**7. Determining the time step**

When calculating the vibration of an electrical machine, it is important to note, as in the case of its measurement, the need to take into account the sampling frequency of the search signal. In the case of finite element calculation, this sampling frequency is represented by the time step of the individual calculations.

Results of transient analysis are linear approximate in the Ansys program. This can cause data loss. Example of choosing a time step or sampling frequency is shown in **Figure 4** on the simple signal. The used signal shows the sinus function with frequency 1 Hz. When selecting a large time step (specifically 3 Hz), this function is approximated by a straight line. There is a complete loss of function. When there is use 5 values on signal sampling and there is use linear interleaving function for his reconstruction, then constructed signal has triangle waveform. The calculated signal is the same as the original function only when using a sampling frequency of 36 Hz (and higher). This example shows how the time step can affect the results of the time waveform

**Figure 4.** Linear approximation of sin signal.

calculations of primitive function. Even greater influence can be experienced on the course of the calculated waveforms of the vibrational signals, which are much more complicated.

For the sampling of the general signal, the so-called Nyquist condition is mentioned in the literature. This indicates that each frequency-limited time function can be replaced by samples taken with the *Tsam* period, which is equal to half of the overturned value of the *<sup>f</sup>* max , the highest frequency contained in the sampling signal. Therefore:

$$f\_{am} = \frac{1}{T\_{am}} \ge \mathcal{D}f\_{\text{max}} \tag{3}$$

where *f sam* is sampling frequency and *<sup>f</sup>* max is maximal frequency included in the signal.

As shown in **Figure 4**, when this state is used for a complex signal (such as the vibration are complex), there has been a strong deformation of the entire calculated waveform. Determining the time step requires some experience and knowledge that leads to a compromise between the quality of the result and the time consumption of the calculation [4].
