US2046735A - Ship stabilizing control system - Google Patents

Description

July 7, 1936. E. FRISCH ET AL 2,046,735

SHIP STABILIZING CONTROL SYSTEM Filed June 9, 1952 2 SheetsSheet 1 may.

WITNESSES: /7 L/ /0 E JVEfl 1 TO R/S1.

4- a r U79 rise 9f 2/ Walfer Scha elchlin and ATTORNEY July 7, 1936. E, FR H ET AL 2,046,735

SHIP STABILIZING CONTROL SYSTEM Filed June 9, 1932 2 Sheets-Sheet 2 7b Conductor 35 WITNESSES: INVENTORS.

WW C [r] 171g Frz'gch,

' Walfer Schaelchlm and John H. flshbauyh.

" ATTORNEY Patented July 7, 1936 I UNITED STATES PATENT OFFICE SHIP STABILIZING CONTROL SYSTEM Application June 9, 1932, Serial No. 616,256

Claims.

Our invention relates to gyroscopic stabilizing systems for limiting or preventing the rolling motion of vessels and it has particular relation to methods and apparatus for controlling the ,1 precession thereof.

One object of our invention is to provide control equipment of the kind referred to wherein energy from the waves, or from the rolling of the ship, is regenerated through the precession motor.

A further object of our invention is to prevent the effective regenerative effect of the precession motor from exceeding a predetermined value.

It is also an object of our invention to keep the speed relation between the spinning motor for the gyroscope and the generator supplying the spinning motor within predetermined limits.

A more specific object of our invention is to provide for a dynamic braking effect in a preces- 2O sion motor and for weakening the excitation of a precession-generator supplying energy to the precession motor to maintain the speed relation between a gyroscope spinning motor and a spinning-generator supplying energy to the spinning motor substantially constant.

A further object of our invention is to stop-a precession motor when the speed of a generator supplying energy to a gyroscope spinning motor exceeds a predetermined value.

It is also an object of our invention to prevent over-travel or hunting of a precession motor.

A still further object of our invention is to decelerate a precession motor in two successively more efiective steps.

It is also an object of our invention to provide a simple control system whereby the entire stabilizing operation may be automatically accomplished with a minimum of apparatus.

With these and other objects in view, our invention consists in the details of construction and in the methods of manipulation described in the following specification and covered by the appended claims, and illustrated in the accompanying drawings, wherein:

Figure 1 is a diagrammatic view of circuits and apparatus embodying our invention in preferred form;

Fig. 2 is a side view, looking from left to right, of the main gyroscope and housing, showing the mechanical bufiers and the limit-switch tripping devices;

Fig. 3 is a diagrammatic showing of a scheme associated with the control gyro to prevent hunting of the precession motor;

Fig. 4 is a modified showing of the directional contactors, showing them provided with mechanical interlocks; V

Fig. 5 shows a graduated dynamic braking circuit for the main motor; and

Fig. 6 shows a connection for the precession motor and direct current generator to produce an artificial roll of the vessel.

Referring more particularly to. Figure 1 of the drawings, l designates a main stabilizing gyroscope which is horizontally pivoted on gudgeon bearings I40 disposed athwartship whereby the stabilizing forces may be transmitted to the frame structure of the vesseh The gyroscope further comprises a casing in which casing is mounted a flywheel 33 which is supported in thrust bearing ME and bearings itl and which is driven by a squirrel-cage spinning motor 2. Connected to the casing is an arcuate gear member M2 which is in operative relation to a pinion M3 mounted on the shaftof the precession motor 5 and which is in operative relation with a pinion 863 for operating the limit-switch mechanism it l The spinning motor 2 is supplied with energy from an alternating current generator or alternator 3, which in turn is driven by a turbine 4. A precession motor 5 is mechanically coupled to the arcuate gear member M2, as above specified, and the precession motor ii is supplied with energy from a direct current generator 6 coupled to the shaft of the alternating current generator.

A mechanical brake l, which may be released by an electromagnetic arrangement, is mechanically coupled to the precession motor and the precession motor is controlled in the directi of rotation by a control gyro 8.

To accomplish the desired controlling effect, an electrical control system is provided consisting of a pair of directional contactors ii and it, a control contactor it, a dynamic braking contactor i l and a generator field neutralizing contactor l5. To further control the operation of the various elements, a power limit relay 63, a limit-switch mechanism IGI and an overspeed mechanism 26 are provided.

