Three phase and IBM
eric at brouhaha.com
Sat Mar 19 18:08:07 CDT 2011
Jochen Kunz wrote:
> No. If you have a symetrical load like a motor you can wire it Y
> without connecting neutral. So the presence of netral is no indication
> of Y or D.
If the power plug or cable has a neutral lead, you cannot assume that it
doesn't need a neutral connection. For instance, a disk pack drive may
use three-phase power for a motor, but you can't assume on that basis
that a delta connection is suitable. It is not uncommon for there also
to be a logic power supply with input wired from a phase to neutral.
Note that neutral is NOT the same thing as ground. Neutral is a
current-carrying lead. While neutral is connected to ground at one
place (usually at the main breaker panel), at ANY other place in the
local power network neutral may be at a significantly different
potential (voltage) than ground, due to current causing resistive losses.
Ground should never carry any significant current; if it does, there is
a fault. The allowable current is varies by jurisdiction and type of
equipment. In the US, for Class I (electrically insulated with
protective earth ground) information technology equipment, the maximum
ground leakage current is 0.75 mA for handheld devices, and 3.5 mA for
Because the ground conductor is not supposed to carry any significant
current, it should be at nearly the same potential (voltage) everywhere
in the local power network.
> Y or D depends on the voltage the equipment needs per "leg"
Not necessarily. Voltage requirements should be determined by the
specifications for the equipment, which ideally are printed on it somewhere.
Here in the US, the most common commercial three-phase power
* 120/208 wye - 120V RMS phase-to-neutral, 208V RMS phase-to-phase
* 208 delta - 208V RMS phase-to-phase
Notice that the phase-to-phase voltage is identical, so if something is
marked as needing 208VAC 3-phase, that tells you NOTHING about whether
it needs delta or wye.
There are certainly other 3-phase voltages used in the US. Probably the
next most common are 277/480 wye or 480 delta.
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