STUDY OF VENTILATION IN ENCLOSED ASYNCHRONOUS MOTORS AND THEIR APPROXIMATE VENTILATION CALCULATION

V. A. Zhadan, D. I. Sannikov

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

In works [1, 2] it is shown that disk-type fans provide greater possibilities for a significant increase in their capacity, although they have somewhat increased ventilation losses in comparison with diskless fans. Therefore, the investigations were carried out, on the basis of motors of the AO2 series of frame sizes 1–4 and D-132, only for disk fans, the dimensions of which were varied over wide limits. The tests were carried out at various rotational speeds. Since the character of the principal relationships is the same at different rotational speeds, Figs. 1–4 show, for illustration, the dependences of air flow rate and ventilation losses on the principal dimensions of the ventilation unit for only one rotational speed—1500 rpm.

Relationships were obtained for the air flow rate \(V\) and the ventilation losses \(P_{\mathrm{v}}\) as functions of the principal dimensions of the ventilation unit: the outside diameter of the fan

\[ D^{*}_{2} = \frac{D_{2}}{D_{k}}; \]

the inside diameter of the fan

\[ D^{*}_{1} = \frac{D_{1}}{D_{2}}; \]

the blade width

\[ b^{*} = \frac{b}{D_{k}}; \]

the number of fan blades \(N\), and the diameter of the outlet grille of the casing

\[ D^{*}_{\mathrm{out}} = \frac{D_{\mathrm{out}}}{D_{k}}. \]

In order to compare motors of different frame sizes, the principal dimensions of the fan were taken in relative units; as the base dimension determining the motor frame size, the diameter of the casing in the cylindrical part, \(D_{k}\), was selected.

With an increase in the outside diameter of the fan \(D^{*}_{2}\) (Fig. 1, a), the flow rate increases almost proportionally, while the ventilation losses increase more sharply. Since the tested motors of the AO2 series, and to an even greater degree the D-132, have a compressed casing, the maximum outside diameter is 0.9. In motors for 3000 rpm, where ventilation losses constitute a considerable value, it is advisable to reduce \(D^{*}_{2}\) to 0.8 in order to reduce ventilation losses and noise.

Changing the inside diameter of the fan \(D^{*}_{1}\) within the limits 0.1–0.6 does not lead to any substantial change in \(V\) and \(P_{\mathrm{v}}\) (Fig. 1, b); however, with an increase in \(D^{*}_{1}\), the air flow rate decreases and the ventilation losses increase, which is explained by an increase in impact losses at the fan inlet owing to the increase in peripheral speed at the inner diameter of the fan. Therefore, the inside diameter of the fan ԥ

Fig. 1.
a) dependence of the air flow rate and ventilation losses on the outside diameter of the fan;
b) dependence of the air flow rate and ventilation losses on the inside diameter of the fan for motor D—132 M—4

one must choose the smallest. In practice, for fans with a hub, it may be recommended that

\[ D_1^* = 0.3. \]

The dependence of the air flow rate on the blade width \(b^*\) (Fig. 2, a) has a maximum at

\[ b^* = 0.25. \]

Fig. 2. Dependence of the air flow rate and ventilation losses on blade width

for all the motors tested. The ventilation losses increase monotonically with increasing blade width. Since the dependence \(V = f(b^*)\) is flat in the optimum region, deviations toward smaller values of \(b^*\) are possible.

The dependences of the flow rate and ventilation losses on the number of blades are shown in Fig. 3, a, b. It has been established that for each frame size there is such a number of fan blades at which an increase in flow rate practically ceases. In Fig. 3, a, the line AA shows the limiting val—

Fig. 3. Dependence of air flow rate and ventilation losses on the number of blades

an appropriate number of fan blades for various motor frame sizes at 1500 rpm. At a rotational speed of 3000 rpm, in order to reduce ventilation losses, the limiting number of blades should be decreased.

The dependence of \(V\) on the inlet diameter of the fan (Fig. 4, a) has a maximum at

\[ D^{*}_{\text{in}} = 0.75—0.80. \]

The ventilation losses (Fig. 4, b) increase with increasing \(D^{*}_{\text{in}}\), and especially sharply at \(D^{*}_{\text{in}} > 0.80\), which is explained by a considerable increase in leakage.

Fig. 4. Dependence of air flow rate and ventilation losses on the inlet diameter of the casing

On the basis of investigations of disk and diskless fans for enclosed asynchronous motors of frame sizes 1—4, a method was developed for a simplified ventilation calculation, consisting in determining the velocity of the air blowing over the motor surface. This velocity is the most important factor determining the heat-transfer coefficient of the enclosure. Since at different portions of the surface the air velocity differs substantially, for the cal-

the calculated velocity is taken to be its mean value at the outlet of the air stream from under the housing,

\[ v_p=\frac{V}{S_e}, \]

where \(S_e\) is the outlet area.

The results of ventilation studies show that the blowing velocity \(V_p\) can be calculated as a function of the peripheral velocity of the fan

\[ v_0=\frac{\pi D_2 n}{60}, \]

\[ v_p=v_0\cdot k_N\cdot k_b\cdot k_f\cdot k_i\cdot k_e\cdot k_T, \]

Here \(k_N\) is a coefficient taking into account the influence of the number of fan blades, which is equal to

\(k_b\) takes into account the influence of the blade width; it is determined by the formula

\[ k_b=1-6(0.32-b_\ell^*)^2 \quad \text{for} \quad 0.12 \leq b_\ell^*=\frac{b_\ell}{D_{\mathrm{st}}}\leq 0.38; \]

\(k_f\) takes into account the influence of the relative free area of the grille \(f_p\), and is determined by the formula

\[ kf=1-1.333(1-f_p)^3; \]

\(k_i\) takes into account the diameter of the inlet grille in the fan housing and is determined as a function of the relative inlet diameter

\[ D_i^*=\frac{D_i}{D_{\mathrm{st}}+D_2}, \]

\(D_i\) is the diameter of the inlet grille,

\(D_{\mathrm{st}}\) is the diameter of the frame, according to the formula

\[ k_i=1-9(0.45-D_i^*)^2-30(0.45-D_i^*)^3 \quad \text{for} \quad 0.3 \geq D_i^* \geq 0.6; \]

\(k_e\) takes into account the relative outlet area

\[ S_e^*=\frac{S_e}{D_{\mathrm{st}}^2}, \]

\[ k_e=0.525 \quad \text{for} \quad S_e^*\geq 0.25 \]

and

\[ k_e=0.775-S_e^* \quad \text{for} \quad S_e^*>0.25; \]

\(k_T\) depends on the fan design and may be taken as:

for diskless fans — 0.9,

for a fan with a small disk

\[ \left(\text{up to } \frac{2}{3}D_2\right) \]

— 1.0,

for a fan with a large disk — 1.1.

Table 1

Results of the ventilation calculation for several asynchronous motors

Conclusions

1. Fans having optimal dimensions make it possible to increase the air flow by a factor of 1.7–2.3, with an increase in ventilation losses by a factor of 1.4–2.2 in comparison with the initial motor of the AO2 series.

2. The proposed method for approximate ventilation calculation has accuracy sufficient for engineering practice (the error of the method does not exceed 10%).

References

1. E. B. Kovalev, A. T. Tokarenko, A. N. Burksovskii. Ventilation calculation of electric motors of the VAO series. “Elektrotekhnika,” 1968, No. 1.

2. V. A. Zhdan, D. I. Sannikov, G. A. Sinailov. Study of the ventilation of enclosed asynchronous low-power motors. Izvestiya TPI, vol. 190, 1968.