Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 102)

100%
Percent
Voltage
Initial
Torque
Kickstart
Start Run Soft Stop
Soft Stop
Time (seconds)
Coast-to-rest
Fig. 3.9-6
Softstop function with adjustable coasting time
3.9.6 Soft starters for pump controls
In the case of rapid changes of the speed of liquids – whether at acceleration or braking –
hydraulic hammer and cavitation effects can arise in large centrifugal pump systems that subject
the systems to heavy mechanical stress and generate corresponding acoustic side effects. The
mechanical impacts result from the fact that liquids cannot be compressed and the pressure
changes in the pipe systems resulting from changes in the speed of the liquid. In the accelera-
tion process the inertial mass of the medium to be accelerated (or decelerated) must be taken
into account.
To avoid the problems described above, a slow change in the flow rate of the fluid is required
during starting and stopping. During starting, this requires control of the motor voltage so that a
small, constant acceleration torque matched to the specific plant requirements is produced that
towards the end of starting makes a gentle transition to the operating point. The characteristics
of normal soft starters with voltage ramp or with current limitation only partially fulfill these
requirements. The special pump control characteristic of Rockwell Automation soft starters offer
an appropriate solution (
Fig. 3.9-7 ).
T/T
e
Fig. 3.9-7
The pump control function ensures that the liquid is accelerated in a gentle manner
When a centrifugal pump is allowed to naturally come to a halt, the counter-pressure of the
pumped medium usually results in abrupt braking and the associated hydraulic surges, which
subject the system to heavy mechanical stress. A normal soft stop by linear reduction of the
motor voltage would reduce the problem somewhat but not completely remove it. The Rockwell
Automation pump control function with soft stop reduces the motor voltage so that the flow-rate
of the liquid continuously decreases and hydraulic impacts are avoided.
LVSAM-WP001A-EN-P - April 2009 3-26