Biggest misunderstandings with 3 phase power......

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Smart $

Esteemed Member
Location
Ohio
I've run into this before:

Motor is pulling 10 amps on all 3 legs. So that means that the motor is pulling 30 amps.

10+10+10=30 is completely wrong but very easy to understand
Easy to understand if it is explained to that level of detail each time. But what if a person that believes that way to be correct comes up to you and just says the motor is pulling 30 amps, and you do not know this person believes incorrectly...???
 

Saturn_Europa

Senior Member
Location
Raleigh, NC USA
Easy to understand if it is explained to that level of detail each time. But what if a person that believes that way to be correct comes up to you and just says the motor is pulling 30 amps, and you do not know this person believes incorrectly...???

It was a tough argument. Hard to tell someone a good deal older than you, that has been working maintenance since before you were in high school that they are dead wrong. I tried to explain 120 degree phase angle and root mean square but it didnt really sink in. I ended up showing him the motor nameplate FLA and explained that 30 amps was completely impossible.

There is a lot of misconception about 3phase power and not just from people that are unfamiliar with the industry.
 

dionysius

Senior Member
Location
WA
100 v
10 A

3 1ph w/N
3 x 100 x 10 = 3000

3 ph
1.732 x 100 x 10 = 1732

(3000-2732)/3000 = 42% less
3 phase carries 42% LESS than 3 1 ph w/N, NOT 73% less
but the 3 x 1 ph needs 100% more Cu
the 3 x 1 ph does carry 73% MORE than the 3 ph

the mechanical model will only get you so far
Talking of Cu usage. Do you have to consider a neutral so 4 wires on a 3 phase Y for a fair comparison.

3 Single phase: 6 wires
One 3 phase : 4 wires
So the 3 x 1 ph needs (6-4)/4 = 50% more Cu NOT (6-3)/3 = 100% more Cu
 

Smart $

Esteemed Member
Location
Ohio
It was a tough argument. Hard to tell someone a good deal older than you, that has been working maintenance since before you were in high school that they are dead wrong. I tried to explain 120 degree phase angle and root mean square but it didnt really sink in. I ended up showing him the motor nameplate FLA and explained that 30 amps was completely impossible.
I bet.

There is a lot of misconception about 3phase power and not just from people that are unfamiliar with the industry.
All too true. :happyyes:
 

dionysius

Senior Member
Location
WA
I've run into this before:

Motor is pulling 10 amps on all 3 legs. So that means that the motor is pulling 30 amps.

10+10+10=30 is completely wrong but very easy to understand
For 3 phase the Amps are calculated as:


I am unclear as to the point here.....one can say motor is pulling 10A not 30A???:?
 

mwm1752

Senior Member
Location
Aspen, Colo
For 3 phase the Amps are calculated as:


I am unclear as to the point here.....one can say motor is pulling 10A not 30A???:?
It appears the premise is the 208v 10 amp 3 phase motor draw will have a running cost the same as 1 - 30 amp 120v loads. The difference is conductor size for the branch circuit -- now we're out of the perverbial box -- :?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
A further enlightenment on my fluid model above is to think of the three wires in 3 phase as a single energy conduit.
You are spending way to much time creating 'simple to understand' analogies for things that are not similar.

I think you would be better off understanding the mathematics behind 3-phase wye connected circuits. After you grasp the formulas you can move onto

In engineering school we spent 9 months on 3-phase AC circuits before we studied single-phase 3-wire, yet that is where you want to start.
 

Ingenieur

Senior Member
Location
Earth
Talking of Cu usage. Do you have to consider a neutral so 4 wires on a 3 phase Y for a fair comparison.

3 Single phase: 6 wires
One 3 phase : 4 wires
So the 3 x 1 ph needs (6-4)/4 = 50% more Cu NOT (6-3)/3 = 100% more Cu
No
a 3 phase 3 w will deliver power to a load
try that with 3 1 ph and no N
 

Ingenieur

Senior Member
Location
Earth
You are spending way to much time creating 'simple to understand' analogies for things that are not similar.

I think you would be better off understanding the mathematics behind 3-phase wye connected circuits. After you grasp the formulas you can move onto

In engineering school we spent 9 months on 3-phase AC circuits before we studied single-phase 3-wire, yet that is where you want to start.
this
 

Saturn_Europa

Senior Member
Location
Raleigh, NC USA
You are spending way to much time creating 'simple to understand' analogies for things that are not similar.

I think you would be better off understanding the mathematics behind 3-phase wye connected circuits. After you grasp the formulas you can move onto

In engineering school we spent 9 months on 3-phase AC circuits before we studied single-phase 3-wire, yet that is where you want to start.

How long ago were you in engineering school? Most of the EE programs near me share curriculum with computer engineering. To double major in Computer Engineering and Electrical Engineering you only have to take an extra 3 or 4 classes.

