  An electric motor is just the reverse of a dynamo; it is a machine for converting electrical energy into mechanical energy. The electrical energy delivered by the dynamo must be obtained from a steam engine, gas engine, or other power; the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature. Ques. What is the construction of a motor? Ans. It is constructed in the same manner as a dynamo.
Principles of the Motor.—All the early attempts to introduce motors failed, chiefly because the law of the conservation of energy was not fully recognized. This law states that energy can neither be created nor destroyed. Early experimenters discovered, by placing a galvanometer in a circuit with a motor and battery, that, when the motor was running, the battery was unable to force through the wires so strong a current as that which flowed when the motor was standing still. Moreover, the faster the motor ran, the weaker did the current become. Fig. 389.—Conductor, lying in a magnetic field and carrying no current; the field is not distorted whether the conductor be at rest or in motion. Ques. Why does less current flow when the motor is running than when standing still? Ans. Because the motor, on account of its rotation acts as a dynamo and thus tends to set up in the circuit a reverse electromotive force, that is, an electromotive force in opposite direction to the current which is driving the motor. Ques. What is the real driving force which causes the armature of a motor to rotate? Ans. The propelling drag, that is, the drag which the magnetic field exerts upon the armature wires through which the current is flowing, or in the case of deeply toothed cores, upon the protruding teeth. The Propelling Drag.—In fig. 389 is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field. The magnetic lines pass straight from one pole to the other. The field is not distorted whether the conductor be at rest or in motion, so long as there is no flow of current. This represents the condition in the air gap of a motor or dynamo, when no current is flowing in the armature. Ques. What happens when a current flows in the conductor of fig. 389. Ans. It sets up a magnetic field of its own as shown in fig. 390. Ques. What is the effect of this magnetic field? Ans. It distorts the original field (fig. 389) in which the conductor lies, making the magnetic lines denser on one side and less dense on the other as in fig. 390. Ques. What is the nature of these distorted magnetic lines? Ans. They tend to shorten themselves to their original form of straight lines. Ques. What effect has this on the conductor? Ans. It produces a force on the conductor tending to push it in the direction indicated by the arrow, fig. 390. Fig. 390.—Conductor carrying a current in a magnetic field. The current flowing in the conductor sets up a magnetic field which distorts the original field as shown, making the magnetic lines denser on one side and less dense on the other. This results in a force upon the wire, which, in the case of a dynamo (fig. 391) opposes its movement, and which forms the propelling drag in the case of a motor (fig. 392).
Figs. 391 and 392.—Action of the magnetic force in a dynamo and motor. In the first instance, according to Lenz' law, the direction of the current induced in the wire is such as to oppose the motion producing it. In the operation of a motor, the current supplied in flowing through the armature winding distorts the field and thus produces rotation. In the figures, the direction of the force is clearly indicated by remembering that the distorted lines of force act like rubber bands tending to straighten and shorten themselves. Ques. What are the essential requirements of construction in a motor? Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular to the field, and 3, provision for motion, of the conductors across the field in a direction perpendicular to both themselves and the field. The Reverse Electromotive Force.—When an electric current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy, according to whether the electromotive force acts with or against the current. Fig. 393.—Force exerted on a current carrying conductor placed across a magnetic field. Let N, S, be the pole pieces of an ordinary electromagnet, having their faces flat and with only a narrow air gap between. In this gap is stretched the vertical copper wire A B, kept taut by a strong spring at A; current can be passed into the wire from the leads C and D. Attached to the wire in the middle of the gap is a horizontal cord passing over a pulley P and kept taut by a weight W; the pulley carries a pointer F which moves in front of a scale s s. If the electromagnet be now excited and have the polarity indicated, it will be found that on passing a strong current down the wire, the index F moves toward the right, showing a similar movement in the wire. The index returns to zero when the current in the wire ceases, and moves in the opposite direction if the current in the wire be reversed and sent up instead of down. The experiment can be further varied by reversing the magnetizing current of the electromagnet.
