How to get Matchcad 14 to calculate in VA instead of W?

[RasmusSkjøtt]

Hi

I just started in calculating on transformers, and i have the problem i cant get Mathcad to calculate in VA (volt*amp) I've searched the net sadly without any results.

I also have the same problem with a method to get the right results for VAR.

S = VA

P = W

Q = VAR

I've tried to use the attached document, but i still only gets it out in W (watts)

Hope someone can help me out, because i've soon given up.

Regards

Rasmus Skjøtt

[VladimirN]

Rasmus, hello!

Can you attach a Mathcad file?

Rasmus Skjøtt wrote:

...Hope someone can help me out, because i've soon given up...

Do not give up.

[RasmusSkjøtt]

Oki here goes a mathcad file

[wayne]

Select the unit, W, and type the unit you want, VAr

To see the predefined units that Mathcad has, select Insert<Unit and see the list. You can use these or one you define, as you did with VAr.
By default, Mathcad will use W, you will need to change each time.

[RichardJ]

Define VA:=W. Then click on the result, and there is a plceholder for units. Type in VA.

Incidentally, you don't need to define j as the square root of -1. It's built in. To get it, just type 1j rather than j.

[RasmusSkjøtt]

hmm oki thanks both of you.

I hoped there was a way to set up Mathcad so it is predefined so i dont have to change everytime since i'm gonna use this the next year.

Changed the help.xmcd. If there's a way to predefine VA / VAr units please let me know, because i read something about defining your own units in "Mathcad\Mathcad 14\units" something something.

[RichardJ · Accepted solution]

In this case the problem is not so simple as just finding a way to define VA as a Mathcad unit. VA and W are dimensionally the same, but they do not have the same meaning (as you are obviously aware), Mathcad cannot know which one you want in any given result. All Mathcad knows about is the dimensions of the unit, not the mathematical/scientific/engineering context.

[RasmusSkjøtt]

Well in some way they're the same yes, but:

S = Re(S) + Im(S) VA

P = Re(S) W

Q = Im(S) VAr

Example:

S = (3 + i5) VA

P = 3 W

Q = i5 VAr

So in my mind it depends if you got an imaginary unit in your formula or not?

I dont know if it's possible for Mathcad to know the difference tho?

[RichardJ]

You are still asking Mathcad to apply units based on the form of the mathematical expression. It has no such capability.

Your rules are also not universal. There are other fluctuating systems that transmit power with a phase lag (e.g. temperature), in which case VA may not apply at all.