1. At the present time the United States Navy uses, to a greater or lesser extent, the following systems for the transmission of fire control information:
1. Sperry (Train only) } Installed in conjunction with each other. 2. Ford (Elevation only) 3. Vickers 4. General Electric (Selsyn) 5. Ford D.C.S. (Direct current, synchronous) 6. Arma.
2. Some of these transmission systems differ only in details of design, whereas others are based upon radically different electrical principles. In this chapter each of these systems is briefly discussed, with the object of enabling the student to group readily its underlying electrical principle, and such other features as will assist in a clear differentiation between the several systems.
3. This system of transmission was developed a few years prior to the World War, as the result of collaboration between the Navy Department and the Sperry Gyroscope Company. Fire con trol installations using this type of transmission were limited to main battery control in train only; but somewhat later the Ford Instrument Company designed a companion transmission system for control in elevation. As a result, these two transmission systems are almost always found installed in conjunction with each other, but as each utilizes a different method of transmis- sion they will be described separately.
4. The Sperry system of transmission is a step-by-step system operating on direct current (20 volts), and is not self-synchron- ous. The operation of the transmitters originates successive electrical impulses which cause motors in the indicators to follow step-by-step, i.e., one step for each impulse. In order that the indicators can duplicate the readings of the transmitters it is necessary, each time the system is placed in operation, first to synchronize the system by setting each indicator manually to the reading of its transmitter. Synchronization must be repeated each time the current supply is interrupted, unless it is certain that none of the transmitters have been moved while current was off; for if a transmitter is moved while the current is off, the indi- cators cannot follow, and when the system is again energized the indicators will be out of step with the transmitter and will remain
so until again synchronized by hand. Similarly, it is necessary to synchronize the system when shifting from one transmitter to another.
5. The accompanying diagram (Plate 1) indicates the electrical operation of the Sperry transmission. It shows the two types of transmitters, the single transmitter at the top and the double transmitter at the bottom. For simplicity, each is shown wired to the motors of only one indicator.
6. The single transmitter has three contact fingers, from one side of which three wires are run to the three pairs of poles of the step-by-step motor. The other sides of the three contacts are connected together and to one of the SO volt supply lines. The other supply line is connected to the common side of the motor field coils. Thus the separate field coils are energized only when the transmitter contact to which they are connected is closed. There is no electrical connection to the laminated iron armature, which is moved simply by the magnetic pull of the field coils.
7. The contact fingers on the transmitter are raised and lowered in proper sequence by revolving cams mounted on the same shaft and spaced at angles of 60 degrees. Starting with No. 1 contact closed and Nos. 2 and 3 open, the motor armature will, since No. 1 coils are energized, take up the position shown, standing opposite No. 1 pole. If now we go forward, the next step will be with Nos. 1 and 2 contacts closed (No. 3 open) and- motor armature in position half way between Nos. 1 and 2 poles; the next step, No. 2 closed Nos. 1 and 3 open; and so on, through the twelve different armature positions (steps) in a complete revolution of the transmitter cam shaft. If the transmitter shaft is turned, continuously in one direction the motor armature will follow at the same speed in the same direction. When the transmitter shaft is reversed the armature will also reverse and follow in the opposite direction.
The double transmitter consists of two sets of contact fingers, actuated by a single cam shaft but connected to two motors.. By means of this arrangement twice the number of steps is obtained per revolution. This is accomplished by gearing the two motor armatures together and having the two motors oppose each other on alternate steps.
8. Double transmitters are used in parts of the system where greater accuracy is necessary, but with both types the gear ratios are usually so designed that one step of transmitter and motors corresponds to 5 minutes of arc.
9. The indicators of this system are of the follow-the-pointer type, that is, the motor armature shaft moves a red pointer
around a dial, whereupon the turret trainer moves his turret until a second (white) pointer, actuated by the train of the turret, is matched up with the red pointer of the indicator.
10. Miscellaneous features of this system of transmission are:
(a) Provision is made for setting all transmitters on an "electrical zero", so that if a transmitter has to be dismounted it can readily be lined up when reinstalled.