Fig. 394.—Showing relative directions of armature current and reverse electromotive force of a motor. When a motor is in operation, the wires around the periphery of its armature "cut" the magnetic lines of force produced by the field magnet exactly as in the case of the dynamo. Consequently, an electromotive force is induced in each wire, as in the dynamo armature. This induced electromotive force is in opposition to the flow of current due to the electromotive force of the supply circuit, and tends, therefore, to keep down the flow of current. The figure shows a single loop of wire, on the armature core connected directly to the source of electricity. With current flowing in the loop in the direction indicated by the arrows marked c, a magnetic field is set up in the direction indicated by the large arrow marked "direction of armature flux." With the field magnet energized so as to produce a field in the direction indicated by the large arrow F, the reaction between the two fields will turn the armature core in the direction indicated by the arrow R. As the core turns, the upper wire of the loop will cut the flux under the south pole of the field magnet, and the other side of the loop will cut the flux under the north pole. The result will be the induction of a reverse electromotive force in the loop, the direction being indicated by the small arrows marked e. The actual flow of current in the armature is that due to the difference between the impressed and reverse voltage; the latter is proportional to the speed of the armature, the number of armature wires and the strength of the magnetic field in the air gaps between the armature and the pole faces. The speed of a motor supplied with current at constant voltage varies directly with the reverse electromotive force, also with other conditions fixed, the stronger the field, the slower the speed. Weakening the field will increase the speed up to the point where the increase in reverse electromotive force due to the increased speed cuts down the armature current below the value necessary to give the requisite pull at the armature periphery. When this point is reached, any weakening of the field will reduce the speed of the armature. The pull or torque of a motor armature is directly proportional to the strength of the magnetic field, and to the strength of the armature current, the number of armature inductors being fixed. In a field of constant strength, therefore, the pull of the armature depends on the amount of current passing through the winding. The torque must be just sufficient to overcome the load; if in excess, the speed will increase until the increase of the reverse electromotive force reduces the current and the increase of speed increases the load to the point of equilibrium between load and torque. If the torque be insufficient for the load, the speed will diminish until equilibrium is established, assuming the motor is running on constant voltage circuit. Ques. Describe similar conditions which prevail in the operation of a dynamo. Ans. When no current is being generated by the dynamo, little power is required to drive it, but when the external circuit is closed and current is forced through it against more or less resistance, work is being done, hence more power is required. In other words, there is an opposition to the mechanical force applied at the pulley which is proportional to the electric power delivered by the dynamo. An opposing reaction or reverse force then is set up in a dynamo when it does work. Fig. 395.—Circuit with generator and motor. Whenever current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy according to whether the electromotive force acts with the current or against it. In the figure, the generator and motor are rotating clockwise, and hence each generates an electromotive force tending upwards from the lower brush to the higher. In each case the upper brush is the positive one. In the dynamo, where energy is being supplied to the circuit, the electromotive force is in the same direction as the current, while in the motor where work is being done and energy is leaving the circuit, the electromotive force is in a direction which opposes the current. Ques. In the operation of a motor what is the nature of the reverse electromotive force? Ans. It is proportional to the velocity of rotation, the strength of the magnets, and to the number and arrangement of the wires on the armature, that is, the reverse voltage depends on the rate at which the lines of force are cut. Figs. 396 and 397.—Water and electric circuits. Diagrams showing comparison between water motor and electric motor.
Fig. 398.—Fairbanks-Morse standard TR type motor. This type is built in the smaller sizes and the design is such that the motor can be installed upon the floor, wall or ceiling, the bearing yokes being attached to the frame by four equally spaced bolts so that they can be turned to provide for proper operation of the oiling devices in either position. A substantial base is provided with a thrust screw for adjusting the belt tension. This base has clamping bolts which permit adjusting the position of the motor while suspended. There is a cast ring type frame having steel side pieces which press firmly together, the steel laminations making up the pole pieces. The field coils armature, and armature coils are illustrated in detail in figs. 399 to 401. The commutator bars are of drawn copper, insulated with mica. The lugs which extend outward from the bars to receive the lead wires from the armature windings are formed in one piece with the bars, and are of the full width of the bars with the insulator extending outward between them, so that when assembled a solid flange is formed to receive the armature connections. Self-oiling bearings are provided and the location of the bearing sleeves in the housing is adjustable so that the armature may be centered in the magnetic field. The brush rigging is carried on a skeleton rocker supported in a groove, turned in the edge of the frame. The brush holders are of the box type with independently adjustable tension spring for each. Standard shunt windings are for 115, 230 and 550 volts. The compound wound motors operate at approximately the same full load speeds as the shunt wound, but the no load speeds will be about 20 per cent. higher than the full load speeds. They have, however, the ability to exert a more powerful starting effort than shunt motors without drawing such a heavy current from the line, and are, therefore, especially adapted for driving apparatus that has to be frequently started and stopped under load and where close speed regulation is not required. Ques. Describe an experiment which shows the existence of a reverse electromotive force in a motor. Ans. The apparatus required consists of a small motor, battery, and ammeter. They should be connected in one circuit and the deflection of the ammeter observed when the armature is held stationary, and when it rotates with various loads.
