Commercial electric circuits furnish electricity in two types, direct current, ordinarily termed D.C., and A.C. or alternating current. The terms direct and alternating current apply to the direction of the electric impulses which constitute the transmitted energy. In the electric dynamo, the generation of the current is due to impulses that are induced in the wires of the dynamo armature as they pass through a magnetic field of great intensity. These electric impulses are directed by the manner in which the wires cut across the lines of force which make up the magnetic field. In the case of the direct current the impulses are always in the same direction through the circuit, while in the other they are induced alternately to and fro and so produce alternating current.
The term electric current is used only for convenience of expressing a directed form of energy. Since nothing really passes through the wires but a wave of energy, the effect is the same whether the electric impulses are in the same or in opposite directions. An incandescent lamp will work equally well on an A.C. or a D.C. circuit of the proper voltage; but in the case of motors the form of construction must be suited to the kind of current. Both A.C. and D.C. commercial circuits are in common use, the units of measurement are the same for each but in ordering a motor it is necessary to state the type of current and the voltage, in order that the dealer may supply the required machine. In the case of an alternating motor it is further necessary to state the number of cycles of changes of direction made per second in the A.C. circuit. All of this information may be obtained by inquiring of a local electrician or of the power station from which the current is obtained.
There is still another item of information necessary to be supplied with an order for a motor, other than those of fractional horsepower. With motors of a horsepower or more it is necessary to state the number of phases included in the circuit. This information to be complete must state whether the motor is to operate on a single-phase, two-phase, or three-phase circuit. These terms apply to a condition made possible in A.C. generation that permits one, two, or three complete impulses to be developed in a circuit at the same time. These phases are transmitted by three wires, any two of which will form a circuit and give a supply of energy at the same voltage. Either one phase or all may be used at the same time and for this reason the phase of an A.C. motor should be given in an order. To make the information complete there should be included the number of cycles or complete electric impulses per second produced in the circuit. Suppose that a 1-horsepower motor is required to work on an A.C. circuit of 110 volts. Inquiry of the electric company reveals that the circuit is three-phase at 60 cycles per second. The dealer on receiving this information will be able to send a motor to suit your conditions. Most A.C. motors of 1 horsepower or less are of the single-phase variety. In the case of D.C. motors it is necessary only to state the voltage of the circuit to make the required information complete.
Fuse Plugs.
—Every electric circuit is liable to occurrences known as short-circuiting or “shorting.” This is a technical term describing a condition where, by accident or design, the wires of a circuit are in any way connected by a low-resistance conductor or by coming directly into contact with each other. In case of shorting, the resistance is practically all removed and the amount of current which flows through the circuit is so great as to produce a dangerous amount of heat in the wires. If the covering of a lamp cord becomes worn so as to permit the bare wire of the two strands to come together, a “short” is produced. Immediately, the reduced resistance permits the electric pressure to send an amount of current through the wires, greater than they are intended to carry. When this occurs an electric arc will form at the point of contact with the accompanying flash of vaporizing metal and the wire will finally burn off. Fires started from this cause are not uncommon.
To guard against accidents from short-circuiting, every electric circuit should be provided with fuses which, in cases of emergency, are intended to melt and thus break the circuit. Fuses are made of lead-composition or aluminum and are used in the form of wire or ribbon-like strips, of sizes that will carry a definite amount of current. They are designated by their carrying capacity in amperes. As an example: a 2-ampere fuse will carry 2 amperes of current without noticeable heating, but at a dangerous overload the fuse will melt and the circuit be broken. Should a short-circuit be formed at any time, the rush of current through the fuse will cause it almost immediately to melt, and stop the flow of current. They are, therefore, the safeguard of the circuit against undue heating of the conducting wires.
When an open fuse blows (melts), the heat generated by the arc, formed at the breaking circuit, is so sudden that there is frequently an explosive effect that throws the melted metal in all directions, and in case it comes into contact with combustible material a fire may result. To do away with this danger, fire insurance companies in their specifications of electric fixtures state what forms of fuses will be acceptable in the buildings to be insured. These specifications are known as the Underwriters Rules and may be obtained from any fire insurance company. The fuses, or fuse plug, as they are commonly called, generally occupy a place in a cabinet or distributing panel, near the point where the lead wires enter the building. The cabinet contains the porcelain cutouts for sending the current through the different circuits; the fuse plugs form a part of the cutouts, one fuse to each wire. The cabinet contains besides the cutouts a double-poled switch to be used for shutting off the current from the building when desired.
Fig. 230.—Electric cabinets.
Cabinets for this purpose are made in standard form of wood or steel to suit the condition of service. These cabinets may be obtained from any dealers in electrical supplies or the cabinet may be made a part of the house since they are only small shallow closets. Fig. 230 represents such a cabinet as is used in the average dwelling. It is made of a light wooden frame set between the studding of a partition at any convenient place. The bottom of the cabinet is made sloping to prevent its being used as a place of storage for articles that might lead to trouble. The cabinet is sometimes lined with asbestos paper as a prevention from fire but this is not necessary as the fuse plugs and their receptacles, when of approved design, are sufficient to prevent accident.