dear professionals;
we have a 2000 ton centrifugal chiller, the supply voltage is 6600 V, three phase star start. the FLA=145 A, the Inrush current is 823 A, LRA=823A. my doubt is how much inrush current it will take when the motor is decoupled, and after the initial inrush duration is passed then how much current the motor will take in decoupled state.
thanks and regards.
"Inrush" is the current that flows for a fraction of a second when a motor winding is first energized and is not yet a magnet, so the current is only limited by the wire resistance. Inrush is typically 1000-2000 % of nameplate FLC dending on a host of factors, none of which have anything whatsoever to do with the connected motor load.
The 500-600% current you might be thinking of that involves the load is called "Starting Current", not Inrush, although many many people use the term incorrectly. Technically, starting current on a motor started Across-The-Line (DOL to you) will also be the same regardless of whether or not the load is coupled to the motor. However what changes is the DURATION of the starting current, because it remains at that high level until the motor reaches about 90% speed. So with the load uncoupled, that may happen in 1 second, but with a centrifugal chiller it will take longer. How long depends on the mass of the chiller impeller, but also the fact that in a centrifugal machine, load increases with speed. So as it accelerates, more and more of the load (flow) is getting coupled to the motor, which increases the torque demand. Depending on how the motor was sized, that can extend the starting time.
Which comes back to the comment by StarCat, only the manufacturer of the machine can accurately tell you that because they chose the motor and know the load torque requirement profile. However they will not know what effect the power system you are connecting to will have on this as well. Voltage drop from starting a large load will also affect the start time.
Sophisticated Transient Motor Starting Analysis software, like ETAP or SKM, have typical load profile models for common machines like Chillers, they can provide you with a fairly accurate picture of what to expect. But those software packages are not inexpensive, it's usually better to find a professional engineer who already has it and just pay them to run the numbers for you.
Motor current draw in an uncoupled state is almost completely meaningless and cannot be predicted beyond about +- 40%, meaning it could range anywhere from 20% to 60% of motor FLC. But that too is meaningless because it is mostly at a very low power factor. It's only value is in recording it over time to trend any gross changes in it to use in determining the health of things like bearings and winding insulation leakage.