Fig. 399.—Fairbanks-Morse field coil and pole piece. The field coils are wound upon iron forms, each layer treated with insulating compound. Afterward they are removed from the forms and baked hard and dry and finally wrapped with insulating materials; all but the three smaller sizes are wrapped with a protecting cord. The series and shunt coils of the compound winding here shown are wound separately, the smaller one being the series coil and the larger the shunt coil. Ques. Explain the action of the current supplied to a motor for its operation. Ans. The motor current passing through the field magnets polarizes them and establishes a magnetic field, and entering the armature, polarizes its core in such a way that the positive Fig. 400.—Fairbanks-Morse armature for 7½ H. P., 1300 R. P. M., TR type motor. The armature core is built up of thin sheet steel laminations with notches in the circumference, which, when the discs are placed together, form grooves or slots to receive the armature coils. The armature cores for the larger machines are mounted on a cast iron spider, which also carries the commutator, making the two parts entirely self-contained, and with this construction, it is possible to remove the armature shaft, without disturbing the core, commutator or windings. Cores of all sizes are provided with ventilating spaces, running from the surface to the central opening of the core, so that air is drawn through the core and blown out over the windings by the revolution of the armature. Direction of Rotation of Motors.—In the case of either a motor, or a dynamo used as a motor, the direction in which the armature will rotate is easily found by the left hand rule, as Ques. How may the rotation of a motor be reversed? Ans. By reversing either the current through the fields, or the current through the armature. Ques. What will happen if both currents be reversed? Ans. The motor will run in the same direction as before. Fig. 401.—Fairbanks-Morse wire wound armature coils. These coils are form wound and are thoroughly insulated and baked before assembling in the slots. Material of great mechanical strength as well as high insulating value is used, and the coils are subjected to dippings in insulating compound and to bakings, thus driving out all moisture and making a coil which is practically waterproof and which will withstand rough handling. These coils, when completed, are placed in the slots, where they are retained by bands on the three smaller sizes and by hardwood wedges on the larger sizes. Ques. What is the effect of supplying current to a series dynamo? Ans. It will run in a direction opposite to its motion as a dynamo. Ques. What is the result of reversing the direction of current at the terminals of a series motor? Ans. It will not change its direction of rotation, since the current still flows through the armature in the same direction as through the field. Figs. 402 to 410.—Diagrams showing relative direction of rotation of motors and dynamos. From figs. 391 and 392, it is seen that the direction of the current in a motor armature must be such as will increase, by the flux it produces, the intensity at the leading polar edge and decrease the intensity at the trailing polar edge. In a dynamo, the armature has to be moved by mechanical force, against a magnetic force, hence the leading polar edge is weakened, while the trailing edge is strengthened. The magnetomotive force in a motor armature is, therefore, opposed to the direction of that in a generator armature, when the direction of rotation and the direction of the field magnetomotive force are the same. Upon this depends all the relations existing between the direction of rotation of a machine when acting as a motor or as a dynamo. Ques. What is the behavior of a shunt dynamo when used as a motor? Ans. Its direction of rotation remains unchanged. Ques. Why is this? Ans. Because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo, the current through the field will be reversed, since the field windings are in parallel with the brushes. Fig. 411.—The "left hand rule" for direction of motion in motors. Place the left hand, as shown, so that the thumb points in the direction of the current, the 3rd, 4th and 5th fingers in the direction of the lines of force, then will the 2nd or forefinger, at right angles to the others, point in the direction in which the conductor is urged. Armature Reaction in Motors.—In the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo. A current supplied from an outside source magnetizes the armature of a motor and transforms it into an Fig. 412.—Principle of the electric motor as illustrated by experiment showing effect of a magnetic field on a wire carrying an electric current. Let a vertical wire ab be rigidly attached to a horizontal wire gh, and let the latter be supported by a ring or other metallic support as shown, so that ab is free to oscillate about gh as an axis. Let the lower end of ab dip into a trough of mercury. When a magnet is held in the position shown and a current from a cell is sent through the wire as indicated, the wire will move in the direction shown by the arrow f, that is, at right angles to the direction of the lines of magnetic force. Let the direction of the current in the wire be reversed, then the direction of the force acting on the wire will be found to be reversed also. The conclusion is that a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current. The relation between the direction of the magnetic lines, the direction of the current, and the direction of the force, is often remembered by means of the following rule, known as the motor rule, and which differs from the dynamo rule only in that it is applied to the fingers of the left hand instead of to those of the right. Let the forefinger of the left hand point in the direction of the magnetic lines of force and the middle finger in the direction of the current sent through the wire, then will the thumb, at right angles to the other two fingers, point in the direction in which the wire is urged.
