Adding seperate KVA's

bassey

Doing phase 6 and wondering if anyone can help me with this question.

I know you can't just sum the seperate KVA values but I can't remember how to add them.

Any help appreciated.

Bruthal

The 35kw at unity would be 35kva in its kva column rather than 0 anyway

sparcocars

As bruthal said, at unity KW = KVA. All your answers were fine just that the KVA total would be 75.

2011

bassey wrote: »

Doing phase 6 and wondering if anyone can help me with this question.

I know you can't just sum the seperate KVA values but I can't remember how to add them.

Any help appreciated.

I don't agree with your answer.

This is the way I see it:

sparcocars wrote: »

As bruthal said, at unity KW = KVA. All your answers were fine just that the KVA total would be 75.

The total KVA is the phasor sum (taking into account magnitude & direction).
They cannot be simply added as they are at different angles.

.G.

I did phase 6 two years ago.I can't remember any of this stuff!

I know this isn't a helpful post but it scares me,I feel like Homer Simpson,everytime I learn something new it pushes some old stuff out of my brain:D

Bruthal

2011 wrote: »

bassey wrote: »

Doing phase 6 and wondering if anyone can help me with this question.

I know you can't just sum the seperate KVA values but I can't remember how to add them.

Any help appreciated.

I don't agree with your answer.

He didnt really give one though, he said he knows you dont directly add the KVAs.

This is the way I see it:

Thats the way it is probably.

Probably be clearer if just have total kw (67) by straightforward addition on horizontal of triangle. Total KVAr (24) on the vertical. Pythagoras to get the total KVA on the sloped side = 71.17 i think anyway. But im getting old now:)

Think I need pen and paper though looking again:

Bruthal

superg wrote: »

I did phase 6 two years ago.I can't remember any of this stuff!

I know this isn't a helpful post but it scares me,I feel like Homer Simpson,everytime I learn something new it pushes some old stuff out of my brain:D

Wait till ye get to my age. I used to be able to do this stuff in the head. Now i struggle with the calculator:D

2011

Bruthal wrote: »

He didnt really give one though, he said he knows you dont directly add the KVAs.

He did.
Look at the 2nd attached jpg in the OP

Bruthal

2011 wrote: »

He did.
Look at the 2nd attached jpg in the OP

What was wrong with it?

2011

Bruthal wrote: »

What was wrong with it?

35KW @ unity = 0KVA is incorrect

it should read
35KW @ unity = 35KVA

2011

I also see that I have mixed up the 35KVA load with the 40KVA load in my diagram. I will pretend that this is a deliberate mistake to see if everyone is awake

Bruthal

2011 wrote: »

35KW @ unity = 0KVA is incorrect

it should read
35KW @ unity = 35KVA

O right

2011

Bruthal wrote: »

O right

Our posts crossed.
I was doing my pretty diagram when you posted that

Bruthal

2011 wrote: »

Our posts crossed.
I was doing my pretty diagram when you posted that

Its nice alright. I was looking at it going, hmm, something strange there:)

Bruthal

Anyway in answer (I think) to BASSEY

You have the 40kva at 0.8pf

So thats 40kva and 32kw and 24 KVAr

35KW at unity is 35KVA 35KW 0 KVAr


Total kw is simply 32+35 = 67 as you had.
total kvar = 24

As per diagram like 2011 had, and drawn below....

So pythagoras

67² + 24² = 5065
√5065 = 71.17 = total KVA

bassey

Bruthal wrote: »

Anyway in answer (I think) to BASSEY

You have the 40kva at 0.8pf

So thats 40kva and 32kw and 24 KVAr

35KW at unity is 35KVA 35KW 0 KVAr


Total kw is simply 32+35 = 67 as you had.
total kvar = 24

As per diagram like 2011 had, and drawn below....

So pythagoras

67² + 24² = 5065
√5065 = 71.17 = total KVA

Just checked the model answer, bang on Bruthal!

Edit: also sick at meself for not redoing the triangle with the angles, ah sure had been at it for a few hours

Thanks for all the help everyone

Bruthal

bassey wrote: »

Edit: also sick at meself for not redoing the triangle with the angles, ah sure had been at it for a few hours

Well it wont have been a waste of time anyway.

cast_iron

bassey wrote: »

Edit: also sick at meself for not redoing the triangle with the angles, ah sure had been at it for a few hours

Might be handy to note that having a calculator that can add complex numbers will allow you to add different magnitudes and different phase angles without the need to use triangles and the like. Though the triangles can be a good visual aid if people are a bit unsure on things.

2011

cast_iron wrote: »

Might be handy to note that having a calculator that can add complex numbers will allow you to add different magnitudes and different phase angles without the need to use triangles and the like.

You are correct.
However I don't think that apprentices are taught to use complex numbers to solve these types of problems (at least they were not when I served my time).

Perhaps arse..biscuits can confirm?

cast_iron

Indeed. Can be a useful check if you are proficient with a calculator all the same. Admittedly, like Bruthal, I struggle a bit to get the hang of using one. But it's a doddle once you master it.

bassey

Nah they don't use the imaginary numbers method with apprentices, I usually double check with it though alright to make sure angles are correct