US2404641A - Control system - Google Patents

Description

y 1946- H. H. LEI GH ETAL 0 CONTROL SYSTEM Filed Feb. 1, 1944 2 Sheets-Sheet 1 lnvewfim sr Henry H. Leigh. Gurdon Ewvafltam Their 'Attcvnqey.

y 1946. H. H. LEIGH ETAL 254047641 CONTROL SYSTEM 7 Filed Feb. 1, 1944 2 Sheets-Sheet 2 27a 27a Figlb.

Inventors: Henry H. Leigh, Gordqn E.Wa?,er-,

by 6&4 "7

Them Attorney.

Patented July 23,1946

CONTROL SYSTEM Henry H. Leigh and Gordon E. Walter, Scotia, N. Y., assignors to General Electric Company, a corporation of New York Application February 1, 1944, Serial No. 520,616

14 Claims.

This invention relates to control systems, more particularly to systems for controlling the starting operations of electric motors, and it has for an object the provision of a simple, reliable, and improved control system of this character.

More specifically, the invention relates to motor control systems in which direct current is supplied to a motor through electric valve apparatus and in which provision is made for reversing the direction of rotation of the motor, and a further object of the invention is the provision of means for effecting the reversal of the motor in a minimum of time without damage to the motor.

A more specific object of the invention is the provision of means for effecting successful operation of the electric valve apparatus as an inverter during the reversal period, thereby to eliminate undesirable operating characteristics such as current surges, sparking at the brushes, and violent changes in shaft torque.

A still more specific object of the invention is the provision of means for effecting successful inverter operation of the electric valves during the stopping operation of the motor.

In carrying the invention into eifect in one during a reversal operation in which the current I limiting means is ineifective to limit the current. Accordingly, means are provided for preconditioning the control of the electric valve apparatus to limit to a predetermined value the current which can be conducted by the electric valve apparatus at the beginning of the inversion period before the current limiting means becomes fully efiective. One form of these means comprises a control valve connected to control the supply electric valve apparatus, a source of standard voltage, reversing switching means for connecting the armature to the supply valve apparatus, and contacts on the reversing switching means for connecting the grid of the control valve in such a manner as to compare the output voltage of the supply valve apparatus to the standard voltage source when the motor is disconnected from the supply valve apparatus, thereby to condition the supply valve apparatus to conduct only a predetermined amount of current when the switching mechanism is reclosed for the reverse direction of rotation. The preconditioning means itself may require a longer time to function than is required by the reversing switching means to reverse the motor connections. Accordingly, a suitable time delay device is provided for preventing the reversing switching means from completing the reversed connections until the preconditioning means has had time to become effective.

Additional electric valve apparatus is provided for supplying direct current to excite the motor field. The control of this additional valve apparatus is arranged to excite the field strongly when the directional contactors are open before starting from rest. Means are provided for preventing the strengthening of the motor field during the interval in which the reversing contactors are open during a reversal operation, thereby to limit the armature voltage and to promote a successful inverter operation. Also, means are provided for delaying the strengthening of the field of the motor during the inversion period following the reversal of the connections of the motor armature to the supply valve means until the speed or the motor has beenreduced to a predetermined low value.

For a better and more complete understanding of the invention, reference should now be had to the following specification and to the accompanying drawings of which Figs. 1A and 1B are a simple, diagrammatical illustration of an embodiment of the invention.

Referring now to the drawings, an electric motor [3 having an armature Illa and a shunt field winding lllb is supplied from a source of alternating voltage H through a supply transformer l2 and suitable electric valve apparatus comprising electric valves I 3, I4, I?) and Hi. The starting, stopping, and reversing of the motor it! are under the control of a suitable controlling accessory such as a push-button station I! and a pair of speed controlling potentiometers l8 and 19. If desired, these speed controlling potentiometers may be mounted on the push-button station control panel.

The secondary winding of the supply transformer I2 is provided with a midtap 12a and this midtap is connected to the bus 20 which becomes the negative side of a D.C. system, i. e., the negative armature terminal, the negative field terminal, and the negative control terminal. Assuming that the motor in is designed for operation on 230 volts, the full secondary voltage of the transformer will be' approximately 620 volts. Two 5'7 -volt taps I21) and I20 either side of center 3 provide a l15-volt center-tapped source of control power for filaments, transformers, phase shift bridges, and relay excitation.

The current supplied to the field winding it?) from the source I I is controlled by electric valves is and M. As shown, the anodes i311 and Ma of electric valves 13 and I l are connected by means of conductors 2! and 22 to opposite terminals of the secondary winding of transformer E2. The filamentary cathodes i322 and Mb are heated by means of current supplied to the cathodes through a filament transformer 23, of which the primary Winding (not shown) is connected to the 1l5-volt taps i2?) and I20 of the supply transformer, and the secondary winding 23a is connected to the filamentary cathodes I31) and Mb. The secondary winding 23a is midtapped and the midtap is connected to the conductor 24 which thus becomes the positive terminal of the field supply. Thus, the field circuit may be traced from the positive conductor 24 through the normally open contacts 25a of a time delay relay 25, the operating winding 26a of a field loss protective relay 26, through field winding ifib to the negative conductorzt.

The supply of current from the source H to the armature 56a of the motor is controlled by means of the electric valves l and it. As shown, the valves l5 and it, lik the electric valves I3 and M, are connected for biphase rectification, i. e., their anodes a and Eta are connected through primary windings 21a and 2% of a special current transformer 2? to the opposite terminals of the secondary winding ofthe supply transformer 52. Th cathodes 15b and E61) of valves l5 and it are provided with suitable heating units which are connected to the secondary Winding 23b of the filament transformer 23. The cathodes i521 and 157) are connected to the terminal 28 which thus becomes the positive side of the supply for the armature. Thus the armature circuit is readily traced from the positive terminal 28 through conductor 29, series field winding lilo, one or the other of the directional contactors 3d, 3 l, a heating element 32a of an overload protective relay 32, through the armature lea, commutating field winding ltd to the negative supply bus 26.