To more clearly understand the operation, as well as the novel features of our invention, a study of the sequence of operation will now be given.

Assuming that conductors l6 and l! are supplied with direct current energy from a suitable source and that the attendant closes the double pole knife switch IBto energize conductors 9 and 10, a circuit is established from the energized conductor 9 through conductor '20, actuating coil 2| oi the main line contactor 22 disposed in the circuit leading from the alternating cur rent generator 3 to the spinning motor 2 through the slip rings 32, rings 23 and 25 bridged by the spring biased contact member 24 of the overspeed device 26, conductor 21 to the energized conductor I'll.

A second circuit is also established from the energized conductor 3 through conductor 28, field rheostat 29, field winding 30 o! the alternating current generator 3 and conductor 3| to the negatively energized conductor 21.

The .field winding 30 of the alternating current generator is provided with a high resistance discharge resistor 30' which prevents damage to the insulation of the field winding when double pole switch I8 is opened. The field rheostat 23, during the starting operation, should be adjusted to a maximum excitation so that the voltage of the generator 3 will be a maximum during the initial stages of its rotation when the frequency of the generator is low.

If the generator has been excited, as hereinabove stated, steam is admitted to the turbine l and the generator will build up a voltage as the speed of the turbine increases. At a speed correspondingto approximately 3 cycles a second a sufliciently large torque is developed at the spinning motor to overcome the static friction of the flywheel 33 which, consequently, will start turning at a slowly increasing speed. As the speed increases the torque margin between the generator 3 and the motor 2 will increase and the generator field can be weakened. Further, as the speed of the spinning motor and generator increases to high values the field winding 36 of the generator 3 should be excited less and less.

When the main stabilizing gyro [J has attained its normal operating speed the double pole knife switch 34 is actuated thereby energizing the buses 35 and 39.

Energization of the buses 35 and 39 establishes a circuit from the bus 35 through field rheostat 36, motor field 31 and conductor 38 to the bus 33. When the buses 35 and 39 are energized the control gyro 3 also starts operation since the motor 40 driving the gyro wheel H is directly connected to the buses 35 and 33. The control gyro is a relatively small unit and the gyro wheel at operates at a comparatively high speed so that its sensitivity may be great. The shaft of the gyro wheel 4i is disposed athwartship so that the casing of the control gyro, which is spring biased in the manner shown, may be caused to oscillate when the vessel rolls. It should be noted that the motor for the control gyro and the field winding 31 for the precession motor 5 cannot be energized unless the switch 08 has been closed, thereby exciting the field winding 30 of the alternating-current generator 3 supplying the energy to the spinning motor 2.

If the ship is rolling the spring biased contact member 43 is brought into contact relation with either contact member 42 or contact member 98, depending upon the direction of roll of the vessel. Assuming that the vessel rolls in such a direction that the spring biased contact member 43 engages the contact member 42, a circuit is thus established from the energized conductor 35 through conductor 4!, contact members 63 and 42, actuating coil 44 of directional contactor ii, limit-switch 45, limit- switches 66 and 41, conductor 48, contact members 43, which were closed when the line contactor 22 operated, conductor 50 and actuating coil 6| of the control contactor I3 to .the energized conductor 33.

It should be noted that operation of the control contactor i3 releases the brake 1 holding the precession motor 5 in place. The circuit for the brake releasing coil 51 may be traced from the energized conductor 35 through contact members 62 of control contactor l3, conductor 53, contact members 54 on the brake releasing mechanism connected in parallel to the resistor 55, resistor 56, actuating coil 51 for the mechanical brake 1, conductors 56 and 59 to the energized conductor 39. While we have shown but a single brake 1 in actual practice, more often a plurality of brakes are utilized so that the stopping oi' the precession motor may be efiected in a. very short interval of time particularly when coacting with the regenerative braking and dynamic braking eifect, more specifically discussed hereinafter, which are set up in the precession motor. The brake releasing coil 51 is provided with a discharge resistor 856, which may be adjustable for varying the time constant of coil 51.

