Induction motor

“ Internal Resistance Induction Motors.—The armature of this type of induction motor differs from the squirrel cage variety in that the winding is not short circuited through copper rings, but, in starting, is short circuited through a resistance mounted directly on the shaft in the interior of the armature. ”

“ An induction motor will operate only on alternating current.
A small motor such as that shown in Figure 267, and having a three-pole armature, is the best type to use in making an experimental induction motor.
Remove the brushes from the motor and bind a piece of bare copper wire around the commutator so that it short-circuits the segments.
A source of alternating current should then be connected to the terminals of the field coil. If you have a step-down transformer, use it for this purpose, but otherwise connect it in series with a lamp bank such as that just described. ”

“ The capacity of the motor is theoretically only half what it would be if it were to convert the whole of the electrical energy into mechanical power because the rating depends upon the speed of the rotating field and not on that of the rotor. If the two machines have a different number of pole, or are connected to run at different speeds, the division of power is at a different but constant ratio. The machine starts up as an ordinary polyphase induction motor and the field of the converter is built up as though it were an ordinary dynamo. ”

“ The resultant flux may be obtained by combining the two fluxes due to coils AA and BB, taking account of the phase difference of the two phase current, as in fig. 1,655.
The construction of an induction motor is very simple, and since there are no sliding contacts as with commutator motors, there can be no sparks during operation—a feature which adapts the motor for use in places where fire hazards are prominent. ”

“ Polyphase Induction Motors.—As many central stations put out only alternating current circuits, it has become necessary for motor builders to perfect types of alternating current motor suitable for all classes of industrial drive and which are adapted for use on these commercial circuits. Three phase induction motors are slightly more efficient at all loads than two phase motors of corresponding size, due to the superior distribution of the field windings. The power factor is higher, especially at light loads, and the starting torque with full load current is also greater. ”

“ The current taken by an induction motor from a constant pressure line varies with the speed as in a direct current motor. When a load is thrown on, the speed is reduced correspondingly and as the self-induction or reactance is diminished, more current circulates in the squirrel cage winding, which in turn reacts on the field coils in a similar manner and more current flows in them from the line. In this manner the motor automatically takes current from the line proportional to the load and maintains a nearly constant speed. ”

“ The induction motor being electrically tied into the circuit, the possibility of a runaway of the water wheel is eliminated, since its speed can only slightly exceed the synchronous speed of the system. ”

“ In efficiency and closeness of speed regulation and good general running properties polyphase induction motors approximate very closely to the best continuous-current practice. They produce, however, a certain amount of lag between primary electromotive force and current, which causes the apparent input to be larger than the real input, as generally happens in alternating-current work. The ratio between the real and the apparent watts input is the power factor of the motor. In well-designed modern machines this is usually from 85 to 90% at rated load ”

“ A synchronous motor will drive a generator at a constant speed during changes in load on it, and by having a field regulating resistance it can be used to improve the power factor of the system.
When an induction motor is used its speed drops off slowly as the load comes on the generator, and it is necessary to regulate the voltage of the generator by means of a field rheostat, or compound wound machines may be used. ”

“ The advantage of this change is that the commutator is eliminated, save at starting, and the motor becomes practically a constant speed machine like any other properly-designed simple induction motor. Such motors can be made to start if necessary with several times the normal running torque and a nearly proportionate increase of current. The short-circuiting of the commutator is generally performed automatically by a centrifugal governor. When at speed, efficiency and power factor are those of the typical motor of class (B) 3. ”

“ These motors of the larger size are generally started through an auto-transformer. For the smaller ones, it is customary to provide a throw-over switch, one side only of the switch being fused. The motor at starting takes very strong currents which would blow the running fuse. To reverse a two-phase induction motor the two wires of one phase must be reversed. ”

“ Neutralized Series Motor.—A chief defect of the series motor is the excessive self-induction of the armature, hence in almost every modern single phase series motor a neutralizing coil is employed to diminish the armature self-induction. ”

“ In lieu of these devices various forms of electro-dynamic induction machines, including the combined motors and generators, have been devised. For instance, a motor is constructed in accordance with well understood principles and on the same armature are wound induced coils which constitute the generator. ”

“ The question of how best to convey a current to the car-motor is a serious one, doubtless at this moment occupying the attention of highly-trained intelligence everywhere. The motor current is one of high power, and as such intractable; and it is in the character of this current, rather than in methods of insulation, that the remedy for the much-objected-to overhead wire is to be found. It will be remembered that all the phenomena of induction are unhindered by insulation. ”