Although the electric valves H3, E4, E5, and It may be of any suitable type, they are preferably grid-controlled, mercury vapor thyratron tubes. The cathodes MD and E61) of the valves l5 and it which control the supply of current to the armature are indirectly heated, and these valves are provided with shield grids 5c and 60 as well as with control grids E511 and ltd, respectively. The valves l3 and it which control the supp-1y of current to the field windings have directly heated filamentary cathodes and have only single grids 33c and Me which are control grids. In thyratron valves, the function of the control grid is only to initiate the flow of current between the anode and cathode during each positive halfcycle of anode voltage. Once current has started to flow in the anode-cathode circuit, the grid exercises no further control until the flow of current through the valve has been interrupted by some means external to the valve itself. Once the current has ceased to flow, the'potential of the grid will again determine the point in the positive half-cycle of anode voltage at which the valve will again become conducting. Thus, by varying the firing point, the average Value of current flowing in the anode-cathode circuit can be controlled. These valves are therefore grid-controlled arc rectifiers.

The purpose of the time delay relay 25 is to allow time for the initial heating of the cathodes of the electric valves before power is applied to the anode-cathode circuit. Since normally open contacts 25a of the time delay relay 25 are included in the energizing circuit of the field loss protective relay 26, which has normally open contacts 26b in the energizing circuit of the directional contactors 3i! and 31, the latter cannot be closed to complete the power circuit of the electric valves until a predetermined interval of time after the connection of the supply transformer l2 to the source of alternating voltage I l.

The purpose of the field loss protective relay 25 is to delay the application of voltage to the armature of the motor until a safe field excitation has been established and to interrupt the armature circuit in case of a field failure.

Although the thyratron valves i3, M, 5, and it may be controlled by any suitable method, it is preferred to use the method of phase shift control of the grid voltage. For the carrying out of this method of control, a pair of phase shifting networks, one for the armature thyratrons and one for th field thyratrons, is provided. The phase shifting network for the armature thyratrons comprises a resistor 33 and the alternatingcurrent winding 34a of a saturable core type reactor 34, and the network for the field thyratrons comprises a resistor 35 and the alternating-current winding 36a of a saturable core type reactor Both these networks are connected in parallel across the low voltage terminals I21), I20 of the supply transformer. The primary winding 31a of a grid transformer 37 is connected between the midtap 2a and the junction point 33a of the resistor 33 and reactor winding 34a. This grid transformer has two secondary windings 31b and 310. The secondary winding 81b is connected between the cathode and control grid of armature thyratron i5, and similarly the secondary winding 310 is connected between the cathode and control grid of the armature thyratron [6. A corresponding grid transfo-rmer 33 is provided for the field thyratrons i3 and I4. It has a primary winding 38a connected between the midtap [2a and the junction point 350: of th resistor 35 and reactance Winding 35a, and a pair of secondary windings 38b and 380 which are connected between the cathode and grid of the thyratrons l3 and M, respectively. The phase shift of the grid voltages is produced by varying the reactance of the saturable core reactors, which is accomplished by varying the D.-C. saturation of these reactors.

The control is such that when the saturable reactors are saturated, the voltages of the grid transformers are advanced to the in phase posi tion with respect to the anode transformer voltage, and when the reactors are unsaturated, the voltages of the grid transformers tend to be out of phase and lagging. Intermediate values of saturation produce intermediate phase relationships. Thus, when the saturable reactors 34 and 36 are fully saturated, the thyratrons l3, l4, I5, and iii are fully conducting, and conversely, when the reactors are unsaturated, the thyratrons are nonconducting. For intermediate values of saturation, the thyratrons have corresponding intermediate values of conductivity.

The push-button station I? is provided with a plurality of push-button type switches 39, MI, and ll for controlling the starting, stopping, and direction of rotation of the motor ii). The pushbutton switch 39 controls the starting of the motor in the forward direction; the push-button switch 49 controls the starting of the motor in the reverse direction; and the push-button switch 4| controls the stopping of the motor.

T the low voltage taps I21) and 120 of the supply transformer is connected the primary winding 42a of a control voltage supply transformer 42. The opposite terminals of th secondary winding 4% of this transformer are connected to the anodes 43a and 53b of a small double diode rectifier valve 43 which furnishes a separate source of low voltage D.-C. from which the control electric valves are energized. The saturating windings 34b and 36b of the saturable reactors 34 and 35 obtain their energization from this source of direct voltage. This direct voltage is filtered by means of a smoothing reactor 44 and a capacitor 45. The voltage across the capacitor 45 is impressed On a circuit comprising a resistance 46 in series with two glow tubes 47 and 48. These glow tubes 41 and 48 are gaseous discharge devices which operate in that region of their characteristic in which the voltage drop across the discharge supporting electrodes is substantially constant over a wide range of current. The voltage across the points 46a and 43a is fixed at a value which is determined by the type of glow tube used, and within the operating limits of this equipment, this voltage is independent of variations in the A.-C. supply voltage. Any difierence in voltage between the voltage drop across the capacitor 45 and the constant voltage across the glow tubes 4'! and 43 is absorbed by the resistor 46.

The voltage drop across the glow tube 41 is used to stabilize the voltage of the amplifier valves which are connected between the points 45a and 41a. The voltage drop across the glow tube 48 is the voltage standard with which signal voltages are compared for controlling purposes.

For the purpose of varying the direct current which flows in the saturating winding 34b, a suitable amplifying electric valve (is is provided. This valve is provided with an anode 49a, a cathode 49b, and a control grid 450. The D.C. winding 34b and the valve 49 are connected in series across the glow tube 4'51 The control of the current through the D.-C. winding of the armature saturable reactor 34 is accomplished by proper choice of the grid-to-cathode voltage of the valve 49. As the voltage of the grid 490 is made less negative with respect to the voltage of the cathode 49b, the current transmitted by the valve increases, thereby increasing the sa"- uration of the armature saturable reactor 34, which, as stated in the foregoing, results in increasing the voltage applied to the armature of the motor l6. Conversely, as the voltage of the grid 490 is made more negative with respect to the voltage of cathode 49?), the current transmitted by the valve decreases and this decreases the voltage supplied to the armature of the motor. An additional amplifying electric valve 3, which is provided with an anode a cathode 55b, and a control grid 530, is provided for the purpose of varying the voltage on the grid 490 so that the speed of the motor IE is maintained constant at a preset value which is correlated with the position of the slider [8a on the speed controlling potentiometer 58, In other words, the electric valve 58 serves as a connecting and amplifying link between the armature speed controlling potentiometer l3 and the electric valve 49 which controls the saturation of the armature saturable reactor 36, and hence, controls the armature,

6 of valve as is connected to a voltage divider com prising resistors 5m, Elb, and Bio. The electric valve 50 is connected between the slider [8a of the speed controlling potentiometer l8 and the junction point of the resistors 51a and SH). When the voltage of the grid 500 is made less negative with respect to its cathode, the current transmitted by the valve 5?) is correspondingly increased, and since this current flows through the resistor sea, the voltage drop across resistor 51a is correspondingly increased and consequently, the voltage of the grid 490 is correspondingly decreased so that the current transmitted by valve 19 is decreased and the armature voltage and speed are correspondingly decreased. Thus, increasing the conductivity of electric valve 56 has the effect of decreasing the voltage supplied to the armature and conversely, decreasing the current conducted by electric valve at has the eifect of increasing the voltage supplied to the armature.

