US1367155A - Electrical system - Google Patents

Description

W. O. LUM.

- ELECTRICAL SYSTEM:

APPUCAYIGN HLED ram/16r 191s.

Patented Feb. 1, 1921.

2 SHEETS-SHEET 1- INVENTOR Walier O. Lum.

mluinok ATTORNEY hl'jklmhop panm/kjibgfedcba w. 0! L UM. ELECTRICM. SYSTEM.

APPUCATION FILED MAY 161 i916.

1,367,155@ Patented Feb. 1,1921.

2 SHEETS-SHEET 2.

INVENTOR W12! Ter 0. L. um.

ATTORNEY unites WALTER O. LUM, 0F PITTSBURGH, PENNSYLVANIA,

ASSIGNOR 'IO WESTINGHOUSE ELECTRIC AND MANUFACTURING COMKANY, A CORPORATION'OF PENNSYLVANIA.

ELECTRICAL SYSTEM.

Specification of Letters Patent.

Patented Felo. 1, 1921.

Application filed May 16, 1916. Serial No. 97,800.

To all whom it may concern:

Be it known that l, ,"Wsnrnn 0. Iron, a citizen of the United States, and a resident of Pittsburgh, in the county or Allegheny and Stateof Pennsylvania, have invented a new and useful Improvement in Electrical Systems, of which the following is a specification.

My invention relates to electrical systems and particularly to such systems as are em ployed to control the voltage of electric generators.

.My invention has :tor its object to provide a simple and eilicient means for temporarily impressing a relatively high voltage upon the field windings of an electric erator in order to cause the voltage of the generator to more quickly reach a value corresponding to the adjustment of the COIL trolling mechanism.

In the operation of reversing rolling mills and similar machines wh re frequent stops and. reversals are necessary, it is desirable that the voltage of the generators which supply energy to the driving motors shall be quicklyresponsive to the adjustments or the controlling devices. It is well known that, if only the normal exciter voltage he applied to the field-magnet windings or a generator, an appreciable amount of time must elapse before the generator voltage reaches a value corresponding to the position of the controller. The period of time is connected to the terminals oil the generator. A controller, which controls the circuits of the actuating coils of the electromagnetic switches, automatically determines the number of such coils that are in circuit with the relay coil.

The relay operates to shunt the field resister when the generator voltage is below the normal value corresponding to the position of the controller. By means of this arrangement, a relatively heavy current traverses the field-magnet windings oi the generator until the voltage reaches the normal value corresponding to the adjustment of the controller, whereupon the relay operates automatically to open a shunt for the resistor.

In the accompanying drawings, Figure 1 is a diagrammatic view of the main circuits of two generators and two motors connected in a closed circuit, and an exciter generator for supplying currentto a part of the motor field-magnet windings. Fig. 2 is a diagrammatic view of the circuits ofthe field-magnetwindings of the motors and generators,

together with the controlling switches there for. Fig. 3 is a chart or table indicating the sequence of operation of the various switches. Fig. 4; is a diagrammatic view of the circuits for controlling the various switches illustrated in Figs. 1 and 2, together with the relay for automatically controlling the resistance of the generator field circuits. Two generators G and two motors M are connected in series in closed circuit, the generators and the motors 'hcingdisposed alternately in the circuit. The field excitation of the generators and motors is controlled hy electromagnetic switches 1 to 85, inclusive. An eXciter E, which supplies current to motor field-magnet windings 37, is provided with a field magnet winding 38 that is in series with the generators G and the motors ll.

Aresistor 39, that is in series with the motor field-magnet windings 37, is controlled by electromagnetic switches 9, 10 and 11. A resistor 40, that is in series with motor field-magnet windings ll, is controlled hy electromagnetic switches 24: and 25 which are caused to operate in the manner of fluttering relays, under conditions to be later described, and by electromagnetic switches 26 to 85, inclusive. The motor field-magnet windings d1 are arranged to he nergized by any suitable source of constant voltage. The voltage applied to the motor field-magnet windings 37 is variable, since the voltage of the exciter E varies in accordance with the current traversing the main circuit. In the illustrated embodiment of the invention, 37 and ll are shown as separately excited windings.

A resistor 43, which is connected in series with the generator field-magnet windings 44, is controlled by electromagnetic switches 12 and 13 which are caused to operate in the manner of fluttering relay switches during deceleration, as will be later described, electromagnetic switches 14 to 21, inclusive, and electromagnetic switches 22 and 23 which may be termed generator fieldd'orcing switches. The direction of the current traversing the generator field-magnet windings 44 is controlled by electromagnetic reversin switches 1, 2, 3 and 4. The connections 01' motor field-magnet windings 37 to the terminals of exciter E are similarly controlled by reversing switches 5, 6, 7 and 8.

