Module 202 revision

[Chrisuk]

Question 1

So A needs to be increased to 10A

10x0.2=2V
5x0.2=1v
Totally 3V
230-3v=227

Answer D

Yet my tutor said it's C

Can someone else see what answer they get please

Question 2 and I'm struggling with this because I don't know the correct formula

The line current of a 400V three phase load of 8kW at 0.8 power factor is

A. 11.54A
B. 14.43A «------ tutor said it's the correct answer
C. 20A
D. 25A

But it was time to leave and I want to know the formula

Isn't the answer the line current
The 400v surely is the line voltage?

 

[Chrisuk]

I have done 8000/400/.8=16
8000/400x.8=25, it's one of the answers
But I have a feeling I need to throw a square root in to work it out, just can't do it

pf= P / VxA

 

[telectrix]

W = sq.r3 x (V x I. x PF)

so I = watts / rt3 x 400 x 0.8 = 14.434A

 

[Chrisuk]

I think I'm having a blonde moment, what's rt3? A ratio of 3?

I still can't get the answer on my calc

Square.r3(VxIxPf) but I don't have I

 

[Simonslimline]

1.732 square roote of 3 mate

 

[Chrisuk]

1.732 square roote of 3 mate

Nice one, from what u and tel said we haven't done this yet, but my tutor did say the practice test was much harder than what we had done

We have just done working out pf

Not anything to do with questions which have the pf in it

 

[Simonslimline]

Do 8000/(1.732 x 400 x 0.8)

 

[Chrisuk]

Do 8000/(1.732 x 400 x 0.8)

Is it still the same if it's a 230V question, I ask because the next question is 230V, 8500W, pf of 0.65

I now know to divide by 1.732 for 3 phase

But how to I with out this ,230V question

 

[Simonslimline]

I had my 309 exam today and lots of questions came up which we actually had not coverd much at all. I passed all the practice papers but the real exam was nothing like the practice papers we have been doing. Fingers crossed on that one I guess

 

[Chrisuk]

I had my 309 exam today and lots of questions came up which we actually had not coverd much at all. I passed all the practice papers but the real exam was nothing like the practice papers we have been doing. Fingers crossed on that one I guess

If the practice exams was nothing like the main exam then all you could of done was your best, I'm sure you will pass

 

[Simonslimline]

Is it still the same if it's a 230V question, I ask because the next question is 230V, 8500W, pf of 0.65

No dont worry about it with single phase, remember that with 3phase they are 120degrees apart so thats why roote 3 is used.

 

[Chrisuk]

No dont worry about it with single phase, remember that with 3phase they are 120degrees apart so thats why roote 3 is used.

I won't lie, only because u told me what to type in the calc is why I got it
So anytime is 400V I can remember to divide by 1.732 and times the rest

But how do I do the,230V question?

Thanks


Is it 8500/(230x0.65) = 56.85?

 

[Simonslimline]

sorry mate scap that I think I muddled that up a bit. now I cant remember if the answer is 24 or 56 . ill have a read through notes tomoz and post. I broke my tibia and fibula the other day and have been on painkillers for it which are making my head cloudy .

 

[Chrisuk]

If you are trying to get the amps for the other question then its 8500/230 x 0.65 .

is one of the answers 24Amps?

Finally found out how to use the calc in case I forget 1.732


Yes
A. 24.02A
B. 36.96A
C. 41.63A
D. 56.86


If I do 8500/230 x 0.65 = 24.02

But 8500/(230 x 0.65) = 56.86

Why are u saying use brackets on the 3 phase question but not on the 230V question?

We were told 56.86 is the correct answer

While u are here can u look at question 1, I've worked out 227V but college said it's 226V

 

[Simonslimline]

Right so Volts x Amps x power factor equals Watts.

So 230V x 56.86 Amps x 0.65 Power factor Equals 8500.57Watts