Ratings And Sizes

Eddy-current couplings are available in a wide range of ratings and sizes, from fractional power units up through 10,000 hp (7500 kW) and beyond. The type of cooling employed is an important consideration; some manufacturers use either water or air exclusively and others use a combination of the two. In addition, the type of load is a factor because thermal capability will vary significantly between water- and air-cooled units.

Centrifugal pump variable-torque loads are usually best handled by air-cooled couplings having high-torque capabilities at low slip values and limited heat-dissipating capabilities. The selection chart shown in Table 1 is representative of one manufacturer's line of couplings designed specifically for centrifugal pump loads. Where full-torque capabilities are realized at 3% slip below motor speed, thermal loads at two-thirds of motor speed will be 16.2% of rated speed load power. Sizes starting at 3 to 5 hp (2.2 to 3.7 kW) and extending up through 3000 to 5000 hp (2240 to 3730 kW) are generally available.

Eddy-current couplings for pump loads requiring constant-torque drives have rather limited usage. In small sizes where thermal capabilities are proportionately greater than the 6:1 ratio encountered on large units, constant-torque loads can be adequately handled by air-cooled couplings. Beyond 100 hp (75 kW), air-cooled units can be impractical because of thermal and starting-torque requirements.

Water-cooled eddy-current couplings have somewhat different characteristics, making them more suitable for constant-torque and large-power variable-torque loading. High starting torque, high minimum slip, and high thermal capabilities all tend to lead to those conclusions. Thermal capabilities are frequently equal to or greater than full-load power ratings. The starting torque is usually the maximum torque, and as a result low slip values are limited.

TABLE 1 Eddy-current coupling selection chart for centrifugal pumps, horsepower output (1hp = 0.746 kW)

Percent of motor full-load speed

TABLE 1 Eddy-current coupling selection chart for centrifugal pumps, horsepower output (1hp = 0.746 kW)

Percent of motor full-load speed

Drive motor input speed, rpm

Unit Size

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