Ques. What effect has the cross magnetizing force on the field? Ans. It tends to shift the field around in a direction opposite to that of the rotation. Fig. 413.—Current commutation in a motor. Considering the coil W which is ascending, current is flowing through it from the top brush, while it is itself the seat of an electromotive force that tends to stop or reverse its current. The condition for sparkless commutation requires that during the interval the coil is short circuited by the brush, the coil should be passing through a field that is not only sufficiently strong but one that tends to reverse the direction of its current. The coil is already in such a field, hence, commutation must take place before it passes put of this field. To accomplish this the brushes must be shifted backward, that is, given negative lead, to overcome sparking. In other words, the commutating plane must be shifted back of the neutral plane in a motor instead of being placed in advance as in a dynamo. Ques. What are the conditions of minimum sparking? Ans. The same conditions must obtain as in a dynamo, that is, the current in the coil undergoing commutation must be brought to rest and started again in the opposite direction. This involves that while the coil is short circuited by the brush, it should be passing through a field that tends to reverse the direction of the current. Since the coil is already in such a Fig. 414.—Railway motor. This type of motor, since it must operate under cars, has taken on the peculiar form under which it is most familiar. As illustrated, the case is of such shape that compactness and water proofing are secured, and the means of attachment to the car axle and support from the axle and truck frame are provided. Method of Starting a Motor.—Although motors and dynamos are practically similar in general construction and either one of them will act as the other when suitably traversed by an electric current, there are certain differences between the connections and accessories of a machine operated as generator and one employed as a motor. For instance, when a machine is operated as a dynamo, it is first driven up to speed until it has excited itself to the right pressure, and then it is connected to the circuit; but when a machine is used as a motor it will not start until it has been connected to the circuit, and this must not be done until the proper precautions have been taken to ensure that the current, which will pass through it when so connected, will not be excessive and thereby result in serious injury to the motor. For this reason a rheostat or variable Fig. 415.—View of railway motor, open. The frame is of cast steel for lightness, and which serves as magnetic circuit and protecting case. It is circular or octagonal in form except in very large motors. Four short magnets project from the case. The armature is large in order to secure the required torque. It is always series wound, requiring two brushes. The brush holders are mounted upon a frame of insulating material which is attached to the upper half of the case. The brushes are adjustable radially, but usually it is not necessary to provide for shifting as they remain in the neutral plane. In motors which receive so little attention as these, special attention must be given to the design of devices for keeping oil and grease out of the case. These would injure the insulation of the coils and produce sparking at the commutator. Oil rings are, therefore, placed on the shaft, and these discharge into chambers connected to the oil wells or allow the oil to overflow on the track. The bearings are made self-oiling or self-greasing by means of rings or wicks and will run for weeks without attention. Classes of Motor.—Motors are classified in the same manner as dynamos. The fields may be either bipolar or multipolar, and with respect to the type of armature winding employed, motors are classed as: 1. Series wound; Fig. 416.—Series motor connections. A series motor on a constant voltage circuit does not have a constant field strength, and does not run at uniform speed. If the load be taken off it will run at excessive speed. To start the motor, the circuit is completed through a variable resistance or rheostat by moving the switch S so that the resistances R, R1, R2, R3, are gradually cut out of the circuit. To stop, the switch S is moved back to its "off" position. Series Motors.—A series motor is one in which the field magnet coils, consisting of a few turns of thick wire, are connected in series with the armature so that the whole current supplied to the motor passes through the field coils as well as the armature. Fig. 416 is a diagram of a series motor showing the connections and rheostat. Ques. What are the characteristics of a series motor? Ans. The field strength increases with the current, since the latter flows through the magnet coils. If the motor be run on a Fig. 417.