Since the lower terminal of resistor 53c and one electrode of glow tube 13 are connected together at point ite, and since the upper terminal of resistor tie is connected to the grid 490, the valve t9 compares the voltage drop across the resistor tile with the voltage drop across the tube 43.

If the armature voltage or a portion of the armature voltage is impressed on the grid 500, an increase in armature voltage will increase the conductivity of valve 50, thereby decreasing the conductivity of valve 49 and decreasing the voltage supplied by the thyratrons I 5 and 18, thereby to correct for the increase in armature voltage. If the armature voltage decreases, the reverse action takes place and the decrease in armature voltage is corrected. The position of the slider 3a on the armature speed control potentiometer determines the percentage of the total voltage drop across the valve 48 which is to be derived and used as a preset indication of speed. The voltage that is so derived and used as a reference voltage is the voltage between the slider i805 and the negative bus 2t. Since the cathode of the valve 50 is connected to the slider, then the position of the slider will determine the voltage of the cathode relative to the negative bus 20. A signal voltage is derived from the armature voltage by means of a voltage divider which comprises resistor 52a, potentiometer 52b, resistor 52c, and that portion of the potentiometer 53 between the slider 53a and the negative bus 20. The signal voltage used is the voltage from the slider 52d to the negative bus 20, and this signal voltage is impressed on the grid 560 of valve 52!. Thus the grid-to-cathode voltage of the valve 58 is the difference between the signal voltage and the voltage from the slider I 3a to the negative bus 20. The tendency of the circuit is to maintain the signal voltage approximately equal to the reference voltage, 1. e., the voltage from the slider 18a to the negative bus 20. Hence, the armature voltage and the speed of the motor will be approximately proportional to the reference voltage tapped off by the slider 18a of the speed control potentiometer.

The voltage selected by the position of the slider [8a is a portion of the constant voltage across the glow tube 48 and is therefore substantially constant. As the slider [8a is moved from position 0 to position 5, the preselected speed rent of the field saturable reactor 35, a pai of electric valves 54 and 55 corresponding, respectively, in function to the valves 49 and 55 of the armature control, is provided. The electric valve 54 has an anode 54a, a cathode 54b, and a, control grid 55c; and similarly, the valve 55 has an anode 55a, a cathode 55b, and a grid 55c. In the control of the field current, the voltage across the shunt field winding lllb is utilized as an indication of field current. The connections and operation of the valves 55 and 55 are similar to the connections and operation of the valves 49 and 55 of the armature control. The electric valves 54 and 55 operate to compare the voltage across the field winding lob, or a selectable portion thereof, with a reference voltage Which is derived from the voltage of the glow tube 48 by means of the slider position on the field control potentiometer is which is connected in series with a rheostat 55 across the glow tube 48. difference between the signal voltage derived from the field winding and the reference voltage derived from the glow tube 58 is impressed on the grid cathode circuit of the valve 55in such a manner that if the voltage across the field increases, the conductivity of the valve 55 increases, thereby decreasing the conductivity of the valve 54 and the saturation of the field saturable reactor 35, thereby to decrease the voltage supplied to the field winding. Conversely, a decrease in the voltage across the field winding has the effect of decreasing the conductivity of the valve 55, thereby increasing the conductivity of the valve 54 and increasing the field voltage.

The armature speed control potentiometer l8 and the field weakening control potentiometer i9 are preferably combined on a common shaft with the resistance portions l8 and l9 arranged circumferentially and with the sliders so oriented that with the speed control knob in the zero position, which may be assumed to be the full counterclockwise position, the sliders Ilia and 59a will also be in the zero position. Thus, when the speed control knob is turned in a clockwise direction fro-m the zero position, the slider Eda of the armature voltage potentiometer taps oil an increasing portion of the reference voltage of valve 45, but the slider Illa of the field weakening potentiometer slides along the contact strip I91) and therefore taps off the full voltage of the tube 58 which corresponds to a condition of full field excitation. However, when the control knob passes position 5, the slider of the armature speed control potentiometer slides on the contact strip 58b so that it taps oif the full voltage of the tube 48 corresponding to full armature excitation, but the slider l9a of the field weakening potentiometer begins to tap oil decreasing portions of the voltage across the tube 48 so that the field is progressively weakened as the movement of the control knob in a clockwise direction continues. In the extreme clockwise position, which. is position 9 on the dial, the armature thyratrons l and it are supplying rated voltage to the armature, and the field thyratrons l3 and M are supplying the minimum voltage to the field winding 151). Therefore, the motor rotates at maximum speed.

For the purpose of limiting the armature current to a maximum permissible value, mean are provided for comparing a signal voltage derivedfrom the anode current of the armature thyratrons with a reference voltage and utilizing the difference of these signal and reference voltages to control both the armature and field thyratrons The 1 in such a manner as to limit the armature cur rent to the desired value. These means are illustrated as comprising the anode current transformer 21, the control amplifier valves 51 and 58 and the double diode rectifying electric valve 59. As shown, the two primary windings 21a and 21b of the anode current transformer are connected in series with the anode circuits of each of the armature thyratron valves, and this transformer is polarized in such a manner that when one of the armature thyratrons conducts, the flux in the core is in one direction and when the other thyratron conducts, the flux is reversed. As a result, an A.-C. voltage is induced across the secondary winding 21c and the magnitude of this induced voltage is determined for a given Value of primary current by the resistance load connected to the secondary and by the turn ratio between primary and secondary windings. This alternating voltage is rectified by the electric valve 59 and appears as a direct voltage across a voltage divider comprising potentiometer resistor 60, fixed resistor 65a, and potentiometer resistor 53.

