a', then the rotational sequence is a-c-b.

Please note, to compare the "angle" displacements with respect to a "neutral" reference point, then one end of all vectors must start at the same origin. For example consider the phasor sequence a-b-c. Connect the 'c', 'a', and 'b' ends of vectors Vca, Vab, and Vbc, respectively, Then their angular displacements would be, 0, 120,
and 240 degrees.

If still unclear, contact me.

Regards,
Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

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#### Curt Wuollet

> I am studying for the NYS PE exam and have 2 questions. > 1. Are the terms "boost" & "Buck" used only for referencing auto- > transformers, and "Step-up" & "Step-down" used for ideal transformers? No > 2. assuming I was given a 3 phase source with a phase sequence > of a,b,c and I can choose Vca to be the reference, what is the > correct angle for the two other voltages? I have solved it as > Vca angle of 0 > Vab angle of 120 > Vbc angle of -120 > Although I would assume it to be correct a past exam had > conjugate angles for the latter two. > What is correct? Both, depending on your point of view. The problem with these answers is that they are matters of convention and different authorities use different conventions. Kinda like current flow. This is positive to negative by convention yet electrons go the opposite way, and then we get into hole flow.... You aren't gonna get em all. Regards cww