Save energy with a Variable Speed Drive

The installation of variable speed drives (VSDs) on pumps can be an effective energy saving measure. Lowering the speed of a motor by just 20% can produce an energy saving of up to 50%. 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, essentially 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. 


Assessing suitability


Pumps that experience highly variable demand conditions are often candidates for VSD’s (variable speed drives). 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 system 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 relative sensitivity of VSDs and their digital control systems to environment conditions. For example, it may be unwise to install expensive VSDs in locations with a high likelihood of lightning strike. 

Cost and retrofitting


In principle, VSDs can be installed to any pump; however, this is not always practical or cost-effective.
The VSD need to be connected to a control signal and may also require the installation of measurement devices or controllers, which typically should be included in the costing of the measure. The financial viability of installing a VSD depends on the motor application and operating hours. VSDs tend to be most economical when fitted to large pumps. 
VSD budget prices are typically $100 to $300/kW for low-voltage units of less than 100kW, and can be 30to 80% higher for higher-voltage ones.
VSDs can be retrofitted on existing motors but limitations related to the recommend minimum speed (e.g. overheating) must be checked. Harmonics can also reduce the motor’s efficiency, so it is recommended you check whether harmonic filters are required to protect your motor.

Mechanical benefits


As opposed to common flow-control methods such as throttling valves or by pass systems, the principal advantage of VSD technology is that it better matches the fluid energy the system requires with the energy 
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 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.

Key factors when evaluating quotes


VSDs result in losses in the form of heat; normally, their efficiency ranges between  95  to  99%. When comparing different devices, check their rated VSD efficiency.
VSDs also cause harmonics that can reduce the efficiency of the motor. Though losses are typically low (approximately 1%), it is recommended that you assess the associated harmonics to check whether harmonic filters will be required to protect your motor.
VSD budget prices are typically $100 to $200/kW low low-voltage units and can be 30 to 80% higher for higher-voltage ones. Installation costs will vary depending on the distance between the motor and the switch room (cable length) and whether a cabinet is required to meet VSD protection and refrigeration  requirements.



Credit

An article by Farm Energy Innovation Program - Energy & Irrigation http://www.nswfarmers.org.au/r-and-d/farm-energy-innovation-program/factsheets/Variable-speed-drives-on-pumps