by Joe Evans, Ph.D
December 17, 2011

Although a magnetic coupling can reduce repair and maintenance costs in an application subject to problematic misalignment, it has another application that may not be as obvious. I mentioned earlier that the air gap causes a certain amount of slip between the magnetic and conducting discs. The gap distance is determined by the application and the torque that must be transmitted. If, however, the gap distance is increased, slip will also increase and rotational speed is reduced. Torque transmission is reduced proportionally, but when the load is a variable torque machine, such as a centrifugal pump, its torque requirement is decreased by a similar amount.

On a properly sized coupling, spacers can be used to increase the air gap, and pump speed can be decreased up to 30 percent without a significant decrease in motor efficiency. A typical EPAC motor will reach its maximum efficiency somewhere between 50 and 100 percent of rated load and will drop only 1 to 2 percent on either side throughout that same load range.

What applications might benefit from speed reduction via magnetic coupling air gap variation? There are several. Suppose an existing pump was sized incorrectly and its flow rate is much greater than required. You could trim the impeller, but hydraulic efficiency might decrease due to increased recirculation. You could also use a throttling valve, but power would be wasted. A magnetic coupling, with the proper air gap, will reduce flow and power while preserving hydraulic efficiency. If the pump were running far to the right of BEP, it would actually increase efficiency. In a new application where flow is expected to increase over time, a properly gapped magnetic coupling can provide start-up flows and then ultimately increase flow as demand increases.

In either application, use of a magnetic coupling will result in operation at or near BEP and a reduction in the BHP required. A cushioned start and far fewer alignment problems will also be bonuses. Figure 2 shows the results when a magnetic coupling was applied to an oversized pump.

Figure 2

The original design called for 800 gpm at 135 ft, but the head was overestimated. After installation, the pump operated at 1,050 gpm at 100 ft (blue curve). The black curve is produced by a magnetic coupling that was gapped to allow about 8 percent slip. This amount of slip allows the pump to operate at its original design flow and reduces the hydraulic HP by about 27 percent.

Pumps & Systems, December 2009