Improvement of the ventilation system of P-series DC machines of frame sizes 1–6
Abstract Generated abstract
This paper investigates inadequate cooling in protected P-series DC machines of frame sizes 1 to 6 with axial exhaust ventilation and centrifugal fans. Using a specially built aerodynamic test stand, the authors measured ventilation characteristics for stationary and rotating machines, compared experimental air flow with conventional calculations, and analyzed the fan’s conditional pressure to identify losses from leakage and circulating flows. The study finds that actual air flow is substantially below calculated values, mainly because of reduced fan pressure, and proposes enlarging outlet openings, replacing the front fan disk with a fixed annular diaphragm, and modifying the rear disk. Experiments indicate that these measures can increase cooling air flow by up to 45 to 70 percent, with corresponding reductions in winding overheating, without substantially increasing fan power consumption.
Full Text
IMPROVEMENT OF THE VENTILATION SYSTEM OF DC MACHINES OF THE P SERIES, FRAME SIZES 1–6
V. A. Zhadan, D. I. Sannikov, M. A. Sannikova
(Presented by the scientific seminar of the Department of General Electrical Engineering and Electrical Machines)
Electric machines of the P series, protected design, frame sizes 1–6, have an axial exhaust ventilation system with an undeveloped air duct and a centrifugal fan.
By agreement with the Elektromashina plant (Prokopyevsk), work was carried out at TPI to improve the cooling of these machines. To conduct ventilation tests, an aerodynamic test stand was designed and manufactured in the form of a sealed chamber, in the outlet opening of which the machine under test is installed, while the inlet is equipped with a flow-measuring device operating on the principle of measuring the rarefaction in the smooth inlet collector. The aerodynamic stand has an auxiliary axial-type fan with smooth speed regulation, allowing the air flow through the machine \(V\) and the pressure in the chamber \(H\) to be varied over wide limits.
The test procedure consisted not only in the usual measurement of flow rate, but also in obtaining the characteristics of the ventilation system \(H(V)\), both with the machine stationary and with the machine rotating. In the first case, the hydraulic resistance of the machine \(z_{\mathrm{e}}\) is determined from the characteristic; in the second, the fan pressure is determined.
The experimental characteristics of the machines are given in [2]. Their intersection with the horizontal axis gives the value of the operating air flow through the machine \(V_{\mathrm{p}}\), and with the vertical axis—the no-load pressure of the fan \(H_{\mathrm{o}}\).
The flow-rate values found experimentally proved to be 1.7–2.4 times smaller than those calculated by the usual method. The small value of the flow rate leads to a reduction in the heat-transfer coefficient on the cooled surfaces and to heating of the air in the machine above the recommended level. However, this circumstance may not be detected during thermal tests, since the temperature of the air stream in the machines studied reaches a maximum before the fan and then rapidly falls as it approaches the outlet opening.
Since the calculated and experimental values of the equivalent hydraulic resistance of the machines \(z_{\mathrm{e}}\) agree well with one another, the discrepancy between the calculated and experimental flow rates must be attributed to a reduction in fan pressure under the influence of leakage flows and circulating flows in the interblade channels.
A convenient method for studying the properties of fans is to construct the characteristics of their conditional pressure \(H_{\mathrm{y}}(V)\) by subtracting the ordinates of the characteristics of the ventilation system \(H(V)\), obtained with the machine stationary and with the machine rotating.
\[ H_y(V)=z_\mathrm{e}V^2-H(V)_{(n\ne 0)}, \tag{1} \]
The form of the experimental characteristics \(H_y(V)\) is shown in Fig. 3.
The conventional head of the fan exceeds the useful head \(H_v\) by the amount of the head loss in the internal resistance of the fan \(z_v\).
\[ H_y(V)=H_v(V)+z_vV^2. \tag{2} \]
In the operating regime (at \(H=0\)) the conventional head is equal to the pressure drop over the total hydraulic resistance of the machine
\[ H_p=V_p^2z_\mathrm{e}. \tag{3} \]
For a fan with an ideal parabolic characteristic
\[ H_v(V)=H_0-z_vV^2 \tag{4} \]
the conventional head is
\[ H_y(V)=H_0=\mathrm{Const}. \tag{5} \]
The difference of the experimental characteristics from case (5) may be explained by the influence of leakage, and also by the influence of circulating flows, which have the same qualitative character. If a fan with a parabolic characteristic
\[ H_v(V)=H_0'-z_v'V^2 \tag{6} \]
(curve 1 in Fig. 1) is shunted in the equivalent circuit of the machine ventilation system by the leakage resistance \(z_y\) (Fig. 1), the characteristic of which is represented by curve 2, then the dependence of the useful head on the air flow through the machine
Fig. 1. Influence of leakage on the fan characteristic.
