ld
Got that part of it and that is all good for the typical NEMA starter. You give up a bit of run protection for the motor to start.
The softstart has a much better, I hope, process for determining overload at run. Is the class just used at startup or is it again used in calculations for run protection?
Tom,
The Soft Starter OL protection curve is for the
Running condition. It may also be for starting, but it is perfectly acceptable (legal) to completely bypass the OL in startup, in fact it is necessary in some cases like centrifuges. I'm not saying it is that way or should be, I'm just pointing out that when we use the term "Over Load Protection", we are ALWAYS referring to "
Running Over Load Protection" whether stated or not.
If it is a submersible pump or an IEC motor (power rating shown in kW is a clue), use Class 10
If it is a NEMA design motor, use Class 20
Class 30 is almost never used, it was for what were called "Mill Duty" motors which, because of efficiency standards, have all but ceased to exist. Some soft starter mfrs allow you to program 2 separate OL curves; one for Starting and one for Running. Motortronics, Toshiba, ABB, Benshaw and a few others are like that, don't know about WEG. In that case, I would often use Class 30 at start-up, then the SS will automatically change to Class 20 when it determines start-up is complete and the motor is running full voltage. That was a handy feature but the starter has to be bilt to take it and many are not.
But another caution, and again I don't know the WEG starter intimately, is that with some soft starters, selecting a different OL class LOWERS the rating of the soft starter. In other words it may say it is a 250HP starter if Class 10 OL is selected, but if you change the OL to class 20, it becomes a 200HP starter and skews the FLC parameters internally so that yo may then find yourself unable to set the motor FLC high enough for your motor. The Cutler-Hammer IT starters are like that, as are the Siemens 3RW40 and 44