(b) The torque on the motor armatures is amply sufficient to hold the indicator pointers steady when the transmitter is at rest.
(c) If a motor armature should become "stuck" or jammed in any manner, it will not affect the electrical impulses sent to the other motors by the same transmitter.
11. The principal advantage of this system is that it is electrically simple. Its disadvantages are that it is not self-synchronous, the necessarily limited number of steps do not permit an absolutely smooth and continuous following of the target, and the design of material is not as rugged as could be desired.
12. The Ford system of transmission is a step-by-step system operating on direct current (20 volts), and is self-synchronous within its-range of operation. Unlike the Sperry transmission, which must operate throughout 360 degrees of arc, the Ford transmission merely has to function within an arc which is limited approximately by the maximum angles of elevation and depression of the main battery guns, or in the neighborhood of 35 to 40 degrees. Because of this limited range of operation it is possible to construct a simple step-by-step system which is also self-synchronous.
13. The electrical features of the Ford transmission are shown on the accompanying diagram (Plate 2) which shows one transmitter unit wired to one motor. The essential difference between this system and the Sperry system is that a wound rotor is used in the motors instead of a plain iron armature. This rotor consists of two iron vanes, one above the field coils and one below, with a coil wound about the rotor shaft between them. The ends of the coil are connected to the supply line in such manner that the upper vane of the rotor is always positive and the lower one negative. This gives the rotor directional characteristics which cannot be obtained by the simpler Sperry armature.
14. The transmitter consists of a four-quadrant contact drum, one quadrant of which is always positive and the opposite one
negative, while the two quadrants in between are cut away in order to "break contact. Five contact fingers, wired to the five motor field coils, are equally spaced about the transmitter drum so as to make contact alternately with the positive and negative quadrants as the drum revolves.
15. Each motor field coil is wound in two sections, the upper half being wound in one direction and the lower half in the opposite direction, so as to make both ends of the pole have the same polarity when the coil is energized.
16. With this combination of a rotor with constant magnetic polarities and field coils in which the polarities can be varied at will, a finer control of steps is possible, and twenty steps are obtained per revolution instead of only twelve steps as with the Sperry transmission. However, as these additional steps are not sufficient to give one transmitter the necessary range of self-synchronous operation, the actual transmitting- units of the Ford system consist of two or three transmitters with their drums interconnected through a mutilated gear and transfer mechanism. In this way the first (units) transmitter, drum makes a complete revolution before the second (hundreds) drum makes one step. For the director-correction transmitter the steps of the units drum represent 5 minutes of arc, and the steps of the hundreds drum represent 100 minutes. For the gun elevation indicators the steps of the units drum are 2-1/2 minutes, those
of the second transmitter drum 50 minutes, and a third (three-position) drum makes one step for each 1000 minutes, Through these arrangements the Ford transmission becomes self-synchronous within its designed range of operation.
17. The indicators of this system are of the follow-the-pointer type, with the addition of windows in which appear numbers showing in minutes the transmitted angle and the actual angle set on the gun or range converter.
18. Miscellaneous features of this system are similar to the Sperry system, i.e., index marks permit setting, transmitters on "electrical zero", the motor torque holds pointers steady while the transmitter is at rest, and a "stuck" motor will not affect electrical impulses sent out by a transmitter.
19. The advantages of the Ford transmission over the Sperry are that it is self-synchronous and the material design is considerably more rugged. Electrically it is almost as simple. In common with the Sperry system it has the disadvantage of being a step-by-step system, "but for the usual conditions of director fire this feature is of little consequence in an elevation system.
20. The Vickers system of transmission was devised by the British. During the World War we adopted it for use in the director systems of the secondary batteries of our battle ships, and later it was extended to the main battery control of light cruisers and gun control of destroyers.
21. It is a step-by-step system, operating on 15 to 20 volts direct current, and is not self-synchronous. The transmitter is very compactly arranged, with three contact brushes and a supply brush grouped in pairs on either side of a rotating drum. The supply brush bears continuously on the drum, but by means of two 180-degree segments cut out of the drum, the other three brushes have their contact made and broken in proper sequence. In the accompanying diagram (Plate'3) the transmitter arrangement is shown somewhat altered for the sake of clearness, but the order of brush contact is identical with-that of the actual transmitter.