—General Electric type CL-B motor for slow and moderate speeds. It is of multipolar construction, having six pole pieces. The advantages of slow speed machinery are generally understood, and in motors the additional outlay to secure slow speeds is warranted, inasmuch as it results in diminished wear and friction losses in gearing, belting, bearings, and commutators, and decreased brush renewals. The comparatively slow speeds of these motors are of importance in that they permit belting or gearing the motors directly to ordinary slow speed line shafting without employing intermediate counter shafting. When motors are geared to heavy duty machines, it is considered better practice to supply an outboard bearing to take up the additional strain that would otherwise be put on the gearing and bearing. Ques. For what kinds of service are series motors unsuited? Ans. Series motors should not be employed where the load may be entirely removed because they would attain a dangerous speed. They should not be used for driving by means of belts,
Ques. For what service are series motors adapted? Ans. For gear drive.
Fig. 418.—Shunt motor connections. A shunt motor runs at constant speed on a constant voltage circuit. In connecting the motor in circuit, the field coils must be placed in circuit first, so that there is a certain amount of field strength to produce rotation of the armature and thus prevent excessive current through the armature. If the field magnets were not put in the circuit first, the armature, at rest on receiving current, would probably burn out, because it is of low resistance, and would take practically all the current supplied, especially since no reverse voltage is generated in the armature at rest. The method of starting is shown in the illustration. To start, the switch is closed, and the rheostat lever pushed over so as to make contact with A and B, thus first exciting the magnets. On further movement of the lever, the rheostat resistances R, R1, R2, R3, etc., are gradually cut out as the speed increases, until finally all the resistance coils are cut out. To stop, the lever is brought back to its original position. Ques. What advantage is obtained with series motors with respect to the connections? Ans. A single wire only proceeds from the rheostat to the motor, so that, with the return wire, only two wires are required. Ques. For what service are series motors specially adapted? Ans. Series motors are used principally for electric railways, trolleys, and electric vehicles, and similar purposes where an attendant is always at hand to regulate or control the speed. They are also used on series arc light circuits in which the current is of constant strength. Very small motors are generally provided with series windings. Fig. 419.—Speed regulation of a shunt motor. The speed of a motor depends on the voltage of the current supplied and the field strength. The motor tends to rotate so fast as to produce a reverse voltage nearly equal to that supplied to the brushes; hence, the speed varies with the voltage supplied. By decreasing this voltage then, the speed is decreased. Accordingly, the speed may be reduced by inserting, by means of a rheostat, a resistance in series with the motor. By inserting this resistance in the field circuit, the voltage at the terminals of the motor is lowered, thus giving the condition necessary to reduce the speed. The arrangement for speed regulation shown in the figure includes a starting regulator and a shunt regulator. Shunt Motors.—A shunt motor may be defined as one in which the field coils are wound with many turns of comparatively fine wire, connected in parallel with the brushes. The Figs. 420 to 422.—Reversing the direction of rotation of a series motor. Fig. 420 shows the connections for counter clockwise rotation. The motor may be reversed: 1, by allowing the current to flow in its original direction (from D to C) in the field magnet coils, and altering the direction of the armature current by changing the two connections on the brushes A and B, thus connecting C to A and B to the return wire as in fig. 421, or 2, by leaving the direction of the current in the armature in its original direction, and reversing that of the field current, as in fig. 422. If the wires leading to the rheostat and motor directly, were reversed there would be no reversal of the motor, because by so doing, both the armature and field magnet currents would be reversed. Figs. 423 to 425.—Reversing the direction of rotation of a shunt motor. Fig. 423 shows the connections for counter clockwise rotation. The motor may be reversed: 1, by allowing the current to flow in its original direction through the field magnet coils (from D to C), and reversing its direction through the armature (from A to B) as in fig. 424, or 2, by allowing the armature current to flow in its original direction (from B to A) and reversing the current through the field coils (from C to D) as in fig. 425. Ques. What may be said with respect to the speed of a shunt motor? Ans. It is practically constant with varying loads.
