Synchronous motor

“ In practice the synchronous motor has a number of pairs of field poles. It is essentially an alternating current generator running as a motor. One of the principal uses of the synchronous motor is that of a converter, receiving alternating current and delivering direct current. Synchronous motors are also used in transmission lines to aid in maintaining constant voltage. ”

“ Fig. 1,610.—Westinghouse self-starting synchronous motor. Motors of this type are suitable for constant speed service where starting conditions are moderate, such as driving compressors, pumps, and large blowers. Synchronous motors can be made to operate not only as motors but as synchronous condensers to improve the power factor of the circuit. The field is provided with a combined starting and damper or amorlisseur winding so proportioned that the necessary starting torque is developed by the minimum current consistent with satisfactory synchronous running without hunting. ”

“ When the speed of the motor has thus been brought to the desired rate the switch is shifted to throw the main current directly through the motor-circuits, and although the currents in both circuits will now be of the same phase the motor will continue to revolve, becoming a true synchronous motor. To secure greater efficiency, the armature or its polar projections are wound with coils closed on themselves. ”

“ In a direct current motor this reversal is brought about by the commutator. In a synchronous motor the armature reaches the 90 degree position at the exact instant at which the current reverses in the line. Thus in the case of a two-pole motor the armature must make exactly one revolution for each cycle; it is, therefore, a constant speed motor. Such motors are frequently employed in converter stations where alternating current is converted into direct current by what are called rotary converters. ”

“ A polyphase synchronous motor is self starting, because, before the current has died out in the coils of one phase, it is increasing in those of the other phase or phases, so that there is always some turning effort exerted on the armature. ”

“ It therefore becomes possible to use a synchronous motor, if the necessary current due to the load be not too great, as a voltage and phase regulator upon an alternating circuit, a function very valuable in power-transmission work. If the excitation be set to produce leading phase at small loads, the phase angle will gradually diminish as the load rises, and then, passing through zero, increase again with the lagging current, thus holding the power-factor near to unity at all working loads. ”

“ Fig. 2,620.—Synchronous motor with vibrating mirror as used with Duddell moving coil oscillograph. Since the motor must run synchronously with the wave form it is required to investigate, it should be supplied with current from the same source. The motor can be used over a wide range of frequencies (from 20 to 120) . When working at frequencies below 40, it is advisable to increase the moment of inertia of the armature, and for this purpose a suitable brass disc is used. ”

“ Inasmuch as neither motor is doing any work while the current is acting upon the other, the two armatures are rigidly connected, both being mounted upon the same shaft A, the field-magnets B of the synchronizing and C of the torque motor being secured to the same base D. The preferably larger synchronizing motor has polar projections on its armature, which rotate in very close proximity to the poles of the field, and in other respects it conforms to the conditions that are necessary to secure synchronous action. ”

“ Fig. 1,612.—Diagram illustrating the use of a synchronous motor as a condenser. If a synchronous motor be sufficiently excited the current will lead. Hence, if it be connected across an inductive circuit as in the figure and the field be over excited it will compensate for the lagging current in the main, thus increasing the power factor. If the motor be sufficiently over excited the power factor may be made unity, the minimum current being thus obtained that will suffice to transmit the power in the main circuit. ”

“ This is done by motor generator sets, the motor taking its current from the low frequency circuit.
Synchronous motors are generally used for such service as the frequency is not disturbed by load changes; it also makes it possible to use the set in the reverse order, that is, taking power from the high frequency mains and delivering energy at low frequency.
Fig. 2,172.—Course of the Schaghticoke-Schenectady transmission line of the Schenectady Power Co. ”

“ When the machines are running at the same frequency and in phase the pointer is stationary at the marked point. In construction, it is like a small, two phase, bipolar synchronous motor, the field being supplied with alternating instead of direct current. ”

“ A theoretically interesting form of induction motor is a modification which runs at absolutely synchronous speed, receiving the necessary energy in the secondary not in virtue of slip behind synchronous speed, but from great difference in wave form between the primary and secondary circuits, so that energy due to harmonics of the fundamental frequency is periodically received by the armature in spite of synchronism in speed. ”

“ If the secondary member is of very low resistance, the slip behind synchronism is very small, even at full load—less than 2% in motors developed for this particular property. An increase of secondary resistance produces increased falling behind from synchronous speed; and if resistance be added to the secondary member by interpolating rheostats in its circuits, the motor can be made to produce uniform torque over a very wide range of speed, as is the case with continuous current motors. ”

“ I. A single phase synchronous motor is not self-starting. The figures show an elementary alternator and an elementary synchronous motor, the construction of each being identical as shown. If the alternator be started, during the first half of a revolution, beginning at the initial position ABCD, fig. 1,585, current will flow in the direction indicated by the arrows, passing through the external circuit and armature of the motor, fig. ”

