**2. Basic concepts of noise**

The sound wave is generated by vibrating objects and can be defined as mechanical interference with the finite speed of advancing through the media. These waves have small amplitude, adiabatic oscillation are characterized by a wave speed, wavelength, frequency and amplitude. Sound has the character of longitudinal sound waves in the direction of propagation in the environment. In other words, it is the movement of individual particles

$$p\_c = p\_\text{b} + p(t) \tag{1}$$

$$p(t) = p\_0 . \cos(\alpha. t + \varphi) = p\_0 . \cos(2. \pi. f. t + \varphi) \tag{2}$$

$$p\_c = p\_0 + p\_0 \cos(2.\pi.f.t + \varphi) \tag{3}$$


$$\text{ip} \frac{p\_0}{\sqrt{2}}\tag{4}$$

$$L\,\mathrm{p} = 20.\log\left(\mathrm{p}/\mathrm{p}\,\mathrm{0}\right)\tag{5}$$


$$\mathbf{P\_{ac}} = \mathbf{p\_c} \cdot \mathbf{v}.\mathbf{A} \tag{6}$$

$$\mathrm{L\_{Pic}} = 20.\log\left(\frac{\mathrm{v}}{\mathrm{10^{-9}}}\right) + \mathrm{k} \tag{7}$$

$$I = \overrightarrow{p(t)} . \overrightarrow{\nu(t)}\tag{8}$$


Dividing the sound by timing:


The interest noise frequency is over 1000 Hz for induction machines. Noise of Electrical Machines is characterized as a set of sounds that are caused by rapid changes in air pressure. These changes cause most commonly:

Noise of Induction Machines 211

the acoustic signal in time. If is this decline immediately, then it is obviously a component of the noise of electromagnetic origin. Another method of investigation is the measurement of

Noise origin of ventilation is crucial observe especially in machines with high rotational speed. Detailed analysis of the fan noise shows that the main source in this case is very fan with its nearest surroundings. It is the decisive exceeds other sources of noise, which can be, for example rotor wings, radial or axial cooling channels in the machine, input and output

Frequency analysis of noise ventilation origin shows that the spectrum has a broadband character, either discrete or vice versa. In the first case, the aerodynamic noise is created from turbulent airflow near fan blade and near the entrance, but also the output edges of blades. These pulsations are uneven both in space and in time, so the frequency spectrum

In contrast, discrete nature of the spectrum, sometimes the siren phenomenon can arise. This phenomenon arise if the fan or behind obstacles (such as a blade with these obstacles) is not profile of velocity uniform air flow around the wheel circumference, leading to periodic

The noise of mechanical origin is primarily inflicted on roller bearings and unbalance of rotating machine parts. Rolling bearings can create multiple frequency components, which have their origin mainly in inequality as part of rolling paths of the bearing rings. In

The influence of magnetic induction in the air gap formed magnetic forces; these forces operate across various directions. They may also have various amplitude and frequency. Their work is split between the rotor and stator of electric machine. Their characteristics

The construction of the rotor is the main radiator noise machine. If the frequency is close to the radial force or equal to one natural frequency of the stator system, resonance occurs which leads distorted stator system with vibration and noise. Magnetostriction noise electric machine can be neglected in most cases due low and high frequency 2f arrangement r = 2p of radial forces, where f is the fundamental frequency and p is the number of pole pairs. However, the radial forces due magnetostriction can reach up to 50% the radial forces

Stator: ���� ���(ɷ�� � � �� � � ��) (9)

Rotor: ���� ���(ɷ�� � � �� � � ��) (10)

created of wind noise is broadband and contains all components of the audible band.

pulsation of pressure. Then the siren noise is produced naturally.

principle, the noise of mechanical origin has a mixed character.

depend on the size and shape of the air gap and a number of other factors.

electromagnetic noise spectrum for different values of power - or even frequency.

caps and the like.

**3.1. Electromagnetic noise** 

produced in the air gap magnetic field.

Magnetic flux density wave


Basic sources of noise are induction motors (see diagram):


**Figure 1.** Division of noise sources in electrical machines

The noise from electromagnetic source is the most typical component noise of electrical machine. Its cause is the vibration of motor body, or other parts of the machine on which work the electromagnetic forces. Frequency Spectrum noise of the electromagnetic source has discrete character, while there is very distinct directional radiation characteristics of this component in many cases.

Determining the influence of this component on the overall noise of electric machine is often simply done so, that after switching off the machine from the network is observed decline in the acoustic signal in time. If is this decline immediately, then it is obviously a component of the noise of electromagnetic origin. Another method of investigation is the measurement of electromagnetic noise spectrum for different values of power - or even frequency.

Noise origin of ventilation is crucial observe especially in machines with high rotational speed. Detailed analysis of the fan noise shows that the main source in this case is very fan with its nearest surroundings. It is the decisive exceeds other sources of noise, which can be, for example rotor wings, radial or axial cooling channels in the machine, input and output caps and the like.

Frequency analysis of noise ventilation origin shows that the spectrum has a broadband character, either discrete or vice versa. In the first case, the aerodynamic noise is created from turbulent airflow near fan blade and near the entrance, but also the output edges of blades. These pulsations are uneven both in space and in time, so the frequency spectrum created of wind noise is broadband and contains all components of the audible band.

In contrast, discrete nature of the spectrum, sometimes the siren phenomenon can arise. This phenomenon arise if the fan or behind obstacles (such as a blade with these obstacles) is not profile of velocity uniform air flow around the wheel circumference, leading to periodic pulsation of pressure. Then the siren noise is produced naturally.

The noise of mechanical origin is primarily inflicted on roller bearings and unbalance of rotating machine parts. Rolling bearings can create multiple frequency components, which have their origin mainly in inequality as part of rolling paths of the bearing rings. In principle, the noise of mechanical origin has a mixed character.
