VFD used as a SS 4

[eengK]

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.

 

[waross]

If you are able to start the motor unloaded you may be able to use a smaller VFD. You may be able to bring it up to speed at less than full load current, it depends.
But wait for the VFD gurus.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

How do you then get off the VFD and on to the system? You can't do a closed transition and any kind of open transition risks an inrush transient larger than FLA.

 

[DickDV]

Two simple questions to size this drive. What is the highest continuous running amps required in normal operation? And, what are the maximum short-term amps required for acceleration or shockloading purposes? As for this second question, the real question is what is the TORQUE required for accel or shockloading? A motor on a VFD will require about the same % amps as % of nominal torque even in the overload range. For example, for 100% torque, that's 100% amps. For 130% torque, that's 130% amps.

Size the drive to supply both the continuous and shortterm amps and you will have the smallest system you can get away with.

 

[waross]

At these voltages and Hps a sync check relay and consultation with the VFD manufacturer may do the job.
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

 

[CH5OH]

I think it could work
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.

 

[eengK]

davidbeach,

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...

 

[jraef]

It might be worth noting at in MV drives, there are minimum sizes and size breaks so most likely, there will be little or no difference in a 4500HP drive vs a 3000HP drive. You would also need to duplicate the switchgear for this scheme, something that will likely cost more than what you might possibly save in the VFD.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

There is a solution that depends on the compressor/motor system you have now. If you have mechanical drive access and the compressor can be unloaded, you use a small LV motor and and drive to spin-up the big motor/compressor. Then using so phase logic you close DOL to the MV motor at the correct phase angle - no dip at all. Then turn off the LV drive control.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Reg the changeover to utility supply and the resultant inrush current, how about the motor is accelerated beyond the rated speed, say to synchronous speed, and then switch off the VFD and changeover to grid supply after a time lapse of, say, 5 seconds. How would that affect the inrush current, both its amplitude and the duration ?

Muthu

http://www.edison.co.in