A second circuit is established by the operation of the control contactor l3 which circuit may be traced from the energized conductor 35 through contact member 60 of contactor i3, conductor 6|, contact members 62 and 64 of the power limit relay 63, actuating coil 65 of the contactor for shunting the generator field resistor 16 and conductor 66 to the energized conductor 39. A further circuit is established from contact member 60 through actuating coil 61 of dynamic braking contactor M, which coil is connected in parallel to a discharge resistor 68, actuating coil 69 of the generator, field neutralizing contactor l5 which latter coil is connected in parallel circuit relation to discharge resistor W, to the energized conductor 66. It will be noted that operation of the contactori l removes the dynamic braking resistor 86 and contact members 81 from the encuit of the precession motor and that operation of contactor i5 removes the circuit through resistor 685, differential series field winding H86 and contact members I81 from the generator 6.

The operation of the directional contactor II also establishes a circuit from the energized conductor 35 through conductor 1i, contact members 12 of directional contactor H, conductor 13, generator field winding M, contact members 15, field rheostat 11, conductors l3 and 19, contact members 80, limit-switch 8| connected in parallel circuit relation to a field circuit resistor 82 and conductor 59 to the energized conductor 39. From the circuits just traced it is apparent that the circuit for the field winding of the generator 6 is established, that the generator 5 is running at normal operating speed, that the field 31 of the precession motor is excited, that the armature of the precession motor 5 is connected in closed circuit relation with the generator 6 by a circuit through conductor 63, commutating field IQ for the precession motor 5, the precession motor armature and conductor 84 back to the generator 6. The field winding 14 is designed so that the excitation builds up slowly thereby limiting the starting current for the precession motor 5.

In the beginning of a rolling cycle with the precessing motor in operation, energy is absorbed by sion system with a result that the precession of step with the spinning motor.

motor will act as a generator and transfer energy to the direct current generator 6 which will then work as a motor. This energy will be mechanically transferred to the alternating current generator 3 and will finally be absorbed in the fly wheel 33 of the main stabilizing gyro I. I

If the energy transferred from the alternating current generator 3 to the spinning motor 2 becomes too great, the generator may be pulled out This must, of course, be avoided and a power limit relay is provided in the precession motor circuit to limit the reverse flow of power. When the power reaches the limit for which the relay has been adjusted, the relay will operate and shunt out coil 61 of the dynamic braking contactor I4 with the result that the contact members .61 will close and insert the dynamic braking resistor 86 in-the circuit of the precession motor. The shunt circuit for the actuating coil 61 extends from the energized conductor 6| through contact members 62 and 93 to the actuating coil 69 of the contactor l5.

The power limit relay 63 is provided with a movable armature pivoted at a given point and spring biased to keep the contact members 62 and 64 in engagement when the coil 92, mounted on the movable armature, acts in opposition to the coil 89 mounted on the magnetic frame of the power limit relay. If it be assumed that the conductor 84 is the positive conductor for the particular operation of the directional contactor ll above described, current flow takes place from conductor 84 through resistor 86', coil 89 of power limit relay 63 and conductor 90 to the negatively energized conductor 83. Since, for normal operation, that is, motor operation for the precession motor 5, the flow of current will be from the lower armature terminal of the precession motor through the commutating field winding l9 and conductor 83 to the upper armature terminal of the generator 6. The coil 92 will thus'be energized from the upper terminal of the commutating field winding i9 through current limiting resistor 9i, coil 92 and conductor 90 to the lower terminal of the commutating field Hi. The magnetic action of the coils 88 and 92 will thus be 'as indicated by the full line arrows shown adjacent these coils. When the regenerative effect of the precession motor 5 exceeds a predetermined value the current through the commutating field 59 is reversed and coil 62 is then energized to act accumulatively with coil $9 which will obviously still be energized in its original direction. The power limit relay is thus operated and establishes the shunt circuit for actuating coil 61 and discharge resistor 68 above described.

The shunting of coil 6'? closes the contact members 81 connecting the dynamic braking resistor 86 in circuit with the precession motor 5.

In order to prevent the direct current generator 6 from feeding back to the resistor 86 and also to prevent a rise in the speed of the precession motor 5, the generator field 14 must be somewhat weakened. Since the power limit relay operates to close contact members 62 and 93, the circuit for actuating coil 65 of the resistor shunting contactor is opened whereupon the contact members 15 open to insert the resistor sections 16 in the field winding 74. As soon as the power flow from the motor 5 to the generator 6 again drops below a predetermined limit, the power limit relay operates in the reverse direction, thereby effecting disconnection of the dynamic braking resistor 86. The precession motor 5,

therefore, is braked only by its regenerative braking effect therein. For the particular'installation we have in mind for this invention, it should be noted that for an operation with a maximum rolling of 5 the regenerated energy will be below the permissible limit and no dynamic braking of the precession motor 5 will be required. It is, of course, obvious, that operation of the power limit relay 63 to the position shown again energizes coil 65, thereby effecting stronger excitation for the field I4 of the generator 6.

