Three phase and IBM

Peter Coghlan CCTECH at beyondthepale.ie
Sat Mar 19 13:57:40 CDT 2011


>
>> Check the power plug or cable.  If the equipment has a neutral (not 
>> ground!) lead, it needs wye.  Otherwise it needs delta.
>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. Y or D depends on the voltage the equipment needs per "leg"
>and the voltage the grid delivers. Remember voltage phase to phase is
>sqrt(3) times voltage phase to neutral.
>

Without documentation indicating that the equipment presents a balanced load,
it should not be assumed that it will. If the equipment power lead has a
neutral then it is probably not a completely balanced load and the neutral
will probably need to be connected. The consequences of not connecting the
neutral could be excessive voltages getting applied to some parts and reduced
voltages getting applied to other parts of the equipment. This is not good.

If the equipment does always present a balanced load, it may allow the option
of connecting it in wye (star) or delta format and the supply voltage required
would be different in each case.

To clarify the voltage specification, there are two different ways of
specifying the voltage in a three phase system. The "phase voltage" is the
voltage between any one of the three live conductors and the neutral conductor.
The "line voltage" is the voltage between any two of the three live conductors.
The line voltage is sqrt(3) times higher than the phase voltage. The line
voltage provided by the supply should match the line voltage required by the
equipment. The same applies to the phase voltage, although if one matches, the
other will also. Phase voltage doesn't really have a lot of meaning for a
system without a neutral and such a system should be specified by line voltage
only.

Regards,
Peter Coghlan.



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