Use of Freewheeling Mode for Automation of Mine Hoisting Installations

V. D. Petunov

(Presented by the scientific seminar of the Department of Mining Mechanics)

For hoisting installations of mines having a shallow depth and a considerable load capacity (Kuzbass, Karaganda), no braking forces are required during the hoisting cycle. The period of main deceleration of these installations is effectively carried out in the freewheeling mode of the moving elements of the hoisting installation. The decelerations obtained during freewheeling amount to \(0.6\text{–}1.2 \, \mathrm{m/sec^2}\) and do not reduce the productivity of mine hoisting. The use of the freewheeling mode makes it possible to dispense with dynamic braking in automation schemes for mine hoists. As a result, the automation scheme is considerably simplified, and the time and means required for installing automated hoisting installations are reduced. However, most of the proposed automation schemes for hoisting installations are oriented toward the use of dynamic braking \([1, 2]\).

The automation scheme of a hoisting installation must be developed with reference to the specific conditions of a mine or group of mines. The use of universal schemes based on dynamic braking should be abandoned for many mines of Kuzbass and Karaganda.

The tension of the loaded branch of the rope during freewheeling for an unbalanced hoisting system with two cylindrical drums is

\[ F_{\mathrm{gr}}=\kappa(Q+Q_{\mathrm{m}})+p(H_{0}-x). \tag{1} \]

where \(\kappa\) is the coefficient of mine resistances;
\(Q\) is the weight of the useful load, kg;
\(Q_{\mathrm{m}}\) is the dead weight of the hoisting conveyance, kg;
\(p\) is the weight of one running meter of rope, kg/m;
\(H_{0}\) is the total freewheeling path, m;
\(x\) is the path traversed by the conveyance during freewheeling, m.

The tension of the empty branch of the rope is

\[ F_{\mathrm{por}}=\kappa Q_{\mathrm{m}}+p[H-(H_{0}-x)], \tag{2} \]

where \(H\) is the total hoisting height, m.

Static force on the drum shaft

\[ F_{\mathrm{st}}=\kappa Q-pH+2p(H_0-x). \tag{3} \]

The differential equation of motion of the hoisting machine in freewheeling is

\[ \frac{d^2x}{dt^2}-\frac{2p}{m}x+\frac{\kappa Q-pH+2pH_0}{m}=0, \tag{4} \]

where \(m\) is the reduced mass, \(\mathrm{kg\ sec^2\ m^{-1}}\).

Bearing in mind that \(\frac{2p}{m}<0.01\text{–}0.002\), the second term of equation (4) may be neglected. Then the deceleration of freewheeling for a given load will be a constant quantity

\[ j=\frac{\kappa Q-pH+2pH_0}{m}. \tag{5} \]

During the freewheeling time \(t_{\mathrm{v}}\), the speed decreases from the maximum value \(v_{\mathrm{m}}\) to the crawling speed \(v_{\mathrm{d}}\), and therefore

\[ t_{\mathrm{v}}=\frac{v_{\mathrm{m}}-v_{\mathrm{d}}}{j}, \]

\[ H_0=\frac{v_{\mathrm{m}}+v_{\mathrm{d}}}{2}\cdot t_{\mathrm{v}}, \tag{6} \]

or

\[ H_0=\frac{v_{\mathrm{m}}^2-v_{\mathrm{d}}^2}{2j}. \tag{7} \]

In formula (7) the square of the crawling speed may be neglected, since \(v_{\mathrm{m}}^2\gg v_{\mathrm{d}}^2\) and \(v_{\mathrm{d}}<1.0\). Then

\[ H_0=\frac{v_{\mathrm{m}}^2}{2j}. \tag{8} \]

Solving equations (5), (6), and (8) simultaneously, we obtain the values of the deceleration, time, and distance in freewheeling of the hoisting machine as functions of the weight of the useful load:

\[ j=A+\sqrt{A^2-B}, \]

\[ t_{\mathrm{v}}=\frac{v_{\mathrm{m}}-v_{\mathrm{d}}}{A+\sqrt{A^2-B}}, \]

\[ H_0=\frac{v_{\mathrm{m}}^2}{2\left(A+\sqrt{A^2-B}\right)}, \tag{9} \]

where

\[ A=\frac{\kappa Q-pH}{2m}; \qquad B=\frac{p v_{\mathrm{m}}^2}{m}. \]

Let us consider the use of freewheeling in the example of a skip hoist of mine No. 8 of the Kuzbassugol combine. Characteristics of the hoisting installation: hoisting machine NKMZ \(2 \times 4 \times 1.7\); total hoisting height \(H = 147\) m; skip capacity for coal \(Q = 6000\) kg; weight per running meter of rope \(p = 5.2\) kg/m; reduced mass \(m = 6600\) kg sec\(^2\)/m; maximum speed \(v_m = 5.35\) m/sec; crawling speed \(v_c = 0.6\) m/sec.

The calculated values of deceleration, time, and distance of freewheeling when the load varies from 50 to 130% of the nominal value are given in Table 1. The increase in the hoisting-cycle time when the load is reduced to 30% of the nominal value practically does not reduce the productivity of the hoisting installation.