22. The motor is a six-pole motor, the field of which is wound in the same manner as the Sperry motors. However, the plain iron armature is made in the shape of a cross, and-in this feature, and in the resulting operation, the Vickers motor differs from the Sperry. The Sperry armature permitted only twelve armature positions per revolution for a single motor, whereas the Vickers armature gives twenty-four steps per revolution without making the motor any more complicated.
23. It can be seen from the diagram that with the cross type of armature the direction of motion of the armature is opposite to that of the direction of energization of the field coils, whereas in the Sperry transmission the armature and field both rotated in the some direction. Another difference is that the Vickers transmitter drum makes four complete revolutions for one revolution of the motor armature, while the shaft of the Sperry transmitter makes only one revolution for each revolution of the motor armature.
24. The Vickers indicators (generally termed "receivers") are of the follow-the-pointer type. Synchronization in train is done by hand, while the elevation system is synchronized by elevating and depressing the director against top and bottom steps, in this way bringing all the red elevation pointers into agreement with the director. The gear ratios used give the following values for steps:
1 step of slewing transmitter = 1 degree of train
1 step of training transmitter = 2 minutes of train
1 step of elevation transmitter = 3/4 minute of elevation
25. The Vickers system is electrically simple; all transmitters are identical arid all motors are Identical, and so are interchangeable throughout the system; the control in both train and elevation is finer (smaller steps.) than with either the Sperry or Ford transmissions; and the material design is quite rugged. In common with the two preceding transmission systems, it has index marks for lining up the system,- sufficient motor torque to hold pointers steady, and freedom from transmitter disturbances due to a "stuck" motor. Its principal disadvantage is that it is not self-synchronous.
26. Although the Ford Elevation system, which has been described, is self-synchronous, the term "self-synchronous system" is habitually used to mean a system which is not only self-synchronous but which can also transmit an infinite number of steps. At present three such installations are in use:
The Ford DCS System: (Not to be confused with the Ford Elevation System)
Manufactured by the Ford Inst. Co.
The Selsyn System: Manufactured by the General Electric . Co. The Arma System: Manufactured by the Arma Engineering Co.
27. Future installations will be of the "Synchro" type. "Synchro" is a trade mark registered by the Bureau of Ordnance, and is used to designate a system which is completely described by Bureau of Ordnance specifications. In principle the "Synchro" system is identical with the "Selsyn" system, but any manufacturer may manufacture it, the G.E. patents having expired. The advantages of this procedure is that in future contracts it will merely be necessary to specify that the "Synchro" system be used: the specifications for the "Synchro" system completely defining the system and the conditions it must fulfill.
28. Zero-Reading Instruments. Prior to the introduction of the first self-synchronous system (GE Selsyn system) indicators were of the follow-the-pointer type, i.e., the indicator had two sets of pointers. One set was positioned by the distant trans-mitter while the other set was moved by the gun. Beginning with the Selsyn transmission system the "zero-reader" type of indicator came into use. In this type a moving dial has a pointer. The arrangement is such that when this pointer is brought to a fixed reference line, the gun is properly oriented. In other words, the zero-reader, when on zero, indicates that the difference between the signal received and the gun's angular position is zero. The zero reader type of indicator was used exclusively for years in all new construction. Now, however, the follow-
the-pointer type is again being used in many places, notably on the fine dials of train indicators.
29. Coarse and Fine Dials. Where accurate indications are necessary and the self-synchronous feature is to be retained, two generators, motors, and indicating dials are used. The "coarse" dial indicates large changes and the "fine" dial small ones. The coarse dial has a different position for every trans mitted signal, thereby retaining the self-synchronous feature. A single position of the fine dial may mean any one of a number of angular positions of the transmitter, but it indicates accurately what the coarse dial shows approximately.
DCS (DIRECT CURRENT SELF-SYNCHRONOUS) TRANSMISSION SYSTEM.
