POWER PE Question For the Week

[spinup]

Week 1 Question For The Week can be found at:

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[supra33202]

Is the following correct?

IA* = S / VAN

IA* = 25KVA angle 41.4 deg / [ (480V angel 0 deg)/(sqrt(3) angel -30 deg)

IA* = 90.25 angle 71.4 A.

IA = 90.25 angle -71.4 A.

The solution shows IA = 90.25 angle -41.4 deg A.

When we try to find the phase (line to neutral) voltage vector from the line (line to line) voltage vector with wye connection, I thought we should account for the -30 phase shift.

It seem like the solution ignore the angle for the phase voltage.

Please advise.

Thanks!

 

[pcboiler]

Is the following correct?

IA* = S / VAN

IA* = 25KVA angle 41.4 deg / [ (480V angel 0 deg)/(sqrt(3) angel -30 deg)

IA* = 90.25 angle 71.4 A.

IA = 90.25 angle -71.4 A.

The solution shows IA = 90.25 angle -41.4 deg A.

When we try to find the phase (line to neutral) voltage vector from the line (line to line) voltage vector with wye connection, I thought we should account for the -30 phase shift.

It seem like the solution ignore the angle for the phase voltage.

Please advise.

Thanks!

Delta/wye or wye/delta transformers will cause a 30 deg phase shift.  I don't think that's applicable in this case.

 

[rg1]

Is the following correct?

IA* = S / VAN

IA* = 25KVA angle 41.4 deg / [ (480V angel 0 deg)/(sqrt(3) angel -30 deg)

IA* = 90.25 angle 71.4 A.

IA = 90.25 angle -71.4 A.

The solution shows IA = 90.25 angle -41.4 deg A.

When we try to find the phase (line to neutral) voltage vector from the line (line to line) voltage vector with wye connection, I thought we should account for the -30 phase shift. Please note that the phase angle is always between the Phase Voltage and phase current not between line Voltage and Line currents or any other combination of V and I. While working at line parameters like V or I we take only sqrt3 factor into account not the angle, angle still remains at phase level. We take into account angle only if the question asks for it specifically; for example if I give you a question in which a wattmeter is connected accross phase for Voltage and in line with Line for current at a delta load ( Think about it). I would like to explain/discuss it with you till this concept is clear to you, if you would like.

It seem like the solution ignore the angle for the phase voltage.

Please advise.

Thanks!

See my explanation in red if it makes sense.

 

[supra33202]

 Maybe I should be more clear.

I am talking about voltage phase shift with Wye connection and current phase shift with Delta connection. Not the angle between voltage and current.

My understanding: (the followings are vectosr (magnitude with angle))

For Wye connection, V (line-line) = V (line-neutral) * sqrt(3) angle 30 deg. I (line-line) = I (line-neutral).

For Delta connection, I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg. V (line-line) = V (line-neutral).

I agree that if the problem only ask for magnitude of the voltage or current, then we can ignore the + or - 30 deg angle.

For magnitude of the line voltage with Wye connection, V (line-line) = V (line-neutral) * sqrt(3). No need to worry about the angle.

For magnitude of the line current with Delta connection, I (line-line) = I (line-neutral) * sqrt(3). No need to worry about the angle.

For this spin-up example, should we worry about the 30 deg angle?

"Delta/wye or wye/delta transformers will cause a 30 deg phase shift.  I don't think that's applicable in this case."

So we only care the  + or - 30 deg angle phase shift if we have Delta/wye or wye/delta transformers?

And of the spin-up example, we can assume we have the load connected to the secondary side of a wye/wye transformer?

Please advise.

Thanks!

 

[rg1]

 Maybe I should be more clear.

I am talking about voltage phase shift with Wye connection and current phase shift with Delta connection. Not the angle between voltage and current. If you compare Vline with V phase in star than V line is sqrt3 times Vphase and is 30 degrees ahead of Vphase. ( Drwa the vectors to find pout) and  in Delta, Line currents lag 30deg to phase currents and are sqrt3 times phase currents. However when we are given KVA or VA with an angle, the angle is between phase V & I and not between Line V & I. However you can solve it without thinking about the angle.

