Good day to all of You

I could use some help, and if anyone can provide me with some I would be very grateful.

I recently got a task in my company (piston and screw compressors factory from Bosnia) that includes developing their first rotary screw compressor running with a variable speed drive. Now, I've got the control all figured out, but there is a thing that's bugging me for almost a week. Does anybody have any expirience with installing VSD's on this type of machines? I'm interested what's the minimal overload capability needed to start the motor without tripping the overload error/alarm on the inverter. We have been given a quotation from OMRON distributer in Bosnia for their E7 series, and I would like to clarify on this:

In the datasheet there is a "120% In over 60s period" stated overload capacity. Now, somebody please correct me if I'm wrong, but as I figured it out this is closely connected with the starting torque the motor can develop (3-phase squirrel cage asynchronous motor btw). 120% In means approx. 120% nominal torque during start-up? Is this enough to get the motor going from locked-rotor state, and if not what happens: a ) motor does not start; b ) overcurrent error/alarm kicks in; c ) I'm in for some serious damage on the motor/drive? The compressor would start with all relief valves open etc., therefore as little load as we can set it up with. If this overload capacity is not "big" enough, does anybody know which one would be? I know the "bigger is better" philosophy would work in this case, but we're a small company from a small country and we have to minimize the costs while coming out with a reliable solution.

Thanks in advance.

Best regards, Miki.

Dear Mr. Miki,
Atlas Copco is manufacturing such type screw compressors using siemens drives. earlier it was Master drive now they have turned to the cheaper version of micromaster . but i hope any drive will be able to support your application only following things to be considered
1.

Hi VST.

Thanx for taking the time to read the post and send a reply, I appreciate it very much. If I understood your post correctly it seems to me you started listing the things I need to consider but you didn't finish the post. Please if you have any experience or knowledge in this area finish your post when you can take some time off your schedule. Thanks for the info on Atlas-Copco, I wasn't sure whose equipement are they using. I know for certain that MicroMaster 440 and Danfoss FC 302 series would get the job done, but I'm looking into the possibility of using a less expensive solution that suits our needs regarding the process control and speed regulation capabilities. And I'm still not sure what's the catch with the overload capacity, so any help you, or anybody else could provide on this would be appreciated.

Best regards, Miki.