When a VFD is used as part of the string, it becomes difficult to obtain an accurate 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. A wattmeter can measure the input power to the VFD. However when the input power to the VFD is measured, the efficiency of the VFD must be known to calculate VFD output power to the motor. This information may be available, but it adds another degree of error since the motor efficiency will change due to the nonsinusoidal waveform of the output power from the VFD. (Although many VFDs provide a measurement of output power, the value of this measurement is only approximate and not accurate enough for acceptance testing. This reading does not consider the reduction in motor efficiency when operated on VFD power.)
A string test with a VFD may be required if the customer specifies that the VFD be used for the string test. It may also be required when the customer wishes to have curves at a number of speeds. In both cases, the suggested procedure is to conduct one test without the VFD, running the motor directly across the line. This allows a complete head-capacity-efficiency curve to 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 any power data measurement.
|Figure 2. Influencing factors for calculating pump efficiency for different configurations.|
Figure 2 contains the factors needed to calculate pump efficiency for different configurations. A string test cannot measure the efficiency of an engine-driven pump. In this case, the pump would need to be tested separately if accurate shaft power measurements are required. The pump manufacturer's curves often only provide the end user with the required power at the pump input shaft. Further investigation may reveal that this information is provided with the pump being sealed by packing, not mechanical seals, which may absorb additional power. From an energy consumption standpoint, these data do not provide the user with 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 Figure 2 by placing a wattmeter at the input to the motor or VFD. These data will allow the end user to know the true power consumption of the pump system and evaluate the true operating cost.