INVESTIGATION OF VENTILATION IN ENCLOSED LOW-POWER ASYNCHRONOUS MOTORS

V. A. Zhadan, D. I. Sannikov, G. A. Sipailov

(Presented by the scientific seminar of the departments of electrical machines and general electrical engineering)

The modern trend in the development of motors of mass-produced series, consisting in an increase in motor power at an unchanged center height, makes it necessary to intensify ventilation. Since the possibility of increasing the fan diameter has practically been exhausted in modern motor designs, other ways of increasing its capacity must be used.

The diskless-type fan used in the AO2 series does not provide sufficient possibilities for increasing the flow rate.

In order to give recommendations on the choice of the design, the number and width of the blades, and the fan diameter, more than 50 experimental fan samples were investigated. In these investigations, an important place was occupied by the search for the best fan design; moreover, two fundamentally different design variants were developed: a plate-type fan (Fig. 1, a) and a diskless fan (Fig. 1, b). The small-diameter disk in the latter is necessary for fastening the blades and in principle does not affect the operation of the fan.

Fig. 1

This design of the fans made it possible to vary the number of blades and the outside diameter during the tests. The fans were tested with numbers of blades 4, 8, and 16. The disk fan had an outside diameter \(D_2 = 250, 240, 230\) mm; the diskless fan had \(D_2 = 239, 223, 210\) mm. All investigations were carried out on an AO2-42-4 motor. The results of measuring the fan pressure \(H_0\), the air flow rate \(V_p\), the air velocity at the beginning and at the end of the inter-rib channels—\(v_n\) and \(v_k\)—and the ventilation losses \(P_v\) are given in Table 1. For comparison of the two investigated designs, dependences of these quantities on the fan diameter were plotted (Fig. 2).

Table 1

Fig. 2. Dependence of the air flow rate \(V_p\) and ventilation losses \(P_v\) on the fan diameter \(D_2\):
○ — for the diskless fan, ● — for the plate fan.

The investigations showed that a disk-type fan design is more preferable, since it provides 8–10 percent greater air delivery and a smaller change in air velocity along the length of the frame in comparison with a diskless fan of the same diameter. However, a disk-type fan has greater ventilation losses than a diskless fan of equal diameter, and a lower energy efficiency, which is determined by the formula

\[ \eta_{\mathrm{e}} = 9.81 \frac{V_{\mathrm{p}} H_{\mathrm{p}}}{P_{\mathrm{v}}}, \]

where \(V_{\mathrm{p}}\) is the operating air flow rate, m\(^3\)/s;
\(H_{\mathrm{p}}\) is the operating fan pressure, kg/m\(^2\);
\(P_{\mathrm{v}}\) is the ventilation loss, W.

The difference in efficiency for disk-type and diskless fans decreases as the outer diameter of the fan increases, tending toward zero.

The air flow rate increases steadily as the fan diameter is increased; consequently, there is no diameter that is optimal with respect to flow rate, and it should be chosen as large as possible, proceeding from the specified center height.

It was established that increasing the number of fan blades to more than 8 for motors of the 4th frame size does not lead to a significant increase in flow rate. If, when the number of blades is increased from 6 to 8, the flow rate increases by 3–6 percent, then when the number of blades is increased from 8 to 16 the flow rate rises by no more than 1–2 percent. It may seem that increasing the number of blades above 5 is inadvisable, since the flow rate then increases only slightly. However, measurement of the dynamic pressure in the interrib channels indicates that, in this range of variation of the number of blades, a substantial increase in the intensity of frame blowing still occurs as a result of increased flow turbulence.

The dependence of the principal ventilation parameters on the width of the fan blade \(v_{\mathrm{l}}\) was determined (Fig. 3). It was established that increasing the blade width by 20 mm in comparison with the existing value \(v_{\mathrm{l}} = 55\) mm leads to an increase in flow rate from 0.103 to 0.105 m\(^3\)/s, i.e., by 2.0 percent. Reducing the blade width by 10 mm in comparison with the existing value leads to a decrease in flow rate to 0.101 m\(^3\)/s, or likewise by 2 percent.

The decrease in air velocity at the beginning \(v_{\mathrm{n}}\) and at the end \(v_{\mathrm{k}}\) of the interrib channels at the greatest blade width is explained by a certain nonidentity of the experiment in connection with displacement of the housing.

Thus, within the limits considered, changing the blade width \(v_{\mathrm{l}}\) has only a slight effect on the air flow rate. The possibility of increasing \(v_{\mathrm{l}}\) in order to raise the flow rate is limited by the overall dimension of the motor, while when \(v_{\mathrm{l}}\) is reduced the resulting saving in fan material does not justify the reduction in air flow rate.