The circuits for controlling the actuating coils of the electromagnetic switches 1 to 35, inclusive, are illustrated in Fig. 4. For convenience, the actuating coils have been designated by the same reference numerals that are applied to the corresponding switches. A controller 45 effects the closing of the several switches in the sequence indicated by the chart shown in Fig. 3.

A relay 47 comprises a pivotally mounted arm 48 for effecting the engagement and dis engagement of contact mcmbers49 and 50. The relay 47 is provided with two act ating coils 52 and which tend to a tuate the lever 48 in opposite directions. The-coil 52 is connected across the terminals of one of the generators G and is, therefore, responsiie to generator voltage. The coil is connected in series with the actuating coils of switches 14 to 21, inclusive. The actuating coils of the switches 14 to 21 are connected in parallel relation, and the coil 53 is, therefore, energized to a degree corresponding to the number of actuating coils that are connected in circuit by the controller 45. By means of this arrangement, the resistance of the circuit comprising the coil 53 varies in accordance with the position of the controller and the number of electromagnetic switches operating to shunt sections of the resistor 43.

The relation of the coils 52 and 53 and the actuating coils in series with the relay coil 53 are such that the ampere turns or" the coils 52 and 53 are equal when the generator voltage reaches a value corresponding to the position of the controller The relay switch 47 is provided, also, with an auxiliary coil 54 that is in series with the contact members 49 and 50. The function of the coil 54, which exerts a comparatively slight pull, is to insure that the contact members 49 and are positively separated upon the opening of the relay. This action prevents continual sparking at the contact members, which would otherwise occur when the coils 52 and are energized to a substantially equal degree. A

The relay switch 47 controls the circuits of the actuating coils of. switches 22 and 23. The latter switches control a shunt circuit for the major portion of the resistor 43 and thus operate to materially increase he electromotive force applied to the generator field-magnet windings 44 when the relay switch 47 is closed by reason of the unbalanced pulls of the coils 52 and 53.

The switches 12 and 13, which control a section of the resistor 43, are controlled during deceleration by a relay 56 having an actuating coil 5. connected across a reactive shunt 58 in the main circuit. Switches 24 and for controlling a shunt circuit for the major portion of the resistor 40 are controlled by a relay 59 having an actuating coil 60 connected across the shunt in the main circuit. The effect of the operation or" the relay 59 is reversed by a reverse-current relay 61 having a shunt actuating coil 62 and a series actuating coil 63 that is also connected across the shunt 58, which relay 61 controls also the effect or the operation of the relay 56.

It may be assumed that the controller 45 is in its illustrated or inoperative position and that the generators are being mechanically driven. The voltage of the generators is substantially zero, since the respective field-magnet windings 44 are not energized. The motor field-magnet windings 41 are onergized, since they are connected between L the terminals of a constant-potential source. The motor field-magnet windings 37 are decnergized because their circuits are open at the reversing switches 5, 6, 7 and 8. It may be assumed further that it is desired to operate the motors in the forward direction.

In the oil? position of the controller 45, the switch 11 is closed to complete a shunt circuit for the major portion of the resistor 39 and thus insure a relatively strong field for the motors M upon the operation of the reversin switches 5 and 7 or 6 and 8, as the case may be. The actuating coils of switches 12 and 13 are also energized by a circuit comprising relay 61. The switches 26 and 27 a e also closed when the controller is in its off position to complete a shunt cir cnit for substantially the entire resistor 40 in order that the field-magnet windings 41 may be traversed by a current of relatively high value when the motors are started.

It may be assumed that the controller 45 is actuated to the right, as viewed in 4. In position a of the controller, the actuating coils of reversing switches 1 and 3 are energized to complete a circuit for the generator field-magnet windings 44 through that portion of the resistor 43 that is not shunted by the switches 12 and 13, and through field-discharge switch 65, which has been previously closed. The reversing switches 5 and 7 are also closed, in the first position of the controller, to connect the motor field-magnet windings 37 and a porengagement of the contact members 49 tion of the resistor 39 (not controlled by the switch 11) in circuit with the armature windings of exciter E.

