On the Problem of Automatic Control of Skip Hoisting Installations

V. S. Udut

(Presented at the scientific and technical conference of the mining faculty)

In the matter of automatic control of mine hoisting machines, the greatest interest is presented by solving the problem of automating the control of skip hoisting installations with an asynchronous drive.

The operating mode of skip hoists is characterized by the uniformity of the operations performed and by an insignificant change in the terminal load, which facilitates automation of their control. The execution of a specified operating mode is complicated by the influence of loading, unloading, and landing devices, which create a certain resistance to the motion of the hoisting system. The insufficient study of these resistances leads to the fact that the choice both of the very method of automatic control of the hoisting machine and of the regulation parameters is made intuitively, without proper justification. Consequently, the solution of the problem of automatic control of a hoisting machine must be preceded by an investigation of the operating mode of the installation, which is determined by the static moments of resistance and by the prescribed law of variation of speed during one cycle. The hoisting installation must here be considered as a single electromechanical aggregate.

The skip hoists of coal mines in the eastern regions of the Soviet Union, in particular of the Kuznetsk Basin, are in most cases equipped with machines having cylindrical drums, asynchronous electric motors, skips with bottom discharge, and large-capacity bunkers with metering loading devices.

At the hoists of Kuzbass, three types of skips with bottom discharge are in operation: 1) with a tilting body; 2) with a fixed body and a lever gate; 3) with a fixed body and a sector gate. In addition, individual rock hoists are equipped with tipping skips.

Loading of skips is carried out by metering devices of the drum type with sector gates. Opening of the gates is effected by skips being brought in for loading, and closing by counterweights. Fixing of the skips during loading in a definite position is ensured by landing (support) beams.

According to the arrangement of the counterweights, loading devices are divided into two groups: 1) with counterweights located on the drum-

not the gate; 2) with counterweights carried out from the gate drum and with ropes connected to them.

Loading devices have two types of carriages: 1) with translational motion and 2) with rotational motion (lever-type carriages).

Owing to the insignificant depth of mines, not exceeding 400 m for the eastern regions of the country, the static resistance on the section before the skip enters the unloading curves will always have a positive value, since the appearance of negative static resistances on skip hoists is possible only at mine depths of more than 800 m. Therefore, the deceleration of the machine on this section of the path can be carried out without using brakes.

The magnitude of the deceleration in the free-running mode is determined by the parameters of the hoisting installation

\[ j = \frac{kQ + p(H - 2x)}{\sum m_i}, \tag{1} \]

where \(x\) is the path traveled by the vessels, m;
\(k\) is the coefficient of mine resistances;
\(Q\) is the weight of the useful load, kg;
\(H\) is the hoisting height, m;
\(p\) is the weight of 1 m of rope, kg/m;
\(\sum m_i\) is the mass of the moving elements of the hoisting installation, reduced to the radius of rope winding, kg·sec\(^2\)/m.

Analysis of the operation of skip hoists in mines of the Kuznetsk Basin shows that the average deceleration in the free-running mode on various installations is equal to \(0.4 \div 1.5\) m/sec\(^2\) and depends to a considerable extent on the useful loading of the skips. Thus, for example, a change in the weight of the useful load by \(\pm 20\%\) from the normal value is accompanied by a change in the magnitude of the deceleration in the free-running mode on the skip hoist of the “Kapitalnaya-1” mine of the Osinnikugol’ trust within the limits \(0.7 \div 0.4\) m/sec\(^2\); on the northern skip hoist of the “Koksovaya-1” mine of the Stalinugol’ trust—\(1.4 \div 0.9\) m/sec\(^2\); and on the skip hoist of the S. M. Kirov mine of the Leninugol’ trust—\(1.0 \div 0.6\) m/sec\(^2\).

As a result, the section of the path on which the speed of the vessels decreases from the maximum to the creep speed also changes. The influence of the change in terminal load on control can be compensated, in the presence of positive static resistances, by combining the free-running mode with a motor mode with resistance fully inserted into the rotor circuit, or by switching the motor from one rheostat characteristic to another.

It is most difficult to control the machine of a skip hoist during the creep period, since the characteristics of the asynchronous motor do not ensure a constant low speed under sharp changes in static resistances and oscillations of the useful loading of the skips.