Ques. How should a shunt motor be started? Ans. To properly start the machine, the field coils must be fully excited.
Influence of Brush Position on Speed.—In the case of a shunt motor supplied with current at constant pressure, the speed is a minimum when the brushes are in the neutral plane, and the effect of giving the brushes either positive or negative lead is to increase the speed, especially with little or no load. Ques. Why does the speed increase? Ans. When the brushes are shifted from the neutral plane, the reverse voltage between the brushes is decreased, speed remaining unchanged. Accordingly, the pressure in the supply mains forces an increased current through the armature thus producing an increased armature pull which causes the speed to increase until the reverse voltage reaches a value sufficiently large to reduce the current to the value required to supply the necessary driving torque. Compound Motors.—This type of motor has to a certain extent, the merits of the series motor without its disadvantages, and is adapted to a variety of service. If the current flow in the same direction through both of the field windings, then the effect of the series coil strengthens that of the shunt coil; this strengthening is greater, the larger the armature current. Fig. 426.—Compound motor connections for starting from a distant point. A compound winding may be used on motors for many different purposes. If the current flow in the same direction through both windings, then the effect of the series coil strengthens that of the shunt coil. This strengthening increases with the load. Thus the motor gets, at increasing load, a stronger magnetic field, and will therefore, if the voltage remain constant run slower than before. Accordingly, for a given current, the starting power will be greater than that of a shunt motor. With a decreasing load the motor will run faster. The compound motor has, to a certain extent, the merits of the series motor without its disadvantages. By means of compound motors the starting at a distance with only two mains may be effected, just as in the case of the series motor. The connections are shown in the diagram. If the motor be regarded as being without the shunt coil, then it is connected up exactly as the series motor in fig. 416. The current coming from the starter enters the series coil at F, flows through the series coil and leaves it at E, flowing from there to the armature brush B, through the armature to brush A, and from there through the second main back to the generator. The shunt winding is connected directly with the armature brushes A and B, and gets at starting, therefore, only a very small voltage, hence its field is nearly ineffective. But on account of the series winding, the motor starts as a series motor. Obviously such a motor will not develop a very large starting power like a real series motor, for, on account of the large space occupied by the shunt coils, there is less space available for the series coils than with a series motor. A compound motor may, however, even with this arrangement, be easily started, provided the load on starting be not too heavy. When once running the armature will produce a reverse voltage and the shunt coil will be supplied with nearly the full terminal voltage. Ques. Mention some characteristics of the compound motor. Ans. Since it is a combination of the shunt and series types, it partakes of the properties of both. The series winding gives it strong torque at starting (though not as strong as in the series motor), while the presence of the shunt winding prevents excessive speed. The speed is practically constant under all loads within the capacity of the machine. Ques. Describe the connections for starting a compound motor at a distance. Ans. Control at a distance can be effected with only two wires, just as in the case of a series motor. In the diagram fig. 426, the current coming from the rheostat enters the series coil at F, and leaves it at E, thence it flows to the armature brush B, through armature to brush A, and from here back to the dynamo. The shunt winding, which is connected across the brushes, gets a very small voltage at starting and is accordingly very ineffective. The motor then starts as a series motor. The starting effect is smaller than in a series motor because of the fewer turns in the series winding, most of the available space being occupied by the shunt coils. Power of a Motor.—The word "power" is defined as the rate at which work is done, and is expressed as the quotient of the work divided by the time in which it is done, thus:
The difference between power and work should be clearly understood. Work is the overcoming of resistance through a certain distance. It is measured by the product of the resistance into the space through which it is overcome, thus:
The unit of work is the foot pound, which is the amount of work done in overcoming a pressure or weight equal to one pound through one foot of space. The unit of power is the horse power which is equal to 33,000 foot pounds of work per minute, that is:
In order to measure the mechanical power of a motor, it is necessary to first determine the following three factors upon which the power developed depends: 1. Pull of the armature, in pounds; 2. Distance in feet at which the pull acts from the center of the shaft; 3. Revolutions per minute.