In position 2) of the controller, a circuit is completed for the actuating coil of switch 14 which extends from the positive line conductor of the exciting circuit through contact segment 66, contact segment 67, which is electrically connected to contact segment 66, contact member 68, conductor 69, actuatin coil of switch 14, and coil. 53 to the negatlve line conductor.

The switch 14: operates to shunt asection of the resistor 43 and thus increases the field excitation of the generators. The shunting of a section of the resistor 4E3 by the switch 14 causes the voltage of the generators G to assume a corresponding value, but an appreciable period of time is required for the voltage to reach the value corresponding to the adjustment of the controller after the switch 14-; has operated.

The resistance of the circuit comprising the coil 53 which, in the oi-i" position of the controller, comprises the actuating coil of switch 26, has been decree ed upon the connection of actuating coil of switch 1% in parallel relation to the actuating coil of switch 26. it. correspondingly strong pull is exerted by the coil 53 to effect the and 50. The contact members are closed, therefore, to complete a circuit for the actuating coils of switches 22 and 23, and the latter operate to complete a shunt for a comparatively large portion of the resistor at?) and thus temporarily permit the generator field-magnet windings 4A to be energized by an abnormallyheavy current.

As soon as the generator voltage reaches a value corresponding to the position of the controller, the coil 52- is energized to exert a pull substantially equal to that of the coil 53, and the relay opens to effect the opening of switches 22 and 23. The generator windings are now supplied with current at normal value, and the generator voltage remains substantially constant. in the same manner, the actuation of the controller to its succeeding positions 0, (Z, c, f, g and it operates to effect the closing or". switches 15,

7 16, 1'7 18, 19, 20 and 21 to increase the voltage oi? the generators and thereby accelerate the motors. 7

It willbe understood that the operation of the controller may be step by step or it may be actuated quickly to either of its open ating positions, whereupon the several switches operate automatically and in sequence. The progressive operation of the several switches is insured by the provision of suitable interlocks which control the actuating coil of the succeeding switch to close. The circuits of the actuating coils are thus controlled by the preceding switch to close and the controller.

During the acceleration of the motors by controlling the generator voltage, the relay switch 47 operates in the manner above described whether the controller is actuated step-by-step or is actuated to either oi its operative positions by a single movement. The completion of the circuits for the respective actuating coils of the switches 15 to 21, inclusive, further decreases the resistance of the circuit comprising the relay coil 53, and the latter is energized by an increasingly heavy currentcorresponding to the adjustment of the controller. The relay remains closed, therefore, until the coil 52 is energized at correspondingly higher values.

When the controller occupies position it, switches 9 and 10 are closed in anticipation of the gradual insertion of the resistor 39. 'in the circuit of the motor field-magnetwindings 37. Switches 28 to 35, inclusive, also close in order that the resistor l0 may be inserted gradually in circuit with the motor field-magnet windings all. The circuit of the actuating coil of switch 28 is closed by the interlock connected to the switch 21. The switches 28 to 35, inclusive, then close in order, the circuits of the actuating coils of the respective switches being completed by the interlock of the preceding switch to close.

In position i of the controller, he switch 26 opens to insert a section of the resistor 40 in series with the field-magnet windings ll, to further accelerate the motor. The

circuits of the actuating coils of switches 27 to 35, inclusive, are respectively controlled by interlocks of the preceding switches. Current may be supplied to the actuating coils of switches 27, 28, 29-, 3 0, 32 and 34 through the controller independently of the interlocks. The actuation of the controller to position j effects the opening of switch 27, and, as the controller is actuated to succeeding po sitions j, 70, Z, m and n, the several switches open in order to gradually insert the resistor 40 and thereby weaken the fields of the motors. The switches 11, 10 and 9 open in the order named when the controller sue-- cessively occupies the positions 2', k and m. The motors now operate at their normal operating speeds, with the generator voltage at a normal value and the fields of the motors weakened to a normal degree.

(Ewing to the motors M being supplied notonly with constant-potential field-magnet windings ll but also with the field-magnet windings 37, which are energized in accordance with the value of the current travers ing the exciter field-magnet winding 38, which latter is in; series with the motor ar-j matures. the motors have the characteristics of compound-wound machines. They are, however, superior to compound-wound inotors in that it is not necessary to reverse the main armature current and because, by means of the resistor 39, the ratio between the no-load speed and the full-load speed may be varied as desired and may, indeed, be maintained practically constant. The constant value would be obtained by providing as many switches 9, 10, 11 for shortcircuiting the resistor 39 as there are switches 26 to 35 and by closing one of the first-named switches simultaneously with the closing of each of the last-named switches. This refinement is not always essential; but an approximation to the desired result is obtained by manipulating the switches 11, 10 and 9 in conjunction with the switches 26 to 35, as above described.

