**3.1. Electromagnetic noise**

210 Induction Motors – Modelling and Control

 Infrasound - up to 20Hz Low frequency - 20Hz to 40Hz

 Electromagnetic source The mechanical source The aerodynamic source

**Noise Sources**

component in many cases.

pressure. These changes cause most commonly:

Vibration of machine parts or the whole of its surface

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

Mechanical

Aerodynamic

Electromagnetic

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

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

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

Self

Load induced

Bearing

Balancing

Non-uniform air-gap

Auxiliaries

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

Aerodynamic phenomena that lead to pulsation of pressure near the machine

 RF - 8kHz to 16kHz Ultrasound - 20kHz over Dividing the sound by timing:

 Steady variable intermittent pulse

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

> 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 produced in the air gap magnetic field.

Magnetic flux density wave

$$\text{Stator:}\ B\_{m1}.\cos(\mathbf{a}\_1.t + k.\,a + \Phi\_1)\tag{9}$$

$$\text{Rotor:}\ B\_{m2}.\cos(\alpha\_2.t + l.a + \Phi\_2)\tag{10}$$


$$P\_{mr} = 0,5.B\_{m1}.B\_{m2}.\cos[\left(\alpha\_1 + \alpha\_2\right).t + \left(k + l\right).a + \left(\Phi\_1 + \Phi\_2\right)]+$$

$$+0.5.B\_{m1}.B\_{m2}.\cos[\left(\alpha\_1 + \alpha\_2\right).t + \left(k - l\right).a + \left(\Phi\_1 - \Phi\_2\right)]\tag{11}$$

$$f\_r = f\_1.\left[\frac{n.Z\_r}{p}.\left(1-\text{s}\right)+2\right]$$

$$f\_r = f\_1.\left[\frac{n.Z\_r}{p}.\left(1-\text{s}\right)\right] \tag{12}$$


$$f\_r = f\_1. \left[\frac{n.N\_{rs}}{p}, (1-s) + 2\right]$$

$$f\_r = f\_1. \left[\frac{n.N\_{rs}}{p}, (1-s)\right]$$

$$f\_r = f\_1. \left[\frac{n.N\_{rs}}{p}, (1-s) + \frac{1-s}{p}\right]$$

$$f\_r = f\_1. \left[\frac{n.N\_{rs}}{p}, (1-s) + 2 + \frac{1-s}{p}\right] \tag{13}$$

$$f\_r = f\_1 \cdot \left[ \frac{nN\_{rs}}{p} . (1 - s) + 4 \right] \tag{14}$$

$$f\_r = f\_1. \left[ \frac{nN\_{rs}}{p}, (1 - s) + 2 \right] \tag{15}$$

$$f\_{\text{stat}} = 2.f\_1 \tag{16}$$

$$
\varepsilon = \frac{e}{g} = \frac{e}{R - r} \tag{17}
$$


$$
\Omega\_{\varepsilon} = \Omega. \left\{ \mathbf{1} - \mathbf{s} \right\} = \frac{\alpha}{p}. \left\{ \mathbf{1} - \mathbf{s} \right\} = 2. \pi. \frac{f}{p}. \left\{ \mathbf{1} - \mathbf{s} \right\} \tag{18}
$$

$$f\_{\rm DYN} = f\_1 \pm (1 - \mathbf{s}) . f\_{\rm S0} \tag{19}$$

$$f\_{\rm exc} \left[ (n\_{rt}.R \pm n\_d). \frac{1-s}{p}.n\_{\rm os} \right].f \tag{20}$$


$$L\_A = 60.\log U\_2 + 10.\log D\_2.b\_2 + \sum k\_1 \tag{21}$$


$$f\_v = 0.185. \frac{v}{n\_\circ} \tag{22}$$

$$f\_f = N\_b. \frac{N}{60} \tag{25}$$


$$L\_{\rm w} = 67 + 10.\log\_{10}(P\_{\rm out}) + 10.\log\_{10}(p) \tag{24}$$

$$L\_{\mathcal{W}} = 40 + 10. \log\_{10}(Q) + 20. \log\_{10}(p) \tag{25}$$

$$L\_{\rm w} = 94 + 20.\log\_{10}(P\_{\rm out}) - 10.\log\_{10}(Q) \tag{26}$$



movement of the whole system and thus increasing vibration and noise of the electric machine.

The main mechanical sources of the noise


### *3.4.1. Rolling bearings*

The noise of rolling bearings depends on the type of bearing and its construction and accuracy of bearing parts. The increase in vibration and noise level of bearings, when the rotational speed changes from ��to�� can be expressed as

$$
\Delta L\_v = 20. \log \frac{n\_2}{n\_1} \tag{27}
$$

Noise of Induction Machines 217

*3.4.3. Load induced noise* 

**4. Noise measurement** 

**4.1. Measurement process** 

limitations of this device.

**Figure 3.** Block diagram of measurement process

pressure.

In certain cases, the vibrations and thus noise transmitted from the load, which is connected to the induction motor. In most cases, this occurs with wrong balance or bad connects of couplings. Uneven distribution load acting on the motor shaft or inappropriate use of gears may also affect noise machine. The only possible protection against these effects is the perfect balance of the whole set and if possible an even distribution of forces acting on the connecting elements. Noise arises too due to coupling of the machine with a load, e.g., shaft misalignment. Next noise arises from belt transmission, from cogwheels and couplings. It may also arise to noise due to mounting the machine on foundation or other structure.

For measurement noise of induction machines can be used several techniques. The basic method for the measurement noise is the sound meter. It is a device which measures sound

Measurement of noise can be divided into three main parts. The first part is data capture. For this purpose, the most commonly used microphones, or specialized equipment to measure noise (sound level meter). Their output is usually an analog signal, which must be further processed. When choosing of microphone is needed careful heed on certain parameters that can affect measurement accuracy. One of the most important parameter is the sensitivity of frequency. Worse microphones not recorded of the entire spectrum of the measured noise. Thanks to this complicates achieve it of accurate analysis results. Other parameters include the microphone sensitivity, which indicates the size of the output voltage (mV / Pa), depending on the pressure acting on the membrane. In addition, the structural dimensions of the measurement microphone and also the type of sound field that which is measured. Computers are most frequently use for Signal processing. For this

Large numbers of types A/D converters is on the market. Some are stand-alone converters; others are integrated to the specialized measurement cards. In both cases, the measurement depends on the three main parameters. The first is the measuring range of the converter. It gives the minimum, respectively maximum, measurable value. Because the signal is weak from a microphone, there should be used an amplifier for its amplification. Another parameter of the A/D converter is the bit depth conversion. This parameter defines the

Record Data processing Analyze

reason it is necessary to convert from analog signal to digital form.

Ball pass frequency – outer race

$$f\_{or} = \frac{N\_b}{2}.n\_m.\left(1 - \frac{d\_b}{D}.\cos a\right) \tag{28}$$

Where


Ball pass frequency – inner race

$$f\_{lr} = \frac{N\_b}{2} \cdot n\_m \cdot \left(1 + \frac{d\_b}{D} \cdot \cos\alpha\right) \tag{29}$$

### *3.4.2. Sleeve bearings*


$$f\_{ov} = k.n\_m \tag{30}$$

k=1, 2, 3, …


$$f\_{gr} = N\_g.n\_m \tag{31}$$

��…number of groove