My understanding: (the followings are vectosr (magnitude with angle))

For Wye connection, V (line-line) VOLTAGE = V (line-neutral) * sqrt(3) angle 30 deg. I (line-line) VOLT AMP =  CURRENT I (line-neutral). Something wrong here equating Volatage with VA with current??/

For Delta connection, I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg. V (line-line) = V (line-neutral). SAME COMMENT AS ABOVE

I agree that if the problem only ask for magnitude of the voltage or current, then we can ignore the + or - 30 deg angle. Not only magnitude, even if the problem asks about angle, it holds true. Please form some questions and we will solve them for you for better understanding. 

For magnitude of the line voltage with Wye connection, V (line-line) = V (line-neutral) * sqrt(3). No need to worry about the angle.

For magnitude of the line current with Delta connection, I (line-line) = I (line-neutral) * sqrt(3). No need to worry about the angle.

For this spin-up example, should we worry about the 30 deg angle? I am not able to see the Question here but I can understand your confusion. Can you write the question here? I am sorry I should have asked you this earlier!!!

"Delta/wye or wye/delta transformers will cause a 30 deg phase shift.  I don't think that's applicable in this case."

So we only care the  + or - 30 deg angle phase shift if we have Delta/wye or wye/delta transformers?

And of the spin-up example, we can assume we have the load connected to the secondary side of a wye/wye transformer? If nothing is mentioned in the question you can safely assume a star problem

Please advise.

Thanks!

 

[supra33202]

"I am talking about voltage phase shift with Wye connection and current phase shift with Delta connection. Not the angle between voltage and current. If you compare Vline with V phase in star than V line is sqrt3 times Vphase and is 30 degrees ahead of Vphase. ( Drwa the vectors to find pout) and  in Delta, Line currents lag 30deg to phase currents and are sqrt3 times phase currents. However when we are given KVA or VA with an angle, the angle is between phase V & I and not between Line V & I. However you can solve it without thinking about the angle."

I agree.

"For Wye connection, V (line-line) VOLTAGE = V (line-neutral) * sqrt(3) angle 30 deg. I (line-line) VOLT AMP =  CURRENT I (line-neutral). Something wrong here equating Volatage with VA with current??/

For Delta connection, I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg. V (line-line) = V (line-neutral)"

The equations match your explanation.

For wye/star connection, V line is sqrt3 times Vphase and is 30 degrees ahead of Vphase -> V (line-line) VOLTAGE = V (line-neutral) * sqrt(3) angle 30 deg

For wye/star connection, line current is equal to phase current (Magnitude and angle are the same) ->  I (line-line) =  I (line-neutral)

For delta connection, Line currents lag 30deg to phase currents and are sqrt3 times phase currents -> I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg

For delta connection, line voltage is equal to phase voltage (Magnitude and angle are the same) -> V (line-line) = V (line-neutral)

"For this spin-up example, should we worry about the 30 deg angle? I am not able to see the Question here but I can understand your confusion. Can you write the question here? I am sorry I should have asked you this earlier!!! "

The problem is at http://spinupexams.com/index.php?pr=Question_For_The_Week

I guess my real question is when do we do + or - 30 deg angle phase shift for voltage and current?

Thanks!

 

[rg1]

"I am talking about voltage phase shift with Wye connection and current phase shift with Delta connection. Not the angle between voltage and current. If you compare Vline with V phase in star than V line is sqrt3 times Vphase and is 30 degrees ahead of Vphase. ( Drwa the vectors to find pout) and  in Delta, Line currents lag 30deg to phase currents and are sqrt3 times phase currents. However when we are given KVA or VA with an angle, the angle is between phase V & I and not between Line V & I. However you can solve it without thinking about the angle."

I agree.