\[ H_v(V)=H_v(V_v-V_y) \tag{7} \]
is determined by subtracting the abscissas of curves 1 and 2 (curve 3). The characteristic of the conventional head can be constructed with the aid of formula (2), where
\[ z_{\mathrm{v}}=\frac{z'_{\mathrm{v}}\cdot z_{\mathrm{y}}}{\left(\sqrt{z'_{\mathrm{v}}}+\sqrt{z_{\mathrm{y}}}\right)^2} \tag{8} \]
the internal resistance of the fan, taking leakage into account.
The calculated characteristics of the conditional pressure for various ratios
\[ \gamma=\sqrt{\frac{z_{\mathrm{v}}}{z_{\mathrm{y}}}} \tag{9} \]
are shown in Fig. 2 in relative units. Their shape is, in its main features, close to that of the experimental characteristics; consequently, the proposed explanation is satisfactory. However, it is only illustrative, since the calculation of the curves (Fig. 2) is based on substantial simplifications.
Fig. 2. Calculated characteristics of the conditional fan pressure.
The considerable development of leakage flows and circulation flows is caused by the large local hydraulic resistance at the exit of air from the machine, by the presence of a gap between the front disk of the fan and the frame, and also by the fact that the rear end face of the working zone of the fan is closed by a disk only partially.
Reducing the influence of leakage makes it possible to increase the fan output without increasing its diameter or the number of blades, i.e., practically without increasing noise and the power consumption for ventilation. For this purpose the following measures are proposed:
- In the existing machine design, the outlet openings located on the lateral surface of the rear bearing shield occupy from 96 to 136° of the circumference; it is recommended that this arc be increased to approximately 270°, i.e., to a value at which splash protection can still be provided by means of louvered grilles. Figure 3 shows the characteristics of the fan and of the air duct for this case (curves 2) in comparison with the characteristics of the existing design (curves 1). In addition to a reduction of the hydraulic resistance of the machine by a factor of 1.4–1.8, the fan pressure increases by 15–30 percent. Some reduction in the pre—
Fig. 3. Experimental characteristics of fans and air ducts.
the initial portion of the characteristic is explained by the greater freedom for circulation flows to pass when the fan is operating at no load. The increase in air flow amounts to 25–40 percent of the initial value.
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It is recommended to replace the front disk of the fan with a fixed annular diaphragm located at a distance of 2–4 mm from the blades; in this case leakage through the gap between the front disk and the housing is practically eliminated, as a result of which the pressure increases by 50–80 percent and the air flow by 20–40 percent (curves 4 in Fig. 3).
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Increasing the diameter of the rear disk to the outside diameter of the impeller eliminates leakage through the rear end surface of the fan and causes some increase in pressure, chiefly in the initial portion of the characteristic, as is seen from a comparison of curves 2 and 3. This effect is insignificant owing to the proximity of the shield wall. Therefore these design changes should expediently be applied only simultaneously with the preceding measure, in order to ensure the mechanical strength of the fan.
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Simultaneous implementation of all the recommendations leads to an increase in air flow by 45–70 percent (curves 5).
Thermal tests carried out with various improvements of the ventilation system showed that the reduction in overheating
of the windings amounts on average to 0.25 percent for each percent increase in air flow.
Conclusions
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The characteristic of the fan’s conditional pressure gives a sufficiently complete picture of its properties and is easily determined experimentally with the aid of an aerodynamic test stand.
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Air leakage and circulation flows exert a strong influence on the fan characteristic.
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The proposed measures for reducing leakage provide, in P-series machines of frame sizes 1–6, a significant increase in the flow rate of cooling air, practically without increasing the power consumed by the fan.
References
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A. E. Alekseev. Design of Electrical Machines, GEI, 1958.
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D. I. Sannikov. Study of the ventilation of DC machines of the P series, frame sizes 1–6, Problems in the Theory and Design of Electrical Machines, Interuniversity Collection of Papers, issue 3, Novosibirsk, 1963.