If, during the acceleration of the motors, by weakening their magnetic fields, the current traversing the main circuit exceeds a predetermined value, the relay 59 operates to complete circuits for the actuating coils of switches 24 and 25, and the latter operate to temporarily complete a shunt for the variable portion of the resistor 40 and thus strengthen the fields of the motor. The relay 59 operates in the manner of a fluttering relay during such time as the current values in the main circuit exceed the predetermined value. In this manner, the current traversing the main circuit is automatically maintained within predetermined limits.

lVhen it is desired to stop the motors, the controller is actuated toward its oil posi tion. The motors then operate as generators when the electromotive forces generated by them exceed the voltage of the generators. As the controller is actuated gradually to its inoperative position, the motor-accelerating switches are closed in an order substantially inverse to that in which they opened and the motor fields are strengthened to increase the voltage of the motors. The generator switches are opened to weaken the field of the generators, the switches 22 and 23 remaining open, and thus permit a correspondingly heavier current to traverse the main circuit which constitutes a dynamicbreaking circuit for the motors.

The coil 63 of relay 61, which normally opposes shunt coil 62, now assists the latter because the current traverses the main circuit in the opposite direction. The relay 61 is accordingly actuated to its upper position to reverse the effects produced by the operation of the relay 59 and to render the relay 56 effective. In case the current traversing the main circuit during the operation of dynamic braking exceeds a predetermined value, the relay 56 operates to establish a circuit for the actuating coils of switches 12 and 13, this circuit having been broken by the relay 61 when the latter assumed its upper position. The switches 12 and 13 temporarily shunt a portion of the resistor 43 to increase the field excitation of the generators. The current traversing the main circuit is accordingly decreased to a corresponding degree. The relay 56 operates as a fluttering relay whenever the current exceeds the predetermined value.

In a similar manner, the relay 59 operates also as a fluttering relay to cause the oscillation of switches 24 and 25 during such times as the current traversing the main circuit exceeds the predetermined value. It will be noted that the relays 56, 59 and 61 operate to prevent the current traversing the main circuit from reaching excessive values during the operation of bringing the motors to rest.

The relays 5G, 59 and 61 thus effectively control the resistors 40 and 43. During acceleration, the relay 61 is in its illustrated, lower position to maintain the switches 12- and 13 closed, and the relay 59 flutters to close the switches 24C and 25. During deceleration, the relay 61 assumes its upper position, and the relays 56 and 59both flut ter, the former to close the switches 12 and and the latter to open the switches 24: and 25.

When it is desired to operate the motors in the opposite direction, the controller is actuated to the left. as viewed in Fig. 4. Reversing switches 2 and 4 close to reverse the direction of the exciting current through generator field-magnet windings 4A, and thereby reverse the direction of the electro motive force applied to the motor-armature windings. The current traversing the fieldmagnet winding 38 of exciter E is also reversed. The reversing switches 6 and 8 operate, however, to reverse the direction of current through the motor field-magnet windings 37. The motors operate in the reverse direction, since the current through their armature windings is reversed while the direction of current through the motor field-magnet windings remains the same." The operation is otherwise as described above in connection with the acceleration of the motors in the forward direction.

It will be observed that, while the motors are being supplied with current, the switches 12 and 13 are maintained closed, as the relay 61 always occupies its lowest position. In the sequence chart, Fig. 3, these switches are, however, shown as being open in positions a to p of the controller. This is because the chart applies in other respects to positions which the controller occupies when the motors are being stopped and the switches 12 and 13 are opened and closed by the action of the relay 56, the relay 61 occupying its upper position, as hereinbefore stated.

It will be noted that I provide a system by means of which the voltage of a generator may be increased much more rapidly than is possible when only the normal exciting voltage is applied to the generator field magnet windings. By means of this arrangement, the loss of much valuable time in the operation of rolling mills and similar machines is avoided.

7 While I have shown and described a system comprising two generators and two motors, which represents an actual installation of a system embodying my invention, it will be understood that this arrangement is. illustrative only and that my invention maybe applied with equal facility and effectiveness to a system embodying a single generator which is connected to a similar intermittent load.