When the end oi the roll of the vessel is reached the contact members 42 and 43 will move to open circuit position, thereby deenergizlng the control contactor l3 and the directional contactor II disconnects the field I4 of the generator 6 from the buses and 39 whereupon the cuit of the generator, the actuating coils of con-- tactors l3, l4, and I5 and the actuating @1165 will be deenergized. Interruption of the circuit, at contact members 52 deenergizes the brakes I whereupon the brakes are applied to the precession motor a predetermined time interval after opening of contact members 52, the time interval depending upon the adjustment of the discharge resistor |56 and the inductive characteristics of coil 51. Opening of the contact member 66 efiects the closing of contact members 81 thereby establishing the dynamic braking circuit for the precession motor and closing of the contact members I81 establishes the circuit through the differential series field I86. The differential series field winding I86 for the generator 6 is designed to substantially entirely neutralize the residual magnetism in the pole pieces of the generator 6, thereby preventing excessive circulating currents through the armature of the precession motor when that armature is locked into position by the brakes i.

From the foregoing description, it is obvious that the voltage of the generator 6 rapidly decreases substantially to zero, during which time, depending on the inductive characteristics of the field i4 and the resistance value of the dis= charge resistor H4, at regenerative braking effect is established in the motor 5, thereafter a low resistance dynamic braking circuit is established for the precession motor, and when the precession motor has come nearly to rest the brake i acts to stop, in fact, substantially lock the precession motor 5 in position. It is thus obvious that the precession motor will be brought to rest very rapidly.

The actuating coil 69 of the generator field neutralizing contactor is providedwith a discharge resistor 10 having a much lower resistance value than the discharge resistor 68 of the dynamic braking contactor I 4. The result is that the current in coil 69 will decrease at a slower rate than the current in the coil 61. The stopping operation of the precession motor 5 is thus effected in three distinct steps, namely, a regenerative braking effect immediately followed by a dynamic braking effect which is thereafter aided by the brakes I.

During the braking period which will last only about .4 second, the vessel starts to roll in the 75 opposite direction and the contact members 49 and 98 will close, thereby starting the precession in the opposite direction. In order to prevent the field contactors II and I2 from being closed while the precession motor 5 may still be running in the reverse direction, the contactors II and I2 are provided with additional coils 96 and 95 respectively, which, when energized, will hold the contactors in open position. Coils 95 and 96 are connected across the armature terminals of the precession motor and will thus remain energized until the voltage of the motor and consequently the motor speed has dropped to a very low value. (After the contactors II or I2 are closed, their respective coils 96 and 95 exert a force which is not sufficient to open the contactors and the contactors will, therefore, remain closed until the main coils are deenergized.) In actual practice, the directional contactors may be mechanically interlocked in a well known manner, as by'bar 200, as shown in Fig. 4, to I prevent simultaneous operation of both contactors and thus establish a short circuit across the buses 35 and 39.

The shunt brake is provided with auxiliary contact members 54 which will open when the brake is released by the energization of the coil 51 thereby inserting resistor 55 in the circuit of the coil 51. The resistance value of this resistor is so selected that-the current through the coil 51 is decreased just sufiiciently to hold th a brake shoes in the released position. Since the current necessary to hold them in released position is but a fraction of the current required to release the brakes, a saving of electricity is made and, what is more important, the time to apply the brakes is decreased to a minimum when it is desired to stop the precession motor. It is very important that a very short time be allowed to bring the precession motor from full speed to a dead stop and the circuit arrangement just discussed provides a means for accomplishing just this purpose.