Note: Although this system is described first, it was developed after the Selsyn system.
30. SINGLE SPEED TRANSMISSION: A typical case of a single speed DCS generator transmitting to several DCS motors is shown in Plate 4. The generator has a single rotating armature with two independent windings - one a motor winding, the other a gen-erator or transmitting winding. The motor is connected to the D.C. supply circuit by means of a commutator and two brushes. The field, which is stationary and common to the two armature windings, is connected across the two motor brushes.
31. The generator or transmitting winding on the armature is connected to the generator commutator on which there are three brushes spaced 120° apart. The brushes are mounted on a gear driven ring so that by means of a mechanical input they can be rotated around the commutator to a position representing the quantity to be transmitted. The three brushes are electrically connected through slip rings to external wires leading to the stators of the DCS motors in the indicators to which the signal is to be transmitted.
32. The motor has a three phase stator winding and a single rotor winding. The latter is connected through slip rings to the D.C. supply circuit. A number of motors can be connected in parallel to one generator. A motor may be made with rotable stator for zero reading indicators.
33. Principle of Operation: In the DCS generator the action of the motor winding and the common field is to drive the armature as a motor. The other armature winding in conjunction with the common field acts as a generator. The generator brushes are rotated around the commutator by an external mechanical input in accordance with the quantity which it is desired to transmit. While the brushes are in motion-the voltage between any two
brushes varies as a sine wave, so that there are three sine wave voltages. These are displaced 120 electrical degrees as in three phase alternating current transmission, but the voltages vary only while the brushes are being moved. For any given position of the brushes the three voltages are constant (not equal).
34. In the motor the three voltages acting in the three phase winding set up a magnetic field which rotates with the rotation of the transmitter brushes and is at all times in a position corresponding to that of the brushes. The rotor of the motor, being excited by direct current follows the magnetic field of the stator. The rotor may carry a dial to indicate its position or it may control electrical contacts in a clutch or motor follow-up circuit by means of which the received signal is given sufficient power to drive the desired mechanism.
35. DOUBLE AND TRIPLE SPEED GENERATORS: In order to increase the accuracy with which a signal can be received, it is frequently desirable to transmit it at so called "high" end "low" speeds. The low speed maintains the self-synchronous feature by making only one revolution or a fraction thereof for the complete range of signals. At the generator the high speed is geared at 36 or 14 to 1 with the low speed. In the indicator there is a motor for each speed, the high speed motor giving a fine reading or a fine control. The double and triple speed generators can also be used in transmitting two or three independent signals at single speeds. These generators are the same as the single speed machines except that there is a set of three generator brushes for each speed, the commutator is larger to accommodate the extra sets of brushes. Provision is made of course for rotating each set of brushes in accordance with the signal to be transmitted.
36. CORRECTOR: In changing the direction of rotation, hysteresis causes a slight lag in the magnetic field of the motor. This is compensated for by a device in the generator called a "corrector". It is placed between the input shaft and the brush gear. It is placed between the input shaft and the rush gear. When the direction of rotation of the input shaft reverses, the pinion driving the brush gear is given an axial motion in- addition to its rotation. This results in a small additional rotation of the brush gear so that the brushes are advanced a slight amount-beyond the correct position of the rotors of the DCS motors to which the generator is transmitting. This is just sufficient to compensate the hysteresis lag in the motor.
37. If the generator is to operate with a DCS differential the corrector introduces sufficient compensation for the hysteresis lags in the differential as well as the motors conneced to it.
38. DCS DIFFERENTIALS. The purpose of a DCS differential is to modify the transmitted signal before it reaches the motors. For example it could be used to apply deflection to relative target bearing to obtain and transmit gun train order. The DCS differential generator is shown schematically in Plate 5. It has a three phase stator winding connected by three wires to the transmitter brushes of a DCS generator. This sets up a magnetic field in the stator represented by the line NS. This field is stationary when the transmitting brushes are stationary and when the position of the latter is changed (as in changing the angle transmitted) the stator field changes accordingly. in other words, the stator field of the differential rotates with the transmitter brushes of the generator.
