33
The data is recorded for the operating conditions listed in Table 3-3. The frequency values in Table 3-1 correspond to the inverter frequency that is used to set the speed of the fan and pump impeller.
Table 3-3 Test matrix for vibration measurement 00100.0 30132.4 60164.5
0010.0
3010.5
6010.9
For all cases, the acceleration vector (i.e., all three orthogonal components) is measured. The signal is sampled at 2048 samples per second. For a sampling rate of 2048, a usable span of 800 Hz results. A total record length of 393216 points is collected. The record length is broken into 192 blocks of 2048 samples, resulting in a frequency bin of 1 Hz, to perform analysis. A hanning window is applied to the data and an overlap ratio of 75%o is used. The estimate of random error with 192 averages is
0.072.
Theoretical Model
The system is mathematically modeled as a two-input / single-output system as shown in Figure 3-11 [Bendat and Piersol 2000]. In Figure 3-11, xi(t) corresponds to the accelerometer data from the fan, X20t corresponds to the accelerometer data from the pump, n(t) is uncorrelated noise (with the inputs xi(t) and X2(t)) and y(t) is the vibration measured at the test section. Since the tni-axial accelerometer gives accelerations in three directions, X1, X2, and y can be only one of the three components. In order to choose the