I claim as my invention:

1. In an electrical system, the combination with a generator having a field-magnet winding, and a resistor in circuit with said winding, of a plurality of electromagnetic switches for controlling said resistor, a controller for said switches, and a relay for controlling said resistor in accordance with the value of the voltage of said generator relatively to the normal voltage value corre sponding to the position of said controller.

2. In an electrical system, the combination with a generator having a field-magnet winding, and a resistor in circuit therewith, of means for controlling said resistor comprising a plurality of progressively actuated electromagnetic. switches, a controller for said switches and a relay having two coils, one of which is connected to the terminals of said generator and the other of which is in circuit with the actuating coils of said switches.

3. In an electrical system, the combination with a generator having a field-magnet winding, and a resistor in circuit therewith, of means for controlling said resistor comprising a plurality of progressively actuated electromagnetic switches, a relay having two opposing coils, one of which isconnected across the terminals of said generator and the other of which is in circuit with the actuating coils of said switches, and a controller for said switches and for controlling the number of actuating coils in circuit with said second relay coil.

4. In an electrical system, the combination with a generator, of means for controlling the field excitation of said generator comprising 'a plurality of electromag netic switches having actuating coils connected in parallel relation, a relay having two coils, one of which is responsive to the voltage of said generator and the other of which is in series with said actuating coils, and means for controlling the circuits of said coils.

5. In an electrical system, the combination with a generator, of means for controlling the lield excitation otsaid generator comprising a resistor, a plurality of electromagnetic switches for controlling said resistor, and means for controlling said resistor in accordance with the relation of the voltage of said generator to the number of actuating coils of said switches that are energized.

6. In an electrical system, the combination with a generator and a motor, oi means for controlling the field excitation of said motor and of said generator, said means comprising a plurality of electromagnetic switches, a relay for increasing the field excitation of the generator while the voltage of the latter is less than that corresponding to the operation of certain of the switches, and means for controlling said switches to successively vary the field excitation of said generator and that of said motor.

7. In an electrical system, the combination with a generator having a field-magnet winding, and a resistor in circuit therewith, oi means for controlling said resistor comprising a plurality of progressively actuated electromagnetic switches, and a relay having two opposing coils, one of which is connected across the terminals of said generator and the other of which is in circuit with the actuating coils of said switches.

8. In an electrical system, the combination with a generator having a field-magnet winding, and resistor in circuit therewith, of means for controlling said resistor comprising a plurality of progressively actuated electromagnetic switches, a relay having two opposing coils, one of which is connected across the terminals of said generator and the other of which is in circuit with the actuating coils of said switches, and means for automatically controlling the number of actuating coils in circuit with said second relay coil so as to makev the ampere turns of the two coils equal when the generator voltage reaches a predetermined value.

9. In a generator provided with a fieldmagnet winding and a resistor in circuit therewith, means for successively shunting portions of the resistor, and means for successively shunting substantially the whole resistor alternately with said first means.

10. In a control system, the combination with an electric generator having a field, magnet winding and a resistor in circuit therewith, of means for shunting increments of said resistor, and a relay for shunting portions of said resistor alternately with the shunting of successive increments.

11. In a control system, the combination with an electric generator having a fieldmagnet winding and a resistor in circuit therewith, of a controller, means operated by the controller for shunting successive increments of the resistor to increase the voltage of the generator, and means for temporarily shunting other portions of the resistor when the voltage of the generator is below that which corresponds to the position of the controller.

12. In an electrical system, the combination with a generator having a field-magnet winding, and a resistor in circuit with said winding, of means for successively shunting portions of the resistor to permit abnormal voltage values in said winding and means dependent upon the voltage of said generator for shunting other portions of the resistor, to permit the current flow in said winding to become normal.

13. The combination with a dynamo-electric machine having a resistor, a switch for controlling a portion of said resistor and a switch for controlling substantially the whole of said resistor, each having an actuating coil, of a relay for energizing the coil of said second-named switch and having opposing coils, one of which is responsive to the electrical condition of said machine, and the other of which is responsive to the electrical condition of the coil of said firstnamed switch.

14. The combination with a dynamoeleetrio machine having a resistor, and means for controlling successive increments of said resistor, of a pair of opposing forces for controlling substantially the whole of said resistor, one of said forces being responsive to the electrical condition of said machine and the other of said forces being responsive to the electrical condition of said resistor.

15. The combination with a generator having a resistor, and means for short-eircuiting successive increments of said resistor, of a relay for short-circuiting substantially the whole of said resistor and having two opposing actuating coils, one of said coils being connected across the terminals of said generator and the other of said coils being controlled by said short-circuiting means.

