This equipment covers two hoists, one of 500 h.p. for hoisting ore, and one of 200 h.p. for hoisting men and lowering materials. The operation of the regulator on this installation is very simple, the three phases of the rotor winding being connected to three stationary plates in separate earthenware pots. Above these plates are the movable electrodes which are connected together. The movement of these electrodes is controlled by an induction motor, this motor being supplied with current through series transformers in the line feeding the motor-generator set. The current flowing in the motor is proportional to that in the main line and the torque of the regulator motor varies as the square of this current. The electrodes of the regulator are attached to an arm coupled to the motor shaft and are counterweighted on the other end of this arm beyond the motor shaft. Under normal conditions the counterweight does not quite balance the weight of the electrodes. At the full load current in the main line, the torque of the regulator motor, which tends to separate the electrodes, will be sufficient to bring the whole of the moving parts. to a state of equilibrium. Any increase in the current in the main line will cause the torque on the regulator motor to increase rapidly and the electrodes will be separated, thereby introducing resistance into the rotor circuit, thus reducing the speed tendency of the motor-generator set, with the result that the flywheel gives up a portion of its stored energy. If the current in the main line then drops below the value for which the regulator is set, the electrodes will then come closer together and the speed of the set will be increased and energy restored to the flywheel. The regulator in this way tends to maintain the input to the equipment at a constant value.

The electrodes are made of iron and the tank is filled with a solution of carbonate of soda (common washing soda). In the lower portion of the tank, cooling coils can be provided, through which water can be circulated and excessive evaporation of the solution is thereby obviated. The value for which the regulator is set can readily be changed by altering the amount of counterweight, the addition of more weight reducing the current at which the regulator will operate, as the torque required from the motor will be correspondingly less.

The field controller for the direct-current generator is so arranged that the field circuit is never open and in this point in particular the control is ideal. When the controller is in the "off" position, the field is connected across the armature ter

minals in such a way as to oppose the residual magnetism and any potential generated due to residual magnetism causes current to flow in the field winding, which will tend to build up the field in the opposite direction, thereby killing the residual field. The advantage of this arrangement is that when the hoist is at rest there will be no current flowing. Altogether this system promises much for the solution of the hoisting problem and the equalization of a very intermittent load, so as to make it commercially desirable for central station power supply.

Additional installations of this type of hoisting equipment in this country are the Hecla Mining Company at Burke, Idaho; the Calumet & Arizona Mining Company in Arizona; the Winona Copper Company in Michigan, and some six or seven similar installations in Mexico, notably those in the mines of the El Oro Mining & Railway Co.

It is recognized that this development is most advantageous where fuel costs are high, and consequently the principal installations up to this time have been made in metal mines. This solution of the heavy hoisting problem is nevertheless one of promise and worthy of careful study as applied to coal mining.

In the complete electrification of existing mines having boilers and steam hoists, a suggestion is now being investigated as to the practicability of making such modifications in the steam hoist as are necessary to operate it with compressed air, using one of the boilers as a receiver, and if practical, a re-heater as well, and installing an electrically driven air compressor of such capacity as to work practically all of the time when steady hoisting occurs, the boiler being used to store air while the hoist is out of service, and to supply the excess demand over and above the capacity of the compressor when hoisting. No definite conclusions have been reached as regards either the economy or practicability of this suggestion but it offers a possible simple solution of the perplexing question of what to do with a steam hoist which is sometimes not readily convertible to electric drive.

The ventilation problem is in no way difficult, although we have met with a great deal of prejudice against the use of motors for this purpose as well, but convincing trials have brought mining men to a general use of motors for this application. The Pittsburgh Coal Company is quoted as having established records for continuity of service of electrically driven fans, better than that obtained with steam engines. The load for this purpose is an attractive application from a power standpoint, as the fan is usually required both day and night.

The other secondary operations are considerably varied but comparatively simple in point of application, and combine to make the mine load very attractive to the central station which is so situated as to be able to serve this field. These problems are greatly simplified if the installation is a new one, in which there is no necessity of considering the adaptation of electric drive to existing machinery, and in which we do not have to-consider converting an old power house into a substation. The principal market for power, however, for present consideration, is the electrification of the existing mines, the new installations being only incidental to the general situation.

The power consumption in the electrical operation of coal mines varies widely with different local conditions and cannot be accurately predetermined from general data, but requires careful local study, as a wet mine which is not self-draining will require considerable power for pumping purposes, whereas a selfdraining mine with water level haulage will be relatively light in its power consumption for both transportation and drainage. The load they get is fairly uniform. In cases of a slope or shaft hoist, additional power is required as against the drift entry.

A number of tests have been made which show as low as 1.22 kw-hr., and as high as 3 kw-hr. per ton of coal mined with. varying conditions. In some cases the service was mixed, using both steam and electricity. In others, rope haulage with steam engines was used. One or two specific cases are illustrated as concrete applications, with the corresponding results. As a general proposition, it is estimated that the power requirements will vary in plants operated entirely by electricity from 1 to 3 kw-hr., depending upon the above-mentioned varying conditions, and in the same mine, the requirements will vary with unusual characteristics where a considerable amount of pumping is involved.

Seven months operation 34,000 tons of coke, 110,000 kw-hr. or 3.2 kw-hr. per ton of coke.

When we consider the vast tonnage of coal in the many localities, we begin to appreciate the possibilities of a power market for its production. Among the most prominent installations of the character described are those on the lines of the West Penn Railways Company in the Connellsville, Pa., district, with which many of the local engineers are familiar. The district served in this case involves some of the worst conditions to be met in the way of drainage and hoisting. I feel safe in expressing the opinion, however, that the installations have proved both profitable to the power company and economical to the mining companies.

Considerable activity is evidenced at the present time in other fields, notably the installation now being made in West Virginia by the Appalachian Power Company, which is developing a hydroelectric plant to furnish power in the Pocahontas coal fields. The Pennsylvania Central Power Company, of Altoona, has recently authorized the construction of something like 75 miles (120 km.) of transmission line to cover the Cambria County field.

One extremely advantageous factor in considering this proposition is the fact that with central station energy available, the investment for the small mine is greatly reduced. An isolated plant costing from $12,000 to $15,000 will in many cases be required where a $5,000 substation will do the work. In some cases it has been found advantageous for the central station to put in the substation machinery and operate it on a rental basis, or sell power converted to low voltage or direct current at the busbars. This is the extreme proposition where the mining companies are not sufficiently capitalized to make the improvements and is comparable with the practise of renting simple fixtures to merchants for their lightning contracts, which has also proved advantageous in many cases.

The distribution of the mine load is such as to prove most

desirable, as the principal load comes on at, or slightly before, 7 o'clock in the morning and as a rule falls off to a low value between 3 and 4 o'clock in the afternoon, at which time the lighting load picks up during the winter months. It is true that

in some cases a comparatively small load comes on again at 6 o'clock and carries through the peak lighting period, but is not of such proportion as to seriously influence the station capacity. The accompanying charts show the general character of the load and while wide fluctuations occur, they will in the main

FIG. 3.-DAY LOAD ON 1092-AMPERE MINE PLANT

balance up fairly well where a considerable number of installations are centralized on one plant.

The question of power contracts calls forth considerable discussion and it is beginning to be recognized that in handling the average mine operator, the same methods cannot always be em