Re: [OT] VA=W?

["Paul Robinson" <ukcueman@xxxxxxx>]

thanks Frank

----- Original Message -----
From: "Frank Mc Alinden" <fmcalind@xxxxxxx>
To: <ukha_d@xxxxxxx>
Sent: Saturday, May 24, 2003 12:39 AM
Subject: Re: [ukha_d] [OT] VA=W?


> Hi Paul
>             VA = W is true for dc ..For  ac from memory VA is the true
power and W is the rms i think?? forgotten my theory..its to do with the
power factor...The closer to unity power factor the closer you get to true
power..The power companies i would imagine would not like customers having
Watt meters as its not indicating true usage especially if inductive loads
are being used......
>
> > actually draws less than 50W on the 240V side. This >seems
counter-intuitive
> >to my primitive knowledge of electricity since I >would have
thought the
>
> Power in = Power out ( well almost not taking efficiency into account)
I
think he means it draws less current on the 240v side....
>
> If your using low voltage for all lites then devide total wattage of
lites
by 12 otherwise by 240.......
>
> HTH
>
> Frank
>
>   ----- Original Message -----
>   From: Paul Robinson
>   To: ukha_d@xxxxxxx
>   Sent: Saturday, May 24, 2003 9:12 AM
>   Subject: [ukha_d] [OT] VA=W?
>
>
>   This has been bugging me a little. It's probably trivially simple to
the
>   electronics people here. I didn't get a satisfactory answer from an
>   electrician I know...maybe somebody can enlighten me.
>
>   Transformers (eg for halogen lights) and uninterruptible power
supplies
are
>   both rated in terms of VA.
>
>   So what is the difference between VA and watts, because I'd always
assumed
>   they were the same. Obviously, they are both a measure of power. I
guess
the
>   answer lies in the fact that I'm talking about AC and the voltage is
>   expressed as an RMS term or something like that.
>
>   So why are transformers rated in terms of VA instead of W? And is
there
any
>   difference?
>
>   Can you work out the current drawn by many lights by adding up their
power
>   rating and dividing by 240? Or is it more complicated than that? If
some
of
>   the lights are low voltage halogens, does the calculation change in
any
way?
>
>   My sparky friend said something to the effect that a 50W 12V halogen
>   actually draws less than 50W on the 240V side. This seems
counter-intuitive
>   to my primitive knowledge of electricity since I would have thought
the
>   power used on the low voltage side would have to equal the power
used on
the
>   240V side plus the power given off as heat (and sound) by the
transformer.
>
>   Can somebody put me out of my misery please?
>
>   Thanks,
>   Paul