Eddy Current Drives Serve Reliably Where VFDs Fear to Go

In recent decades, when engineers contemplated the need for variable speed pumping, often the only choice considered has been a variable frequency drive system. However, some planned installations can prove to be technically or economically daunting. In such cases, the time-tested eddy current drive might be a more desirable solution. This article describes several such situations.

Many early shortcomings experienced with VFDs have been overcome or improved upon, but not completely. VFDs are still often equipped with bypass starter schemes to enable the pump when the VFD may fail. Many are equipped with air conditioning to maintain safe operating temperature. Some designs still require custom-designed harmonic filters to meet regulatory harmonic distortion limits. Each of these solutions comes at a cost for the additional hardware. In addition, there is often a substantial cost to make room for and install all this equipment, even to the extent of adding new construction to existing facilities or designing added space to new facilities. The additional power necessary to operate this additional hardware is often ignored when calculating the presumed efficiency of the system.

These factors are especially prevalent in applications requiring medium voltage motors (typically 2300 and 4160 volts). Motor horsepower ratings are usually 300 hp and larger; motors above 500 hp are almost always medium voltage.

Limited space on a Mississippi River levee

A fresh water supply district in Louisiana operated a pumping station originally built in 1954 with three pumps and a fourth pump had been added in 1960. The pumps draw water from the Mississippi River and discharge on the other side of the levee into the adjacent bayou. Two of the vertical axial flow pumps were equipped with constant speed electric motor drive, with an alternative to operate the pumps with diesel engines through a right-angle gear when electric power failed. The other two pumps were equipped only with constant speed electric motor drive.

The pumping station as a whole was evaluated for whether to be used as a continuous duty pump station or merely a backup for a proposed new pump station. It was decided to refurbish the pump station with two new pumps and to retain the engine/electric drive pumps. The station itself was designated for continuous duty.

A consulting engineer and the District undertook a study to assess the condition and future use of the aging pump station.

This fluctuation of the river level and the resultant effect on TDH and flow capacity seen by the pumps suggested that variable speed drives should be considered for the two new pumps.

The new pump required 300 hp, 2400 volt motors for the rated design point of 45000 gpm @ 17.1 ft. TDH at a nominal speed of 490 rpm.   However, expected operating demand suggested that the pumps would operate at an average speed of 85% speed (416 rpm), reducing the pump’s bhp requirement to 111 hp = 83 kw. 

Eddy current drives and variable frequency options were considered and compared by the Engineer for this 85% average operating speed:

                                Drive Eff’y           Motor Eff’y         Combined Eff’y                 kW input

Var. Freq.                 95%                       95%                  .95 x .95 = 90%                  83/.9 = 92.2                      

Eddy Current          83%                        95%                 .83 x .95 = 79%                  83/.79 = 105

“Annual Excess Energy Consumption” was calculated on the basis of continuous operation (24 hrs, 365 days):  (105 – 92.2) kW x 24 hrs x 365 days = 112,000 kW-hr. 

Using an average electrical cost of 8.5 cents/kW-hr, this resulted in an expected additional cost of operation of $0.085 x 112,000 = $9,530 per year, per pump. For two pumps, this would come to $19,076 per year, and apparently the VFD solution would be favored. 

However, additional considerations resulted in choosing the Eddy Current Drive solution:

This pumping station is situated on the river side of the levee, which is seasonally dry land, but is regularly inundated by the river itself.  It is built atop a steel and concrete structure which keeps above the water level, but is not environmentally controlled nor regularly attended. Space limitations and environmental concerns (ambient temperature, humidity, and exposure to weather) presented serious challenges to using VFDs in this application.

2400 volt VFDs and required infrastructure improvements would require $600,000 higher cost to implement.  Using a simple payback analysis, assumed energy savings would take over 30 years to recover the original extra investment.  The expected remaining life of the pump station itself is approximately 10-12 years, before it will be replaced with a new facility.  This became a strong factor in choosing the Eddy Current Drive option. 

The Engineer noted that a lower average speed would increase the energy cost differential in favor of the VFD, while a higher average speed would increasingly favor the eddy current drive. 

