A few questions

[gsurface]

I am an instructor for a new electrical program and though I have experience in the residential wiring, my main background is Engineering (Degree in Electrical Engineering). I just have a few questions that have crossed my mind. Here they go and hopefully I can have them asnwered .

1. What are advantages and disadvantages between single-and three-phase supply systems?
2. What are the differences between Wye and Delta connections? Why use one over the other?
3. In a 3-phase motor, how come it has no neutral connected to it? Doesn't the current need to return somehow?

Hopefully these questions aren't hard to answer. Cheers.

 

[iwire]

Re: A few questions

Originally posted by gsurface:
I am an instructor for a new electrical program and though I have experience in the residential wiring, my main background is Engineering (Degree in Electrical Engineering). I just have a few questions that have crossed my mind. Here they go and hopefully I can have them answered .

Welcome and you came to the right place, all your questions can be answered by the various members here.

I am going to let you know you are bound to get some grief about being an instructor for an electrical program and needing answers to some of these questions.

I also wonder what the requirements are to be obtain a Degree in Electrical Engineering?

Some short answers, others will surly elaborate.

Originally posted by gsurface:
1. What are advantages and disadvantages between single-and three-phase supply systems?

Three phase is more cost efective.

As an example a 10 HP single phase motor draws 55 amps @ 208 volts.

A 10 HP three phase motor draws 30 amps @ 208 volts.

This results in smaller conductors, raceways, disconnects and ultimately a smaller service size.

Keep in mind the motor still uses about the same energy but it is now dived by 3 conductors not two.

There are also other advantages to 3 phase motors and systems.

Originally posted by gsurface:
2. What are the differences between Wye and Delta connections? Why use one over the other?

Do you need a neutral or not?

There is more to it than just that and I am sure others will bring them up.

Originally posted by gsurface:
3. In a 3-phase motor, how come it has no neutral connected to it? Doesn't the current need to return somehow?

This is the same as asking why a diesel car does not need gas.

If I had a 240 volt single phase motor would you expect a neutral was needed?

This is hard to explain without graphics hopefully someone will post a graphic.

But a neutral is not a 'return' conductor.

What exactly will you be teaching?

I think it's great your trying to find the answers but it in my opinion you are not ready to teach about commercial electrical systems.

Good luck and I hope you can take some ribbing, let the ribbing slid by as in between some ribbing you can get some very concise answers.

Bob

[ February 12, 2005, 06:12 AM: Message edited by: iwire ]

 

[cpal]

Re: A few questions

There are many avenues to get the information you are looking for. Certainly opinions will be available on this site but my experience would suggest self study. The URL I have listed is just one of many commercially available publishers that provide excellent texts. You may select the subjects you are concerned with and get the information you need. Delmar for example will address the same subject across server al volumes of text. DC Fundamentals (eg.) is covered in their workbook #1 as well as a complete book Titled DC Fundamentals. my experience is that not one publisher nor volume will address all the concerns of a class room instructor. Research is key. Mc Graw Hill is a focused publisher I like the Singers Math for Electricity and Electronics, in provides numerous practice questions with in each subject category. Tom Henry puts out some good information and general overviews as well as the book authored by Hermans, and of course Mike Holt at this web site.

Good Luck
Charlie


http://www.delmarlearning.com/electrical/

 

[rattus]

Re: A few questions

gsurface,

Obviously you got your BSEE after I did. Many of us old heads had to take a lot of power courses whether we wanted to or not. I understand this is no longer the case, but that is progress.

I would hope though that you had to take math through differential equations in which case you should have no trouble self teaching yourself AC circuits and AC machines.

 

[rattus]

Re: A few questions

iwire,

Seems to me that your 3-phase motor drawing 30A would use 10,800VA while the 1-phase motor would use 11,440VA. That is, the improvement, although significant, is not as dramatic as your numbers imply. Tell me if I am missing something.

Rattus

[ February 12, 2005, 10:56 AM: Message edited by: rattus ]

 

[roger]

Re: A few questions

Rattus, not trying to answer for Iwire, but I would think the point was, the reduction of wasted natural resources (I would include fuel for generation in this) and the cost difference of equipment and service sizes.

