Electric machines are important devices that convert electrical energy into mechanical energy and are extensively used in a wide range of applications.
1. Mechanical and Aerodynamic Noise
Several authors have analyzed the aerodynamic noise in electric motors
[1][2][3][5,6,7]. It is expected that the noise level will increase as the speed of the motor’s cooling fan increases. According to
[2][6], electromagnetic noise is more dominant at low speeds, while aerodynamic noise is more dominant at high speeds. To demonstrate this, the motors are slowly run up to 3300 rpm and the sound pressure is measured. The same measurement is repeated once the motor is running at 3300 rpm, but with the power turned off.
This type of noise is unaffected by motor load. The level of aerodynamic noise remains the same whether the motor is running at full load or with no load at all.
The International Electrotechnical Commission (IEC) has defined the degree of protection against mechanical damage and environmental factors, such as moisture or corrosive vapors, in the standard IEC60034-5. The designation for the degree of protection is denoted by the letters “IP” followed by two numbers. For example, a motor labeled as IP23 indicates that it is open to the surrounding environment, while a motor designated as IP55 suggests that it is enclosed and protected. In this regard, noise levels differ depending on whether the motor is open (IP23) or closed (IP55). The ventilation noise emitted by open motors is significantly higher than that of closed motors, as the rotor generates less noise when enclosed. This also applies to noise generated within the motor, whether of electromagnetic or mechanical origin. Therefore, in order to minimize noise levels, it is recommended to design motors that are as closed as possible (IP55).
There are several sources of mechanical noise, such as bearings, eccentricity between rotor and stator, and mechanical unbalance. In
[4][8], the influence of manufacturing tolerances has even been analyzed. It should be noted that the noise caused by rotor eccentricity can also be considered as a type of electromagnetic noise, since the imbalance of electromagnetic forces caused by the eccentricity leads to increased noise.
In
[5][6][7][9,10,11] the influence of rotor eccentricity on motor noise has been analyzed. According to
[7][11], four types of eccentricities are distinguished (as can be seen in
Figure 1):
Figure 1.
Main mechanical sources of air gap eccentricity: (
a
) shape deviation, (
b
) parallel eccentricity, (
c
) inclined eccentricity and (
d) curved eccentricity [7].
) curved eccentricity [11].
When the stator and rotor are not regular cylinders, their shape deviation can produce air gap eccentricity, as in Figure 1a. Another very important, and widely analyzed, type of air gap eccentricity is that the rotor axis does not coincide with the stator axis, as in Figure 1b–d. When the rotor axis is straight, the eccentricity can be divided into parallel eccentricity, Figure 1b, and inclined eccentricity, Figure 1c. When the rotor shaft is bent, the eccentricity type is called curved, as in Figure 1d.
Test measurements
[5][6][9,10] demonstrate that eccentricities between rotor and stator lead to an increase in the noise level generated by motors. Another interesting conclusion obtained by
[8][12] is that the minimum achievable noise is highly dependent on the eccentricity value, but the dimensions required for the motor to have the lowest noise level are independent of any type of eccentricity.
2. Electromagnetic Noise
Electromagnetic fields result in forces that cause structural vibrations. The magnitude of the vibrations depends on the amplitudes and especially on the frequencies of the forces, and they must not coincide with the natural frequencies of the system. These surface oscillations then cause a conversion from vibrational energy to sound energy.
2.1. Electromagnetic Forces
There are two types of forces behind noise of electromagnetic origin occurring in electrical machines: magnetostriction forces and Maxwell forces. In radial-flow rotating machines, the main structural modes of interest for acoustic noise are those that involved radial vibration waves of the yoke (circumferential modes), which are generally the most efficient ones for radiating acoustic noise. Both magnetostriction and Maxwell forces can generate radial deflections of the stator yoke
[9][13].
According to
[10][2], obtained from Maxwell’s Tensor, the radial magnetic forces per unit area are given by Equation (4):