Table 1

Calculated values of deceleration, time, and distance of freewheeling for the skip hoist of mine No. 8

The impulse for beginning the period of principal deceleration when freewheeling is used can be effected in various ways. The simplest is the method of switching off the hoisting motor regardless of the weight of the load being lifted. In this case the limit switches send an impulse to switch off the motor each time at one and the same point of the path. As shown by calculations carried out for the conditions of the skip hoist of mine No. 8, the use of freewheeling with this method of switching off the motor and a final load of 80% gives a skip entry speed into the unloading curves of approximately 1.7 m/sec, i.e., considerably higher than permissible. Therefore such a scheme can be adopted only with a load of not less than 90% of normal. Overloading of the skips may be permitted up to 130%; in this case the crawling period is lengthened by only 7–10 sec. It may be considered acceptable to apply the indicated method when the load varies within ±10–20% and to adjust the limit switches for switching off the motor at 110% of the nominal load. For greater assurance, the crawling distance should be taken greater than the length of the unloading curves. This method of automating the period of principal deceleration does not require significant capital expenditures, and therefore it can be tested on an industrial installation. The indicated method has, however, the following main disadvantages: 1) it is necessary to have protection that excludes automatic operation of the hoisting

of the installation at loads below the permissible values; 2) at loads greater than the calculated value, the productivity and the efficiency of the hoisting installation decrease because of the increase in the time required for retarding.

In 1958, Engineer B. V. Shushtannikov introduced, on the skip hoist of mine No. 13 of the “Kiselevskugol” trust of the “Kuzbassugol” combine, a scheme for automating the period of the main deceleration of the machine, with regulation of the disconnection of the hoisting motor depending on the magnitude of the load being lifted, by means of an electronic time relay. A similar automation scheme was developed by the design-and-engineering institute “Giprougleavtomatizatsiya” for the skip hoist of the “Polysaevskaya 3” mine of the “Leninugol” trust of the “Kuzbassugol” combine. Certain theoretical and experimental studies on the automation of skip hoisting installations using freewheeling were carried out at the Siberian Metallurgical Institute by Docent A. L. Vinogradov [3].

The use of regulated freewheeling will make it possible to automate the operation of the skip hoist of mine No. 8 when the load varies from 50 to 130% of the nominal value. The dependence of the time-delay setting of the freewheeling time relay on the terminal load is shown in Fig. 1. At loads of less than 30–50%, and especially when empty skips are being moved, trouble-free operation of the scheme cannot be guaranteed, since the kinetic energy stored by the hoisting

Fig. 1. Time-relay setting as a function of useful load.

machine during starting is sufficient for the conveyance to travel a distance greater than half the full hoisting height. After the conveyances meet, the acceleration produced by the system will be greater than zero, and it is not possible to stop the hoisting machine without using braking devices. Therefore, as in the first method, it is necessary to provide protection that excludes the automatic cycle at a load of less than 50%.

Below, by way of discussion, a principle is proposed for automating skip hoisting installations using freewheeling. For controlling the hoisting motor it is proposed to use a liquid rheostat, the movable electrodes of which are set in motion by a two-speed asynchronous motor with a squirrel-cage rotor. The two-speed motor and the rheostat electrodes are connected to each other by a mechanical transmission, with the lower

the motor speed corresponds to the motion of the conveyance in the unloading curves, and the higher speed to motion outside the curves. In this case, during the starting of the hoisting machine, practically constant accelerations are obtained regardless of the final load. To prevent overloading of the motor, the starting time must be selected according to the maximum load, taking into account the overload capacity and heating of the motor. The use of a liquid rheostat makes it possible to carry out periods of starting and creeping without jerks in speed and acceleration. Experience in operating liquid rheostats at the mines of the Leninugol and Anzherougol trusts in the Kuzbass confirms many positive qualities of the latter. However, for purposes of automatic control, a constant temperature regime and electrolyte composition must be achieved. After reaching a speed of approximately 95% of the maximum, the contactor closes the rotor circuit and the motor operates on its natural characteristic. Uniform travel occurs with the motor operating on the rigid natural characteristic and does not require speed regulation.

The pulse for the beginning of the main deceleration when using freewheeling is supplied by a limit switch to a time relay, which disconnects the motor from the network. Creeping is carried out by a pulse method at a speed of 0.3–0.6 m/sec. At a speed below 0.3 m/sec, the voltage relay in the tachogenerator circuit switches on the drive motor, and the rotor resistance is cut out by the two-speed motor when it is operating at the lower speed. At a speed above 0.6 m/sec, the second voltage relay inserts the rheostat resistance and, if the speed does not decrease, disconnects the motor from the network.

Braking of the machine when the conveyance reaches the normal unloading position is performed by a mechanical brake, which is switched on by limit switches.

To prevent an automatic cycle at small loads, provision should be made for the appropriate protection, or braking of the driving motor should be used in the plugging mode [4, 5]. The considered principles of automation of skip hoisting installations can also be used with motor deceleration.

REFERENCES

1. Ivanov A. A. Automation of mine hoisting machines with an asynchronous drive. Ugletekhizdat, 1957.

2. Katl, Metcalf. Automatic mine hoisting. Translated from English. Ugletekhizdat, 1957.

3. Vinogradov A. L. Automation of a mine skip hoisting installation with a drive from an asynchronous motor. Kemerovo Book Publishing House, 1957.

4. Trop A. E. Obtaining rigid characteristics in the plugging mode of an asynchronous hoisting motor. Izvestiya MVO SSSR, Gornyi zhurnal, No. 4, 1958.

5. Trop A. E. Braking of hoisting machines by the plugging method of an asynchronous hoisting motor. Izvestiya MVO SSSR, Gornyi zhurnal, No. 6, 1958.