Motor Power and Motor Efficiency

Motor power is the power that the pump's motor receives from the electrical utility and is related to the pump brake power by:

In other words, the motor efficiency represents that ability of the motor to transfer power from the electrical lines to the pump itself. For most pumps, the motor efficiency can be considered to be constant over the whole operating range of the pump.

Note: In the case of variable speed pumps, the efficiency of the variable speed drive needs to be accounted for. This efficiency varies with pump speed among other things. You are encouraged to correct the motor efficiency to include the variable speed drive efficiency. For variable speed pumps, there is a drive mechanism between the motor and the pump itself. There are also energy losses associated with this drive, which may be significant in some cases.

For example, if a motor has an efficiency of 90% (0.90) and the variable speed drive has an efficiency of 85% (0.85) at the speeds being used, then the motor efficiency should be entered as 76.5% (0.765).

Note: The variable-speed data is merely presented as an example and should not be construed as representative of any particular pump.

You are encouraged to find the drive efficiency data for the specific drive that is being used. See the Variable Speed Drive Efficiency table below for some typical data for variable speed drive efficiency found in the report, "Operations and Training Manual on Energy Efficiency in Water and Wastewater Treatment Plants," TREEO Center, University of Florida, 1986.

Variable Speed Drive Efficiency

Percent of Full Speed	Variable Frequency Drive	Eddy Current Coupling	Hydraulic Coupling
100	83	85	83
90	82	78	75
70	81	59	56
50	76	43	33

These corrections should not be made to alternatives with constant speed pumps. If you are performing an analysis to compare constant and variable speed pumps, you should set up two alternatives: one for the constant speed pump and a second for the variable speed pump.