Inrush Current After Soft Starter

http://lmgtfy.com/?q=max.+inrush+current+after+the+soft+starter+in+%25

thanks for the googling but unfortunately it's not work

Technically, "Inrush current" is the current to establish the magnetic fields in the motor windings and is very brief. While a soft starter will also have an effect on that as well, it's minimal because it takes about as long for a soft starter to react and control the phase angle as the inrush current lasts anyway, i.e. 1-2 cycles.

Most likely you are asking about the "Starting Current", what is typically 600% of FLC until the motor reaches 90% speed or so. If so, then that is exactly what a soft starter is going to do for you, control the starting current. In that endeavor, there are multiple ways to accomplish the task and without knowing what you are going to use for a soft starter or how you are going to program it, there is no way to predict the starting current level. Starting current can be directly limited (called "Current Limit starting") with a soft starter. It can also be indirectly reduced via a "Voltage Ramp" starting method that emphasizes the smooth acceleration over current limiting, and in some cases you can have linear acceleration of speed or torque, which also emphasizes that linearity over current limiting.

In all cases though, the peak starting current will depend on the load profile, the starting method just deals with how you get there. So as a gross generalization, gas compressors typically will need somewhere between 350 and 400% FLC at some point in the acceleration cycle. Sometimes you can get away with less, but then the acceleration time becomes extended and it's always a game of finesse to accelerate with as little current as possible without exceeding the thermal capacity of the motor (as defined by the overload protection curve). In other words you can set the current limit as low as you like, but that does not mean the motor will finish accelerating with that amount of current. My 350-400% value comes from experience.

But because different generators have different peak loading capabilities, it's also difficult to predict what you will need. Again as a gross generalization, the rule I have used is 2.5-3X the motor kW as generator kVA capacity. So for you, 3750kVA of generator capacity may be the ticket. But to do it right, you should take all of your known data and input it into a Transient Motor Starting Analysis program such as SKM or ETAP. This is serious enough and expensive enough to warrant the expense in my opinion.

Sir based on your experience are you trying to say that starting current for gas compressor only reduced 200% by using soft starter ? is there any device rather than soft starter can reduced it up to FLC level ?

secondly what is the difference between starting current and inrush current ? as far as I know both of them are the same, they happen while acceleration stage.

The only two ways of accelerating with FLC is going to be an LCI or a VFD, both of which are going tpo be very very expensive at 6.6kV.

I just explained to you the difference between Inrush and starting Current.