The electric valves 51 and 58 are provided with anodes 57a and 58a, cathodes 51b and 58b, and grids 51c and 580, respectively. The anode 51a is connected to the junction point between the sections 5m and 5lb of the voltage divider to which the grid of valv 59 is connected, and the cathode 51b of valve 57 is connected to the point dla to which the cathod ldb of valve 59 is connected. The anode 53a is connected to the conductor GI and the cathode 58b is'conn'ected to the grid 540 of the field control valve 54 which, as shown, is connected to the junction point between the resistance sections 52b and 620 of a voltage divider comprising the resistance sections 62a, 62b and 520 connected across the D.-C. control system buses 5i and 28. The potential at the junction of resistance sections 62b and 520 when valve 55 is regulating the field voltage, is such that the potential of the cathode 58b is slightly more negative than the potential of cathode 511).

: connected to slider b are very much negative With respect to their cathodes which are connected to the conductor 41a and to the junction point between the resistance sections 6% and 620, respectively. An increase of armature current causes the voltage across the secondary of the current transformer 27 to increase, and the voltage rectified by the valve 59 increases correspondingly so that the voltage between the slider 55b and the negative bus 20 ultimately reaches a value approximately equal to the voltage across the valve 48, and the negative grid voltages of valves 5'! and 53 are reduced to the value at which these valves begin to conduct current.

When valve 51 conducts current, it has the same efiect as if Valve 55 were conducting current, which is to decrease the current conducted by valve 49 and thereby decrease the voltage supplied to the armature.

The operation of valve 58 is slightly different in that its cathode is connected to the grid circuit of the valve 54 instead of to the cathode of valve 41, and its anode is connected to the positive bus 61. When the grid of valve 58 is made sufliciently less negative to cause current to flow in the anode circuit, the efiect of current flow through valve 58 is to make more positive the potential of the junction point of the resistors 52b and 520 to which the grid 540 of valve 54 is connected. This has the effect of increasing the conductivity of valve 54 with the result that the field of the motor is strengthened, if it has been in a field weakened condition. Since the cathode 58b is more negative than the oathode 'lb of valve 51 for low values of armature current, the valve 54 will be controlled slightly ahead of the valve 49 as the armature current increases, with the result that the field will be strengthened before the armature voltage is decreased. To provide for a range of adjustment, the resistor 59 is made in the form of a potentiometer.

When a motor is operated at rated armature Voltage and at maximum rated field current and is carrying rated full load, it is said to be operating at base speed. If the motor I0 is being operated in the field weakened range, e. g, three times base speed, and if then the speed controlling potentiometers I8 and [9 are suddenly changed to a position of lower speed, c. g., one-half base speed, the control would function to decrease the voltage supplied by the armature thyratrons l5 and I6 and to increase the voltage supplied to the field winding by the field thyratrons i3 and 14, as a result of the action of the electric valves 49 and 50, and 54 and 55. With the motor running at three times base speed and full field applied as quickly as the time constant of the magnetic circuit will permit, the tendency is for the armature countervoltage to increase to a value which is approximately three times full terminal voltage, i. e., 750 volts in the case of a 250-volt motor. To prevent such an undesirable increase in the armature voltage, an additional electric valve 54 is provided. This valve is provided with an anode 54a, a cathode 54b, and a grid 640. The anode 54a of this valve is connected to the unction point of the resistors 52a and 62b of the voltage divider to which the grid 540 of Valve 54 is connected, and the cathode 64b of valve 64 is connected to the conductor 47a, The grid 640 of valve 84 is connected to the junction point 55a of two resistors 55 and 55 which, together with the potentiometer 53, constitute a voltage divider connected across the armature of the motor H). As thus connected, this valve 64 measures a fixed portion of the armature voltage, i. e., a portion between the junction point 55a and the negative bus 25, and when this portion exceeds the voltage drop across the reference voltage valve 48, the valve 54 becomes conducting, thereby to increase the voltage drop across the resistor 52a and decrease the potential of the grid 540 of valve 54, to prevent the saturation of the saturabl reactor 36 which controls the field thyratrons l3 and M. The action of valve 64 upon valve 54 is very similar to the action of valve 55 except that the valve 54 receives its voltage signal from the armature circuit, whereas the valve 55 receives its voltage signal from the field circuit.

In order to promote a rapid and smooth transition from rectifier operation to inverter operation of armature thyratrons I5 and I5 during the reversal period. a preconditioning circuit is provided for controlling the armature thyratrons.

This preconditionin circuit connects the grid 510 of control valve 51 to the junction point 61a of resistors 61 and 6B which constitute a voltage divider connected between the positive side of the standard voltage, bus BI, and the positive side of the output voltage of the armature thyratrons I 5 and I5. Included in this circuit are th normally closed interlock contacts 55a and Bla of reversing contactors 30 and 3!, respectively, This preconditioning circuit is effective, therefore, nly when both reversing contactors are dropped out as during a reversal operation, or when the motor is at rest or is bein dynamicall braked to rest.

The point 5 1a on the voltag divider is selected so that at the instant of reconnection of the armature to the thyratrons I5 and I6 during a reversal operation, the phase of the grid voltage of the thyratrons is retarded to a point such that the armature current will not exceed a predetermined safe value, such approximately full load current. The time required for the preconditioning circuit to effect the retardation usually exceeds the time required for the reversing contactors to reverse the armature connections to the thyratrons. For the purpose of preventing the reversing contactors 38 and 3| from completing the reversing of the armature connections to the thyratrons before the preconditioning operation is sufficiently complete, a suitable time delay device such as the relay i0 is provided. This relay has normally open contacts 10a included in circuit with the operating coils 30c and 3lc of the reversing contactors 35 and 3!, respectively, and the operating coil Nb of the relay h'! is included in series with normally closed contacts 301) and 3!!) of the reversing contactors 30 and 3!, respectively. As a result of this interconnection, neither of th reversing contactors 3i) and 3| can be reclosecl during the time following its dropout required for relay H! to pick up.

Another conditioning circuit is completed by an electric valve ll of which the anode Ha is connected to the conductor 41a and thus to the oathode 57b of valv 51, and the cathode 'Hb is connected to the grid 510 of valve 51. The function of valve H is to prevent the grid 510 from becoming excessively negative at light motor loads and delaying the response of the current limit control to sudden increases in the armature cur rent.

In order to facilitate motor reversal at field weakened speeds, means are provided to maintain the previous strength of the field during the interval in which both reversing contactors are open during the reversal period and to provide full field strength for starting the motor from rest. To this end, an additional control relay 1?. is provided. This relay has a normally closed contact 12a included in circuits between the cathode 55b of field control valve 55 and conductor 41a, and a normally open contact 12?) between cathode 55b and slider l9a of the field control potentiometer IS. The relay 12 is also provided with normally open contacts which, when the relay is picked up, complete a sealing-in circuit.

