Re: VA vs. Watts (was Calculating VA)

• To: tesla-at-pupman-dot-com
• Subject: Re: VA vs. Watts (was Calculating VA)
• From: Ed Phillips <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Mon, 01 Nov 1999 18:19:09 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

"For most the term V*A is interchangeable with Watts. However,
as far as I know VA is used for devices which do not *consume*
power (like a transformer), whilst Watts are used for devices
that *consume* power (like a power drill)."

	Not correct.  Volt amperes are just that: the product of the voltage
and the current flowing into (or out of) a circuit.  Watts are the
"real" or heating power of the circuit, and are the product of the
voltage and the "real" or in-phase component of the current.  Watts are
the product of the voltage times the current times the cosine of the
phase angle between them.  The "power factor" of a circuit is the cosine
of the angle between voltage and the current if expressed in per unit,
or 100 times that if expressed in percent.

	Example:  An induction motor can have a power factor of the order of
70%.  (It is an inductive load.) If the voltage is 100 and the current
10 amperes, the motor would be "drawing" 1000 VA, but the real power
would be 10 times 100 times 70/100, or 700 watts. 

Ed

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