AC Drives and Soft Starter (8) - Finish

Sample Applications

Provided here are four sample applications. Two will be for pumps, and two will be for conveyors. These examples do not require variable speed or precise speed regulation, so a VFD or soft starter could be used.

Application 1) A pump is being started on full voltage. There is significant water hammer and the pipe bracing needs constant maintenance.
Answer: A soft starter will fit the application. It provides controlled torque during acceleration and has been shown to minimize and in many cases eliminate water hammer. There is no concern about current limitations as the application is now being started on full voltage.

Application 2) A new irrigation pump is being installed in a rural location. Because of this, the maximum current draw from the utility line without significant voltage drop has been calculated as 200 percent of the motor nameplate reading.
Answer: An inverter is preferred over a soft starter. In some instances soft starters can accelerate pumps with as little as 200 percent current. Application experience indicates that more often 250 – 300 percent current is required. The VFD can provide the torque required to accelerate the pump within the current limit restrictions of the distribution system.

Application 3) An overland conveyor requires 100 percent torque to accelerate when starting fully loaded. The maximum current draw from the utility is limited to 500 percent of the motor full load amperes. The conveyor will normally be started unloaded; however, on occasion it may need to be started when it is loaded. Rate of acceleration is critical to prevent the conveyor belt from being damaged

Answer: Initially a soft starter seems to be the correct choice. The soft starter can provide 101 percent torque with 450 percent current (table 1). However the rate of acceleration, which equates to starting time is critical. The load also varies from unloaded to fully loaded. In this case a VFD would be the correct solution.

Application 4) A 20 horsepower motor drives an overhead plastic chain conveyor through a gearbox. It starts and stops frequently. Full voltage starting could be used, but if the conveyor starts too quickly the product will swing and may be damaged or the chain may break.
Answer: A soft starter would fit the application. There is no time constraint and no limitation on current. Ramp start would typically be used to allow for minor load variations reflected back to the motor. If the gear reduction is high enough, a current limit start could provide a smoother start.

Conclusion
These examples were designed to show how slight application variations can change the type of motor starting that is required. Each application must be evaluated on its on merits. Neither soft starters nor VFD’s are the perfect solution for all situations.