VENTILATION NOISE IN ENCLOSED FAN-COOLED ASYNCHRONOUS ELECTRIC MOTORS

E. A. Knipenberg

(Presented by the scientific seminar of the Department of Electrical Machines and General Electrical Engineering)

The level of noise and vibration in electric motors depends on many different causes that are interconnected. In each individual case it is advisable to find and analyze separately each component of the noise, so that it is clearer which of them must be reduced first. It is generally accepted to divide the main sources of noise and vibration in electric machines into three classes:

a) ventilation noises;
b) mechanical noises and vibrations;
c) magnetic noises and vibrations.

For cooling asynchronous electric motors, radial fans are usually used. Analysis of the noise of motors equipped with such fans shows that, when rotating, the fan element emits vortex noise with a continuous frequency spectrum. This noise is especially annoying at the place where the electric machine is installed, since it is generated and transmitted directly through the air, and not through vibration of the mechanical parts of the machine. The frequencies of the noise produced by the elements of a fan with \(Z\) blades and rotation \(n\) rpm are determined by the relation

\[ f = kZ \frac{n}{60}, \tag{1} \]

where \(k\) is an integer indicating the order of the harmonic.

In addition to these frequencies, the fan creates vortex noise with a continuous spectrum in the high-frequency band because of changes in the velocity of the air flow along the blades. When air flows around the shield and the motor housing, a large number of vortex lines arise on the surface, at irregularities and projections, producing noise over a wide frequency range.

Especially unpleasant noise (of the siren type) may appear in some fans when air strikes fixed obstacles, if the distance between the fan and the stationary part is too small.

This critical distance increases with increasing peripheral speed. Such phenomena must be taken into account in design.

For example, for an electric motor at 12,000 rpm, instead of the usually recommended distance, equal to approximately 15% of the external diameter, taking the indicated effect into account, this distance must be increased—

to increase to 20–25%, or, correspondingly, to reduce the outside diameter of the fan.

In addition, vortex formation at the inlet opening of the casing has a very great influence on the nature of the generation of ventilation noise. The correct choice of the diameter and configuration of the casing inlet opening is an important factor in reducing ventilation noise.

When designing and developing electric motors, for an approximate estimate of the level of ventilation noise produced by a centrifugal fan it is advisable to use various empirical relations found from practice. It may be assumed that the noise produced by a centrifugal fan is approximately proportional to the rotational speed to the sixth power and to the blade area to the first power.

It is sometimes assumed that, at peripheral speeds up to 50 m/s, the output sound power is proportional to the number of revolutions to the fifth power [1]. For a given diameter \(D_0\) and speed \(n_0\) of the motor fan, the change in the sound-pressure level produced by the fan of another similar machine is expressed by the equation:

\[ \Delta L_p = 10 \lg \left( \frac{D}{D_0} \right)^{\alpha} + 10 \lg \left( \frac{n}{n_0} \right)^{\beta}, \tag{2} \]

where \(D_0\) is the fan diameter, \(\alpha = 7\); \(\beta = 5\) for speeds up to 50 m/s, and \(\beta = 6\) or 7 for peripheral speeds greater than 50 m/s.

In order to study the possibility of reducing the ventilation component of noise by changing the design of the fans, experiments were carried out on motors of the AO2 series.

The investigations were arranged so that the required heat removal was ensured with the minimum fan diameter, and the overheating of the stator windings of the motors remained unchanged with different fans. The development of the fan design was based on the fact that the air flow in multiblade plate-type fans is more ordered and the aerodynamic losses are smaller than in the 3- and 4-blade fans currently used. Smaller aerodynamic losses lead to a reduction in the noise level.

The fans for the investigations were made of the plate type (plate inclination angle \(70^\circ\)) with an increased number of blades.

The investigations were carried out on two- and four-pole electric motors of the AO2 series, frame sizes 2, 3, and 4.

From the test materials the following conclusions can be drawn:

1. The experimental data showed that the use of a plate-type fan gives a reduction in the ventilation component of sound pressure at certain frequencies of up to 7 dB on two-pole electric motors (Figs. 1, 2, 3).

In terms of sound level, a reduction of up to 3–2 dB is observed. The insufficiently effective reduction in sound level, as compared with the significant reduction in sound pressure at individual frequencies, is explained by the fact that the design of the end shield and casing was not changed; consequently, the components of vortex noise caused by the stationary projections of the bearing end shield and by the configuration of the casing inlet opening remained practically unchanged.

1. From the standpoint of thermal characteristics it may be concluded that plate-type fans with an increased number of blades provide the required heat removal with improved acoustic characteristics.

2. Similar investigations for four-pole and six-pole electric motors gave a considerably smaller effect, since in absolute magnitude the ventilation component of the noise proved to be

Fig. 1. Spectrograms of the airborne noise of the AO2-42-2 electric motor at \(U=380\) V, s.f. — Y, \(f=50\) Hz. 1 — with the fan of the standard design (3 blades), 2 — with a disk-type fan (8 blades)

Fig. 2. Spectrograms of the airborne noise of the AO2-32-2 electric motor at \(U=380\) V, s.f. — Y, \(f=50\) Hz. 1 — with the fan of the standard design (3 blades), 2 — with a disk-type fan (6 blades)

approximately the same as the bearing and magnetic components. This indicates that, in order to improve the acoustic characteristics of electric motors with lower speeds, it is necessary to combat all noise components.

Recommendations for reducing ventilation noise

On the basis of experimental data for AO2 electric motors of frame sizes 2, 3, and 4, as well as studies carried out in the development of low-noise electric motors (elevator motors, motors for driving Linotype machines, etc.), and also literature data, the following measures may be recommended for reducing ventilation noise:

Fig. 3. Spectrograms of the airborne noise of the AO2-21-2 electric motor at \(U = 380\) V, star connection, \(f = 50\) Hz. 1 — with a fan of the basic design (3 blades), 2 — with a plate-type fan (8 blades).

1. When centrifugal fans are used, the most effective are plate-type fans with a plate inclination angle from 65 to 80°. The larger of these values corresponds to higher rotational speeds. (For example, when refining electric motors for the woodworking industry for 12,000 rpm, an experimentally selected inclination angle of 78° proved to be the most acceptable.) The fan blades are best arranged along the direction of flow.

2. Blades that guide the air flow, and such parts as may resonate, are best made of plastics possessing greater internal damping than steel and aluminum.

3. Sharp corners and edges that fall into the jet of the air flow must be rounded. A smooth surface and rounded corners effectively help reduce vortex noise.

4. To avoid strong noises (of the siren type), it is necessary to choose the distance from the rotating edges of the fan to stationary parts sufficiently large—15–25% of the fan diameter. In this case, the larger distance corresponds to the higher rotational speed.

5. It is advisable to guard ventilation openings not with a stamped metal grille, but with wire mesh.

6. In all cases, in order to reduce the amount of cooling air, it is necessary to strive to provide minimum temperature drops from the winding to the cooling surfaces.

LITERATURE

1. The problem of noise in electric machines. Elektro-Revue, 1962, May/June, pp. 9–10.

2. Proceedings of VNIIEM. Vol. 20, “Energiya,” 1965.

3. Folkman Kh. Design and technological prerequisites for the production of low-noise three-phase asynchronous electric motors with low vibration in the power range up to 10 kW. Elektrie, No. 11, 1968.