I think conservation should be a design consideration.

Roger

 

[gsurface]

Re: A few questions

Originally posted by rattus:
gsurface,

Obviously you got your BSEE after I did. Many of us old heads had to take a lot of power courses whether we wanted to or not. I understand this is no longer the case, but that is progress.

I would hope though that you had to take math through differential equations in which case you should have no trouble self teaching yourself AC circuits and AC machines.

You are correct on this, I finished my degree in BEE not BSEE(which requires us to take more electrical engineering classes than a BSEE) two years ago. Now, in engineering schools, they tend to steer clear from Power and focus more on electronics and networking protocols.

I also wonder what the requirements are to be obtain a Degree in Electrical Engineering? [Confused]

Plenty. The people who believe that Electrical Engineeering and Electrical technical programs are within the same areas are totally off. When someone who goes to an electrical program uses algebra to study AC circuits, engineers use LaPlace transforms (which makes it very easy to add voltages) and tons of Fourier transforms. And thats If you don't like calculus, you should steer clear of Engineeering since it all relys on that. They teach us what we need to know to design things.

Not all intructors know everything, the reason why I am asking here. I have seen an instructor at my school with tons of experience not be able to explain to the students the differences between RMS and Peak-to-Peak, and they have a hard time explaining series and parallel circuits.

Thanks for all the info guys.

 

[Ed MacLaren]

Re: A few questions

I have seen an instructor at my school with tons of experience not be able to explain to the students the differences between RMS and Peak-to-Peak, and they have a hard time explaining series and parallel circuits.

That leaves me wondering how he kept his job. How long did he last?

Ed

 

[gsurface]

Re: A few questions

Originally posted by Ed MacLaren:

I have seen an instructor at my school with tons of experience not be able to explain to the students the differences between RMS and Peak-to-Peak, and they have a hard time explaining series and parallel circuits.

That leaves me wondering how he kept his job. How long did he last?

Ed

He is still there. I have other instructors that all they do is wiring. For example, during the Electrical Codes class, I asked his students what they learned, they said that they just did a lot of wiring. I guess the instructor didn't want to bother atleast reading one chapter ahead so he decided to go with wiring.

We have another class called conduit systems, where the syllabus clearly states the way the course should be broken down. 20% - Video cabling, 20% Telephone cabling, 20% Networking cabling, 20% conduit bending. This instructor took it upon himself to only teach them conduit bending. I made him know that the syllabus states that you should teach other subjects. His responce, "oh, we both know they will not use those in the field". Should I talk to the dean about it? Because we do have other courses that depend on some patching (Patching RJ-45 cables), the class is Fiber Optiocs installation, which requires that the student have some knowledge, the minimum at least, of netowrking protocols.

 

[rattus]

Re: A few questions

Roger,

I agree with you that conservation is important, but plant managers look first at cost, and I was trying to quantify the increased efficiency of the motor alone.

iwire's example though implies that 208V x 30A is the apparent power drawn by the motor. But that is only 6240VA which is less than 7460W which is equivalent to 10hp.

Certainly the absence of special start windings, etc. make the 3-phase motor a cleaner, meaner machine, and the distribution costs should be lower as well. Maybe someone well versed in this area can give us some ballpark figures?

Rattus

 

[laidman]

Re: A few questions

I honestly can't believe what I'm reading here if there are schools out there teaching this way and allowing instructors to pick and choose and not supply a well rounded course because they don't have the knowledge.Then it is no wonder that many electricians in todays market don't have the knowledge they need.

[ February 12, 2005, 12:24 PM: Message edited by: laidman ]

 

[rattus]

Re: A few questions

My understanding is that the BEE, BSET, ASET, stress the practical aspect of engineering, whereas the BSEE is more theoretical.

Of course, there is a place for both disciplines.

 

[iwire]

Re: A few questions

Originally posted by rattus:
iwire,

Seems to me that your 3-phase motor drawing 30A would use 10,800VA while the 1-phase motor would use 11,440VA. That is, the improvement, although significant, is not as dramatic as your numbers imply. Tell me if I am missing something.

