Study of the Heating of Electrohydraulic Thrusters

L. N. Veresh, E. M. Guselnikov, V. A. Zhadan, D. I. Sannikov

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

In connection with the expanding use of electrohydraulic thrusters (EGT) in industry, construction, etc., the problem arises of improving their technical and economic indicators, in particular by improving the degree of utilization of the drive electric motors. This is associated with the need to study the heating conditions of EGT and to determine possible ways of improving their thermal state.

Thermal studies of EGT were carried out to determine the thermal loading of the motor, the temperature field of the stator winding, the housing, and the working fluid in the motor and the hydraulic thruster.

For this purpose, during the manufacture of the EGT, more than 30 thermocouples were embedded in various parts of the electric motor and hydraulic thruster. Thermal studies of electrohydraulic thrusters were carried out on a segment of the TGM series with forces on the rod of 80, 50, and 25 kg. The heating of the EGT to a steady-state temperature was monitored by 20 thermocouples located in various parts of the EGT.

The results of thermal tests of the EGT in continuous operating mode, with the rod in the upper position and at nominal voltage, are given in Table 1, where the average values of temperature rises in individual parts of the EGT are indicated.

An analysis of the temperature field of AMM-type motors included in the set of electrohydraulic thrusters TGM-50, TGM-80, and TGM-25 shows that the average temperature rise of the stator winding corresponds to the class A insulation adopted in this motor, and the root-mean-square scatter of temperature rises at individual points from the average does not exceed 0.5°C.

This is explained by good contact between the sources of heating losses—the stator winding and the stator iron—and the working fluid cooling them—transformer oil. The distribution of oil temperature rises at various points of the internal volume of the motors and hydraulic thrusters, found by means of thermocouples, is uniform. The root-mean-square scatter of the temperature rise does not exceed 0.5°.

The distribution of temperature rises of the motor housing is also uniform. The root-mean-square scatter of temperature rises at individual points of the housing from the average value given in Table 1 is 2%.

The main temperature difference (80–90% of the average temperature rise of the stator winding) is the temperature difference between the housing and the surrounding medium. Therefore, the main direction for improving the thermal state of EGT motors should be considered to be improvement of heat transfer from the surface of the motor housing.

Table 1

Average temperature rises of various parts of the EHT, deg.

From this point of view, it is important to establish the dependence of the heat transfer from the casing on the finning parameters. For this purpose, studies were carried out of the heat transfer of the motor and thruster casings both on an actual electrohydraulic thruster and on thermal models. The thermal model consists of a motor or thruster casing filled with transformer oil and having a built-in heater, which makes it possible to vary the heating losses over a wide range. The dependence of the overheating of individual elements of the electrohydraulic thruster on the fin height for an actual TGM-50 electrohydraulic thruster is given in Table 2.

Table 2

Dependence of the average overheating of various parts of the TGM-50 on the fin height of the motor casing

The values of the coefficient of convective heat transfer from the motor surface, $\alpha_k$, as a function of the average surface overheating $\Theta_k$, obtained on the basis of tests of AMM motor models and of the TGM-50 hydraulic thruster, are given in Fig. 1.

Fig. 1. Dependence of the coefficient of heat transfer by convection on the fin height $h_p$ and casing overheating $\Theta_k$

The coefficient $\alpha_k$ was calculated from the experimental data by the formula

\[ \alpha_k = \frac{P_k}{\theta_{pk} - \theta_0}, \tag{1} \]

where

$\theta_{pk}$ is the average surface temperature of the model casing;

\(\Theta_0\) — temperature of the surrounding air;
\(P_k\) — losses removed by convection from the metallic surface of the housing.

\[ P_k = P_{\mathrm{gr}} - P_l - P_{\mathrm{kr}}, \tag{2} \]

where
\(P_{\mathrm{gr}}\) — power of the model heater;
\(P_l\) — heat flux removed from the metallic surface of the housing by radiation, determined by calculation on the basis of a surface emissivity of 0.72 [2];
\(P_{\mathrm{kr}}\) — heat flux removed through the cover of the motor and hydropusher housings, likewise determined by calculation.

The data obtained indicate that the existing finning has practically no effect on the thermal state of the individual elements of the EHT. It may be expected that increasing the number and height of the fins will not lead to any substantial improvement in heat transfer from the surface of the housing.

In this case, such a method of improving the thermal state of the motor as reducing the overheating of the most highly heated parts of the winding by various measures to equalize the temperature distribution is also inapplicable, since, owing to the small dimensions of the motor and the equalizing action of the oil, the temperature distribution is already uniform.

One of the realistic ways of improving the thermal state of the EHT is to separate the winding of the drive electric motor from the working fluid [1]. The results of thermal studies of a prototype TGO-50 electrohydraulic pusher with a winding isolated from the working fluid by means of an end seal are given in Table 1. In this EHT, owing to the elimination of disk and cylindrical friction losses of the rotor against the working fluid, a significant reduction in heating losses was achieved; therefore, the overheating of individual parts of this specimen, in comparison with series-produced hydropushers, proved to be smaller.

A comparison of the temperature field of the series-produced TGM-50 electrohydropusher and of the experimental TGO-50 with a winding isolated from the working fluid is shown in Fig. 2. Here the upper number corresponds to the overheating of the TGM-50 hydropusher, and the lower number to that of the TGO-50. It can be seen from Fig. 2 that abandoning the oil-filled design of the motor leads to an increase in the nonuniformity of heating of the stator winding; the temperature difference between individual points along the length of the stator winding reaches 1.7 degrees, whereas for the usual EHT the difference is practically absent. At the same time, the nonuniformity of heating of the housing over its height also increases somewhat.

However, taking into account that the thermal state of the motor as a whole has improved, and that the temperature difference of the stator winding does not exceed 4%, this EHT design should be considered promising from the thermal standpoint.

Conclusions

The investigations carried out on the thermal state of electrohydraulic pushers show:

1. The thermal field in series-produced EHTs with oil-filled electric motors is practically uniform: the root-mean-square scatter of overheating at individual points does not exceed 2%.

2. The finning present on series-produced EHTs has practically no effect on the thermal state of their elements.

Fig. 2. Temperature field of the electrohydraulic thruster

1. Through optimal ribbing of the housings of electrohydraulic thrusters with oil-filled electric motors, it does not appear possible to substantially improve the thermal state of the EHT. Further improvement of the thermal state of hydraulic thrusters should be carried out by reducing the losses generated in them, separating the winding of the electric motor from the working fluid, and introducing the housing with the aid of a fan mounted on the motor shaft.

REFERENCES

1. E. M. Gusel’nikov, V. F. Rott. Electrohydraulic thrusters. “Energiya,” 1968.

2. V. A. Zhadan, L. L. Pantyukhov, D. I. Sannikov, G. A. Sipailov, S. A. Shelekhov. Influence of rib dimensions on the heating of electric motors of the A series. — Elektrotekhnika, 1970, No. 11.