"For Wye connection, V (line-line) VOLTAGE = V (line-neutral) * sqrt(3) angle 30 deg. I (line-line) VOLT AMP =  CURRENT I (line-neutral). Something wrong here equating Volatage with VA with current??/

For Delta connection, I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg. V (line-line) = V (line-neutral)"

The equations match your explanation.

For wye/star connection, V line is sqrt3 times Vphase and is 30 degrees ahead of Vphase -> V (line-line) VOLTAGE = V (line-neutral) * sqrt(3) angle 30 deg

For wye/star connection, line current is equal to phase current (Magnitude and angle are the same) ->  I (line-line) =  I (line-neutral)

For delta connection, Line currents lag 30deg to phase currents and are sqrt3 times phase currents -> I (line-line) = I (line-neutral) * sqrt(3) angle - 30 deg

For delta connection, line voltage is equal to phase voltage (Magnitude and angle are the same) -> V (line-line) = V (line-neutral)

"For this spin-up example, should we worry about the 30 deg angle? I am not able to see the Question here but I can understand your confusion. Can you write the question here? I am sorry I should have asked you this earlier!!! "

The problem is at http://spinupexams.com/index.php?pr=Question_For_The_Week

I guess my real question is when do we do + or - 30 deg angle phase shift for voltage and current? I have understood your doubt clearly. Many have it, no worries. Now in this question the transformer does not have any meaning for calculating the current. They have given a 1-phase of load 25KVA which has a pf  of .75 lagging. This lagging pf means current in this load lags the Voltage across this load by a angle 41.4 degrees. So if I take the Voltage across the load as reference ( that is Phase Voltage & angle as zero), the current will have  a value of 92A and the angle of the load will be -41.1 degrees. This is not the complex load you are taking in your solution nor you have to take 30 degrees into account. I hope this has made some sense.

Thanks!

 

[rg1]

In the same question if I give you Line Voltage Vab=480 angle 0 and ask you value of current for 25KVA  load having pf .75 lag then the current found will have -71.4 deg angle. Could you get the difference.

 

[supra33202]

I think I understand.

Since the given line voltage Vab doesn't have an angle, I can set the phase voltage Van at angle 0 deg. Then, the line voltage Vab will be 480 V at angle +30 deg.

Ia (phase current) = S (single phase) / Van (phase voltage)

25KVA / 277 V = 90.2A

And since this is wye connection,  line current vector = phase current vector (Ia) = 90.2 A at angle -41.4 deg.

Basically the question is not clear. It doesn't give us the reference angle for either the line voltage or phase voltage. We have to make assumption.

If I set the phase voltage Van at angle 0 deg, then Ia = 90.2 A at angle -41.4 deg.

If I set the line voltage Vab at angle 0 deg, then Ia = 90.25 A at angle -71.4 deg.

The two Ia values are correct depending on how I set my reference voltage angle.

Is that correct?

Thanks!

I guess my real question is when do we do + or - 30 deg angle phase shift for voltage and current?

If we are dealing with vectors, we have to worry the angles for all vectors (voltage, current, apparent power). If the question asks magnitude only, don't worry about angles.

Is that correct?

 

[rg1]

I think I understand.

Since the given line voltage Vab doesn't have an angle, I can set the phase voltage Van at angle 0 deg. Then, the line voltage Vab will be 480 V at angle +30 deg.

Ia (phase current) = S (single phase) / Van (phase voltage)

25KVA / 277 V = 90.2A

And since this is wye connection,  line current vector = phase current vector (Ia) = 90.2 A at angle -41.4 deg.

Basically the question is not clear. It doesn't give us the reference angle for either the line voltage or phase voltage. We have to make assumption. Once you do a few questions it will start coming naturally to you. When no reference is given phase Voltage is taken as reference.

If I set the phase voltage Van at angle 0 deg, then Ia = 90.2 A at angle -41.4 deg.

If I set the line voltage Vab at angle 0 deg, then Ia = 90.25 A at angle -71.4 deg. 