Fig. 427.—General Electric type CQ Motor. These motors range in capacity from 1/6 to 20 horse power. The small sizes are bipolar, and the larger sizes have four poles. For installations where the motor is exposed to dust, mechanical injury or moisture, it may be partially or entirely enclosed by means of hand hole covers. The standard voltages are 115, 230 and 550. The work done in one minute is expressed by the following equation:
Hence, the power developed is 3,141,600 ÷ 33,000 = 95.2 horse power. Ques. What is "brake" horse power? Ans. The net horse power developed by a machine at its shaft or pulley; so called because a form of brake is applied to the pulley to determine the power. Ques. Describe the apparatus used in making a brake test. Fig. 428.—Prony brake for determining brake horse power. It consists of a friction band ring which may be placed around a pulley or fly wheel, and attached to a lever bearing upon the platform of a weighing scale in such a manner that the friction between the surfaces in contact will tend to rotate the arm in the direction in which the shaft revolves. This thrust is resisted and measured in pounds by the scale. In setting up the brake the distance between the center of the shaft and point of contact (knife edge) with the scales must be accurately measured, the knife edge being placed at the same elevation as the center of the shaft. An internal channel permits the circulation of water around the interior of the rim as shown, to prevent overheating. Ans. Tests of this kind are usually made with a Prony brake as shown in fig. 428. It consists of a band of rope or strip iron—the latter is the arrangement shown—to which are fastened a number of wooden blocks, several carrying shoulders to prevent the contrivance from slipping off the wheel rim. The brake band is drawn tight, as shown, so that the blocks press against the surface all around. The brake thus formed is restrained from revolving with the pulley by two arms attached near the Ques. Why must the point of contact of the brake with the scales be level with the center of the shaft? Ans. In order to determine the force acting at right angles to the line joining the point of contact and center of the shaft. Ques. What is the distance between the center of the shaft and point of contact with the scales called? Ans. The lever arm. Ques. What three quantities must be determined in a test in order to calculate the brake horse power? Ans. The lever arm, the force exerted on the scales, and the revolutions per minute. Ques. How is brake horse power calculated? Ans. From the following formula:
in which
Substituting the given values in the formula,
Now, if the voltmeter and ammeter readings be 220 and 65 respectively, what is the efficiency of the motor at this load? The amount of power absorbed by the motor, or in other words, the input is
and since the output is 17.1 horse power,
Speed of a Motor.—The normal speed at which any motor will run is such that the sum of the reverse electromotive force and the drop in the armature will be exactly equal to the electromotive force applied at the brushes. The drop in the armature is the difference between the applied voltage and the reverse voltage. Mutual Relations of Motor Torque and Speed.—The character of the work to be done not only determines the condition of the motor torque and speed required, but also the suitability of a particular type of motor for a given service. There are three general classes of work performed by motors, and these require the following conditions of torque and speed: 1. Constant torque at variable speed;
Fig. 429.—Two path method of speed regulation of series motor. A rheostat is connected in shunt to the field coils as shown. The current passing from a to b divides between the magnet coils and the rheostat coils; the higher the resistance of the rheostat the less current passes through it, and the more through the magnet coils, hence the stronger the field magnet. 2. Variable torque at constant speed;