When the control gyro closes the contact members 43 and 98, a circuit is established from the energized conductor 35 through conductor 4i,

- contact members 43 and 98, actuating coil 99 of directional contactor I2, limit switch I00, limit switches 46 and 47, conductor 48', contact members 49, conductor 50,.actuating coil 5| of the control contactor I3 to the energized conductor 39. Operation of the directional contactor I2 estabishes a circuit from the conductor 35 through contact members IOI, conductors I9 and 18, field rheostat Ii, contact members I5 which will, of course, be immediately closed afterthe operation of the control contactor I3, field winding I4, conductor I3, contact members I02, conductor I03, limit switch I05, to the negatively energized conductor 59. Connected in parallel circuit relation to limit switch I05 is a field discharge resistor I06 which may be included in the field circuit of the generator 6 when limit switch I05 is operated in the manner discussed more in detail hereinafter. It will be noted that the operation of the directional contactor I2 energizes the field "I4 in reverse direction and the precession motor will be actuated in the reverse direction by the'control elements heretofore discussed which operate in every detail in the manner described in the foregoing discussion of the operation of the precession motor for the direction of roll of the vessel first assumed.

Ilhe precession angle of the main gyroscope from a vertical position must not exceed a cervertical position shown. When limit switches 0| or I05 are opened, depending upon the direction of overtravel, resistor sections 82 and I04, as the case may be, will be inserted in circuit with the field winding I4, thereby reducing the generator voltage and, consequently, the speed of the precession motor by approximately 20%. Another set of limit switches 45 and I00 is located in the main control circuit and will cause the precession motor to stop when opened. These switches are located at about 60 of precession angle.

As an additional safety, the gyroscope is provided with mechanical buffers I09 which will limit the travel to 72. When the buffers are utilized to limit the travel the cutout switches 46 and 41 in the main control circuit are caused to operate, and the gyroscope will be latched in this position and must be released by hand. In this connection, attention is called to Fig. 2 showing an end view of the main gyroscope I. This end view shows the resilient mechanical buffers I09 which coact with the stops IIO on the housing of the gyroscope. Cooperating with the mechanical buflers are the latching members 45' and 41 which are designed to cooperate with latching dogs III to latch the main housing of the gyroscope in its extreme position.

When the ship is subjected to excessive roiling, the successive braking effects may not be sufficiently efi'ective to prevent an over-traveling oi.

the precession motor. The precession motor may, in spite of the fact that it becomes deenergized when the casing of the gyroscope has moved through 60, continue to rotate until the casing has moved to about 72. When such is the case, injury may result to the equipment and it is best to lock the casing in position by the latching means 46' or 41'. To assure that the precession motor remains deenergized, switches 46 or 41, depending upon the direction of excessive oscillation, are opened and in consequence the portion of the system controlled by the direction contactors II and I2 remains deenergized.

In order to prevent excessive speeds of the gyroscope an overspeed device 26 is provided which will open the circuit for the main line contactor 22 disposed in the circuit intermediate the spinning motor 2 and the alternating current generator 3. The opening of this circuit will deenergize the coil 2| and thus cause disconnection of the spinning motor 2 from the generator 3. Since contact members 49 are mounted on the main contactor 22 the circuit for the control system of the precession motor and the generator, therefore, is interrupted and the precession mo-, tor is also caused to stop in a manner similar to the stopping operation heretofore discussed.

In order to shorten the time needed to slow down the flywheel 33 when stabilizing of the ship is no longer required a special three phase dynamic braking resistor, having resistor elements I06, I01, and I08 is provided. This braking resistor may be connected, through knife switches H2 to the slip rings 32 for the spinning motor 2. When energy is no longer supplied to the spinning motor 2 from the generator 3 the spinning motor will act as an induction generator receiving its magnetizing current from the syn- 75 chronous generator. The energy generated by the spinning motor is thus dissipated in the resistor sections I06, I81, and I88 and the flywheel 33 of the gyroscope I is brought to rest. The. resistor sections I06, I01, and I08 may be provided with a plurality of switches 2I2 as shown in Fig. 5, whereby the resistance value thereof may be successively reduced to provide a graduated dynamic braking for the flywheel 33.

It is, of course, obvious that if it is desired to create an artificial roll of the ship a small double throw knife switch 300, as shown in Fig. 6, may be provided in the main control circuit which switch may be operated to successively operate the precession motor 5, first in one direction and then in another direction until the desired roll of the vessel is accomplished.

. To prevent hunting of the precession motor by reason of the control action of the control gyro 8 the additional features shown in Fig. 3 may be provided. Reference character II5 shows a pendulum suspended from a pivot at H6 and l which pendulum through a link III and a bell crank lever i I8 actuates a piston I I9 in the cylinder I20. A substantially non-compressible liquid is placed in the cylinders I20 and I23. With reciprocations of the piston I.I9, liquid is transmitted to the cylinder I23 through tubes I2I and I22 and in so moving between the cylinders piston I24 is reciprocated in a vertical direction and by rod I25 actuates the lever I28 pivoted at I29. The cylinder 923 is provided with a bypass conduit I26 provided with a valve I2'I. When the anti-hunting feature shown in Fig. 3 is not to be utilized, valve 521 may be opened, whereupon movement of the pendulum H5 will be ineffective to move the pivoted lever I28.

