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What Is The Best Initial %voltage Setting For Soft Starter


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hi all,


i'm new here, i am in referigeration industries which is use compressor as the heart of our system,

so my question is what is the best setting for inital start up % voltage for the screw compressor 250hp and 300hp. my customer ask me to chek the setting back if it can reduce their demand charge. what i know star connection is 58% so is it beter i put 58% , now my current setting is :


line voltage :415V

Ramp time : 10

initial %voltage is :70% default

curr limit level : 300%




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Hello Azham


Welcome to the forum.


The best start voltage for the soft starter, is dependent on the start torque required by the compressor, and the current required for the motor to develop that torque.

There are variations in start torque required, and large variations in the torque developed by the motor.

You can set by trial and error. If the start voltage is too low, the motor will take some time before it begins to rotate.

If the start voltage is too high, the start current will be higher than required and the motor will accelerate rapidly from rest.


From my experience, the start current for a screw compressor can range from 300% to 450% depending on the motor characteristics and the loading conditions of the compressor at start.


A 70% start voltage will cause a start current in the order of 450% which is higher than required in most cases.

I would suggest that you require an initial start voltage of around 50%.

The current limit at 300% is probably too low and I would increase this to 400%. If the compressor does not start easily at 40%, you may need to increase this to 450%.


Reducing the start current, will probably have no effect on the demand charge. Most demand charge meters are based on either a 15 minute time integral or a 30 minute time integral. The start time of the motor at around 10 seconds, is so short that it will hardly be seen by the meter.


Best regards,

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