THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CX. FRIDAY, SEPTEMBER 10, 1915. No. 2854. Explosion=proof Electric Motors for Mines. In Technical Paper 101, issued by the United States Bureau of Mines, H. H. Clark deals with the conditions and requirements for test and approval that have laid down by the Bureau in connection with the institution of a “ permissible ” list of explosion-proof electric motors for use in mines. Particulars are also given of the first machine to pass the test. Requirements for Approval of Motors. Design and Construction.—The design and construc- tion cf permissible explosion-proof motors and their accessories must be especially durable. This require- ment will be applied consistently to all the details of the machine, as well as to its principal parts. The protective devices used with permissible explosion-proof motors must not only be capable of preventing the passage of flames from the interior to the exterior of the motor casing, but such devices must also possess sufficient mechanical strength to insure against the accidental destruction of their protective qualities. If there are moving parts in connection with such devices, these parts should be so designed that there can be no interference with their movement. Starting Rheostats.—Starting rheostats and other necessary equipment that may cause an ignition of gas must be protected as adequately as the motor itself. The casings of starting rheostats must be explosion- proof. The resistances and contacts of the starting rheostats used with portable motors of not more than 50-horse power capacity should be enclosed in the same box, unless enclosed in separate boxes connected by approved piping through which all leads are carried. All leads entering the explosion-proof casing of a starting rheostat should pass through the casing in the form of properly protected insulated studs of approved design. The use of rubber bushings will not be approved because the bushing may become displaced and thus destroy the explosion-proof quality of the casing. The casing of the starting rheostat should be mounted on the motor casing if possible, and the inter- communicating openings for the passage of leads should be made large in order to prevent the rise of pressure that always attends the propagation of an explosion through a small hole from one compartment to another. If it is not possible to mount the starting rheostat on the motor frame, all leads connecting the starter with the motor should be carried in rigid metallic conduit. Unless means for opening the circuit both automatically and by hand are provided in a separate explosion-proof casing, they should be incorporated in the design of the starting rheostat. If the starting rheostat is mounted on the frame of the motor, provision should be made for entirely disconnecting the electric circuit from the starting rheostat. Motor Casing.—All joints in the casing of a motor or of any of its accessories must be metal-to-metal joints with faces not less than lin. wide, and if the pressure developed in the motor casing by explosions can exceed 50 lb. per square inch the faces must be not less than 1-2 in. wide. All bolt holes in casings must be bottomed or so arranged that the accidental omission of a bolt will not give an opening through the casing. All openings in the motor casing other than those provided with protective devices by the manufacturers must be tightly closed. It is desirable that such openings be as few as possible. There should be no exposed terminals or contacts outside the motor casing. If there are glass-covered openings in the casing of a motor, the glass should be of ample thickness and should be protected by strong metal covers that close automati- cally unless held open by hand. Armature bearings must be so designed that under no circumstances can an explosion be propagated from the interior of the motor casing around the armature shaft or through the oil wells. Cable Reel and Trailing Cable.—If there are any sliding or rubbing contacts in connection with the cable reel, such contacts should be provided with explosion- proof protection, and any plug connections should be constructed so that they will be explosion proof. At the point where trailing cables enter the frames of Pig. 1.—Cable Reel. .Insulated binding posts J Cabin connecting with power supply I w Expicsion proof enclosure for dip rings and brushes Cable ciamp. Flexible armor Resistance box enclosed in machine frame beneath this point Vent at p’Otectnre device at commutator end Insulated Ixithi.-ig I I Fig. 2.—Motor and Starting Box. Fig. 3.—-View of Protective Device at Commutator End. ■ n Fig. 4.-—View of Protective Device at Gear End. portable motors, the cable should be protected with suitable armour or flexible metallic conduit, securely fastened to the frame of the motor, and of a sufficient flexibility to prevent short bends from occurring in the cable. The cable should not be fastened to this armour but there should be provided inside the frame of the motor an insulated clamp of approved design for securely fastening the cable and taking all mechanical strains that may be put upon it. Tests. In testing a motor to establish its permissibility, the motor casing will be filled and surrounded with the most explosive mixture of Pittsburg natural gas and air. The motor will then be operated at its rated speed and the mixture within the casing ignited by a spark plug, by a spark from the motor brushes, or by any other means that stimulates the conditions of actual practice. Similar tests will also be made with greater and with less amounts of gas in the explosive mixture and with coal dust sifted into the motor casing or into the protective devices. Tests will also be made to determine the point of ignition that gives the greatest pressure, and tests will be made by igniting from such a point. Not less than 50 tests of all kinds will be made, and more than that number may be made if, in the opinion of the Bureau’s engineers, more tests are necessary to prove the per- missibility of the motor. In order for a motor to pass these tests it shall in none of them cause an ignition of the gas surrounding the motor or discharge flames from any part of the motor casing. Neither shall the motor develop dangerous afterburning* or excessive pressure in the casing of the motor or its starting rheostat. As part of the protection of a permissible motor, the manufacturer shall be required to attach to the motor frame a metal plate inscribed as follows :— CAUTION. The permissibility of this motor depends upon the absence of any openings in the casing other than those provided with pro- tective devices by the manufacturer. Cover plates should be screwed on tight and the casing frequently inspected for I improper openings. An Approved Motor. Approval has been issued to the Sullivan Machinery Company covering an explosion-proof coal-cutting equipment, illustrated herewith. Fig. 1 shows the motor and starting box completely assembled. Fig. 2 shows the cable reel completely assembled with the explosion-proof plug and covering for the slip rings. Fig. 3 shows the protective devices which are mounted at each end of the motor casing. Motor.—The motor is rated CY-24-B—30 H P.—30— 250—1130, and is a compound-wound machine designed to deliver 30-horse powei- for 1 hour without exceeding a temperature rise of 75 degs. Cent. The protective devices (fig. 3) used on the motor are of the plate type and are protected from mechanical injury by strong non- corrosive metallic gratings. There are no openings in the motor frame, except one over the commutator, which is closed by a solid cover bolted to the frame. Starting Rheostat and Fuse.—The casing of the starting rheostat is made part of the machine frame. The controller plate is mounted on one side of the machine and the resistances on the other. They are however, in the same box, and all leads between them are completely enclosed and protected. There is a cover plate over the resistances and one over the con- troller. Both cover plates are made with broad flanges and are fastened to the machine frame with stud bolts. The electrical connection with the trailing cable passes through the casing of the rheostat in the form of studs insulated with fibre washers and bushings. There is mounted in the shaft of the controller a totally-enclosed refillable fuse, having practically the dimensions * The term “ afterburning ” as used in this report, is applied to the combustion, immediately after an explosion within an explosion-proof casing, of a gaseous mixture that was not within the casing at the time of the explosion, but was drawn in subsequently while the products of the explosion were cooling.