In recent decades, engineers contemplating variable speed pumping for their systems often only consider a variable frequency drive (VFD) setup. Some planned installations, however, can be technically or economically daunting. In such cases, an eddy current drive might be a more desirable solution.
While many early VFD shortcomings have been overcome, some challenges persist in certain applications. Some VFDs are still equipped with bypass starter schemes to enable the pump if the VFD fails, and many are equipped with air conditioning to maintain a safe operating temperature. Some models still require custom-designed harmonic filters to meet regulatory harmonic distortion limits. The additional hardware for each of these solutions comes at a cost. Making room for and installing this equipment may lead to additional costs. When calculating system efficiency, many managers ignore the extra power necessary to operate this hardware.
These factors are especially prevalent in applications requiring medium-voltage motors (typically 2,300 and 4,160 volts). Motor horsepower (hp) ratings are usually 300 hp and larger; motors above 500 hp are almost always medium-voltage.
1. Installation at a Mississippi River Levee
A freshwater supply district in Louisiana operated a pumping station originally built in 1954 with three pumps. A fourth pump was added in 1960. The pumps draw water from the Mississippi River and discharge on the other side of the levee into the adjacent bayou. All four of the vertical axial flow pumps were equipped with a constant-speed electric motor drive, with two having an alternative to operate the pumps with diesel engines through a right-angle gear when electric power failed.
In 2008, a consulting engineer and the district conducted a study to assess the condition and future use of the aging pump station. The pumping station was evaluated to determine whether it should be used as a continuous-duty pump station or as a backup. The district and its engineering firm decided to refurbish the pump station with two new pumps and to retain the engine/electric drive pumps. The station was designated for continuous duty.
Fluctuation of the river level and the resultant effect on total dynamic head (TDH) and flow capacity suggested that variable speed drives should be considered for the two new pumps.
The new pump required 300-hp, 2,400-volt motors for the rated design point of 45,000 gallons per minute (gpm) at 17.1 feet TDH at a nominal speed of 490 revolutions per minute (rpm). However, expected operating demand suggested that the pumps would operate at 85 percent speed (416 rpm), reducing the pump's brake horsepower (bhp) requirement to 111 hp, or 83 kilowatts (kW).
The engineer considered and compared eddy current drives and variable frequency options for this 85 percent average operating speed (see Table 1). Annual excess energy consumption was calculated on the basis of continuous operation—24 hours for 365 days (see Equation 1).
Using an average electrical cost of 8.5 cents per kilowatt-hour (kW-hr), the engineers calculated an expected additional operational cost of $9,520 per year ($0.085 x 112,000) per pump. For two pumps, this would equal $19,040 per year. Based on these calculations, it would seem that the VFD solution would be favored. Additional considerations, however, led the team to choose an eddy current drive solution.
This pumping station is on the river side of the levee, which is seasonally dry land but is regularly inundated by the river. It is built on a steel and concrete structure that stays above the water level, but it is not environmentally controlled or regularly attended. Space limitations and environmental concerns—temperature, humidity and exposure—presented serious challenges to using VFDs in this application.
Required infrastructure improvements and 2,400-volt VFDs would require $600,000 more to implement. Based on a simple payback analysis, assumed energy savings would take more than 30 years to recover the original extra investment. The expected remaining life of the pump station is approximately 10 to 12 years. This became a major factor in choosing the eddy current drive option.