39. The armature of the differential has two continuous windings each connected at numerous points to its own commutator. Three stationary input brushes, spaced 120° apart on one of the commutators, are connected to the same three wires that feed the stator. The magnetic field thus set up in the armature is rep resented by the line N'S' and is always 90° displaced from the stator field. The forces of magnetic attraction and repulsion between the armature and stator magnetic field cause the armature to rotate. But due to the fact that different commutator bars are continuously coming under the brushes, the magnetic field of the armature remains fixed at 90° displacement from the stator magnetic field.
40. The second winding on the differential armature is wound in the same slots as the first. Spaced 120° apart around the commutator of the second winding are three output brushes. They are mounted on a common brush holder ring so they can be rotated, and they are connected through slip-rings to one or more DCS motors.
41. If the output brushes are in the same position as the three input brushes, the voltages of the output current will be the same as that of the input current, and the connected DCS motor will indicate the same angle as transmitted from the generator. But, if the output brushes are rotated so as to be displace say 30° from the position of the input brushes, the resulting angle transmitted by the differential will be 30° greater or 30° less than the angle sent out by the generator. The output angle v/ill be greater if the output brushes are rotated in the same direction as the generator brushes are rotated for increasing angles and vice versa. Plate 6 shows schematically how a differential is connected in the system.
42. Advantages claimed for the system.
(a) The System operates on direct current which is available on all ships. The present instruments are designed for 120-125 volts. The DCS system can be designed for any DC voltage available and for different supply voltages for motor and transmitter.
(b) The operation is self-synchronous in all respects.
(c) If one motor is held out of synchronism by mechanical means the remaining, motors, even though operating from the same transmitter, will not be affected.
(d) The motors are very strongly self-damping so that oscillations of the dial are damped out at once without the use of a mechanical damper. There is 'no vibration point at which vibration or oscillation is continuous.
(e) Since the fields of the motors are energized with direct current there is no speed at which the motors when given a rapid acceleration will continue to spin as single phase synchronous motors, and therefore no special damping means need be provided to prevent such action.
(f) The stator current is the same whether the rotor is in synchronism or not. Hence in throwing a number of motors on a transmitter there is no current surge. This makes it possible to protect the circuits by fuses, which can be small enough to offer protection since they do not require added capacity to withstand current surges. The current taken by a DCS differential while it is coming up to speed is greater than when it is at full speed. Consequently if a differential is connected between a generator and the motors, the fuses protecting the differential circuit must be larger than required for the normal load.
(g) The absence of current surges makes it possible to throw over from the normal DC supply to an emergency supply by automatic or other quick-acting means without providing overload capacity in the supply circuit.
(h) The transmitting circuit is independent and completely insulated from the D.C. supply circuit, so that grounds on transmission circuit will not affect the D.C. supply line; also a single ground on the transmission line will not cause short-circuits or other troubles.
(i) The two-speed and three-speed transmission is obtained from a single small transmitting unit, thus saving the cost and space requirements of using two or three individual transmitters.
(j) The rotors of indicators need not be energized from the same source that is supplying the transmitter so that the number of wires from the transmitter to the indicator may be reduced to minimum as the indicators can be energized from a local source of supply.
43. Disadvantages of the System:
(a) Both the "generator" and the "differential" are in motion continuously when the system is energized, regardless of whether a change in signal is taking-place or not. This causes relatively short life and more chance of trouble with these units as compared with the Selsyn and Arma units, in which there is mechanical motion only when a change in signal is taking place.
(b) The "generator" and "differential" have commutators, which give more trouble than slip rings.
(c) The system is not quite as accurate in indicating trans mitted signals as the Selsyn and Arma systems.
(d) More power is required to operate the system because current flows from the "generator" to the "motors" constantly, while in the other systems this is true only when a change in signal takes' place.
THE SELSYN SYSTEM.
44. "Selsyn" is the trade name applied to the General Electric transmission system and means "self-synchronous". The system operates on A.C. using about 110 volts, single phase, 60 cycle alternating current.
