Control principles for variable speed pumping:Speed variation for positive displacement pumps

Speed variation for positive displacement pumps

Consideration of starting torque

The speed torque relationships of Positive Displacement (PD) pumps are explained and illustrated in Section 4.3. The starting torque of a PD pump has a major influence on the selection of the motor and the VSD, which

VARIABLE SPEED PUMPING-0768

Use of sensors

The VSD controller can use a signal from sensors such as flowmeters , pressure transducers, temperature detectors etc, these sensors must be capable of working at the relief valve accumulation pressure. The accuracy of the signal must be suitable for the application. Many control systems will be similar to rotodynamic applications as described in Section 8.2. However, the major difference is that PO pumps have a direct relationship between speed and flow irrespective of pressure . This makes them particularly suitable for dosing applications.

Control of positive displacement pump by algorithm

An algorithm can be used to control pump speed . The algorithm will have to consider different variables for each specific design of pump and application, see Table 8.1.

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  Implementation of control systems

In many cases there will be an external control system, such as a PLC or a distributed control system (DCS), which will provide the control by sending analogue or digital signals to the drive. In other cases the drive may have adequate on-board intelligence.

All modern drive systems rely on microprocessor control, and this allows the manufacturer to integrate the basic signal processing functions into the drive. In some instances, manufacturers have installed bespoke control software to the drive to allow specific requirements to be met.

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