The construction just described applies to what are generally known as standard, open, horizontal motors. In order to take care of special conditions surrounding the work, it is sometimes necessary to make mechanical modifica tions, such as arranging motors with the shaft in a vertical plane instead of horizontal, or arranging for back gearing as a part of the motor, to secure comparatively low driving speeds. As a protection against mechanical dust or moisture or acid fumes, the openings in the frame and brackets are sometimes closed with covers Such an impediment to the ventilation results in some re duction in the amount of power the motor can deliver without overheating, but this handicap is offset by the longer life of the winding and insulation and other parts susceptible to breakage or corrosion. In very special cases, as sometimes occurs in trac tion work, such combinations as the twin motor are worked out, in which two separate motors are mechanically connected through gears to one driving axle. Functionally, an electric motor, which converts electric energy into mechanical power, may be looked upon as the direct antithesis of an electric generator (q.v.) which develops electric energy by the application of mechanical power to a suitable structure. From this point of view, direct current machines and alternating current synchronous machines may be used reversibly as motors or generators, but alternating current induction motors, as described later, may not be reversed and used as alternating current generators, unless at the same time there are electrically connected in parallel with them suitable alternating current synchronous motors or generators. Fundamental Mechanical Relations.—The electric motor, after converting electric energy into mechanical force, develops power by means of rotary motion. A consideration of this action involves the torque or turning effort and the revolutions per min ute at which the torque is delivered. From fundamental physical relations, work is measured by the product of a force and the distance through which it acts, or W = Fs. Work is therefore expressed in foot pounds and may be considered as the work done in raising a weight against gravity. Mechanical energy is also measured in the same units. Further, power is the time rate of doing work and is expressed by dividing Work by Time, or P
The power of electric motors is usually expressed by a t unit called a horsepower. This unit is the power required to lift a weight of 33,000 lb. through i f t. in i min., and the horsepower of a motor is calculated from the torque or turning effort at i f t. radius and the revolutions per minute by the formula: Horsepower. X revolutions per minute X 2 X 3.14 • —33,000 In this formula the torque is expressed as the pull exerted at the end of a lever arm or crank 1 ft. long, and the factors 2 and
are introduced in order to calculate the number of linear feet that this pull travels in one revolution. Multiplying by the r.p.m. and dividing by the equivalent of one horsepower gives the horsepower which the motor will develop. The simplest concep tion of torque is that the electric current creates two magnets, one in the stator and one in the rotor. In the case of direct current the stator magnet stands still and attracts to itself the rotor mag net, whose poles are not directly in line with it. At the first glance it would appear that the rotation produced by this attraction would be limited and that as soon as the south poles of the rotor were opposite the north poles of the stator rotation would cease. This would be the case if it were not for the action of the com mutator and brushes, which introduce current successively into different coils on the rotor in such a way that as rapidly as a magnetic pole on the rotor is attracted to the nearest stator pole, the current is shifted into the next coil behind, with the result that the poles on the rotor are continually approaching the poles on the stator but never reaching them, since they are continually shifted back automatically by the action of the commutator. Structurally,
the commutator consists of wedge-shaped copper bars built up to form a complete circular arch, as illustrated in Plate, fig. 5. These copper segments are separated and electrically insulated from one another by thin strips of mica, which permits attaching the ends of each rotor coil to separate commutator bars. The stationary carbon blocks or brushes make contact with the outer surface of the commutator bars, which, during the rotation, slide under them, and in this manner the current is always led into the proper coils to provide a continuous mechanical pull on the rotor, and this is the torque. As the mechanical work or load which the motor is called upon to do is increased or decreased within the normal rated torque of the motor, the torque developed and the speed at which the motor runs automatically adjust them selves until a stable running condition is reached. This variation in speed with changes in load is known as speed regulation. In the alternating current induction motor, the device of commutator and brushes is not used, but in their place advantage is taken of a principle discovered independently by Nikola Tesla and Galileo Ferraris in 1886. By this principle, if the windings of the stator of such a motor are supplied with polyphase alternating currents, a magnetic field of alternate north and south poles is set up in the stator iron core and this rotates in space independently of the mechanical rotor. This rotating magnetic field sets up or induces in the windings on the rotor electric currents, which in turn create magnetic poles in the rotor and, as a result, the rotor magnetic field is pulled around by the rotating stator field at a slightly lower speed than the rotation of the stator magnetic field. The r.p.m. of the stator field is obtained from the expression cycles X 120 ; e.g., on a 4 pole, 6o cycle motor the r.p.m. of number of poles the rotating magnetic field would be 6oX12o= 1,800 r.p.m. 4 The no load r.p.m. of the physical rotor is very close to this so called synchronous speed, but the full load r.p.m. of the physical rotor drops a few per cent below this. In the alternating current synchronous motor, Plate, fig. 1, which is built like an a.c.
generator, the magnetic field of the rotor is separately set up by direct current from an independent source. In this case the rota ting magnetic field of the stator which is set up by polyphase alternating currents locks in magnetically with the rotor field and pulls it along synchronously. For this reason the no load and the full load speed of a synchronous motor are given by the expres sion : revolutions per minute = • The electric number of poles and magnetic relations in both direct current and alternating current motors and their theory and method of calculation are quite analogous to the same conditions in electric generators (q.v.).
is one fundamental principle to which special reference is here made as it is most useful in understanding the action of motors. Counter electro motive force is a back voltage generated by any motor all the time that it is operating as a motor and is of such a direction and such a value that it is directly opposed to and nearly equal to the voltage which is applied to the motor from the supply circuit. The slight margin between the applied line voltage and the generated back voltage (perhaps of the order of 3% to 5% of the line voltage) is just sufficient to send through the resistance of the windings a current which is automatically exactly sufficient to produce the torque required to drive the mechanical load. If the load increases the motor slows down a trifle and generates a little less back voltage, so that the difference between the line voltage and the back voltage is a little greater and hence an increased current flows in the motor, producing an increased torque to balance and carry the increased load. If the load decreases, the motor speeds up and less torque results.