45. There are four types of selsyn units, named according to their uses, as follows:
(a) Selsyn Generator - which transmits.
(b) Selsyn Motor - which receives and indicates.
(c) Selsyn Transformer - which may be inserted between a "generator" and a "motor" in order to modify the transmitted signal. For example, a transformer may be used for applying deflection to relative target bearing in order to obtain gun order.
(d) Selsyn Differential Motor - an indicator which shows the difference between signals transmitted by two generators.
46. The terms "generator" and motor" are misleading because they do not function electrically like generators or motors, in spite of the fact that they are constructed like two pole 3-phase alternators. Electrically the "generator" and the "motor" are the same. The only difference between them is that in the "motor" the rotor is free to turn, while in the "generator" the rotor is mechanically restrained and is placed in a definite position .corresponding to the signal it is desired to transmit.
47. Electrically the two instruments follow the principles of transformer action. When a pair of them are connected and energized, their rotors are the primaries and are both excited by the same power source. Their stators are the secondaries and are connected to each other. If the rotors are in the same positions relative to the stators, the same voltages will be induced in both stators. In this case the voltages "buck" each
other and no current flows in the secondary (stator) windings. But if the rotors are not in the same positions the secondary (stator) voltages will be unequal and current will flow in the stator windings of both instruments. Because these windings carry current and are in the fields set up by the primaries (rotors), there will be a force between the stator and the rotor of each instrument, i.e., a torque will act on both. The "generator's" rotor is constrained and can not be moved by this torque, but the "motor's" rotor is free to turn and will turn in such a direction as to reduce the torque. When the "motor's" rotor is matched with the "generator's" rotor, the "motor" is in synchronism and the stator voltages are balanced, so that no current flows and consequently there is no torque.
48. The student who has had little schooling in alternating current theory should-not throw up his hands in dispair of understanding the selsyn principle. He should reread the last paragraph and tnen study the following paragraphs which describe in detail how the instruments function.
49. Plate 7, figure (a) shows two equal transformer windings parallel to each other. In this case the secondary voltage equals the primary voltage, i.e.(using the subscript 1 to designate the primary, and 2 the secondary):
Y1 = V2
In Figure (b) the windings make an angle of 45° with each other. In this case the secondary voltage will be the primary voltage times the cosine of the angle which the windings make with each other, i.e.:
V1 V2 = V1 cos 45° = √2
V1 = √2 V2
If the angle between the coils is, 90° as in figure (c), then:
V2 = V1 cos 90° = 0
In other words, there will be no induced voltage.
50. In Figure (d) two sets of three transformer secondaries are arranged in the form of equilateral triangles, the corresponding secondaries being connected together. A coil (R,' R') is placed in the center of each triangle and connected to" a single phase A.C. -source. If R and R' are not parallel when the A.C. current is turned on, the voltages set up in the coils, S1, S2,
S3,S1', S2', S3' will not be equal (although they will be in phase), and currents will flow in the leads connecting the two sets of transformers. These currents would set up a flux in the secondary coils which would conflict with the magnetic fields set up by R and R' and therefore induce a torque on the coils R and R* so that if they were free to turn they would do so. When R and R' had turned to a position in which the direction of their flux fields were parallel, the voltages V1, V2, V3 would equal V1', V2', V3' respectively, and no current would flow in the coils, so that no further torque, would be induced on Coils R and R' and they would cease to turn.
51. Figure (e) (Plate 7) shows the actual arrangement of the Selsyn System. One of the sets of transformers is called a generator, the other set is the motor. The secondaries are wound on the stator and connected "Y" so that only three leads instead of six are required to connect the secondaries of the generator and motor. (The windings could be connected "A"). The coils R and R* are wound on the rotors of the generator and motor and are connected to the same A.C. supply. The generator rotor is mechanically geared to a transmitting instrument and its motion is thereby restrained by the transmitting mechanism. The rotor of the motor is free to move and follows in synchronism with the rotor of the generator.
