Short Circuit Model Question
Short Circuit Model Question
(OP)
For those of you that do short circuit studies, I have the following. What has been your experience with the accepted convention for assuming motor contribution when specific motor data is unavailable?
Is it common to assume a value (say 25% of the main switchgear size) and put a lumped motor model at the main switchgear. This is more simplistic than a motor model at each panel, but for small systems may lead to similar results.
I always try to model large motors based on actual data, but for a planning study, that data is not present.
Thanks
Is it common to assume a value (say 25% of the main switchgear size) and put a lumped motor model at the main switchgear. This is more simplistic than a motor model at each panel, but for small systems may lead to similar results.
I always try to model large motors based on actual data, but for a planning study, that data is not present.
Thanks






RE: Short Circuit Model Question
RE: Short Circuit Model Question
RE: Short Circuit Model Question
RE: Short Circuit Model Question
But if you lump motors below 50 hp in one group and 50 hp and above in another group, this will comply with ANSI requirements, provided you apply the correct reactances to each group.
If you are doing short circuit studies, I would recommend purchasing Conrad St. Pierre's "A Practical Guide to Short Circuit Calculations".
RE: Short Circuit Model Question
RE: Short Circuit Model Question
Take a realistic look at the motors which can operate at a any given time, ignore standby units and lumping smalller motors.
If using VSD's I tend to use Xd"=1 (thyristor commutation failure ocurrs resulting in negligible regen). Otherwise Xd" varies between approx 0.2 for larger motors and 0.167 for smaller units using DOL or assisted start.