If the strength of the motor field is increased too rapidly following the reconnection of the armature to the supply thyratrons [5, IS in a reversal from a field-weakened speed, successful inverter operation may be defeated. To prevent this too rapid strengthening of the motor field by the action of valve 58, which operates on valve 54 at high armature currents, a reverse snubbing circuit is provided. This reverse snubbing circuit comprises an electric valve 713 having its anode 53a connected to the grid 54a of the valve 54 and having its cathode 13b connected to the junction point Ma of two resistors M and I which, together with resistor 69, constitute a voltage divider connected across the standard voltage valve 48. The junction point 75a of the resistors 15 and 69 is connected to the common point I6 of'the two reversing contactors by means of a conductor H. The common point 16 of the tw reversing contactors is connected to the positive side 28 of the armature supply. The grid 130 of valve 13 is connected to the negative bus 24] and consequently, except when the armature is rotating in a direction opposite to that which corresponds to whichever of the two directional contactors is closed, the grid 130 is so negative with respect to its cathode that valve 13 is non-conducting and has no efiect on valve 54 which directly controls the field saturable reactor 36.

During the interval in which both directional contactors are dropped out during a reversal from a given speed in one direction to the same given speed in the reverse direction, the field strength remains constant at the value set on the field potentiometer I9. However, when reversing from a high speed in one direction to a relatively lower speed in the opposite direction, the slider l9a of the field potentiometer I9 is moved to a lower speed position in which current conducted by valve 55 is decreased. This tends to increase the current conducted by valve 54 thereby to strengthen the motor field to such an extent that upon completing the reversal of the armature connections, the voltage generated by the motor is so high as to interfere with successful inverter operation. To overcome this tendency of the control to strengthen the fields during the operation of the directional contactors for a reversal from a higher to a lower speed, an additional electric valve I8 is provided for supplying a voltage to control valve 64 in such a, manner as to prevent valve 54 from responding to the potentiometer IE! to strengthen the motor field. Valve I8 is illustrated as a double diode valve having anodes 18a and 1% connected to opposite terminals of the dynamic braking resistor I9 which, when the directional contactors 30 and 3| are dropped out, is connected across the armature terminals by the normally closed contacts 30d and Bid of the directional contactors. The cathode 180 of valve 18-is connected by means of a conductor 80 to the point 65a of the voltage divider to which the grid 640 of the valve 64 is connected.

With the foregoing understanding of the elements and their organization, the operation of the system itself will readily be understood from the following detailed description.

To place the system in condition for operation, the switch 8| is closed to connect the primary winding of the supply transformer I2 to the supply source II. As a result, the operating coil of the time delay relay 25 is energized and after an interval of time which is determined by. the setting of its time delay device, and which is sufiicient to provide for initial heating of the cathodes of the electric valves, the time delay relay 25 closes its normally open contacts 25a to complete the field circuit from the cathodes of the field thyratrons I3 and I4 through the operating coil 26a of the field loss protective relay 26 and the field winding Iflb to the negative bus 20. As a result of the closing of switch 8 l, the low voltage control transformer 42 is energized and the full wave rectifier valve 43 supplies a rectified voltage to the buses GI and 20 which voltage is maintained constant by the standard voltage regulator valves 4'! and 48. Since the directional contactors 3i anad 3! are open, the operating coil 10b of control relay it! is connected across the standard voltage conductors 6! and 20 through the normally closed contacts 39?) and 3l-b of the directional contactors. Responsively to energization, relay 10 picks up and closes its normally open contacts 10a and 700. At this point in the operation, the grid 510 of the current limit control valve 51 is connected through conductor 82 and the normal- 1y closed contacts 30a and am of the reversing contactors to the point'iila on the voltage dividers El, 68 such that the current conducted by the valve 57 produces an IR drop across the resistor section cm which lowers the voltage on the grid 490 of valve 29 to a value which regulates the voltage output of the armature thyratrons I5 and I6 to a low value, c. g. 50 volts, such that when a directional contactor is subsequently closed, the current supplied to the armature Illa of the motor is limited to approximately full load value.

Also at this point in the operation, the cathode b of control valve 55 is connected through the normally closed contacts 5211 of relay I2 and conductor 83 to the conductor Ma of the standard voltage source, with the result that the cathode 55b is so positive with respect to its grid that the valve-55 is nearly nonconducting. As pointed out in the foregoing, this causes the valve 54 to be nearly fully conducting, thereby to saturate the field reactor 36 and advance the phase of the grid voltage of the field thyratrons 3 and M so that these thyratrons supply full rated current to the shunt field winding Iiib, thereby to provide maximum starting torque. Since the field current flows through the operating coil 26a of the field protective relay 2%, this relay picks up and closes its normally open contacts 26b. In closing, contacts 26b complete a circuit for the operating coil Eliia of a relay 85 which is controlled by the stop push button switch M to effect the stopping operation of the motor. This relay is known as the stop relay. The energizing circuit is traced from the low voltage tap I20 through conductor 8%, operating coil of stop relay, normally closed contacts 12d of relay I2, normally closed contacts of stop push button 4|, contacts 260 of field protective relay 26 to the low voltage tap I217. Responsive to energization, the stop relay 85 picks up and closes its normally open contacts 852) and opens its normally closed contacts 8510, 85d and 856. Contacts 85?) in closing complete a holding circuit in parallel with contacts 72d of relay "i2.

Assuming that it is desired to operate the motor at a speed above base speed within the field weakened range, the knob of the speed controlling potentiometer is moved until the sliders 18a and I911 are in positions between 5 and 9 which correspond to the desired operating speed. This has no immediate efiect upon the thyratron valves I3, 54, I5, and I5, since at this point in the operation, these valves are under the control of the preconditioning circuit.

