> I have someone describing an opportunity to me that requires an
> understanding of what he calls 'watless current' in a three-phase line
> connected to a delta transformer primary with SCR switches on each of the
> three lines.
> Apparently, when there are no loads connected to the secondary, current is
> measured on each of the primary legs.
> The same current exists with inverse parallel SCRS's as with SCR's paired
> with inverse rectifiers.
> I thought that as long as the delta primary load is balanced that there is
> supposed to be no current between phases. I do however expect there to be
> current on each line with the SCR/Diode pair configuration. Why would there
> be current flowing with a 6 SCR configuration?
> Is there such a thing as watless current, is this what I am seeing, or is my
> leg being pulled?

> TIA,
> Chris

        This is probably a reference to "reactive" current which is 90 degrees out
      of phase with the voltage and therefore does not draw power from the source.
      If you connect an inductor across an AC source, there will be current flow
      given by I = V / (XL), where XL is the inductive reactance of the inductor. This
      reactance is XL = 2 * PI * F * L, with F = frequency, L = inductance,
      PI = 3.14159. If the voltage is a cosine, the current will be a sine. If the product
      of a sine and cosine is integrated over one cycle, the result is zero, showing
      that the total energy transferred in that cycle is zero. (Energy flows from the
      source for one half-cycle, and back to the source in the second half-cycle.)
      In your case, the inductance is that of the transformer primary. Even though
      there is no average power transferred to the inductor, the current which does
      flow must be taken into account for two reasons.

      The first concerns the current capacity of the source, and the second involves
      the switching behavior of SCRs when the voltage and current are not in
      phase. When using SCRs with inductive loads, you must use "snubber
      networks" which allow the SCR to turn off at the current zero.

      John Power

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