Pumping water is an application where system dynamics play a big part in the selection of what type of pump control is needed.
The ultimate goal of a pumping application is to move the fluid, but it is just as important to reduce the amount of damaging water hammer while quickly detecting issues within the pumping system.
So what is the most effective way to round up the control for the pumping application?
The scenario is the need to pump water from one source to another using a centrifugal pump while reducing water hammer.
In this case, the application calls for the pump to start smoothly and run the pump motor at full speed until the application calls for a stop command.
This is a good fit for a soft starter versus a variable frequency drive (VFD) because no speed control is needed between the start and stop.
Here are three common methods for the starting portion:
- The soft start method is used in soft power source applications where current spikes are not desired when starting. This is the traditional way to use a reduced voltage to start an application.
- The pump control method is used to follow the natural S-curve of the pump to provide enough control and torque when needed by application, conserving energy. Sometimes the current in this method is higher compared to soft start method, depending upon system dynamics.
- The linear acceleration method is used to provide the smoothest start. Linear acceleration is not as load dependent as the previous two starting methods. Most applications require an external tachometer to achieve this method.
The common stopping methods for pump applications are just as easy:
- Soft stop to reduce voltage when stopping, which extends the ramp time beyond that of a coast to rest.
- Pump stop, similar to the pump start, follows the system dynamics when stopping.
- Finally, there is linear deceleration, which will provide a smooth, controlled stop of the pumping application that is not as load dependent.
Below illustrates the torque and speed of a pump load when stopping by the three methods. By controlling the motor torque, the water hammer is controlled.
Now that the starting and stopping is working and water hammer is under control, other aspects of the pumping application diagnostics can be examined.
For example, what about pump cavitation, plugged inlet/outlets or low flow?
Some standalone pump cavitation detection devices only utilize current to detect an issue.
The problem with this method is current is not linear on a motor as the curve below illustrates for this 150 Hp motor. Utilizing this alone, does not tell the whole story.
So if the motor is lightly loaded, is current alone a good way to detect pump issues? No.
In unison, power measurements such as power factor, real power and current can indicate what the system is doing.
Both power factor and real power go towards a zero value, where motor current does not.
There is always motor magnetizing current even at no load.
Utilizing the combination of power factor, real power and current can assist in indicating if a pump issue is taking place.
For example if real power decreases, this may indicate pump cavitation or a clogging issue on the pump intake, which may result in the pump running dry.
As real power increases, this may indicate overload condition such as bad bearings or a rupture in discharge line.
Another way to monitor a pumping application is to use a flow meter.
Typically this device would be fed back to an analog card in a PLC rack that would be monitored in a control program.
The question is: is there a good way to round up the different methods for controlling and monitoring a pumping application and reduce the number of devices?
Round up the pump application with one device such as a complete, fully functional smart motor controller, which can:
- Reduce water hammer during the start and stopping including sensorless linear acceleration and deceleration meaning no external tachometer required
- Provide power monitor capabilities
- Offer programmable alarms and faults
- Include analog input/output expandable that could be connected to a flow meter
- Expand communication
You can learn more about the Allen-Bradley SMCTM-50 Soft Starter.