VFD used as a SS
VFD used as a SS
(OP)
Hello,
Here is the situation, I am having a debate and am trying to determine if it is possible to use a smaller rated VFD then one that would typically be installed for a given load to be used solely as a soft-start for a large compressor.
The utility requires a small inrush current that must be around 100% of the FLA, so therefore a VFD would be required. (as I don't think there are any SS alone that meet this requirement)
An idea floating around is that a smaller VFD (when I say smaller I mean smaller then one that would typically be sized for controlling the large motor) could be used to soft-start a 4500hp 4160v Sales gas compressor. The reason behind thinking this way is due to the cost, as it is only needed for starting the compressor.
Now, I don't think that it is possible to use an undersized VFD, but there is some debate around that on it's own. Their logic is that a smaller VFD could be used during start up by going overloaded by say 150% for a short time thus acting as a soft starter.
My question: Is this a bunch of garbage? I believe that an appropriate sized VFD is required to ensure adequate break away current of the motor is supplied and I didn't think that an undersized VFD would be able to achieve this...
VFD's are not our area of expertise so any input would be appreciated.
Here is the situation, I am having a debate and am trying to determine if it is possible to use a smaller rated VFD then one that would typically be installed for a given load to be used solely as a soft-start for a large compressor.
The utility requires a small inrush current that must be around 100% of the FLA, so therefore a VFD would be required. (as I don't think there are any SS alone that meet this requirement)
An idea floating around is that a smaller VFD (when I say smaller I mean smaller then one that would typically be sized for controlling the large motor) could be used to soft-start a 4500hp 4160v Sales gas compressor. The reason behind thinking this way is due to the cost, as it is only needed for starting the compressor.
Now, I don't think that it is possible to use an undersized VFD, but there is some debate around that on it's own. Their logic is that a smaller VFD could be used during start up by going overloaded by say 150% for a short time thus acting as a soft starter.
My question: Is this a bunch of garbage? I believe that an appropriate sized VFD is required to ensure adequate break away current of the motor is supplied and I didn't think that an undersized VFD would be able to achieve this...
VFD's are not our area of expertise so any input would be appreciated.





RE: VFD used as a SS
But wait for the VFD gurus.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD used as a SS
RE: VFD used as a SS
Size the drive to supply both the continuous and shortterm amps and you will have the smallest system you can get away with.
RE: VFD used as a SS
Your point is valid David, and I overlooked it. My bad. You get the star. However for an installation this size the manufacturer may be willing to add a bypass circuit, or advise on a procedure to safely go to bypass.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD used as a SS
the only thing that needs to be accelerated is the mass of the rotor, since the compressor would be running below nominal speed, not much work is done by the impeller(s):
P1 / P2 = (n1 / n2)^3 (affinity law)
so if the VFD is switched off at its maximum power and the related rpm, the main contactor can then be swiched on, starting current will be limited, since the rotor would have already an initial speed.
RE: VFD used as a SS
That is an excellent point, I would have to say that there would be no way to transition away from the VFD that I could see that would prevent what we are trying to solve in the first place. Essentially they want a VFS (variable frequency starter) but that doesn't really exist AFAIK.
DickDV,
Agreed, seems like a properly sized VFD is really the only option. But then again, further consultation would be required with the manufacture to see if they have a solution.
For the most part right now, this is all talk. We know the HP, V, and compressor flows expected but not much else and we are tasked with basically "suggesting" a solution to optimize cost but our "suggestions" have to valid choices...
RE: VFD used as a SS
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RE: VFD used as a SS
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD used as a SS
Muthu
www.edison.co.in