We are undertaking the replacement of DOL conveyor starters with soft starters - 30kW 55A motors direct coupled to grain conveyors.
The starters are oversized AuCom EMX3s - nominal current 105A and thermally they have a nominal FLC of 60A at 50 degrees C at a duty of 4.5xFLC for 30s with 570s off time (105A:AC53b 4.5-30:570)
The technical risk I am trying to manage, and it comes up all the time in DOL to RVSS projects, is the potential for the soft starter to not be able to produce enough starting torque to get the loaded conveyor away.
I am not versed at all with conveyor calculations and I am not confident (without a very clear how-to) of doing conveyor calculations to determine the minimum breakaway torque required and relating this back to the motor's reduced voltage torque characteristic. Also the project does not have the budget for me to school myself on these calcs. Dont get me wrong I would love to learn how to do these calcs accurately but it just doesn't fit into the bounds of this project budget or timing.
So the current limit on this starter is 600% of FLA, but the starter has an instantaneous trip of 6x nominal rating = 600A. It seems the only thing I can do is to design this system to be able to do as close to a DOL start as I can - as I know the conveyors get away fine at the moment with a DOL start.
So, if I wanted to get the starter to do 700% or 800% FLA on a current limit start, 800% would be 440A which is under the starter's instantaneous trip level of 600A. However, this would require me to 'fake' the motor FLA setting in the soft starter, which would adversely affect the motor and cable protection. This could be fixed by installing an external overload relay.
So really the question I have to answer is can I justify the installation of an external overload relay on this starter.
Another option that has been suggested is to install a maintenance-only switch in the MCC which would activate secondary motor parameters sized to provide a DOL start - for clearing blockages.