Identify Accuracy Challenges in String Tests

Q. What is a string test, and when should one be specified? A. Generating a pump curve requires the measurement of head, capacity and power. From this information, the efficiency of the pump can be calculated. The efficiency shown on the pump curve is always related to the shaft input power. The published efficiency is the pump’s hydraulic power divided by the mechanical input power to the pump shaft. It is only the efficiency of the pump, not of any other component. The most accurate way to obtain the power data is by direct measurement of the shaft torque and rpm with a torque transducer and tachometer. These values are then used when calculating the power input to the pump. A string test uses the complete assembly—motor, pump and drive. The power measured is the input power to the motor. The input power to the pump shaft is then calculated from the published motor and drive efficiencies. A string test is less accurate because these efficiencies are not precisely known compared with the direct measurements of the shaft torque and rpm. A variable frequency drive (VFD) further lowers the accuracy of a string test because of the difficulty in finding a precise value of input power to the pump shaft. A wattmeter cannot accurately measure the power from the VFD to the motor because of the VFD’s nonsinusoidal waveform. When measuring the input power to the VFD, the VFD’s efficiency must be known to calculate output power to the motor. The motor efficiency will change because of the nonsinusoidal waveform. Although many VFDs provide a measurement of output power, it is generally not accurate enough for acceptance testing. This reading also does not include the reduction in motor efficiency when operated on VFD power. The suggested procedure for string testing is to conduct one test without using a VFD by running the motor directly across the line. A complete head-capacity-efficiency curve will be produced at nominal speed. The VFD can then be connected to the motor, and head-capacity curves can be produced at the required speeds without measuring power data. A string test cannot accurately measure the efficiency of an engine-driven pump. The pump should be tested separately for shaft power measurements. The pump manufacturer’s curves often only provide end users with the required power at the pump input shaft. From an energy consumption standpoint, these data do not provide the true cost to operate the pump.

Wire-to-water efficiency and power consumption curves are more useful but are rarely requested. Wire-to-water performance can be measured with all the configurations in Table G-1 by placing a wattmeter at the motor’s or VFD’s input. These data allow the end user to know the true power consumption of the pump system and to evaluate the true operating cost of different component options. For more information, see ANSI/HI 14.6 Rotodynamic Pumps for Hydraulic Performance Acceptance Tests.