The nature of the change in the static resistances of the hoisting system is influenced above all by the type of skip used and by its partial unloading during creep. In the literature [1, 2], the partial unloading of the skip by the moment of stopping is taken as equal to 40% of the weight of the useful hoisted load. Observations of the process of skip unloading in Kuzbass mines, carried out in order to determine the unloading coefficient, gave somewhat different results. Thus, for skips with unloading through the bottom and with a tilting body,

and likewise for skips with a sector gate this coefficient proved to be equal to 25%, and for skips with a lever gate, owing to the greater path of motion in the unloading curves, 40% of the useful load weight. In addition, opening the skip gate is accompanied by considerable pressure of the roller on the unloading curves, which leads to a change in the rope tension and to a disturbance of the static balance of the dead weights of the vessels. During motion in the unloading curves the skip is in equilibrium under the action of active and reactive forces; therefore the rope tension can be determined from the condition of equilibrium of forces. The simplest way to do this is by applying a grapho-analytical method of solution.

Fig. 1. Rope tension during motion of a skip with a tilting body in the unloading curves. 1—loaded skip; 2—empty skip.

To clarify the influence of loading, unloading, and landing devices on the tension of the rope branches and the change in the static resistance of the hoisting installation, let us consider the case of using six-ton skips with a tilting body and loading devices of various structural designs.

Figure 1 presents calculated curves of the change in the tension of the rope branch during motion of the skip in the unloading curves. As can be seen from the diagrams, the rope tension differs considerably from that usually adopted in calculations and shown by dash-dotted lines. The displacement of the empty skip is accompanied by a smaller change in rope tension.

A similar picture is also observed for other skips unloading through the bottom. The rope tension, in relative units of the terminal load, during the motion of loaded skips in the unloading curves is given in Table 1.

Table 1

Opening the gate of the measuring hopper under the weight of the descending skip causes a reduction in the tension of this rope branch. The static resis-

…of the gate when opening depends on the arrangement of the counterweights and their weight, the type of carriage, the resistance of the friction forces between the coal and the opening sector of the gate, the presence of coal in the feeder tray, and the frictional resistance in the shaft journals.

The weight of the counterweights, when there is a translational-motion carriage, is taken, for six-ton skips, as equal to up to 3000 kg, and with lever-type carriages up to 4250 kg. Since the dead weight of the skips is 4100–5250 kg, it becomes clear that the loading device has a considerable influence on the rope tension when the skip is moved on the carriage of the feeder gate.

The static resistance to opening the feeder gate, from the equilibrium condition of the system, is determined by the expression

\[ F'_{\mathrm{st.o}} = \frac{i}{r_c \cos(\alpha+\beta_1)} \left[ G r_{\mathrm{gr}} \cos(\alpha+\varphi_1) + G_y r_0 f \cos\alpha -\right. \]

\[ \left. - G_m r_m \cos(\alpha+\varphi_2) + G_b r f_0 - G_l r_l \cos(\alpha+\beta_2) \right] - G_k, \tag{2} \]

where \(i\) is the coefficient of the lever transmission of the carriage;

\(G\) — weight of the counterweights, kg;

\(G_y\) — force of pressure of the coal on the sector of the feeder gate, kg;

\(G_m\) — weight of the coal located in the feeder tray, kg;

\(G_b\) — weight of the gate drum (if the counterweights are located on the drum, then also their weight), kg;

\(G_l\) — weight of the feeder tray, kg;

\(G_k\) — weight of the carriage, kg;

\(r_{\mathrm{gr}}\) — distance from the axis of rotation of the gate drum to the center of gravity of the counterweights, or to the point of application of the action of the forces of their weight, m;

\(r_0\) — radius of the feeder-gate drum, kg;

\(r_m\) — distance from the axis of rotation of the gate drum to the center of gravity of the coal located in the tray, m;

\(r\) — radius of the journals of the feeder-gate shaft, m;

\(r_l\) — distance from the axis of rotation of the gate drum to the center of gravity of the tray, m;

\(r_c\) — distance from the axis of rotation of the gate drum to the point of attachment of the pull rod from the carriage, m;

\(\alpha\) — angle of rotation of the gate drum, degrees;

\(\varphi_1\) — angle determining the position of the center of gravity of the counterweights with the gate closed, degrees;

\(\varphi_2\) — angle determining the position of the center of gravity of the coal located in the tray with the gate closed, degrees;

\(\beta_1\) — angle determining the position of the hinge of the carriage pull rod on the drum with the gate closed, degrees;

\(\beta_2\) — angle determining the position of the center of gravity of the tray with the gate closed, degrees;

\(f\) — coefficient of friction of the coal on the material of the gate sector;

\(f_0\) — coefficient of friction of the journals in the bearings.

