Supply Network/vsd/motor Power Ratio

Hi guys.

I have a certain problem, and I was wondering if anyone could help me?

I need to select a proper VSD and a motor for a constant torque application. Since it would involve lowering the speed to approx. 25% (~750rpm, 12.5 Hz) I contacted our motors supplier to request a special motor with improved insulation&cooling, and he just gave me a headache.

The problem is he is trying to convince me to take a 132 kW motor instead of a 110 kW one we are using to run the same load in a constant speed application (star-delta or softstarter starting) because he is assuring me that I would have a smaller power output on the shaft when using a VSD instead of a classical starter. Now I know that in order to get my motor to reach its nominal power when using a VSD I would have to draw more power from the supply network meaning I would have to slightly oversize the VSD (nominal current of the VSD has to be bigger than the nominal current of the motor in order to avoid the VSD tripping due to an overload error, right?), but I can't get to understand why would I need a bigger motor in this case? If a VSD can substand a high enough current drawn from the network in order to supply the nominal current to the motor, I should have the same power output on the shaft as I would have if I was using a Star-Delta or a softstarter, right?

If anyone could clear this up a bit for me I would greatly appreciate it. Thanx in advance to all of You.

Best regards, Miki.

Vsd On Crew Compressors

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.

Choosing Contactors For Star-delta Starter

Hi guys, need a hand

I'm trying to find an optimal solution for starting a 90 kW three-phase asynchronous motor with rated current of 164 Amps. It would be a star-delta starter and I'm not sure on the following issue:

Is it necessary to use contactors rated for 90kW ( 180 Amps if I remember correctly)? This ammount of money is almost enough to get an adequate softstarter from Moeller electric. I seem to remember hearing somewhere that when the power circuit changes from star to delta, the two closed contactors don't draw the full rated current. If i remember correctly one would draw 0.58*164= around 90 Amps. Does this mean I could use contactors rated for, say 110 Amps without the fear of damaging them? If there is an online reference regarding this, I would appreciate it very much for someone to share it with me.

Thanx up front