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A water-treatment station in South Carolina, US, has slashed its energy bill by GBP12,000 a year by implementing Parker SSD variable speed drives in place of a 40-year-old eddy current clutch.

The Wildcat Lift Station in Rock Hill, South Carolina, was constructed in the 1970s to accommodate rising population and includes three 150kW centrifugal pumps that lift 11 million gallons per day a total of 143 feet.

Until recently, pump flow was controlled by an eddy current clutch from the original installation.

However, when maintenance supervisor Jon White had to deal with the failure of one of the pump’s drive systems, he realised that the repair cost and time were impractical.

Instead, he contacted Parker SSD to discuss a new variable speed drive.

Parker estimated cost savings and a calculation on the return of investment (ROI) which showed that in energy savings alone, without consideration for the cost of ongoing maintenance of the existing system, the return period would be less than 12 months.

Based on this the decision was made to proceed with the upgrade.

The inefficient eddy current clutch was removed and the existing 150kW, 460 volt, 447T frame motor was directly coupled to the pump shaft, requiring only a simple motor mount modification.

The 150kW high-power modular AC variable-speed drive was then connected through the existing control panel and pump sequencing logic.

The available bottom power entry configuration on the drive allowed a neat installation with no visible external wiring.

The need for an expensive additional external harmonic filter was eliminated as the drive includes a three-per-cent line reactor built into its enclosure, as well as line fuses and a local power-disconnect switch with door interlock and lockable handle.

Replacing the eddy current clutch with a solid-state drive has provided several tangible benefits to Rock Hill besides the energy savings.

For example, eddy current clutches rely on slip to adjust the speed of their load and dissipate energy through heat.

Their efficiency also drops off significantly as speed increases and, with a 150kW motor, this soon adds up.

However, by using a variable-speed drive to regulate the speed of the pump directly there is no energy lost to slippage and the current draw of the motor declines dramatically with reduction of speed.

With a centrifugal pump, simply reducing the speed to 80 per cent will result in nearly a 50-per-cent decrease in energy consumption.

In addition, the motor is now gently ramped up to speed with current limited by the drive.

The old Direct-On-Line starting system inflicted significant stresses on the motor, clutch and pump, resulting in increased wear and reduced service life.

On top of this, the high peak starting current led to higher peak demands which in turn resulted in additional surcharges from the utility company.

In the event of failure, the modular design of Parker SSD means parts are quickly and easily replaceable.

Parker Hannifin – Electromechanical and Drives

Parker Hannifin is the world’s leading diversified manufacturer of motion and control technologies and systems, providing precision-engineered solutions for a wide variety of mobile, industrial and aerospace markets. With a comprehensive portfolio of electromechanical actuators, servo motors, servo drives, operator displays, communications devices and variable speed AC and DC drives, the Electromechanical and Drives Division are uniquely placed to meet all of your motion control and drives requirements.

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