Of definite interest is the study of the aerodynamics of the fan and the ventilation duct. Centrifugal fans with inclined blades and axial-type fans are more refined than centrifugal fans with radial blades; however, they cannot be used in reversible machines with self-ventilation. At the same time, the prospects for using the most refined fan types have led to the creation of various designs with rotating blades, one of which was investigated in this work (Fig. 4). The blades are rotated during reversal under the action of the reaction of the air flow captured by the fan. The investigations were carried out with fans having 3 and 4 blades at a velo-

Fig. 3. Dependence of ventilation parameters on blade width \(b_l\)

Fig. 4

Air motion

rotational speeds of 3000 and 1500 rpm, respectively. Analysis of the results shows that there is an optimum value of the blade turning angle. For a rotational speed of 1500 rpm, the greatest value of the air flow rate \(V_p\) and of the air velocity at the beginning and at the end of the inter-rib channels, \(v_n\) and \(v_k\), occurs when the blades are turned by \(13\text{–}20^\circ\). At 3000 rpm, the optimum angle increases to \(30\text{–}40^\circ\). An increase in the turning angle is accompanied by a gradual decrease in ven-

ventilation losses. At the optimal angle of rotation the losses are less than with a radial blade by 5–10 percent for 1500 rpm and by 15–20 percent for 3000 rpm, while the flow rate increases, respectively, by 3 percent and by 16 percent. Since the blade rotation angle is considerably smaller than the blade setting angle in an axial fan, it may be assumed that the improvement in the fan characteristics occurs because of a reduction in pressure losses when the air enters the blade zone.

The operation of the fan is significantly affected by the equivalent aerodynamic resistance of the air duct. A considerable part of the aerodynamic resistance of the ventilation path is the resistance of the inlet into the casing, which is determined by the area of the inlet openings and the shape of the edges. Therefore, studies were carried out on casings with different diameters of the inlet grille, the results of which are shown in Fig. 5. The optimal value of the flow rate \(V_p\) and of the air velocity in the inter-rib channels at the beginning \(v_n\) and at the end \(v_k\) of the frame occurs at the relative diameter

\[ D_{\mathrm{in}}^{*}=\frac{D_{\mathrm{in}}}{D_{\mathrm{tr}}}=0.8, \]

where \(D_{\mathrm{tr}}=280\) mm is the diameter of the casing in its cylindrical part.

Fig. 5. Dependence of ventilation parameters on the relative diameter of the casing inlet opening \(D_{\mathrm{in}}^{*}\)

From the results of the study it is evident that, as \(D_{\mathrm{in}}^{*}\) increases, there is simultaneously a decrease in aerodynamic resistance due to the increase in inlet area and a decrease in the fan pressure under the influence of leakage of the air flow. At first the first factor predominates, and at large values of \(D_{\mathrm{in}}^{*}\) the second predominates; consequently, the flow rate has a maximum at \(D_{\mathrm{in}}^{*}=0.8\).

The maximum value of the flow rate, attained by optimizing the inlet diameter for various designs of the casing and the fan, is 0.140–0.154 m³/s, with an initial flow rate of 0.101 m³/s.

For convenience and to simplify preparation for the experiment, the studies on optimizing the inlet opening were carried out without a grille. Installing a grille with a free area of 0.8–0.85 in the opening with the optimum diameter reduces the flow rate by 2–5 percent. However, this reduction can be fully compensated by rounding the sharp edges of the inlet to the casing with a radius of 5–7 mm. This requirement can be met if the casing is made as a casting or stamped with rounded edges.

Along with the study of the inlet section of the casing, the configuration of the outlet section was examined; it usually has a cylindrical shape and directs the air along the frame. In order to press the air more strongly against the frame, this part was made in the form of a truncated cone in two variants: with the generatrix directed at angles of 4° and 14°, while the diameter of the casing at the outlet remained unchanged. Although the air flow rate in this case increased by 14.8 percent and 16.8 percent as a result of the increase in the diameter of the fan chamber, the overheating of the frame increased correspondingly by 9.4 percent and 17.4 percent. The cause of the deterioration in cooling was that, instead of the flow being pressed against the frame, it was reflected and dispersed. This confirms I. Khak’s well-known conclusion that a cylindrical casing is the most advantageous.

Conclusions

1. To ensure maximum fan output, it is necessary to use a disk-type fan with 8 blades at 1500 rpm.

2. Increasing the width of the fan blade beyond the existing value is inadvisable, since it does not lead to a noticeable increase in flow rate.

3. A fan with adjustable blades is considerably more complex than a conventional one and gives only a slight increase in air flow rate; therefore, its use in motors up to 3000 rpm is inadvisable. The prospects for using them at higher speeds for the purpose of reducing losses are connected mainly with the reliability of the design.

4. The optimum diameter of the inlet opening in the casing is equal to 0.8 of the outside diameter of the casing. With this diameter and rounded edges of the opening, the air flow rate increases by 40–50 percent.

5. A casing with a narrowing outlet section does not produce a positive effect.