Drive rating

The drive rating is the drivepower available at the shaft coupling and is measured (see Unit) in watts (W). For high drive ratings more convenient units are used, such as kW or MW.

Series-produced pumps are typically assigned a fixed choice of drive. Their drive rating is selected based on the maximum power input within the permissible operating range and, in the case of standardised motors (in accordance with IEC), is rounded up to the next highest motor rating level. The rated power specified on the motor rating plate of standardised motors must not be exceeded by more than 3 % during continuous operation. Additional safety margins to account for potential long-term wear or sedimentation are not common for series-produced pumps, since their power input as a function of flow rate typically changes but slightly at the upper limit of the operating range (see Characteristic curves).

Engineered centrifugal pumps are designed to match the targeted operating point. Their drive rating must always exceed the power input (P) required by the pump to accommodate increased power input due to, for example, operation under off-design conditions (see Design point), variable operating points, changes in speed (see Pump affinity laws), changes in the density of the fluid handled, manufacturing tolerances, long-term pump wear or sedimentation.

A margin of 10 to 20 % will be sufficient to account for these factors in engineered clean water pumps.

The safety margins can be specified by the customer ordering the pump or in codes and standards. See Fig. 1 Drive rating.

Safety margins may also be necessary for dimensioning a waste water pump drive to accommodate potential problems at start-up, varying density of the fluid handled, high wear, single-pump operation of parallel pumps (see Parallel operation), environment-related factors (e.g. operation in a tropical environment), or closed-loop control involving super-synchronous speeds (see Synchronous speed).