When is it worth the Cost to use a Soft Starter?

[kwired]

Wouldn't a soft start be a good idea where high power factors are a problem.
I can understand not using them on part winding starts. I would think a PW would have problems.
There was a young hvac tech that burned out a compressor that was part winding start because he held in a contactor.

I don't know what you may be getting at with high power factors. AC induction motors naturally have low power factor for one thing. Maybe you meant demand factor? Motor starting doesn't last long enough to be a problem with typical demand factor charges other than maybe for some cases with a high inertia load that takes a very long time to accelerate.

Part winding starting is nothing more than one possible equivalent of reduced voltage starting. Not exactly reduced voltage but you lessen the surge current by only energizing part of the windings for a brief time, once the surge current has dropped off and hopefully you have some rotation in the motor, energizing the second part of the winding won't have such a high surge effect either. Placing soft starter in line with PW controls is rather pointless when the soft starter alone would provide overall better control. The young tech likely held in part of the winding which accelerated the motor, maybe or maybe not to full speed depending on loading conditions and the second winding never was energized leaving the motor trying to drive the entire load on only part of the windings. The delay between part and full windings on normal startup is like only a couple seconds at the most but often much less than that, usually just a bang bang between the steps.

 

[a.bisnath]

Soft starters can help a lot with long conveyer belts as loaded belts will require very high starting currents that will lead to belt bursting, all depend on the need, especially where reduced voltage starting is required. Elevators for materials are another example , but then a momentary full voltage kickstart feature may be need to overcome initial high starting torque. Over the years they have reduced in size and cost. Bypass contactor is a good idea.
Depending on geographical location however it may be easier and faster to obtain electromechanical contactors than wait a long lead time
to get a replacement soft starter. I remember really large Westinghouse soft starters from 30 years ago now being replaced with much smaller ones
with a lot more features. one important thing to note is properly programming and setting the parameters of the soft starter as this may cause the starter to trip or stop when there no real motor or load fault

 

[Jraef]

Sounds similar to where I retired from. They had 4 roof top cooling towers with 75 HP motors. They changed a pulley on one tower to have it run at least 25% faster. Destroyed the gear box .

I have seen that way too many times. People mess with mechanical designs without understanding the physics. 25% faster on a fan results in the load increasing by the CUBE of the change in speed. So 1.253 is 195%, so that 75HP motor was trying to drive almost a 150HP load…

a.bisnath

one important thing to note is properly programming and setting the parameters of the soft starter as this may cause the starter to trip or stop when there no real motor or load fault

Yes, many digital soft starters (which is almost all of them now) will have a minimum load requirement of around 20-30% of their maximum rating, triggered after the ramp time has expired. An unloaded motor often can fall that low. Some allow you to disable that, others do not.