Variable speed drives on pumps

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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.

Costs and retrofitting

In principle, VSDs can be installed on any pump; however, this is not always practical or cost-effective.

The VSD needs 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 100 kW, and can be 30 to 80 percent higher for high-voltage ones[1].

VSDs can be retrofitted on existing motors but limitations related to the recommended minimum speed (e.g. overheating) must be checked. Harmonics can also reduce the motor’s efficiency, so it is recommended that 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 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.

Technical explanation

Figure 1 (overleaf) shows that as the motor’s speed is reduced, the pump curve moves downwards to the left and flow and head are reduced, resulting in lower power consumption. If a VSD was not used in this flow-control application, the flow would need to be reduced by throttling, bypassing or switching the pump on and off.

VSD efficiency
Figure 1: The effect of reduced drive speed on head, power and efficiency
Energy Technology Support Unit (ETSU), Harwell, 1998

This relationship between pump performance (flow, head and power) and speed is explained by the Affinity Laws.  

For example, reducing the speed of a pump by 10 percent results in a 27 percent reduction in power:

 

These laws apply only to systems with no or very low static head. With significant static-head contribution, operating points move to the left. At slower speeds, the pump doesn’t produce enough head to overcome static head and ‘hydraulic shut-off’ occurs.

Reduced drive speed
Figure 2: Shifting of operating point with reduced drive speed.
NSW Farmers

Worked example

A water pump runs continuously, delivering water at a flow rate of 350 L/min and a head of 30 m. Flow is controlled via
a throttling valve and it has been estimated that process
flow requirements are as shown in table 1. The motor is 92 percent efficient.

Table 1: Estimated operating hours and flow for example pump.
Percentage of annual operating hours percentage of design flow
25% 100%
25% 80%
50% 70%

Estimate the energy savings if a VSD is retrofitted to a pump to control flow as per process requirements.

The input power of the pump at 350 L/min and 30 m is calculated as:

Where:

Therefore:

With this power requirement, the annual electricity usage is:

Using the Affinity Laws to calculate flow and head at different speeds, the following operating points are determined:

Illustrative operating points
Figure 3: Illustrative operating points.
NSW Farmers

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. The annual electricity usage is calculated below.

Table 2: Annual electricity use calculations
Time % Flow Flow (L/min) Head (m) Pump Effic. Power (kW) Elec. (kwh/yr)
25% 100% 350 30 84% 2.2 4,865
25% 80% 250 23 77% 1.3 2,906
50% 70% 180 20 70% 0.9 4,003
Total           11,775

The electricity savings are therefore:

Key factors when evaluating quotes

Variable speed drives result in losses in the form of heat; normally, their efficiency ranges between 95 and 99 percent. 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 one percent), it’s 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 for low-voltage units and can be 30 to 80 percent higher for high-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.

Further information

Technology guide: variable speed drives (VSDs)

Pumps and fans, opportunities, case studies and key resources

References

Agricultural Resource Management, 2009. Farm and Irrigation Energy Efficiency. [Online]

Anon., 2006. Improving Pumping System Performance. [Online]

Energy Technology Support Unit (ETSU), Harwell, 1998. Good Practice Guide 249, 'Energy savings in industrial water pumping systems', s.l.: UK Department of the Environment, Transport and the Regions.

US Department of Energy, 2004. Variable Speed Pumping  
A Guide to Sucessful Applications, Executive Summary.
[Online]


[1] Schneider Electric 2011 Pricing Guide, AC Drives and Softstarts.


This content was produced with assistance from Energetics
www.energetics.com.au

 

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