To start the motor in the forward direction, the forward push-button switch 39 is depressed to bridge its normally open contacts 39a, thereby to complete an energizing circuit for the operating coil of the forward contactor 30. This circult is traced from the low voltage tap lZb of the supply transformer, through contacts 10a of relay 10, contacts 39a of the forward pushbutton switch, normally closed contacts of the reverse push-button switch, normally closed contacts 86a of a reverse control relay, normally closed interlock contacts of reverse contactor 3|, operating coil of forward contactor 39 to the low voltage tap l 20. In response to energization, the forward contactor 3i! closes its main contacts 30f and 30g and its interlock contacts 30h and 301' and opens its interlock contacts 30a, 30b, 30c, 30d and 397'. lhe main contacts 30f and 30g in closing complete the armature circuit from the positive terminal 28 of the thyratrons through conductor 29, series field winding lDc, contacts 30g, heating element 32a of the overload relay, armature [a, and contacts 35) to the negative bus 20. At the instant of the closing of forward directional contactor 33, the armature thyratrons l5 and I5 are phased to supply approximately full load current to the armature, owing to the connection of the grid 510 of control valve 51 to point 61a of the voltage divider 61, 68 between bus 6! of the standard voltage source and the positive side 11 of the armature thyratrons l5 and 16,. However, the simultaneous opening of interlock contacts 32a interrupts this preconditioning circuit so that for the remainder of the accelerating period, the current supplied by thyratrons I5 andv Hi to the armature is controlled by the setting of the slider 60b of the current limit potentiometer to which the grid 5110 is connected. Interlock contacts 3th in closing complete an energizing circuit for the operating coil of control relay 12. This circuit is traced from low voltage tap through conductor 84 and the operating coil of relay [2, contacts h of the forward contactor, contacts 39a of the forward push button, contacts 10a of control relay iii, and thence to the low voltage tap l2b. In response to energization, relay [2 picks up and closes its contacts 12c to complete a holding circuit for the operating coil 300 of the forward contactor and a sealing-in circuit for its own operating coil in parallel with the contacts 39a of the forward push-button switch which now may be released. Relay 12 in picking up opens its normally closed contacts 12a to interrupt the field preconditioning circuit and closes its contacts 12b to connect the cathode 55b of field control valve 55 to the slider |9a of the preset field potentiometer 29, to provide for weakening the field to the value preset on the potentiometer 19 as the armature current decreases near the end of the acceleration. Following the interruption of the armature preconditioning circuit by the opening of interlock contacts 36a, the armature current builds up at a rate partially determined by the inductance of the D.C. winding of the armature saturable reactor 34. Contacts 3% in opening interrupt the energizing circuit for the operating coil 10b of control relay H! which, in response to deenergization, drops out and opens its normally open contacts 19c.

As a result of the completion of the armature circuit, the motor begins to accelerate to a speed determined by the setting of the speed control potentiometer. During acceleration, before the armature oountervoltage has built up to a value corresponding to the preset speed which it is de sired to maintain, the phase of the grid voltage of the armature thyratron tends to be fully advanced and hence, the armature thyratrons tend to supply a current to the armature which is many times full load value. However, the current limiting control acting through valves 51 and 58 decreases the output of the armature thyra trons to the value determined by the setting of the slider of the current limiting potentiometer 60. If, as assumed, the field control has been set for a speed in the field weakened range, then the current limit control acting through valve 53 will tend to maintain full field until the armature current tends to fall below the present limiting value. Thus, during acceleration to a present speed within the field weakened range, the armature voltage is first allowed to build up at a rate determined by the load on the motor and by the value of the armature current which has been set upon the current limiting potentiometer 61! until full armature voltage i reached. At this point, there is a tendency for the armature current to decrease. However, this tendency will make the grid voltage of the valve 58 more negative, thereby decreasing the conductivity of the valve and making the voltage of the grid of valve 54 more negative, with the result that the field thyratrons l3 and I 4 will supply less current to the field. This results in maintaining the armature current substantially constant until the preset field weakened speed is reached, at which point the armature current will drop to that value which is necessary to drive the load. Since the system operates to maintain the maximum permissible value of armature current during the acceleration, the load is accelerated to the preselected speed in the minimum possible time consistent with the armature current limit at which the control is set to operate.

At a predetermined low armature voltage, the control valve 81 which is connected acros the armature in series with the operating coil 88a of relay 88 becomes conducting. Responsively to energization, relay 88 picks up and closes its contacts 88b in parallel with the normally closed contacts 86b and 89b of the reverse and forward control relays 86 and 89, respectively. Once picked up, relay 88 remains picked up until the positive component of armature voltage is subsequently reduced approximately to zero. Negative components of armature voltage introduced by reversals will not cause the relay to drop out since they are blocked by the rectifying action of valve 81.

If it is desired to operate the motor in the reverse direction at the same speed, it i only necessary to depress the reverse push button 49 to open the normally closed contacts Md and close the normally open contacts 4231). Contacts 49a in opening interrupt the energizing circuit for the operating coil 3G0 of the forward contactor which, responsively to deenergization, drops out to the open position in which it is illustrated. Contacts 30] and 39g in opening disconnect the armature from the thyratrons l5 and i5, and contact 3001 in closing completes the connection of the dynamic braking resistor 79 across the armature terminals, thereby to produce a dynamic braking torque which is effective in decelerating the motor. Contacts 353a in closing again complete the preconditioning circuit between the grid 510 of the armature control valve 57 and the point 61a on the voltage divider between the standard voltage bus 6! and the positive side of the output voltage of the supply valve apparatus, line H. Whenever the preconditioning circuit is made effective by the reversing contactors being deenergized, valve 51 operates in conjunction with Valve 49 to control the output voltage of thyratrons I and 16 in much the same manner that valve 59 operates when the thyratrons l5 and it are con nected to the armature of the motor. An increase in the output voltage of thyratron-s i5 and is makes the voltage of junction filo of resistor 6'1 and 68 less negative with respect to the cathode 51b of valve 51 which increases the conductivity of valve 57, thereby decreasing the conductivity of valve 49 and the saturation of the saturable reactor which decreases the output voltage. Conversely, a decrease in the output voltage of thyratrons l5 and i6 makes the voltage of junction point 61a more negative With respect to the cathode Slb of valve 5?. This decreases the conductivity of valve 51 thereby increasing the conductivity of valve 49 and the saturation of the saturable reactor with the result that the output voltage of the thyratronis correspondingly increased. Since during the preconditioning period the only load on thyratrons l5 and i6 is a resistance load consisting of resistor 62-) and the various resistance voltage dividers, controlling the output voltage at a constant low value means holding the phase angle of the thyratron grid voltage at a definite retarded angle, which may be predetermined by proper selection of the circuit components. Thus, the phase of the grid voltage of the armature thyratron-s i5 and i6 is regulated to a point that will permit armature current in the order of full load current to flow at the first instant after the reverse contactor 3! 131855 up to initiate inverter action. Since the sealing-in circuit of the control relay i2 is independent of the contacts of the reversing contactors, relay i2 remain picked up during this reversing period and consequently the cathode 55b of field control valve remains connected to the slider i900 of the field control potentiometer, and thus there is no tendency for the control to strengthen the field prior to the reclosing of the reversing contactor-s. Thus, the countervoltage of the armature and the conductivity of the armature thyratrons i5 and iii are both regulated to values which will be favorable to successi'ul inverter operation when the reverse contactor Si is closed.

