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Petezel

Mon May 14 2007, 12:42PM

Registered Member #211 Joined: Sun Feb 19 2006, 05:33PM
Location:
Posts: 27

Hi peoples
The end of the college year is rapidly approaching, which is great aside from the 7 or so assignments that have to be done and in before that time. I have gotten stuck on most of them, and running out of options I would like to ask here that if anyone has done BTEC NC or HNC or anything higher than that if they would be willing to lend a hand. Being the second year of a two year course it will make a mess if I do *gulp* fail.
I have poked around on the internet for information of course, but many things are left unclear. For those who frequent the chatroom, even better - a good medium to use there.

Motors in particular, Three Phase Systems, analogue devices + noise are some of the topics I have to tackle. Soon I will scan/copy some of the assignment documents and upload them somewhere to reference.

I can't thank enough for those that have helped out in the past and will be evermore appreciative of anyone who can help me out in this matter

Kind regards,
Petezel

Thomas

Mon May 14 2007, 01:54PM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Sorry but we don't do your homework here. I'm sorry but if you need to resort to these measures you don't deserve to pass. I've worked in tutorial, and have been a professor's assistant. I see students like you all the time who have a c average and don't care about it. You have to realize that college isn't high school, you can't procrastinate and wait till the last minute to do work. Maybe your grades will reflect this wake up call? I never understood why students never ask their professors for help, even when they are failing. I don't know why students don't go to tutorial center for help. Then it hit me, it's because they don't care, and this is why your go to this forum. To detach yourself from doing the work.

AndrewM

Mon May 14 2007, 02:40PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Thomas, quit being a self righteous asshole.

I'd love to help Pete, but it sounds like the subject matter is out of my circle.

Steve Conner

Mon May 14 2007, 04:09PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hey Pete,

You should know better than to post pleas for help here, it's probably against the forum rules... just ask in the chatroom instead

I did all that stuff in college, and though I'm pretty busy, I am sometimes around #hvcomm especially at the weekends.

In the end though, it's all about you. My math skills used to suck, and when I saw my math grades getting down to 25% and realised I wouldn't graduate with honours unless I did something about it, that certainly motivated me to study harder! I got them back up to around 65% in time for final year. You do have to spend a certain amount of time staring at stuff that you just plain don't understand, and more or less banging your head off the table until it becomes clear.

Petezel

Tue May 29 2007, 01:03PM

Registered Member #211 Joined: Sun Feb 19 2006, 05:33PM
Location:
Posts: 27

Hi,
Mixed and varied response there o_O
I'm afraid some of those are what drove me away from the forum, again.
If you feel there is no need for useful engineers in the world and almost actively set against it, so be it. You shall not drag me down.
Perhaps it is conner, I'll ask ChrisR.
One question I am screwed on is having a single phase motor which requires a PFC capacitor to bring it to unity, but only give the power factor (0.7 lagging), the line current/voltage/frequency, 60A 250V 50Hz respectively.
I have seen at least 3 different methods, but they all seem to require extra parameters that I do not have. Am I going mad, or is something missing ?
Regards,
Petezel

Steve Conner

Tue May 29 2007, 09:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi pete, Yes you are going mad. here's how I would do it:

Power factor=real power/apparent power= cos phi

But, you need to know reactive power, so what you need is sin phi=reactive power/apparent power

Now if cos phi=0.7 then phi=44.4 degrees hence sin phi= 0.7 too

Also, apparent power=60A*250V = 15000VA
So, reactive power= 15000*0.7 = 10500VAr

To get unity power factor, you must pick a capacitor that generates an equal amount of reactive power. So you need 10.5kVAr worth of capacitors. If you want to know what that is in uF:

Reactive power=(voltage squared)/reactance

hence the required reactance is (250^2)/10500= 5.95 ohms

and the reactance of a capacitor=1/(2*pi*f*c)

hence the capacitance is 1/(2*pi*50*5.95) = 535uF

I think. See here

for a better explanation.

thom

Tue May 29 2007, 10:36PM

Registered Member #239 Joined: Thu Feb 23 2006, 03:15AM
Location: canada
Posts: 23

http://home.earthlink.net/~jimlux/hv/pfc.htm

Looks like enough information to do the equations on this page?

Its funny, I had a power class a while ago and there were a bunch of oddball equations which could only be found in the text. If you didn't have those (of course I was cheap and didn't buy the text), it was a bit of a challenge to figure out the answer :)

Steve Conner

Wed May 30 2007, 07:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

let me guess, the textbook was written by the lecturer?

AndrewM

Wed May 30 2007, 02:45PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Steve Conner wrote ...

let me guess, the textbook was written by the lecturer?

ha. perhaps equally bad, I once had a text that quoted my professor on the back cover: "Bravo!"

Carbon_Rod

Thu May 31 2007, 11:49PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

A competent prof will not give you assignments without purpose. And will often post practice question sets from prior terms.

One prof I had busted 6 Asian exchange students for plagiarism as they solved an impossible task for their level of study (unknown PSK.) These types are weeded out eventually... Sometimes years later...

Sometimes you need to learn how to handle failure, life can be difficult for students... But again, belief in yourself and cooperation with your peers will allow you to learn the real skills needed later on. Not to mention you may gain friends with similar interests that later go on to work at lame places like Google, Microsoft, Apple, RIM, and IBM.
;oP

If you don't like the prof, transfer sections early.

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