VMX
2
Series Digital Solid State Soft Starter User Manual
74 |
P a g e
Appendix 2: Pump-Flex
Decel Mode Application Considerations
Pump-Flex
Deceleration (Soft Stop) is a unique feature of Motortronics
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 tuned off.
The primary use of this function is with centrifugal pumps as a means to
reduce the sudden changes in pressure that are assoc
iated 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 uphill is ca
lled the “Head
Pressure”. The pump is sized to provide enough Output Pressure to
overcome the Head Pressure and move the fluid up the hill. 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.
SOLUTION:
By using the Pump-Flex
Deceleration feature of the
VMX2 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
VMX2 Series
will end the Pump-Flex
Deceleration cycle and turn
itself off.
Caution!
Deceleration is THE OPPOSITE of braking. Enabling the
Deceleration feature will make the motor take LONGER to
stop than if it were simply turned off.