52. Selsyn Transformer. The Selsyn Transformer is a device for introducing an angular change into the output of a generator before it enters the motor, so that the reading of the motor indicator will be that of the generator + the change introduced by the transformer. This is accomplished by having the rotor as well as the stator wound with three circuit windings. See Plate 8.
53. The set of three voltages from the generator impressed on the transformer rotor produces exciting currents which produce an alternating magnetic flux. The direction of this flux with respect to the transformer rotor leads R1", R2", R3", is the same as the direction of the generator rotor flux with respect to the generator stator leads S1, S2, S3. This flux induces a set of three voltages in the three transformer stator circuits. When the transformer rotor windings are in the same positions as the transformer stator windings (i.e., when R1", R2", R3" are adjacent respectively to S1", S2", S3"), the voltages across S1", S2", S3" are the same as those across S1, S2, S3. In this case the output signal from the generator is not modified by the transformer.
54. The transformer stator voltages maybe changed either by turning the rotor of the generator with the transformer rotor mechanically restrained, which turns the transformer flux by
electro-magnetic means, or by turning the transformer rotor, which mechanically changes, the direction of the direction of the flux.
55. The angular signal, as represented by the set of trans-former stator voltages, is therefore the algebraic sum of the generator rotor angle and the transformer, rotor angle. The transformer stator voltages, being impressed on the stator of a motor, cause it to operate in synchronism with the modified signal, as indicated.
56. The Selsyn Differential Motor is simply a Transformer with its rotor free to turn, which is connected between two generators, so that the sets of voltages of one generator are impressed on the stator and those of the second generator on the rotor. As long as both generators are transmitting the same angular signal, the rotor of the Differential Motor remains fixed at its zero position, but as soon as there is any disagree-ment in the angular signals the rotor moves off the servo position an amount equal to the angular difference between the transmitted signals. A dial attached to the rotor shaft indicates this difference.
57. Exciters. When a single generator transmits to a number of motors it carries a heavy load. In some cases it would be necessary to use an inconveniently large generator to carry the load, were it not for an "exciter" which: is connected to the circuit. There are two types of "exciters". One is a large selsyn motor immersed in oil which is connected on the line like any other motor; but it is so wound that, under load, it produces a slightly higher set of voltages and therefore carries most of the load. The other type of exciter consists of a set of condensers which are so arranged as to improve the power factor, thus reducing the load. The first type of exciter is not very satisfactory principally because they do not follow perfect-ly and thus cause errors. However in later installations the indicating motors themselves are so wound as to function like exciters of this type, and it is believed that they will prove to be satisfactory.
58. "Stuck" Motor. If the rotor of a Selsyn motor, in a circuit in which there are several motors actuated by a generator, becomes "stuck", so that it cannot rotate freely, the motor will then operate as a generator and cause the readings of all the other motors in that circuit to be in error. (They will indicate half way between the generator signal and the reading of the stuck motor.) If the "stuck" motor is removed from the line the other motors will synchronize and the circuit will be able to operate correctly again.
59. Damping. Motors are provided with oscillation dampers. One type is a kind of fly-wheel, attached to the shaft through a friction device. The other type is a magnetic drag.
60. Advantages of the Selsyn System. Various advantages of this system over other self-synchronous systems have been claimed.
The thing most in its favor is that it has proved satisfactory in service.
(a) If one motor sticks, it throws out the other motors.
(b) The first installations have many more units than necessary.
(c) The torque is zero when the motor indicates correctly. In spite of this, however, the indicators follow very closely the transmitted signal.
ARMA. TRANSMISSION SYSTEM.
62. The Arma system works on the same principle as the Selsyn system. In fact an Arma unit (generator, motor, etc.) could be substituted for a selsyn unit and vice versa, provided the unit substituted was designed for the same voltage, etc., and that it would mechanically fit in place.
63. Types of Units. The Arma system has units corresponding exactly with the four selsyn units (see Par. 45), and in addition it has an additional kind of transmitter. The Arma nomenclature is as follows:
(a) "Synchronous Transmitter". This is the equivalent of the selsyn "Generator". It transmits signals.
(b) "Synchronous Indicator". This is the equivalent of the selsyn "Motor". It receives and indicates signals.
