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Softstarter Selection


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#1 anu_rags

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Posted 12 September 2008 - 10:53 AM

Dear All,

I have an application of Reciprocating Gas compressor installed at a CNG gas filling station. Earlier we have installed soft starter on similar application and achieved starting current less than 200%. For my present application, customer has an existing Genset of 365kva installed. The details of genset are as below:

Alternator voltage: 415V
Alternator effi: 80%
Engine overload capacity: 110% for 1 hour every 24 hours
Alternator overload capacity: 110% for 1 hour every 24 hours

We have 2 3 phase motors of 160kw, 415V, 278A rated connected on this genset. The motors are presently started with Star Delta Starter. Inrush current kick goes upto 700-750A when the motor is started. Normal running current is around 200A. When the 1st motor is running, quite often it gets difficult to start the 2nd motor on the genset. We want to determine if by connecting the softstarter, we will be able to get the 2nd motor started while the 1st motor is already running.

If not by softstarter, is it possible by installing VFD on one of the motor.

Regards,
Anurag

#2 marke

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Posted 14 September 2008 - 06:40 PM

Hello Anurag

Looking at the figures that you have quoted, I would be surprised if you can run both motors on that generator.
You quote 160KW motors. If we allow for a motor efficiency of 91% and a power factor of 0.85, then the KVA required for each motor is 206 KVA. To run both motors under load, would require 412KVA which is higher than the genset rating!!

Your start current of less than 200% on a similar machine, is highly unusual. Normally I would expect to see 300% - 400% start current for this load if well offloaded and god start torque motor.

To calculate the starting capability of a genset, you need to know the 15 - 30 second overload capacity of the engine and the alternator.

Have a good day,
Best regards,
Mark.




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