Load Characteristics

In the application of slip couplings for continuous pump loads, both variable- and constant-torque requirements are encountered.

Variable-Torque Loads Variable-torque loads are those where the torque increases with the speed and varies approximately as the square of the speed while the load power varies approximately as the speed cubed. The centrifugal pump fits into this classification. To be specific, the above torque-power-speed relationship exists only where the friction head is the total system head, as would be the case if a centrifugal pump were pumping from and to reservoirs having the same liquid levels or in a closed loop.

The various relationships of load power, motor power, and slip loss applying to a typical variable-torque load are shown in Figure 2—again frictional and windage losses have been disregarded.

Static heads, which usually exist in centrifugal pump systems, do not significantly affect the selection of a suitable slip coupling for a specific requirement. Pump efficiency, on the other hand, can be quite an important factor when it decreases significantly with speed reduction. Pumps with relatively flat efficiency curves are most desirable for adjustable-speed duty.

Centrifugal pumps usually operate against some static head. This causes the pump torque to follow a closed discharge characteristic until sufficient speed is reached to cause the resultant discharge head to equal or exceed the static head. This is equivalent to a closed discharge or normally unloaded condition. The pump load then follows a different curve to the full-load condition at minimum slip. These characteristics are illustrated in Figure 3. Curve OAB indicates the closed discharge power. At point A the static head is overcome, and

pump speed, percent of motor rpm

FIGURE 3 Percent efficiency, motor load loss power versus pump speed in percent of motor speed for a static head and friction pumping system

pump speed, percent of motor rpm

FIGURE 3 Percent efficiency, motor load loss power versus pump speed in percent of motor speed for a static head and friction pumping system

FIGURE 4 Power, torque, and losses for a constant-torque load

the pump load then rises along curve AC; this causes a significant difference in slip loss as shown. Under conditions as indicated, where static head is not exceeded and water does not start to flow until 80% of full speed is reached, the slip loss never exceeds 10% of the full-load rating. In general, this is true of a pump with high static head.

Constant-Torque Loads Constant-torque loads are those requiring essentially constant torque input regardless of operating speed. Positive displacement pumps generally are of this type. The load characteristics showing the division between slip loss and load power are illustrated in Figure 4. Note that the driving motor output does not change, regardless of load speed and power.

The slip loss characteristics make slip couplings undesirable for large power loads if any appreciable speed range is required. However, in relatively small units, the simplicity and ease of speed control will frequently justify the use of slip couplings instead of more efficient but more complex speed-control systems.

The constant-torque-load capacity of a slip coupling is largely limited by slip loss and, to a lesser degree, by breakaway torque and minimum slip. Because slip loss is directly proportional to slip, the desired speed range will definitely limit the torque, and therefore the power, that can be transmitted. In some cases, breakaway torque is important as the static friction may be quite high. It is usually recommended that at least 150% of starting torque be available for any constant-torque load.

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