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TD Series Digital Solid State Soft Starter 48 - 1250A
Appendix 2: Decel Mode Application Considerations
Deceleration (Soft Stop) is a unique feature of Solid State Soft Starters such as
the TD. It provides a slow decrease in the output voltage, accomplishing a
gentle decrease in motor torque during the stopping mode. This is the
OPPOSITE OF BRAKING in that it will take longer to come to a stop than if
the starter were just tuned off. The primary use of this function is with centrifugal
pumps as a means to reduce the sudden changes in pressure that are
associated with “Water Hammer” and slamming of check valves. Decel control
in pump applications is often referred to as Pump Control.
In a pump system, liquid is being pushed uphill. The force exerted by gravity
on the column of liquid as it goes up hill is called the “Head Pressure” in the
system. The pump is sized to provide enough Output Pressure to overcome
the Head Pressure and move the fluid up the pipe. When the pump is turned
off, the Output Pressure rapidly drops to zero and the Head Pressure takes
over to send the fluid back down the hill. A “Check Valve” is used somewhere
in the system to prevent this (if necessary) by only allowing the liquid to flow in
one direction. The kinetic energy in that moving fluid is suddenly trapped when
the valve slams closed. Since fluids can’t compress, that energy is transformed
into a “Shock Wave” that travels through the piping system looking for an
outlet in which to dissipate. The sound of that shock wave is referred to as
“Water Hammer”. The energy in that shock wave can be extremely damaging
to pipes, fittings, flanges, seals and mounting systems.
By using the Soft Stop deceleration feature of the TD, the pump output torque
is gradually and gently reduced, which slowly reduces the pressure in the
pipe. When the Output Pressure is just slightly lower than the Head Pressure,
the flow slowly reverses and closes the Check Valve. By this time there is very
little energy left in the moving fluid and the Shock Wave is avoided. When the
output voltage to the motor is low enough to no longer be needed, the TD will
end the Decel cycle and turn itself off.
Another common application is on material handling conveyors as a means to
prevent sudden stops that may cause products to fall over or to bump into one
another. In overhead crane applications, Soft Stopping of the Bridge or Trolley
can prevent loads from beginning to over swing on sudden stops.
Volts
100%
60%
30%
BDL
Begin Decel Level
F026
Stop Command
from control
system
Decel Ramp Time
F028
Decel Shut-Off
Level
F027
Output to Motor
PUMP FLOW
BACK FLOW
SHOCK WAVE
NO SHOCK WAVE
NEUTRAL
PRESSURE
PUMP
ON
PUMP
OFF
PUMP
OFF
PUMP
OFF
PUMP
SOFT
STOP
Check Valve
Open
Check Valve
Slams
Check Valve
Closed
Check Valve
Closes Slowly
Check Valve
Closed
Pump Control Soft Stop with RVSS
Coasting Stop
(using Electro-Mech. starter)