The majority of EE recent graduates I've worked with did not know how to use a multi-meter or read ladder logic diagrams and lacked basic understanding of common electrical theory. It was rather surprising.
 

Ingenieur

Senior Member
Location
Earth
How long ago were you in engineering school? Most of the EE programs near me share curriculum with computer engineering. To double major in Computer Engineering and Electrical Engineering you only have to take an extra 3 or 4 classes.

The majority of EE recent graduates I've worked with did not know how to use a multi-meter or read ladder logic diagrams and lacked basic understanding of common electrical theory. It was rather surprising.
Yes
same at my school
but core courses are the same for both if you want ce and ee
ckt theory 1 & 2
power 1
machines 1
fields
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
How long ago were you in engineering school? Most of the EE programs near me share curriculum with computer engineering. To double major in Computer Engineering and Electrical Engineering you only have to take an extra 3 or 4 classes.

The majority of EE recent graduates I've worked with did not know how to use a multi-meter or read ladder logic diagrams and lacked basic understanding of common electrical theory. It was rather surprising.
My area of emphasis was 'power systems and rotating machinery'. Back then calculators were optional, slide rules weren't.
Luckily some schools still have this emphasis option.

My point is that we learned 3-phase as a single entity, it was what it was. Sure we spent some time on 2-wire DC circuits once we got that concept down it was on to 3-phase. It is not that we ignored single-phase but rather it wasn't treated as the starting point of anything. After all the most common faults on 3-phase systems are L-G 2-wire ones.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The most common misunderstanding regarding three phase power I run into is the impression that in a balanced line the current in a phase conductor is 1/3 of the total current. That is, that if 150A of OCPD is needed, then it's a 50A fuse on each phase.
 

Ingenieur

Senior Member
Location
Earth
My area of emphasis was 'power systems and rotating machinery'. Back then calculators were optional, slide rules weren't.
Luckily some schools still have this emphasis option.

My point is that we learned 3-phase as a single entity, it was what it was. Sure we spent some time on 2-wire DC circuits once we got that concept down it was on to 3-phase. It is not that we ignored single-phase but rather it wasn't treated as the starting point of anything. After all the most common faults on 3-phase systems are L-G 2-wire ones.
we learned single ph (not called that) first
single sinusoidal signal
made ckt math/analysis easier
time constants, RLC ckts, etc
kcl, kvl, equvilent ckts
phase relationships, composite signals, harmonics (essential for 3 ph study)
matrix math, networks
they use the same book today as 30 years ago
2 part course, 4 cr each, pre req was physics 1 and 2 and calc 1 & 2 (differenital and integral)

intro to 3 ph end of 2nd term
CHAPTER 12 Three-Phase Circuits 568

text 9th! Edition http://www.wiley.com/WileyCDA/WileyTitle/productCd-EHEP002507.html

current couse info
actusl test/solutions at bottom
http://www.pitt.edu/~qiw4/Academic/MEMS0031/Syllabus.htm
 
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Ingenieur

Senior Member
Location
Earth
Here's the funny thing
3 ph is typically reduced to a single line equivilent for most basic analysis
symetrical faults, power flow, etc
 

Ingenieur

Senior Member
Location
Earth
A 3 phase ckt can be looked at as 3 1 ph for some purposes
power
10 A
480 ph V

va = Vlna x i + Vlnb x i + Vlnc x i
assuming V and i are equal/balanced
va = 3 x Vln x i = 3 x 480/sqrt3 x 10 = 8310
same as
va = sqrt3 x Vll x i
Where Vll = sqrt3 x Vln
substitute
va = sqrt3 x (sqrt3 x Vln) x i = 3 x Vln x i
 

dionysius

Senior Member
Location
WA
You are spending way to much time creating 'simple to understand' analogies for things that are not similar.

I think you would be better off understanding the mathematics behind 3-phase wye connected circuits. After you grasp the formulas you can move onto

In engineering school we spent 9 months on 3-phase AC circuits before we studied single-phase 3-wire, yet that is where you want to start.
Hello Mr. Dungar and all of the great contributors on this stellar forum.

I do understand the math I have been exposed to. However I believe if you really really grasp the subject it should be a wonderful mental challenge to develop a lucid abstraction for non analytical laymen that would enhance their depth of knowledge. That is the challenge I am putting out there for smart people like you who do understand their subject. I would be honored if you would go through the analogy I gave above and tell me why it is not useful or correct and lend me some guidance and help. If you can improve on it or present a totally different one I would be doing cartwheels.

My feeling is since no one gave me feedback directly on it, it must be totally deficient and even crazy. I have searched the web but cannot find a good answer. Feel free to start a new thread if you wish and I will contribute. Most electricians in the field are lost once you descend into the second or third level of detail. This I find very sad. This is just a genuine request, you do not have to reply since you owe me nothing. This is just a fun mental challenge that may help us all.

Do have a great Fourth......lots of celebration.
 
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