Previous in Forum: Merging Current and Voltage Sources - How Can I Do This?   Next in Forum: how to calculate multiplying factor if meter ctr is -/5?
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
5 comments
Rate Comments: Nested
Oak
Member

Join Date: Jan 2013
Posts: 7

Balanced Three Phase Delta Circuit Problem

02/25/2015 10:13 PM

Doing some studying for my FE exam and I've been going over some practice problems I found online for single phase and three phase type problems. (courtesy to Dr. Ravel F. Ammerman)

Question: A three-phase circuit is shown (above). Load resistors (66ohms) are connected in delta and supplied by a 220V balanced three-phase source through three lines of 2ohm resistance. The magnitude of the RMS, Line to Line voltage across each 66 ohm load resistor is most nearly:

a) 198V b)110V c) 201V d) 220V e) 120V

So, the formulas that I have at hand are:

Vline = Vphase, Iline = sqrt(3)* Iphase

Also have some other formulas involving load impedance. In a normal circuit situation I would calculate the voltage drop across the 2 Ohm resistor then use kirchoffs to find the voltage across the load. However, this doesn't work for a 3 phase delta. I need the thought process explained behind calculating the line-to-line voltage.
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Good Answers:

These comments received enough positive votes to make them "good answers".
SolarEagle
Guru

Join Date: Mar 2007
Location: by the beach in Florida
Posts: 33392
Good Answers: 1817
#1

Re: Balanced Three Phase Delta Circuit Problem

02/26/2015 12:26 AM

In a resistive divider, power dissipated is equal to the square of current times resistance. The power is divided as is voltage, so the fraction of power dissipated in the line is the same as its voltage drop or the ratio of its resistance to total resistance.

__________________
 All living things seek to control their own destiny....this is the purpose of life
Register to Reply
3
spades
Guru

Join Date: Aug 2014
Location: Port Macquarie N.S.W. Australia
Posts: 1071
Good Answers: 225
#2

Re: Balanced Three Phase Delta Circuit Problem

02/26/2015 4:55 AM

I would first convert the delta section to a star equivalent which will give you a load resistance of 66/3 = 22Ω (forget the 2Ω series resistance for the moment)

Therefore the equivalent circuit for each phase will be 220/√3 volts across 22Ω + 2Ω, assume phase angle is 0°

Then use the formula VL = (((220/√3)*22) / (22+2))* √3 = 201.667V.

So I would calculate C to be the correct answer.
Register to Reply Good Answer (Score 3)
Go to Next Good Answer
Oak
Member

Join Date: Jan 2013
Posts: 7
#3
In reply to #2

Re: Balanced Three Phase Delta Circuit Problem

02/26/2015 1:20 PM

Thanks for the reply. This makes sense to me now, I don't know why I was having trouble with this. I know the 2 Ohm resistor would have to be accounted for some how but I didn't realize I could just convert this in a star config and can solve a series resistance.

Thanks!
Register to Reply
spades
Guru

Join Date: Aug 2014
Location: Port Macquarie N.S.W. Australia
Posts: 1071
Good Answers: 225
#4
In reply to #3

Re: Balanced Three Phase Delta Circuit Problem

02/27/2015 12:23 AM

Actually it's a parallel resistance calculation first and then a series one.
Register to Reply
2
horace40
Guru

Join Date: Jan 2006
Location: Tamworth, UK.
Posts: 1782
Good Answers: 45
#5

Re: Balanced Three Phase Delta Circuit Problem

02/27/2015 8:17 AM

Unless it is a trick question meant to test your ability to get you involved in complex 3 phase circuit calculations, just look at it as a simple resistor network. Leave the arithmetic till last.

By simple inspection the resistance across any two points of the delta can be seen as 44 ohms. ie. 66 ohms in parallel with 2 x 66 ohms.

Then in series with 2 + 2 ohms. A total of 48 ohms

The current will be 220 / 48 amps.

Voltage across the delta will be 44 x 220/48 volts

This gives 201.667 v

Which by inspection C = 201 is the answer.

__________________
 When arguing, remember mud-slinging = lost ground.
Register to Reply Good Answer (Score 2)
No more Good Answers.
Register to Reply
Have a Different Question?
5 comments

Good Answers:

These comments received enough positive votes to make them "good answers".
Copy to Clipboard

Users who posted comments:

horace40 (1); Oak (1); SolarEagle (1); spades (2)

Previous in Forum: Merging Current and Voltage Sources - How Can I Do This?   Next in Forum: how to calculate multiplying factor if meter ctr is -/5?

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Electrical Engineering:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (68 right now)
Show Members
Advertisement