3. Variable torque at variable speed.
Speed Regulation of Motors.—The speed of motors connected to constant voltage circuits is usually regulated by the two following methods: Fig. 430.—Variable field method of speed regulation of series motor. The field winding is divided into a number of sections with leads connecting with switch contact points as illustrated. The speed then is regulated by cutting in or out of the circuit sections of the field winding thus varying the strength of the field. 1. By inserting resistances in the armature circuit of a shunt wound motor; 2. By varying the strength of the field of a series motor. The first method is sufficiently explained under fig. 418 and the second method is illustrated in fig. 430. The controller switch S is so arranged that a greater or lesser number of field coils can be inserted in the field circuit. When the switch arm is Ques. What results are obtained by this method of regulation? Ans. The speed of a series motor may be nearly doubled, that is, if the lowest permissible speed of the motor be 250 revolutions per minute it can be readily increased to 500 revolutions per minute by changing the field coil connections from series to parallel. It is on this account, as much as on their powerful starting torque, that series motors have been until recently almost exclusively employed for electric traction purposes. Series Parallel Controller.—When two motors are used in electric railway work, their armatures are connected in series with each other and an extra resistance which prevents the passage of an excessive current through the armature before the motor starts. As the speed of the car increases, the extra resistance is gradually cut out of circuit and the field winding connections changed from series to parallel by means of a series Efficiency of a Motor.—The commercial efficiency of a motor is the ratio of the output to the input. As a rule, the power developed by a motor increases as the reverse voltage generated by it decreases, until this voltage equals one-half of the voltage applied at the brushes. After this point is reached, the power developed by the motor decreases with the decrease of the reverse voltage. Therefore, a motor performs the largest amount of work when its reverse voltage is equal to one-half the impressed voltage. Fig. 431.—Double-throw, double-pole switch for reversing direction of rotation of a motor. The direction of rotation can be reversed by changing the direction of current in either the armature or the field coils. It is preferable, however, to reverse the direction of rotation by changing the direction of current through the armature. The switch is wired as shown, means of reversal being provided by running the wires as indicated by the dotted lines. The efficiency of a motor as just stated is the ratio of the output to the input; this is equivalent to saying that the efficiency The electrical horse power is easily obtained by multiplying the readings taken from volt meter and ammeter, which gives the watts, and dividing the product by 746, the number of watts per horse power. That is:
Fig. 432.—Wiring diagram, showing electrical connections between the armature, field, and interpoles of an interpole motor. As the name implies, an interpole motor has in addition to the main poles, a series of interpoles which are placed between the main poles, and whose function is to assist in the reversal of the current under the brushes. They provide a separate commutating field of a correct value at all loads and speeds, and their windings are for this purpose connected in series with the armature. The proper functioning of the interpoles is independent of the direction of rotation of the armature, also of the load carried over the whole speed range. In an ordinary motor without interpoles, commutation is assisted by a magnetic fringe emanating from the main poles, but as the value of this fringe is altered by the load of the motor and by rheostatic field weakening, if higher speeds be desired from such a machine, commutation becomes imperfect and sparking results, making a readjustment of the brushes necessary. Interpole Motors.—An interpole motor has in addition to the main poles, a series of interpoles, placed between the main poles. The object of these poles is to provide an auxiliary flux or "commutating" field at the point where the armature coils are short circuited by the brush. Figs. 433 to 437.—Parts of the type S interpole motor built by Electro Dynamic Co. They are as follows: 1. yoke—commutator view; 2. interpole coil; 3. top R. H. main coil; 4. bottom R. H. main coil; 5. main pole; 6. interpole; 7. armature shaft, R. H. bearing; 8. commutator; 9. armature wedge; 10. armature coil; 11. brush ring; 12. brush carrier insulation; 13. brush carrier; 14. brush guard; 15. carbon brush; 16. brush holder; 17. cross connecting cable; 18. oil ring; 19. commutator end bearing bushing; 20. pulley end bearing bushing. Ques. What is the object of the commutating field produced by the interpoles? Ans. Its object is to assist commutation, that is, to help reverse the current in each coil while short circuited by the brush, and thus reduce sparking. Fig. 438.—Interpole motor as built by the Electro Dynamic Co. This type of motor is devised to prevent sparking at all loads by the use of interpole magnets, that is, small magnets placed between the field magnets. The interpoles set up a field in a direction to stop and reverse the current in the armature coils while they are short circuited by the brushes. Ques. What is the nature of the commutating field? Ans. The excitation of the interpoles being produced by series turns, the field will vary with the load, and will, if once adjusted to give good commutation at any one load, keep the same proportion for any other load, provided the iron parts of the circuit be not too highly saturated. Ques. State briefly how sparking is reduced or prevented by the action of the interpoles. Ans. Sparking is due to self-induction in the coil undergoing commutation, which impedes the proper reversal of the current. The action of the interpoles corrects this in that they set up a field in a direction that causes a reversal of the current in the coil while it is short circuited. Thus, the coil at the instant it leaves the brush, is not an idle coil, but has a current flowing in it in the right direction to prevent sparking. Ques. Mention some of the claims made for interpole motors. Ans. Constant or adjustable speed, and momentary overloads without sparking; constant brush position; operation at adjustable speeds on standard supply circuits of 110, 220, and 500 volts; constant speed with variable load; reversal without changing the position of the brushes. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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