If the counterweights are carried outside the drum and connected to it by a rope, then for any gate position \(\cos(\alpha+\varphi_1)=1\).

The change in the tension of the descending branch of the rope during opening of the gate of the loading device is shown in Fig. 2. The determination of the rope tension is performed from the expression

\[ F_{\text{o.v}}=Q_{\text{m}}-\frac{k-1}{2}Q+px-F'_{\text{st.o}}, \tag{3} \]

where \(Q_{\text{m}}\) is the dead weight of the skip, kg;
\(x\) is the distance traveled by the descending skip, m.

The rope tension when the skip is set onto the carriage (point K in Fig. 2) drops sharply, since the resistance to opening the gate of the loading device has its maximum value. Then, as the carriage moves and the gate opens, the rope tension gradually increases and in certain cases (curve 3) may exceed its tension before the skip was set onto the metering-bin carriage, since the gate tray, under the action of the force of the coal coming from the bunker, rests against the front wall of the skip and transmits an additional force to the rope.

Fig. 2. Rope tension under the action of the end load during the process of opening the metering-bin gate: 1—when the counterweights are located on the drum and the carriage is of the lever type; 2—with counterweights removed from the drum and a carriage with translational motion; 3—when the counterweights are located on the drum and the carriage has translational motion.

Setting the skip onto the support beams completely unloads the rope from the action of the end load, and its tension with the machine stopped and the given branch running onto the drum is determined by the rope’s own weight (point P in Fig. 2)

\[ F_{\text{o.k}}=pH. \tag{4} \]

The closing gate of the metering bin acts on the skip being raised with a force equal to:

a) if the carriage acts on the skip,

\[ F'_{\text{st.z}}= \frac{i}{r_c\cos(\alpha+\beta_1)} \left[ G r_{\text{gr}}\cos(\alpha+\varphi_1) - G_y r_b f\cos\alpha - \right. \]

\[ \left. - G_m r_m\cos(\alpha+\varphi_2) - G_b r f_0 - G_l r_l\cos(\alpha+\beta_2) \right] - G_k; \tag{5} \]

b) if the gate tray acts on the skip,

\[ F'_{\text{st.z}}= \frac{1}{r_k\cos(\alpha+\gamma)} \left[ G r_{\text{gr}}\cos(\alpha+\varphi_1) - G_y r_b f\cos\alpha - \right. \]

\[ \left. - G_m r_m\cos(\alpha+\varphi_2) - G_b r f_0 - G_l r_l\cos(\alpha+\beta_2) \right] - G_k \frac{r_c\cos(\alpha+\beta_1)} {r_k\cos(\alpha+\gamma)}. \tag{6} \]

Here \(r_k\) is the distance from the axis of rotation of the gate drum to the point of contact of the tray with the front wall of the skip, m;
\(\gamma\) is the angle determining the position of the point at which the tray contacts the skip when the gate is closed, deg.

Knowing the force with which the feeder gate acts on the skip, we determine the rope tension

\[ F_{\text{l.b.}} = \frac{k}{2}Q + Q_{\text{n}} - p(H-x) = F'_{\text{s.g.}} . \tag{7} \]

Diagrams of the change in rope tension during closing of the gate of the loading device are given in Fig. 3.

The use of a carriage with translational motion and the placement of counterweights on the drum causes a considerable increase in rope tension at the beginning of hoisting (curve 3), but in the remaining cases (curves 1 and 2) it does not exceed the tension of the branch after the skip leaves the carriage

\[ F_{\text{l.b.}} = \frac{k}{2}Q + Q_{\text{n}} + r(H-x). \tag{8} \]

Fig. 3. Rope tension due to the action of the end load during the process of closing the feeder gate. 1—when the counterweights are located on the drum and with a lever-type carriage; 2—with counterweights removed from the drum and a carriage with translational motion; 3—when the counterweights are located on the drum and with a carriage with translational motion.

Comparison of the diagrams presented in Figs. 2 and 3 makes it possible to conclude that the influence of the loading devices on the rope tension is manifested mainly when the feeder gate is opened.

