Page B–6 Stellar
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SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix B: Soft Starter Application Considerations
b.2.7 – sofT sTopping
Soft stopping can reduce positive surge pressures in pipelines on shutdown. It is necessary to
make sure that the ramp-down time is long enough to remove the energy from the fluid before
the firing of the thyristors is stopped. Otherwise the surge pressure may still be present.
Soft stopping can also be successfully applied to loads such as conveyor belt systems where
sensitive items such as bottles are being transported.
b.2.9 – replaCemenT of fluid Couplings
Soft-starters can replace fluid couplings yielding benefits of higher efficiency running and
lower costs to the user. If the coupling is used to magnify the available breakaway torque, it
may be necessary to replace the installed motor with another of a larger size, or one with a
high starting torque characteristic before a soft starter can be employed.
b.2.12 – overhauling loads
Certain applications can overspeed the motor as part of normal operation. Power then flows
from the motor to the power supply. It is important to disable the iERS optimizing mode
during over-speed conditions, and reinstate the optimizing mode during normal conditions.
(External control or communication is required to disable and reinstate iERS.)
b.2.13 – appliCaTion Table
The table on the following page shows many common motor applications that suit the SR55
soft starter. It lists typical breakaway torque requirements as a percentage of motor full-load
torque (FLT). For the most satisfactory soft starter in a given application, the motor should
have a full-voltage locked-rotor-torque (LRT) that is at least twice the breakaway torque
(e.g. for a reciprocating compressor the FLT is normally in the region of 50% motor LRT.) As
a general rule, the higher the motor LRT is above the load breakaway torque, the greater the
control over the starting process.
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