“ We then see the engine going through all its motions but much more slowly, and can follow them easily. In another form devised by Callendar, [2] a revolving contact disk is placed on the shaft of an alternator, or of a synchronous motor driven by the alternating current under test. A pair of contact springs are slowly shifted over so as to close the circuit at successive assigned instants during a complete phase. ”

“ When very close regulation is necessary, motor generators are often preferred to synchronous converters. Series arc lighting from transmission circuits is a much more serious problem. At the present time two methods are in vogue: first, the operation of continuous-current series-arc machines by synchronous or induction motors driven from the transmission system; and, secondly, series alternating apparatus for feeding alternating arcs. ”

“ The true synchronous speed of this form of motor is that of the generator when both are alike—that is to say, if the number of the coils on the armature and on the field is x, the motor will run normally at the same speed as a generator driving it if the number of field magnets or poles of the same be also x. ”

“ Synchronous motors are seldom used except for special purposes. They are exactly similar to the ordinary synchronous generator or alternator in construction, and the field system is almost invariably the rotating part. ”

“ This machine which is only to be used for converting from alternating to direct current, consists of an ordinary induction motor with phase wound armature, and a dynamo. The revolving parts of both machines are mounted on the same shaft and from the figure it is seen that the armature of the motor and the armature of the dynamo are also electrically connected. The motor converter is a synchronous machine, but the dynamo receives the current from the armature of the motor at a frequency much reduced from that impressed upon the field winding of the motor. ”

“ Tesla has discovered a much more simple method or plan of operating synchronizing motors, which requires practically no other apparatus than the motor itself. In other words, by a certain change in the circuit connections of the motor he converts it at will from a double circuit motor, or such as have been already described, and which will start under the action of an alternating current, into a synchronizing motor, or one which will be run by the generator only when it has reached a certain speed of rotation synchronous with that of the generator. ”

“ 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. ”

“ This motor can be used as a generator also, in the sense that if connected to the line and driven above synchronous speed by some external means, it will act as an asynchronous generator and return power to the line. For variable speed service, an armature having a winding upon it similar to that on the frame must be used. External resistances inserted in the armature circuit may be used to produce, first, a reduction of starting current, second, an increase of starting torque, or third, a variation of speed. ”

“ If through overloading, or for any other reason, the motor is unable to maintain its synchronous speed, it immediately falls out of step and stops.
Alternating-current Commutator Motors.—These motors are in general appearance similar to the induction motor, but the rotor is fitted with a commutator. According to the electrical connections, these motors may be given characteristics similar to direct-current series or shunt motors. Single-phase commutator motors with series characteristics are used on the L.B. & S.C.R. electric trains. ”

“ The field slots are overhung or partially closed, affording mechanical protection to the coils.
Ques. Why are induction motors called "asynchronous"?
Ans. Because the armature does not turn in synchronism with the rotating field, or, in the case of a single phase induction motor, with the oscillating field (considering the latter in the light of a rotating field) .
Ques. How does the speed vary?
Ans. It is slower (more or less according to load) than the "field speed," that is, than "synchronism" or the "synchronous speed."
[Pg 1288]
Figs. ”

“ In some cases, as when a large motor is used and when the number of alternations is very high, there is more or less difficulty in bringing the motor to speed as a double or multiple-circuit motor, for the plan of construction which renders the motor best adapted to run as a synchronizing motor impairs its efficiency as a torque or double-circuit motor under the assumed conditions on the start. ”

“ When the motor is thrown in circuit, a very low starting current is drawn from the line due to the added resistance in the armature. As the motor comes up to speed, this resistance is gradually cut out, and at full speed the motor operates as a squirrel cage motor, with short circuited winding. ”

“ 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. ”

“ Every machine, while it is being driven by an electric current as an electric motor, is, at the same time, acting as a dynamo to generate a current in the opposite direction. Notice [37] in the third place that this is a shunt-wound instrument. The current which is sent into the instrument divides, and part of it goes through the field, while part goes through the armature. Motors, as well as dynamos, are either shunt-wound or series-wound. But notice finally that the axle on which the armature is carried has a cyclometer arrangement which keeps account of the number of revolutions. ”

“ For a constant-current transmission the magnet and armature windings of both dynamos and motors are usually connected in series with each other and the line so that the same current passes through every element of the circuit, except that each motor may have some current shunted out of its magnet coil for the purpose of speed regulation. ”

“ 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. ”

“ The solution was found by making the two wheels move by means of a timing fork, which vibrates with the same speed everywhere, and under all conditions.
Electric Motor.—The direct current electric motor depends for its action on the principle that likes repel, and unlikes attract. The commutator so arranges the poles that at the proper points, in the revolution of the armature, the poles are always presented to each other in such a way that as they approach each other, they are opposites, and thus attract, and as they recede from each other they repel. ”

“ The employment of multipolar motors secures in this system an advantage much desired and unattainable in the continuous current system, and that is, that a motor may be made to run exactly at a predetermined speed irrespective of imperfections in construction, of the load, and, within certain limits, of electromotive force and current strength. ”