I spent the day at a rural water system in north Texas. They are trying to use a couple of VFD's as soft starters for two 40 HP Berkeley centrifugal pumps. 5,000 feet from the pump station to a stand pipe, through a 6" main, with distribution all along the way. Pump has got to build 48 PSI before it starts moving water against the static head of the standpipe. A 3500 RPM pump doesn't build 48 PSI until it reaches 2700 RPM.
Programmed to take 30 seconds to start up, the VFD spins the motor for 25 seconds before it reaches 2700 RPM. Then the last 5 seconds gets it to 3500 RPM and the pressure gauge pegs out over 100 PSI, and slowly comes back to about 78 PSI.
We reprogrammed it to take 2 minutes to reach full speed. The pump rattles through about 3 critical speeds during the minute and 40 seconds it takes to reach 2700 RPM. The last 20 seconds is really all the soft start there is during 2 minutes of ramp up. 20 seconds is really not enough to stop a pressure spike. The pressure quickly went to about 90 PSI before settling back to 78 PSI.
Then we tried ramping it down slowly. Told to take 60 seconds to ramp down, nothing happened for about 10 seconds. Then the pressure dropped off fast. There wasn't much difference from just cutting off the power. The system dropped from 78 PSI to 10 PSI, bounced back to 80 and then hit 0 or lower. The gauge bounced back and forth, subsiding a little each time for about 2 minutes until it steadied out at 48 PSI, which is tower pressure.
The VFD or soft starter causes the pump to vibrate considerably, spending too much time going through several critical speeds. The whining noise as the pump ramped up, was so loud that it drove the water operator out of the building each time. The pressure bounces around so much, that pressure control to start and stop the pumps and alternate between the two would not work so, we were operating the pumps on manual. The customer was advised to purchase a telemetry system to operate the pumps from the standpipe 5,000 feet away, to eliminate the bouncing controls.
This is typical of the problems with soft start or soft stop. Head is lost or created by the square of the pump speed. A pump does not produce enough pressure to open the check valve until the RPM is at 80 to 90% of full pump speed. When a pump is ramped up slowly to full speed, it is only the last couple of seconds that actually produces enough head to move water. This makes ramping up the pressure a delicate procedure. First you must determine at what RPM the pump will actually build enough pressure to buck the static head. Then you must convert this calculated RPM into hertz. Now you must bring the pump up to this speed very quickly, then slowly ramp up from this speed to the required speed. The slightest miscalculation of RPM, either to high or too low, or any change in the static pressure, will cause the pressure to surge. Any delay in producing the flow required after start up, will cause a negative pressure wave, and a subsequent pressure surge. The same thing happens with a soft stop, only in reverse.
A Cycle Stop Valve has a hydraulic soft start and soft stop. It starts and stops the pump at 5 GPM, regardless of the static pressure. This eliminates pressure surges or water hammer, that can't be eliminated with electronic soft start or ramping up of a VFD. After start up, the CSV quickly opens up to maintain a constant pressure, before a negative pressure wave can accumulate. Hydraulic soft start and stop are just that. After all, isn't the fluid what needs to be started or stopped softly, not the motor?