The closing of contacts Sill) completes an energizing circuit for the operating coil ill!) of control relay ill which, responsively to energization, picks up after a short time delay and closes its contacts We to complete an energizing circuit for the operating coil 3 lc of the reverse contactor ii i. This circuit is traced from the conductor 35, through coil 35c, normally closed contacts tile of the forward contactor, normally closed contacts 89a of forward control relay 89, normally closed contacts 3% of the forward push button, contacts diib of the reverse push button, and thence by the circuit previously traced to low voltage tap l2c of the supply transformer. The relay ll! has sufficient time delay in its operation to delay the closing of the reverse contactor 3i until the retardation of the phase of the grid voltage of armature thyratrons l5 and it is sub stantially completed.

Responsively to energization, reverse contactor Jl picks up and opens its normally closed interlock contacts Slot, 3H), std, 3H6 and 3M, and closes its main contacts 3i) and Big and its interlock contacts 3th and 3h; Contacts 3th. in closing complete a holding circuit for the operating coil Bic independent of the contacts of the reverse push button which may now be released. Contacts 3ld in. opening interrupt the dynamic brakin circuit, and main contacts 3h and My in closing connect the armature Illa to the thyratrons :5 and is for the reverse direction of rotation, and since the armature is still rotating in the forward direction, inverter operation ensues. The opening of interlock contacts 312) interrupts the energizing circuit for the operating coil of control relay it which responsively to deenergization, drops out and opens its contacts Ilia and 7&0. Simultaneously, interlock contacts 31a in opening interrupt the preconditioning circuit between the grid 570 of control valve 51 and the point Kilo on the voltage divider across the standard voltage source. The control of the armature thyratrons I5 and it during the inverter operation is now under the control of the current limit potentiometer 60 to the slider Ellb of which the grid Bic of the current limit control valve 5'! is connected. Thus, the armature current during inverter operation is limited to the value which is preset upon the current limit potentiometer 60.

The closing of main contacts Sig of the reverse contactor 3i connects the lower armature terminal through conductor "H to the cathode 13b of the valve '13 in the reverse snubbing circuit. Since the armature Illa is still rotating in the forward direction, the polarity of the countervoltage is unchanged and the lower armature terminal is therefore negative, and this negative voltage is applied to point 15a between which and line did is connected a voltage divider consisting of resistors M and 15. The values of resistors M and it: are so chosen that the potential of the cathode 13b of valve '13, which is connected to their junction point Ma, reaches the potential of the negative conductor 2i] at the same time that the lower terminal of the armature attains its maximum permissible negative value with respect to the negative bus 26. The application of negative voltage to the cathode 13b renders the valve 113 conducting and the resultant voltage drop across the resistor Si, lowers the voltage of the grid 540 of valve 5 thereby decreasing its conductivity and weakening the field of the motor. As a result, the armature countervoltage is kept below the safe limit for inverter operation and at the same time the field strength is maintained as high as possible, consistent with successful inverter operation, to obtain strong reversal torque. Thus, the tendency of the field current limit control valve 58 to strengthen the field as the armature current approaches the preset current limit is counteracted by the reverse snubbing circuit valve I3 during inverter operation.

As a result of the inverter operation, a strong regenerative braking torque is set up, and the speed of the armature is rapidly decelerated to rest. As the armature approaches zero speed, the countervoltage decreases and consequently, the voltage of the cathode 73?) again becomes positive and renders the valve 13 nonconducting so that during the ensuing acceleration in the reverse direction, the field control valve 54 is con trolled by the field potentiometer I9 and the current limit potentiometer (ill acting through the control valve 58.

A reversal of the operation of the motor from a high field weakened speed to a lower speed is accomplished by depressing the reverse push button M3 and rotating the knob of the preset speed control potentiometers l8 and 19 to a position corresponding to the desired speed for the reverse .direction of rotation. In other words, the slider lea of the field potentiometer i9 is moved into 17 engagement with the contact strip I92), with the result that the voltage of the cathode 55b of the valve 55 becomes so positive with respect to the grid voltage that the valve 55 becomes nonconducting and the valve 54 tends to become fully conducting. If the valve 54 were permitted to become fully conducting, the field saturable reactor 35 would be fully saturated and the field thyratrons l 3 and I4 would supply maximum current to the shunt field winding IIib. In the period following the closing of the reverse contactor 3I, this strengthening of the motor field is prevented by the action of the valve I3 in the reverse snubbing circuit as described in the foregoing. However, in the interval in which both forward and reverse contactors 3B and SI are dropped out preceding the closing of the reverse contactor, the valve 13 is not connected to the armature I Get and is therefore inactive. However, the strengthening of the field during this period in which both forward and reverse contactors are dropped out is prevented by action of the valve I8, the anodes of which are connected through normally closed contacts 30d and 3Id, respectively, to opposite terminals of the armature during the period when both reversing contactors are open. During this period, the valve 78 supplies a voltage to the junction point 65a of the resistors 65 and 66 to which the grid 640 of Valve 54 is connected. As a result, the valve 64 is rendered conducting, thereby producing a voltage drop across the resistor 62a which lowers the voltage of the grid 540 of the field control valve 54 sufiiciently to counteract the tendency of the preset potentiometer I9 to strengthen the field during this period.

As the forward speed of the motor approaches zero, the inverter action ceases and the motor is accelerated in the reverse direction in a manner similar to the forward acceleration described in the foregoing.

