Re: Input power measurement

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 3/10/03 8:28:39 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:


>Now the question becomes...how does one measure the actual (RMS ?) input
>current in order to calculate the wall plug watts? Must one purchase or
>borrow a multi-buck wattmeter?
>
>Again thanks in advance for any help.
>


Skip,

The only ways I know of to measure the true watts without
using a wattmeter is to use a power factor meter along with
an ammeter and voltmeter.  The power factor can be measured with
a cheap dual trace scope if the waveforms are not distorted.
However the current waveform usually *is* distorted.  Measuring
the current using an RMS ammeter won't help, because
the ammeter still won't tell you anything about the power factor.

Nice old rather accurate wattmeters can be often found at
hamfests for $10 to $20 dollars.  I use a Weston model 310.
Dave Sharpe has completed his design for a low-cost electronic
wattmeter also.

John