Toshiba 18 - 1250 A - Manual (Page 76)

TE Series Digital Solid State Soft Starters 18 – 1250A
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Appendix 2: Pump-Flex
®
Decel Mode Application Considerations
Pump-Flex
®
Deceleration (Soft Stop) is a unique feature of Toshiba
Solid State Soft Starters. 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 turned 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.
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.
SOLUTION:
By using the Pump-Flex
®
Decel feature of the TE Series, the pump
output torque is gradually and gently reduced, which slowly reduces the
pressure in the pipe. When the Pump Output Pressure is just slightly
lower than the System 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 TE Series will
end the Pump-Flex
®
Decel cycle and turn itself off.
CAUTION
Decel is THE OPPOSITE of braking. Enabling the Decel
feature will make the motor take LONGER to stop than if it
were simply turned off.
Pump On
Check Valve
Open
Pump Off
Check Valve
Slams
Pump
Soft Stop
Check Valve
Closes Slowly
Neutralized Flow
Pump Off
Check Valve
Closed
((( SHOCK WAVE )))
Coast-to-Stop
(using Electro-M ech. starter)
Pump-Flex ®
Soft Stop
(using VMX Series starter)
Pump Off
Check Valve
Closed
NO SHOCK WAVE
Pump Flow
Back Flow