The two Ia values are correct depending on how I set my reference voltage angle. You are right but if nothing is given then it is phase Voltage as reference

Is that correct?

Thanks!

I guess my real question is when do we do + or - 30 deg angle phase shift for voltage and current?  I am not able to answer this question outrightly but you can form a few questions, I will answer with explanation. However you should know this concept for PE exam 

If we are dealing with vectors, we have to worry the angles for all vectors (voltage, current, apparent power). If the question asks magnitude only, don't worry about angles. Let us not generalize it .We should always use phasors else it may give erroneous results. Let us talk about it question by question.  A very good idea will be to draw the phasor diagram for better understanding of the issue.

Is that correct?

 

[spinup]

Week 2 Question For The Week can be found at:

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[spinup]

Week 5 Question For The Week can be found at:

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[pcboiler]

Has anyone bought the Spinup practice exams?  Any thoughts on how well they represent actual exam questions?

I much prefer practice problems to reading dry material, but I've already got all the Complex Imaginary books, the practice exam at the end of Graffeo, and the NCEES practice exam.  Trying to decide if it's worth buying these as well.

 

[spinup]

Week 6 Question For The Week can be found at:

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Under the "Question For The Week" tab

Previous weeks posted in separate tab

 

[Stephen2awesome]

Has anyone bought the Spinup practice exams?  Any thoughts on how well they represent actual exam questions?

I much prefer practice problems to reading dry material, but I've already got all the Complex Imaginary books, the practice exam at the end of Graffeo, and the NCEES practice exam.  Trying to decide if it's worth buying these as well.

Spinup is really for making sure you understand the basics. CI and the PPI practice exam has a much more Exam feel. Of course, the NCEES practice as well.

 

[spinup]

Week 7 Question For The Week can be found at:

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Under the "Question For The Week" tab

Previous weeks posted in separate tab

 

[Surf and Snow]

Has anyone bought the Spinup practice exams?  Any thoughts on how well they represent actual exam questions?

I much prefer practice problems to reading dry material, but I've already got all the Complex Imaginary books, the practice exam at the end of Graffeo, and the NCEES practice exam.  Trying to decide if it's worth buying these as well.

I just bought the Lanza Spin-Up book of 5 practice exams. I havent started it yet, but I will tomorrow night as I just finished the PPI 2 exam book and Graeffeo book exam. Just thumbing though it I can say that I dont like the choice "e" None of the above on all the questions. While yes, it helps you really learn the topics, lets be honest here; right now I just want/need to pass the damn PE exam!  :angry:  Thus, I want my practice exams to mimic the real exam. :withstupid:  On questions I'm struggling, I want to learn how to reverse engineer the answer, or at least be able to see how I can eliminate 2 options and narrow my chances to a 50/50. When I'm trying to zone in a 6-minute OR LESS average on all questions, I dont want to work a problem thoroughly thru and arrive at a CORRECT answer in 6 minutes or less, then see its not an a-d option and sit there second guessing myself and wasting time checking my work needlessly. I want it to mimic the real exam where if I get say 69.8A and the options are a.) 50A, b.) 60A, c.) 70A, d.)80A I mark the C and move on. not sitting there going "Crap! the closest answers are 30A or 90A, I mustve done something wrong!" and then wasting 10 minutes redoing everything meticulously to double check. I'm trying to pass the test, on a 6-minute/question time, not question everything I know to revisit it all and redo all my work!  :angry:

 

[Zach Stone P.E.]

Has anyone bought the Spinup practice exams?  Any thoughts on how well they represent actual exam questions?

I much prefer practice problems to reading dry material, but I've already got all the Complex Imaginary books, the practice exam at the end of Graffeo, and the NCEES practice exam.  Trying to decide if it's worth buying these as well.

We (Electrical PE Review) recommend Spin Up in addition to a few others.  If you'd like to see why, check out our article on our recommended references for the Electrical PE Exam here:

Electrical PE Review - Recommended References for the Electrical PE Exam

 

[spinup]

Week 9 Question For The Week can be found at:

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