The static resistances, defined as the difference between the tensions of the loaded (ascending) and empty (descending) branches of the rope, are presented in Figs. 4 and 5. The actual value of the static resistances at the beginning of motion (Fig. 4) is somewhat lower than the values usually used in calculations. In the case of using a loading device with counterweights located on the drum and with a carriage of translational motion, an increase in static resistance is observed at the beginning of motion; this is explained by the resistance of the feeder gate to the motion of the ascending skip. During the process of closing the gate, the carriage begins to push up the ascending skip, reducing the static resistance to the motion of the hoisting system. With contactor starting of the hoisting motor, the time variation of the static resistances will not introduce any substantial difficulty into the automation of control of the hoisting machine.

Fig. 4. Influence of loading and unloading devices on the static resistance to skip motion at the beginning of hoisting. 1—when the counterweights are located on the drum and with a lever-type carriage; 2—with counterweights removed from the drum and a carriage with translational motion; 3—when the counterweights are located on the drum and with a carriage with translational motion.

A considerably greater influence on the change in static resistances is exerted by the loading and unloading devices at the end of the hoist (Fig. 5). First of all, attention is drawn to their sharp change when the skip is seated on the carriage of the loading device (point K), and then on the landing beams (point P). The static resistances during the creeping of skips vary over wide limits and have higher values than are usually assumed in calculations [1, 2]. The kinematics adopted in automated skip hoists provides for creeping of the skips at a constant speed; therefore the value of the driving force developed by the motor at the rope-winding radius may be taken equal to the value of the static resistances.

Fig. 5. Influence of loading, unloading, and landing devices on the static resistance to motion at the end of the hoist.

1—when the counterweights are located on the drum and on a lever-type carriage; 2—with counterweights carried off the drum, and a carriage with translational displacement; 3—when the counterweights are located on the drum and the carriage has translational displacement.

Fig. 6. Diagrams of current and speed of skip hoist of mine No. 8.

1—hoisting speed; 2—stator current.

The current and speed diagrams (Fig. 6), taken on the skip hoist of mine No. 8 of the “Prokop’evskugol’” trust, confirm the above-described nature of the change in static resistances at the end of the hoist. Point a on the current and speed curves corresponds to the moment when the loaded skip enters the unloading curves. The slowing of the hoisting system before the unloading curves was produced with the rheostat in the rotor circuit at full resistance, and the creeping was carried out on the second mechanical characteristic of the motor, without applying the mechanical brake. The brake was switched on only for the final stopping of the hoisting machine. The speed of motion during creeping changed insignificantly and averaged 0.7 m/sec. Consequently, in this case the motor developed a force close to the static resistance of the hoisting system. Processing of the recorded diagrams (Fig. 6) with

taking into account the parameters of the installation and of the drive motor makes it possible to conclude that the average value of the resistance of the hoisting system when the skips move in the unloading curves and on the feeder carriage at the end of hoisting is 30% higher than the resistance before entering the unloading curves.

The resistances to motion change similarly for other types of bottom-discharge skips used in the Kuzbass. This necessitates drawing up the skips in the motor mode on the second or third rheostat characteristics. Maintaining the speed within the required limits is possible by means of pulse switching of the motor from one rheostat characteristic to another [3], or by using the mechanical brake while the electric motor operates with resistance in the rotor circuit. The latter method of control is undesirable, since it is accompanied by excessive consumption of electric power and wear of the brake shoes. Therefore, for drawing up, the pulse method of control may be recommended.

A scheme of automatic control of a hoisting machine at the end of hoisting, constructed on this principle, will be serviceable only if the range of regulation of the driving torques developed by the motor covers the range of variation of the resistances during the motion of the skips in the unloading curves and on the feeder carriage, with an allowable deviation of the speed from the specified value.

Summarizing the above, the following conclusions may be drawn.

1. In solving problems of automating the control of skip hoisting installations in mines of small depth \((100 \div 400\ \text{m})\), for slowing the hoisting machine before the skips enter the unloading curves, the free-running mode should be adopted.

2. Drawing up the skips in the unloading curves and on the feeder carriage, taking into account the increase and abrupt changes in the resistances of the hoisting system, should be carried out with the hoisting motor operating in a pulse mode.

REFERENCES

1. F. N. Shklyarskii. Physico-mechanical Foundations of Electric Mine Hoisting. Ugletekhizdat, 1952.

2. A. P. German, F. N. Shklyarskii. Mine Hoisting Installations. Ugletekhizdat, 1947.

3. B. D. Kudin. Automatic Skip Hoisting Installation. Ugletekhizdat, 1956.