16. The combination with an electric generator having a field-magnet winding and a resistor in circuit therewith, oi an electric motor, means for maintaining said resistor short-circuited during the acceleration of said motor, and means for inserting said resistor into circuit and short-circuiting said resistor in accordance with current conditions during the deceleration of said motor.

17. The combination with a pair of dynamo-electric machines, each having a resistor, of a switch for controlling each of said resistors and a switch for controlling the effect of the operation of each of said switches.

18. The combination with a motor and a generator each having a field-magnet winding and a resistor in circuit therewith, of a fluttering relay for controlling each of said resistors, and a switch for controlling the effect of the operation of each of said relays.

19. The combination with a pair of dynamo-electrie machines, of three switches for controlling said machines, one of said switches, when it occupies one position, controlling one of said machines irrespective of the position of a second of said switches, and, when it occupies another position, controlling said one machine when said second switch occupies a predetermined position, and said one switch controlling the other machine in accordance with the position of said third switch.

20. The combination with a dynamo-electric machine having a resistor and a reactive shunt, of a single-throw svitch and a double-throw switch for controlling said resistor, each havi a oil across the terminals oi? said reactive shunt, said double-threw switch having an additional shunt coil and being adapted to control said positions, irrespective resistor in one or it's of the position of said singie-throw switch, and to control said resistor in its other position when said single-throw switch occupies its clo ed position.

21. An electrical system comprising a dynamo electric machine, a resistor in the fieldmagnet circuit thereof, means for successively shuntir increments of said r sister, and means for temporarily shunting the remainder of said resistor alternately with the shunting of the increments.

in electrical. system comprising a dynamo-electric machine, a resistor in the field magnet circuit thereof, means for successively shunting increments of said resistor, and means for temporarily shunting supplementary and variable amounts of said resistor alternately with the shunting of the increments.

23. In an electrical system, the combination with an electric generator having a field-magnet 'incling and a resistor in circuit therewith, of means for controlling the effective portion of said resistor, said means comprising a controller, a plurality of switches controlled by said controller, and a relay operable upon each actuation of said controller to a succeeding operative position to shunt a portion of said resistor and to remove said shunt when the generator voltage reaches a value corresponding to the adjustment of the controller.

24. In an electrical system, the combination with an electric generator having a fieldmagnet winding and a resistor in circuit therewith, of means for controlling the effective portion of said resistor, said means comprising a manually operable controller, a plurality of switches controlled by said controller, and means operable, when the voltage of the generator has a value below that for which the controller is adjusted, for shunting a portion or" said resistor and for varying said shunted portion in accordance with the position of said controller.

' 25. In an electrical system, the combination with an electric generator having a fieldmagnet winding and a resistor in circuit therewith, of means for controlling the effective portion of said resistor, said means comprising a controller, means controlled by said controller for shunting a portion of said resistor, and means controlled in accordance with the value of the generator voltage relatively to that for which the controller is adjusted, for shunting a second portion of said resistor.

26. In an electrical system, the combination with an electric generator having a fieldmagnet winding and a resistor in circuit therewith, of means for controlling the efifective portion of said resistor, said means comprising a controller, means controlled by said controller for shunting a portion of said resistor, and means controlled in accordance with the value of the generator voltage relatively to that for which the controller is adjusted, for shunting a second portion of said resistor and for varying said second portion.

27. In an electrical system, the combination with an electric generator having a field-magnet winding and a resistor in circuit therewith, of means for controlling the effective portion of said resistor, said means comprising a controller, means controlled by said controller for shunting a portion of said resistor, and means controlled in accordance with the value of the generator voltage relatively to that for which the controller is adjusted, for shunting a second portion oi said resistor and for varying said second portion in accordance with the ad.- justment of said controller.

28. In an electrical system, the combination with an electric generator having a lield-magnet winding, of means for applying an abnormal Voltage to said winding when the generator voltage is below its normal value, said means comprising a controller and means for varying the value of the voltage applied to said winding in excess of that for which the controller is adjusted in accordance with the position of said con troller.

29. The combination with a motor and a generator having armatures connected in series and separately excited field-magnet windings, and an eXciter having a field-mag net winding in series with said armatures, said motor having a field-magnet winding in series with the armature of said exciter, of resistors adapted to be inserted into circuit with said motor field-magnet windings, and means for simultaneously inserting portions of each of said resistors into circuit.

In testimony whereof, I have hereunto subscribed my name this 11th day of May, 1916,

WALTER O. LUM.

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