When the ship rolls the pendulum, which is disposed to swing athwartship, operates the lever I28 to bring either of the contact members G2 and 98 in contact relation with contact member 63 depending upon the direction of roll of the vessel. The levers .iBl and I32 pivoted at i35 are biased to engage the stop I30 by the spring I34. From the arrangement shown it is olevious that after any given oscillation the lever 92% will always be repositioned to a neutral position so that the main controlling efiect of the control gyro B is not impaired. With the arrangement shown in Fig. 3 the contact members 52 and 93 may be positioned more closely to the contact member d3 than for theshowing in Fig. 1 thus aiding in preventing the control gyro e from effecting a hunting of the precession motor.

It is to be understood that the description of our invention herein given is merely illustrative and that the circuit arrangements may be modifield and the controlling effect modified Without departing from the spirit of this invention as defined in the appended claims.

We claim as our invention:

1. In a control system the combination including an electric motor having an armature winding connected to drive a gyroscope, and a field w nding, a generator having an armature and a field winding, means for energizing said field windings, means for connecting the said armatures in closed circuit relation, means to deenergize the said generator field winding, thereby establishing a regenerative braking action in the motor, two means operable successively for initiating a. dynamic braking action and a mechanical braking action. and means for reducing the field flux of the field winding of the generator to substantially zero during such braking actions, thereby preventing the circulation of a large amount of current through the armature of the motor when it is brought to rest by the braking actions.

2. In a control system for a gyroscopic stabilizer comprising a spinning motor for driving the gyroscopic wheel, a precession motor mechanically coupled to the gyroscope wheel and adapt-' ed to vary. the position of the gyroscopic wheel to counteract'the force tending to disturb the balance of the stabilizer, a generator electrically connected to the spinning motor for delivering electrical energy thereto, a second generator electrically connected to the precession motor, means for driving the said generators, means for mechanically interconnecting the said generators, means for changing the relative circuit arrangements between the spinning motor and the generator therefor, for regeneratively arresting the rotation of the. precession motor, thereby, through said second generator, operating as a motor, and the mechanical interconnection between the generators, utilizing the force tending to disturb the balance of the stabilizer to drive the spinning motor, and means for utilizing a predetermined amount of the force tending to disturb the balance of the stabilizer to drive the spinning motor, thus preventing the overload! ing of the second mentioned generator when operating as a motor.

3. Control means for controlling the operation of a gyroscopic stabilizer for a vessel, comprising, in combination, a direct current precession motor having an armature, a generator permanently connected to the armature of said motor, field windings for said motor and generator, respectively, means for driving the generator or for absorbing energy therefrom during regenerative operation of the motor, means to supply exciting-current to said motor and said generator, means whereby the field windings oi. the motor and the generator may be excited, a reversing switch and circuits connecting the field winding of the generator across said source to energize the same in the one direction or the other, a dynamic braking circuit operably associated with the motor, a power limit relay connected to said generator and motor and being operable when the regenerated energy exceeds a predetermined value, and means independently responsive to the operation of the reversing switches or the power limit relay for connecting the dynamic braking circuit to the precession motor.

4. In a control system for a gyroscopic stabilizer, the combination including a main gyro wheel, a spinning motor operably connected tosaid wheel, a generator supplying energy to the spinning motor, a prime mover for driving the generator, a direct current generator mechanically coupled to the prime mover, a direct current precession motor operably connected to the main gyro wheel, a control gyro, means cooperadapted to deerierglze the said generator field winding whereby a regenerative braking eflect is produced, a dynamic braking clrcuit,'a mechanical brake, means for successively initiating a regenerative braking effect, a dynamic braking effeet, and a mechanical braking effect for the motor, and means for reducing the field flux of the field winding of the generator to substantially zero during the action of such braking effects, thereby preventing the circulation of a large amount of current through the armature of the motor when it is brought to rest by said braking effects.

ERLING FRISCH.

WALTER SCHAELCHLIN.

JOHN H. ASHIBAUGH.

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