Smartmulti > Agriculture
SMI INVT Variable speed drives on pumps
The installation of variable speed drives (VSDs) on pumps can be an effective energy-saving measure. Lowering the speed of amotor by just 20 percent can produce an energy saving of up to 50 percent. Variable speed drives can be installed on all pumps, including those associated with HVAC systems. The VSD needs to be connected to a control signal and may also require installation of measurement devices or controllers, which typically are included in costing. The financial viability of installing a VSD depends on the motor application and operating hours. VSDs tend to be most economical when used on large pumps.
NSW Farmers recommends that all farmers with significant
pumping expenses investigate the costs and benefits of
installing variable speed drives (VSDs). Pumps are everywhere in agriculture, essential both for irrigation and for heating, ventilation and cooling (HVAC). In the intensive sectors, VSDs can halve energy consumption in some applications, with savings rapidly covering the costs of the technology. Energy savings can vary significantly
depending on the characteristics of the system and the type of
operation. Typical savings range from 30 to 50 percent (US
Department of Energy, 2004). Assessing suitability
Pumps that experience highly variable demand conditions are
often good candidates for VSDs. In such cases, electronic
controls vary the frequency and voltage supplied to the motor,
which regulates the motor speed and, in turn, adjusts the
pump’s output. Not all pumps and applications will benefit from having VSDs, however. VSDs are not recommended for systems with high static head or pumps that operate for extended periods under low-flow conditions. A further consideration is the operating environment of the pump and the relative sensitivity of VSDs and their digital control systems to environmental conditions. For example, it may be unwise to install expensive VSDs in locations with a high likelihood of lightning strike.
As opposed to common flow-control methods such as throttling valves or bypass systems, the principal advantage of VSD technology is that it better matches the fluid energy the system requires with the energy the pump delivers to the system.The pump’s power is proportional to the cube of the motor’s speed; therefore a significant reduction in power (and energy savings) can be achieved by reducing the speed of the motor. A VSD can alleviate the need to throttle the flow (and lose energy), or allow the water or refrigeration fluid to be pumped more slowly, reducing frictional energy loss. Soft start and stop capabilities also reduce mechanical and electrical stress as well as the risk of water hammer.
Reducing the flow rate would decrease pump efficiency. It is assumed that the efficiency of the pumps at 75 percent flow would be 77 percent and the efficiency at 50 percent flow would be 70 percent. Motor efficiency does not decrease significantly until the motor loading drops below 50 percent; thus it is assumed the motor remains 92 percent efficient.