Energy Saving-Variable Speed Drives for Motor Pump Control

source : Courtesy from ABB Review (Special Report)
Dancing With the Dragon

Generally speaking, AC drives are the most energy-efficient method for the control of pumps. ABB provides

an optional software package for water-pump control in conjunction with its ACS800 industrial drive: intelligent pump control (IPC). In this application, every water pump is controlled by one drive. The power range of the drives can be between 0.55 kWand 5,600 kW (depending on the application). An example with three drives controlling three parallel water pumps is shown in 1 . The adoption of communications between the drives (using fiber-optical cables) eliminates the need for an external PLC1), thus permitting energy savings, shortening repair times, and preventing blocking of the rotation of the water pump and jam (blocking of the flow).The operation of a multi-pump system at efficient speed therefore offers potential for significant savings.

Energy-saving principles

Pump control using general-purpose  drives is mainly implemented through the control of flow rate. As in many  other variable-speed drive applications, this enables notable energy saving

Fig. 2: Power applied to the shaft is reduced significantly when drives rather than valves are used to control speed

a and b = lift flow curve of the pump at different rotation speeds , n1 and n2

c and d= characteristics of pipe for difference resistance.

Reducing flow by reducing the speed is much more energy efficient than reducing it by increasing the pipe resistance using valve.

Explanation:

1. With the pump running at the rotation speed n1, and the pipe resistance set to its lowest value 2c , the working  point of the pump is at A (intersection of 2a and 2c ). The output flow rate Q1 is maximal and the power P1 applied to the shaft is proportional to the area Q1H1.

2. If the flow rate must be reduced from Q1 to Q2, the traditional method is to change a valve setting, increasing the  resistance of the pipe from 2c to 2d .The working point hence changes to B. This increases the pump lift from H1 to H2. The power P2 now applied  to the shaft is proportional to the area Q2H2.

3.Variable Speed Drive Way
The pipe resistance remains at 2c but pump rotation speed is decreased from n1 to n2. The new  head-flow curve is thus 2b and the working point moves to C. The pump head is decreased substantially to H3.The power P3 applied to the shaft is directly proportional to the area  Q2H3. This represents a reduction (compared to P2 ) proportional to the  area Q2(H2-H3 ). The energy savings  achieved will be similary impressive

To be continue...