The primary factor in choosing variable speed pumping of any kind was for stabilizing operational control, as opposed to energy saving considerations. Nonetheless, an overall energy saving was achieved.  After the station was restored to operation, experience shows that the pumps operate at an average of 90% speed, with an estimated reduction in pump bhp from approximately 250 hp at 490 rpm to about 170 hp at 441 rpm.  Assuming the pumps run continuously in both scenarios, the power consumption comparison is:

490 rpm:              250 hp = 187 kW x 8760 hr/yr = 1,638,120 kW-hr x $0.085 = $139,240/yr

441 rpm:              170 hp=  127 kW x 8760 hr/yr = 1,112,520 kW-hr x $0.085 = $ 94,564/yr

Energy Cost Savings per year:                                                                                            $44,676 per pump                    

The two new pumps, motors, and eddy current drives, including digital excitation/controller units were installed in 2012.  The motors were specified with TEFC enclosure, and the Eddy Current Drives received a NEMA Weather Protected Type II enclosure, a design which allows ventilation using ambient air, but restricting the entry of airborne moisture and contaminants.  The eddy current drives required no more floor space than the motors themselves, as they are vertical machines interposed between the drive motor and the pump.  The digital controllers were easily accommodated within the existing structure, requiring approximately 30” x 36” space within a NEMA 1 wall mount enclosure. 

Replacing  Existing Eddy Current Drives

A Midwest consulting engineer was tasked to evaluate options and specify equipment to replace 35 year old variable speed eddy current drives at a water plant in Southern Indiana.  The original equipment included two 200 hp 900 rpm eddy current drives and one 500 hp 900 rpm eddy current drive, driving vertical turbine type pumps installed in 1979.  Motor voltage was 4160.  In the more recent past, one of the 200 hp units had been replaced by a 480 volt motor and VFD operating through a transformer to enable operation from the 4160 volt source. 

The user had experienced good reliability and service life from the eddy current drives, and was equally satisfied with the newer VFD that replaced one of the eddy current units.  However, they were eager to integrate the control of the new variable speed equipment with their central control system via Ethernet communication. 

The Engineer compared new eddy current drives to VFDs for each of the ratings on the basis of first cost, installation considerations, and operating cost expectations.   They elected to replace the 200 hp eddy current drive with a 480 volt VFD, similar to the arrangement implemented some years earlier on the other 200 hp.  However, the 500 hp unit was considered too much power to be operated at 480 volts, and they chose to keep that system at 4160 volts.  Based on this consideration, the VFD option would have cost considerably more for the equipment, and would need more space than was conveniently available.  The 500 hp unit was replaced with a new eddy current drive and new drive motor. 

The digital exciter/controller was supplied with an Ethernet IP interface capability that was easily integrated into the user’s central control system

A large Ohio municipality engaged a consulting engineer, to explore options at water pumping station.  This station was originally equipped in 1976 with two 1250 hp 514 rpm vertical eddy current drives with synchronous drive motors operating at 4160 volts.  Although they operated satisfactorily for 35 years, they had begun to experience overheating in the exciter/controller cabinets.  Temporary air conditioned cooling air was implemented to keep the controllers operating while a permanent solution was studied and implemented.

It was originally assumed that the eddy current units would be replaced by VFDs.  However, it was soon learned that new VFDs would occupy much more space than available at the aging facility.  Also, the harmful effects of heat were a concern.  The existing equipment was already in danger of failure due to the need to operate in 100 degree F ambient temperature conditions.  It became clear that new VFDs, confined to a relatively small room would be in danger of overheating from their own heat discharge in such conditions.  Furthermore, it was feared that this expensive new solution might last only a decade or so before needing significant repair or complete replacement at even more expense. 

The choice was made to replace the aging eddy current drives in kind with new drives and induction motors.  This solution proved to be significantly less expensive on a first cost basis, and was a proven solution by virtue of the previous 35 years of reliable service by the old equipment.  The eddy current drive losses are successfully absorbed by the air in the motor room floor without the need for air conditioning, and the new controllers are rated to operate safely below their maximum rating with a substantial thermal margin for the electronics.  Mechanical installation, wiring and cabling of the electrical and instrumentation connections were largely a matter of replacing old with new. 

This solution was estimated to save the client about $2 Million in initial installation costs. 

Want to learn more? Dynamatic’s team of engineers can help achieve true system efficiency with a Dynamatic variable speed system and digital controls.