Intrinsically Safe Circuit for Remote Control of Explosion-Proof Starters

A. I. Zaitsev

(Presented by the scientific seminar of the Mining Faculty)

The presence of inductance in electrical circuits significantly increases the danger of ignition of mine gas and dust. Investigations carried out by Professor P. P. Pirotsky [1] show that, in order to reduce the danger of gas ignition, one must strive to reduce the energy of the electric arc, which depends on the inductance and the magnitude of the current flowing through this inductance. At present, in order to reduce the explosion hazard, magnetic starters are controlled at reduced voltage by alternating current (starters PMV—1331, PMV—1344, PMV—1357, PMV—1365, etc.). Because of the considerable inductive resistance of the holding coils (starter PMV—1344) or intermediate relays (starters PMV—1357, PMV—1365, etc.) and the large current, the existing control circuits cannot be classified as explosion-proof.

To increase intrinsic safety, a number of measures have been developed. Such measures are mainly reduced to the use of electronic and ionic valves, whose grid currents are very small and quite intrinsically safe, and also of direct-current relays with shunting of the source by an active resistance or by semiconductor valves [2, p. 3; 3, p. 48].

However, the first method has so far not found practical application, apparently because of the insufficient mechanical strength and short service life of electronic and ionic tubes having a heated cathode. The second method is used in the reversible starter PMVR—1451 and in the new magnetic starter PMV—1358 (VUGI). Intrinsic safety is achieved by shunting the inductance with a semiconductor valve.

Intrinsically safe control circuits can be obtained if electromagnetic amplifiers, which are widely used in other fields of engineering, are employed for this purpose. Electromagnetic amplifiers are saturation chokes whose inductance depends on the magnetic induction in the core. The basic operating circuit of an electromagnetic amplifier is shown in Fig. 1. Alternating-current windings 1 and 2, located on the outer limbs, are connected in series with the load. These windings are connected to each other in parallel (Fig. 1a) or in series (Fig. 1b), with such a calculation that the resultant alternating-current magnetic flux in the middle limb would be equal to zero. If through the control wind—

If winding 3 is allowed to pass a direct current, then the direct magnetic flux thereby produced in the middle core, closing through the outer cores, during one half-period will add to the alternating flux of one of the outer cores, and in the other will be subtracted. As is known, the inductive reactance of the outer cores will depend on the resultant magnetic induction. Therefore such an amplifier will have the greatest resistance when the control current is equal to zero. As the control current increases, the inductive reactance decreases, and it will be the smaller, the greater the control current. The phenomenon of the dependence of inductance on the magnitude of the direct current in the control circuit is used in electromagnetic amplifiers. At the present time it is possible to manufacture electromagnetic amplifiers with a large power gain, and to make the control circuit intrinsically safe. Therefore the action of an electromagnetic amplifier may be likened to the action of an electronic tube, where the role of the control grid is performed by the control winding. In comparison with electronic or ionic tubes, such amplifiers have undeniable advantages, consisting in simplicity of design, durability, insensitivity to mechanical shocks, readiness for operation at any moment, etc.

Fig. 1.

The use of electromagnetic amplifiers in remote-control circuits for mechanisms operating in mines hazardous with respect to gas or dust makes it possible to obtain intrinsically safe control circuits. The proposed circuit is shown in Fig. 2. In this case the electromagnetic amplifiers are placed in the explosion-proof enclosure of the magnetic starter. The power windings of amplifiers 1 and 2 are connected in series with the holding coil of contactor 3, and control is effected by means of control winding 4, which has intrinsically safe parameters. The control circuit is supplied from step-down transformer 5, which transforms the primary voltage to 6–12 V, through rectifier 6. For greater reliability the control circuit is shunted by valve 7. In order to obtain the required power-gain coefficient of the electromagnetic amplifier with the calculation that a current of the order of 5–10 mA flows in the control circuit, feedback may be used. For this purpose, on the middle core, together with the control winding, a second winding is placed, through which a direct current flows equal to the current in the power circuit. By changing the number of turns of the feedback winding, the gain coefficient of the amplifier can be varied to a considerable extent. The circuit of an amplifier with feedback is shown in Fig. 3. The value of the current in the control circuit should not be taken less than 5 mA, since, owing to the condition of the insulation in a humid environment, leakage currents may reach values of several milliamperes. For example, at a control-circuit voltage of 12 V and an insulation resistance of 5000 ohms, the leakage current will be 2.4 mA. Under actual conditions, however, the insulation level is often below the figure cited. This circumstance is also a serious obstacle to the use of electronic and ionic valves, whose grid control current is many times larger.