“ What I wish to [Pg 234] show you is that this motor rotates with one single connection between it and the generator; that is to say, one terminal of the motor is connected to one terminal of the generator—in this case the secondary of a high-tension induction coil—the other terminals of motor and generator being insulated in space. To produce rotation it is generally (but not absolutely) necessary to connect the free end of the motor coil to an insulated body of some size. The experimenter's body is more than sufficient. ”

“ The magneto itself generates electricity at low pressure, which is transformed by means of a special induction coil into a high-pressured current. An ingenious arrangement enables the current to be distributed to any number of cylinders without the necessity of more than one induction coil, and moreover allows the advancement of the spark throughout a complete revolution of the engine. ”

“ The weakest point in these polyphase induction motors is the importance of employing a very small clearance between armature and field, in order to increase the power-factor by making the structure more efficient, considered merely as a transformer. The clearances in ordinary use are seldom greater than 1/16 in., even in motors as large as 100 h.p., and in smaller machines are frequently not more than 1/32 in.. Induction motors, however possess many valuable properties, and are the mainstay of long-distance power-transmission work at the present time. ”

“ The plant efficiency of such a three-wire system would be increased by using, in connection with the ordinary type of my motor, other types which act more like inert resistances. The plant efficiency of the whole system would, in all cases, be greater than that of each individual motor — if like motors are used — owing to the fact that they would possess different self-induction, according to the load. ”

“ There are a good many more things to be said about the remarks contained in Industries. In regard to the magnetic time lag the writer says: " If a bar of iron has a coil at one end, and if the core is perfectly laminated, on starting a current in the coil the induction all along the iron corresponds to the excitation at that instant, unless there is a microscopic time lag, of which there is no evidence." Yet a motor was described, the very operation of which is dependent on the time lag of magnetization of the different parts of a core. ”

“ Induction motors and other inductive apparatus take a component of current which lags behind the line pressure, and thereby lowers the power factor of the system, while a non-inductive load, such as incandescent lamps, takes only current in phase with the voltage and operates at unity power factor. ”

“ He may get a kick from the handle which will give his arm a nasty jar, or possibly sprain his wrist. These 'back fires' are the result of what is called 'premature ignition,' and therefore belong strictly speaking to the chapter on 'Ignition.' I may merely remark that they are due to the spark-timing gear being too much advanced, the platinum tubes being too long, the burners being too close in, faulty opening of the induction valves, overheating of the motor, or ignition tubes being too hot ”

“ This mixture reaches the combustion chamber through the induction pipe i p, and the induction or inlet valve i. It is fired by an electric spark occurring in the combustion chamber, or a red-hot platinum tube protruding into same, and the exploded charge is ejected through the exhaust valve e, as will be hereafter explained.
THE SUCTION STROKE
Let it be supposed that the fly-wheel has been set rapidly revolving, that the piston has been up at the top at t, and has just descended to the bottom of its stroke (b) . ”

“ Assuming a vehicle with running gear complete and motors mounted and geared to the driving-wheels, the motors have to be fed with energy in order that the car may be propelled— that is to say, we must in addition to the motors have a source of electric current present. A car with body, running gear, and motors geared in position, is a potentiality only. ”

“ A rapidly revolving armature will take more current, without being overheated, than one not in motion. A motor at full speed acts like a dynamo, and generates a current which tends to flow from the machine in a direction opposite to that which produces the motion. It is evident, then, that when the armature is at rest, all the current turned on passes through it without meeting with this opposing current.
Fig. 235 shows a starting, stopping, and regulating box, inside of which are a number of German-silver resistance coils properly connected to contact-points at the top. ”

“ The great majority of motors in actual use have four poles.
The principle of separate excitation has some advantages. It enables great torque or rotary pull to be obtained, without as much loss in the windings of the field magnets as is involved in the case of the series motor. In the separate excitation method of arranging, two or four cells are usually connected separately to the windings of the field magnets, the main battery supplying current to the armature. ”

“ The motors are so light that they develop two horse power for every pound of their weight; while, to keep the frames thin, the necessary power is obtained by terrific speed of the moving parts, as though a steam engine, to avoid great pressure in its cylinders, had a long stroke and ran at great piston speed, which, however, is no disadvantage to the rotary motion of the electric motor, there being no reciprocating cranks, etc., that must be started and stopped at each revolution. ”

“ With our present knowledge of alternating currents it is useless to attempt to calculate from the simple though misleading assumptions ordinarily made, the output, conditions of maximum work, &c., of this machine. Experiment alone can determine its value, and one properly conducted and interpreted set of experiments should enable us to judge both the merit of the invention and its best possible form. I cannot see, however, how, in the form described in the last issue of this journal the motor can work under conditions of a suddenly varying load as satisfactorily as continuous current motors. ”

“ 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 ”