Rattus

I guess your missing other parts of that same post.

I was also careful to say cost effective not more efficient although it actually is a little more energy efficient.

Bob

 

[Ed MacLaren]

Re: A few questions

I looked on the web for comparisons between single phase and three phase motors with regard to efficiency, but wasn't able to find any.
I believe the reason is that there is "no contest". Three phase motors are always used where three phase power is available.

The example below illustrates the power factor and efficiency for a typical three phase, 10 HP, 460 volt, normal-speed SCI motor.

Note that both the efficiency and power factor of an induction motor is reduced as its load decreases, so it is important not to oversize motors.
A typical 10 HP, 1800 RPM, 3-phase, Design B motor with a full-load power factor of about 80 percent decreases to about 65 percent at half-load.

Ed

 

[iwire]

Re: A few questions

Originally posted by rattus:
but plant managers look first at cost, and I was trying to quantify the increased efficiency of the motor alone.

I wish I could provide some figures but I can not. I also can not imagine any plant managers choosing single phase over three phase unless forced to by the utility.

 

[Ed MacLaren]

Re: A few questions

I agree with you that conservation is important, but plant managers look first at cost

That may be so, but perhaps they should take another look at the newer "high efficiency" motors, even though they cost more to buy.

Ted Jones, of the Consortium for Energy Efficiency, states-
"The cost of a big industrial motor typically amounts to just 3 percent of the total cost to operate it over its lifetime, which can extend up to 20 years."
"So what you're paying for energy over that time is 97 percent of your total cost".

Ed

 

[cpal]

Re: A few questions

rattus

Your post concerning the HP Vs VA for the three phase 30A motor requires another multiplier of 1.73 for the formula to work.

Also with out Power Factor and % of eff the factors applied will only yield a VA analysis.

VA= V.A.1.73 for 3-phase the multiplier is 1.4142 (I think) for 2-Phase and it is 1 for single phase.

HP = V.A.Phase.PF/746 Times %Eff.


The 10 HP 3-P 30A motor has a Apparent input of of?6240VA X 1.73 = 10795.2 VA. Which is about OK, rule of thumb I've used is 1kVA per HP for feeder loads(note I did not say demand).

HP ratings of motors are indicators of the shaft output.

There are other issues involved, such as Dia of shaft, torque and service factor. I don't get into those variables often, but I understand that they may come into play.

Three phase systems are generally more economical to install because the equipment is smaller and lighter per kVA or HP depending on what you are comparing.
The motors for example are smaller per HP and have fewer parts to fail Vs single phase. The distribution is convenient in so much that you get a 75% increase in available power by adding a third conductor to the feeders. this is a great savings in raceway, wire size and voltage drop. Less amps same amount of power and smaller wire. Copper loss !! well you know the story.

Sorry I got gabby


Charlie

[ February 12, 2005, 03:54 PM: Message edited by: cpal ]

 

[rattus]

Re: A few questions

cpal,

That is my point. iwire's example is incomplete; one could infer that this 3-phase motor is more than 100% efficient. Apparent power should have been computed for both motors.

We don't know the PF or the efficiency of either motor, so we can't really apply those factors here. We all agree too that three phase is the way to go with large motors.

I believe that 1KW per hp is a good rule of thumb for either single or three phase.

 

[cpal]

Re: A few questions

I like your motto I can't tell you what we used in ASW.

Sorry I missed your point in the previous post

Charlie

 

[rattus]

Re: A few questions

Ed, your comment about the high efficiency motors reminds me of my first job. At age 16, I was a clerk for Utility Rate and Engineering Co. We audited utility bills for commercial establishments and recommended ways to reduce their bills.

We recommended rate changes where possible, combining services, spreading out demand, and installation of capacitor banks and synchronous motors to improve PF.

As a mere child, I often did the whole job. The boss merely checked the report for accuracy. No engineers were on the staff, and none of us understood PF. That was almost 60 years ago, and sometimes I wonder if I understand it now!