(c) "Differential Transmitter". This is identical with the selsyn "Transformer". It receives a signal from a transmitter, modifies it, and transmits the resultant to a "synchronous indicator".
(d) "Differential Indicator". This is identical with the selsyn "Differential Motor". It indicates the difference between the signals transmitted by two transmitters.
(e) "Commutator Transmitter" which is different from the "synchronous transmitter"(or selsyn "generator") in construction, although it has the same electrical and mechanical inputs and outputs.
64. The only difference between the "synchronous transmitter" and the selsyn "generator" is that the rotor and stator windings are interchanged. That is, the three-circuit winding is on the rotor and the single-circuit winding is on the stator. Electrically the action is the same. Similarly, the only difference between the "synchronous indicator" and the selsyn "motor" is that the rotor has the three-circuit winding and the stator the single circuit winding.
65. There is no difference (except of course in minor details of design) between the Arma units "Differential Transmitter", and "Differential Indicator", and the corresponding Selsyn units ("Transformer" and "Differential Motor").
66. Commutator Type Transmitter. In the selsyn "generator" and in the Anna "synchronous transmitter" the primary and secondary circuits are separate, one being on the rotor and the other on the stator. In the commutator transmitter, transformer action is also used, but it has only one winding, i.e., it is an "auto-transformer".
An "auto-transformer" is a transformer in which part of the same winding is employed as both primary and secondary. The difference between an auto-transformer and a separate coil trans-former is indicated in the accompanying figure. The voltage relationships are as indicated. It can be seen that in an auto-transformer any desired voltage can be obtained by connecting with taps at the proper points, such as k, l , m.
68. In the arma "commutator transmitter" the exciting voltage is put across a long coil (which is bent into circular form for compactness). At various points this coil is tapped and each lead is connected to a segment of a cormnutator. The number of coils between adjacent commutator segments is not constant however. In fact the number of coils between commutator segments
is so arranged that if two brushes spaced 120° apart are moved around the commutator the voltage across the brushes will vary as a sine wave* If a third "brush is spaced 120° from the other two brushes, there will be three voltages. If the three brushes are rotated around the commutator, the three voltages will vary as sine waves which are spaced 120° apart. This is precisely what happens when a selsyn "generator" or an Arma "synchronous transmitter" is rotated, in changing the transmitted signal._Consequently this transmitter will function like the other types. The signal is transmitted by positioning the set of three brushes,
69. Plate 9 illustrates the method of tapping the auto-trans former to obtain the sinusoidal voltage characteristic as the brushes are rotated. It will be noted that there are 360 segments on the commutator, with opposite members connected in parallel. To these have been brought 157 taps and the two end leads of the auto-transformer. Of course more or fewer segments could be, used, but the fewer the segments the larger the voltage step in shifting the brushes from one commutator to the next.
If there could be an infinite number of commutator segments, the three output lsecondary) voltages would vary as smoothly as is the case with a selsyn "generator" or Arma "synchronous transmitter". However with a 360-bar commutator there are 720 steps in one revolution (one step for each bar, and one for the position when the brushes each contact two bars). These steps are small enough for the purpose.
70. There are two kinds of commutator transmitting units, a "one-speed transmitter unit" and a "high and low speed transmitter unit." Then one speed transmitter has but one set of three brushes bearing on the commutator segments and transmits readings at one speed, (i.e. coarse signals). The high and low speed transmitter has two sets of brushes bearing on the same commutator segments, both obtaining their voltage supply from the same transformer. The brushes are geared together at a ratio of 1 to 36 or 6 to 72, according to the particular requirement of the instrument in which it is used, so that readings can be transmitted to two separate motors, one from each of the two sets of brushes, for coarse and fine, respectively.
71. Advantages claimed for the commutator type transmitter are:
(a) If one motor is "stuck", the other motors will be affected only to a negligible degree, because the auto-transformer type of transmitter can carry the extra load with very little effect on the secondary voltages.
(b) One unit is capable of transmitting two independent signals, thus taking the place of two transmitters of the other type.
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