Power for Iris 220S

[Art Hays - PSTAFF]

"Robert G. Brown" <uunet!phy.duke.edu!rgb> writes:

> This wiring uses two out of three legs of a "three phase 220"
> circuit.  This is typical output of a "Wye" transformer and is common
> in Universities and offices.  Again, there may or may not be a current
> carrying neutral allowing it to be split into three 120V lines.  The
> potential difference is:
> 120V sin(wt) - 120V sin(wt + 2Pi/3) = 207.8V sin(wt + Pi/3)

	While on the subject of power, another interesting
topic is neutral heating due to switching power supplies.
The typical three phase wye transformer has the same guage wire for
the phases and neutral.  The assumption is that if the loads on
all the phases are equal, there will be very little current flow in the
neutral.  Panels have all three phases in them, and the breakers alternate
which phase they connect to.  

	The current waveform of a switching power supply is far from
a sine wave.  It draws current in brief periods near the peak of the
voltage waveform.  For various reasons (which I dont claim to fully understand)
this creates currents in the neutral of the three phase wye.  I believe
this problem is being addressed in the electrical codes now.  There are
various derating factors to apply in calculating loads on the distribution
transformer to prevent neutral heating when much of the load is
from switching power supplies.

	Even measuring load isnt easy, as I recently discovered.  I wanted
to measure our current load in the computer room to buy a UPS.  The normal
clamp on ammeter isnt sufficient.  Inexpensive ones read out in RMS current,
but measure AVERAGE current.  They assume the current waveform is a sine
wave, and apply the factor to convert average to rms.  If the current
waveform is not a sine wave (such as with a switching power supply) this
type of meter will read low.  One must use a true rms meter with a
wide frequency response (I used a Fluke true rms handheld with their
widest freq. response current clamp.  Amprobe has a computerized meter
that works also).  What I measured in my computer room was:

	phase 1: 21 amps
	phase 2: 17.3 amps
	phase 3: 17.0 amps
	neutral: 30 amps

	Note how the neutral is carrying much more than would be expected
from the phase imbalances.  An average reading meter would have been off by
more than a factor of 2.

Art Hays, Nat. Eye Institute,			uunet!lsr-vax!art
Nat. Institutes of Health, Bethesda, MD		(301) 496-7143