**6.2. Noise of induction machine**

On Fig. 7 is an analysis of the measured noise using MATLAB. Specifically, was carried Fast Fourier Transform (FFT). Dominant frequency is 600 Hz. This frequency is multiple of power supply frequency. It is a frequency of radial forces. In measurement signal can be involved many harmonics frequencies of radial forces. Than we can write equation

$$f\_v = 6.k.f\tag{48}$$

Where

220 Induction Motors – Modelling and Control

�(�) <sup>=</sup> � �

Then

�

**6.1. Disturbed surroundings** 

� ��(�) ��(��)

relations given by (43) can be proven by using (44), and writing (38) as

� .���(�) � ���(�)�. ��.�.�.�.��� � � �

**6. Measurement noise of induction machines** 

can be created undesirable frequencies in the frequency band.

�=�1 1

Equations (44) and (45) are very useful to convert from one representation to the other. When �(�)is real, ��(�) and ��(�)are also real. Then, (44) shows that ��(�) and ��(��) are complex conjugates of each other. Equations (44) and (45) are also valid in the case of the discrete time Fourier transformation. In addition, they are valid for Fourier series and the discrete Fourier transforms with the replacement of fby the frequency index n. The RFT

�(�) <sup>=</sup> � ���(�). ���(�. �. �. �) � ��(�). ���(�. �. �. �)��� �

Surrounding noise sources have an impact on the measurement of electrical machinery. It is not always possible to perform measurements in specialized laboratories, which are perfectly sound-insulated. To laboratory measurement can penetrate the noise from nearby sources (see Fig. 4), which is inaudible to the human ear. The interference from other sources

 a) Noise detected in the laboratory b)Fast Fourier Transformation of laboratory noise

Interference of other sources in the neighborhood of workplace cannot be directly prevented, but you can minimize their impact on analysis of the measured signal. Before the measurements it must be made measurement ambient noise before the main measurements. It is necessary to determine whether the background noise is random, or it is periodically repeated. In the case of random noise is preferable to wait to other time of measurement or it must count with errors in the measurement. In the event that can be measurement of noise repeated. Can be recorded the extent of spectral interference with which will be calculate when evaluating the measured results. From Spectral analyses of interference is possible to

**Figure 4.** Noise measurement in the laboratory when the machine is switched off

1 ���.� ��(�)

� . �� �

� ���(��) � ���(��)�. ��.�.�.�.��� (46)

� (47)

��(��)

� (45)



For ݂ ൌ ͷͲݖܪ are frequencies of radial forces݂௩ ൌ ͵ͲͲǡ ͲͲǡ ͻͲͲǡ ǥ ݖܪ.

Noise of Induction Machines 223

**Figure 9.** Rotor Eccentricity - Mechanical measurement

**7. Conclusion** 

main sources of noise are.

deviations of movement in rotation of bearing.

Analyze of noise was performed on the one rotation of rotor. The Fig 5 shows the noise levels depending on the position of the rotor. As the graph shows it is to generate greater

Diagnosis of induction motors is a very complex issue that has many components. One of them is the analysis of motor noise. Noise measurement asynchronous machines are the commonly used diagnostic method. This method is relatively simple. You need to be near an electrical machine quality microphone and recording equipment. Analysis itself can be the

Subsequent analysis of the signal can then indicate whether the machine operates as required, or whether there was damage to electrical equipment. Based on the fast Fourier analysis of noise can be determined which components of the signal are dominant. Based on knowledge of layout design of the engine is then possible to determine what is causing individual harmonics. According to the frequency it is possible to determine which there the

A major problem in measuring the noise may be interference from nearby sources. To avoid

During measurements realized appeared possible link between noise and rotor eccentricity of electrical machinery. In the analysis of noise is dominant skew in the range of 120 in one rotation. In the same range (120 ) was measured the dominant deflection of rotor eccentricity this rotor machine. Given that the machine has not a cooling system, there is not source of aerodynamic noise; there are only two possible causes of this deviation. Source of electromagnetic noise would not cause deviation only at certain rotor position, but in the whole rotation. Displacement of noise in a certain position the rotor it cannot assign too resources source of mechanical noise. This group includes vibration bearings. During the measurement was verified that the bearings are not damaged. There are not larger

the external influence of external noise is possible only in specialized laboratories.

levels of noise in the position of the rotor from 300 to 60 degrees (about 120 degrees).

performed on specialized software, either on the spot or later in laboratory.

**Figure 7.** Fast Fourier Transformation of induction machine noise

It was done measurements eccentricity of rotor. Eccentricity of rotor is shown in Fig. 9. From the measured values it was found that the largest deviations occur in the range of approximately 120 degrees.

When comparing the noise of induction machines recorded on one rotation and values of rotor eccentricity can see a connection. In both cases (Fig. 8 and Fig. 9) appeared larger deflection in the range of 120 degrees. Extreme deviation is in a different quadrant in each graph. This is due to the different measurement principles. Noise measurements done digitally, while measuring the eccentricity was used mechanical method. It was therefore not possible to accurately determine the initial rotor position in both measurements.

it can be argued that the noise of induction machines is generated of the rotor who has eccentricity. Given that the, that machine is equipped with a ventilator, there are two sources of noise. The influence of the fan but will not cause displacement of only a specific part of one rotation.

Given that the measured induction motor was not equipped with cooling system (fan) can be assumed, that the vibration and thus the noise are produced only by electromagnetic source and mechanical source.

**Figure 8.** Noise envelope – 1 rotation

**Figure 9.** Rotor Eccentricity - Mechanical measurement

Analyze of noise was performed on the one rotation of rotor. The Fig 5 shows the noise levels depending on the position of the rotor. As the graph shows it is to generate greater levels of noise in the position of the rotor from 300 to 60 degrees (about 120 degrees).