several times smaller than leakage currents. To eliminate false tripping from leakage currents, the voltage on the control grid will have to be taken from a resistance connected between the grid and the cathode, through

Fig. 2.

which it is necessary to pass a current of the same order, 5–10 mA. The latter condition will greatly reduce the sensitivity of electron and ion tubes.

Fig. 3.

Under otherwise equal conditions in the construction of control circuits for electron and ion valves and electromagnetic amplifiers, the clear advantage will lie with electromagnetic amplifiers.

Existing magnetic starters can also be converted quite easily to intrinsically safe control. For this purpose it is neces-

one amplifier to be connected into the circuit of the intermediate relay. The amplifier is designed for a voltage of 12–36 V. A circuit for converting existing magnetic starters to intrinsically safe remote control with the aid of electromagnetic amplifiers is shown in Fig. 4. New starters can also be equipped according to a similar circuit. Owing to the introduction of the intermediate relay, we can obtain a considerable gain in the dimensions of the electromagnetic amplifier.

In order to reduce the number of control conductors, the grounding conductor may be used, since the circuits are intrinsically safe; and the “run” pushbutton can also be shunted by an active resistance. Thus, for remote control of mechanisms only one conductor will be required. The shunting resistance is selected with such a calculation that

Fig. 4.

with the “run” pushbutton open, a current flows through the control winding of the amplifier at which the holding coil of the contactor or intermediate relay does not release the movable armature when switching-on has been performed by the “run” button, and switching-on does not occur when, for some reason, the voltage in the supply network has disappeared and then been restored.

The minimum value of the current in the control winding at which disconnection will not occur depends on the return coefficient of the entire control system. This coefficient is determined mainly by the parameters of the magnetic system of the contactor or intermediate relay.

The value of the shunting resistance may be determined from the following equation

\[ R_{\text{sh}}^{1}=\frac{U_y}{I_y \cdot k_v}, \]

where \(U_y\) is the voltage in the control circuit;
\(I_y\) is the magnitude of the control current when the “run” button is closed;
\(k_v\) is the return coefficient of the system, equal to the ratio of the control current at disconnection and at switching-on of the contactor or intermediate relay.

In practice it is necessary to take the resistance \(R_{\text{sh}}^{1}\) somewhat smaller, in view of a possible voltage drop within the permissible limits. This condition may be represented by the reliability coefficient \(k_n\). Taking into account—

In view of the foregoing, the value of the shunting resistance must be

\[ R_{\mathrm{sh}}=\frac{k_{\mathrm{n}}\cdot U_{\mathrm{u}}}{k_{\mathrm{v}}\cdot I_{\mathrm{u}}}. \]

For more reliable operation of the control circuit over two wires, it is necessary to have the return coefficient as small as possible. For this purpose it is desirable to supply the intermediate relay with direct current from a rectifier connected into the power circuit of the amplifier.

Thus, the proposed control method has the following advantages in comparison with existing ones.

1. Complete intrinsic safety of the control circuit.
2. Practically unlimited service life.
3. Simplicity of design and insensitivity to mechanical shocks.
4. Practically unlimited control range owing to the small control currents at a direct-current voltage in the control circuit of 6–12 V.
5. Broad possibilities for automation of production processes using apparatus of normal design.
6. Because of the potentially large control range, dispatcher control of mechanisms can be implemented.

REFERENCES

1. Pirotskii P. P. Intrinsically safe systems of electrical signaling and communication and the ignition of mine gas. DNTVU, 1937.
2. Intrinsically safe signaling systems for coal mines. BTI MUP, 1948.
3. Ozernyi M. I. Mining Electrical Engineering. Ugletekhizdat, 1951.