Assuming the motor is operating in the forward direction, and that it is desired to stop the motor: The stop push-button switch M is depressed to open its normally closed contacts thereby to deenergize the stop relay 85. Responsively to deenergization, the stop relay drops out and opens its normally open contacts 85b and closes its normally closed contacts 85c, 85d, and 85e. Since relay I2 is picked up and will remain picked up until the motor comes to rest, the stop relay cannot be picked up again until the motor comes to rest. Contacts 85d in closing complete an energizing circuit for the operating coil of the reverse relay 86 which extends from low voltage tap I20 through coil of reverse relay 85, normally closed contacts 890 of forward relay 89, normally closed contacts 3Iy of reverse contactor 3I, contacts 85d of stop relay, contacts 120 of relay I2, contacts 8% of relay 88 to low voltage tap I2b. Responsively to energization, reverse relay 85 picks up and opens its normally closed contacts 86a, 86b, and 860 and closes its normally open contacts 85d and 86e. Contacts 86min opening deenergize the forward contactor which thereupon drops out and disconnects the armature from the thyratrons I5, I 6. As a result, the relay I is again energized over the circuit previously traced and picked up after a small delay to close its contacts 10a and lilo. This completes an energizing circuit for the reverse contactor which is traced from low Voltage tap I21) through contacts 70a and 86e, normally closed contacts of forward push button, normally closed contacts 85a of forward relay, normally closed contacts 30c of for- 18 ward contactor and coil of revers contactor to low voltage tap I20. In response to energization, reverse contactor picks up and closes its main contacts 3! and Mg to connect the armature to the thyratrons I5 and I6 for the reverse operation. However, at this point the armature is still rotating in the forward direction and consequently, inverter action ensues, as explained in the foregoing in connection with the reversal operation. The inverter operation produces a strong braking torque which rapidly decelerates the motor. The reverse contactor in picking up energizes control relay at. The circuit is traced from low voltage tap 120 through coil 90a, contacts IE0 of relay 75, contacts 3Ii of reverse contactor, normally closed contacts 850 of stop relay to the low voltage tap I217. Responsively to energization, relay 9t picks up and closes its contacts 901) to complete a sealing-in circuit in parallel with contacts Hi0 and 3| 2', and also closes its contacts 580 and opens its normally closed contacts 90d.

In the closed position of the reverse contactor, contacts 3129 are open and deenergize relay 10 which drops out responsively to deenergization. Contacts 99d in opening disconnect the cathode 50b of the armature control valve 59 from the slider I8a of the preset speed potentiometer I8, and contacts 900 in closing connect the cathode 50b to the negative bus 20. This is equivalent to setting the slider l8a at the zero speed position, and if the grid voltage, which is a measure of the armature voltage, remained unchanged at this point the valve 50 would become fully conducting with the result that the armature thyratrons I5 and It would be rendered nonconducting before the motor had been braked to rest. However, when the reverse contactor 3| closed, the grid 500 of valve 50, which was previously connected to the positive terminal of the armature, is now connected to the negative terminal and the conduction of valve 50 is maintained at a low value with the result that the grid voltages of thyratrons I5 and I6 are controlled to maintain the inverter operation, thereby to continue the rapid deceleration of the motor. As the deceleration continues, the negative grid voltage of valve 50 is correspondingly decreased so that at zero speed the thyratrons I5 and I5 are phased off and cease conducting.

As soon as the thyratrons I 5 and I6 are phased off, the valve 87 ceases to conduct, thereby deenergizing relay 88. Responsively to deenergization, relay 38 drops out and deenergizes relay I2 which thereupon drops out and opens its contacts 12b and I20 and closes its contacts 12a and 12d. Contacts I20 in opening deenergize the reverse contactor 3| and the reverse relay 86. Responsively to deenergization, the reverse contactor drops out to disconnect the armature from the thyratrons I5, I 6. Contacts 1201 in closing complete the energizing circuit for the stop relay 85, which picks up to close its contacts b and to open its contacts 85c, 85d, and 85e. Contacts 85b in closing complete a sealing-in circuit, and contacts 850 in opening deenergize the relay 9B which drops 1 out to open contacts 90b and 950 and to close contacts d, thereby leaving the control in a reset condition.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle thereof has been explained, together with the best mode in which it is now contemplated applying that principle, it will be understood that alterations and modifications will readily suggest themselves to persons skilled in the art Without l9 departing from the true spirit of this invention or from the scope of the annexed claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Control apparatus for an electric motor comprising in combination, means for controlling the supply of direct current to an electric motor comprising electric valve apparatus provided with a control grid, a standard voltage source, means for presetting an operating speed for said motor comprising means for deriving a predetermined reference voltage from said source, means for producing a signal voltage proportional to the speed of said motor, and means responsive to the difrerence of said derived voltage and signal voltage for varying the conductivity of said valve means in accordance with said difference, means for pmiting the current supplied to said motor to a predetermined value comprising a control circuit carrying current proportional to the current supplied to said motor and a control electric valve connected to control the grid circuit of said valve means and having a grid connected to said control circuit, reversing switching means in the output circuit of said valve means, and means responsive to said switching means when said switching means is open for directly connecting the grid of said control valve to the output circuit of said valve apparatus to adjust the conductivity of said valve means for inverter operation in response to operation of said switching means to effect reversal of said motor.

2. Control apparatus for an electric motor comprising in combination, electric valve means for controlling the supply or" direct current to an electric motor, means for limiting the current supplied to said motor to a predetermined value comprising a control circuit carrying current proportional to the armature current of said motor, a control electric valve for controlling said electric valve means and having a control grid connected to said control circuit, a standard voltage source, switching means in the output circuit of said valve means, means responsive to said switching means when said switching means is open for directly connecting the grid of said control valve to the output circuit of said valve means to adjust the conductivity of said valve to a predetermined value, and an auxiliary electric valve having an anode connected to the cathode of said control valve and a cathode connected to the grid of said control valve to render said auxiliary valve conducting and thereby limit the negative voltage applied to the grid of said control valve at low armature current of said motor.

3. A control system for an electric motor comprising in combination, electric valve means provided with a control grid for controlling the supply or direct current to an electric motor, a standard voltage source, means for presetting an operating speed for said motor comprising means for deriving a predetermined reference voltage from said source, means for producing a signal voltage proportional to the speed of said motor, and means responsive to the difference of said derived voltage and signal voltage for varying the firing point of said valve means in accordance with said difierence, means for limiting the current supplied to said motor to a predetermined value comprising an auxiliary electric valve having a grid connected to be responsive to the current supplied to said motor and having its out put circuit connected to control the firing point of said valve means, reversing switching means for selecting the polarity of the voltage supplied to said motor to control the direction of rotation, and means responsive to said switching means when said switching means is open for directly connecting the grid of said auxiliary valve to the output circuit of said electric valve means to retard said firing point to limit the current supplied to said motor before said current limiting means becomes fully eiiective, and relay means operatively associated with said reversing switching means for preventing the reversal operation of said reversing switching means for the period of time required for said retarding means to become effective.

4. A control system for an electric motor comprising in combination, first electric valve means for controlling the supply of direct current to the armature of said motor, additional electric valve means provided with a control grid for controlling the supply of direct current to the field of said motor, forward and reverse contro1 devices and reversing switching means contro1ledthereby in the output circuit of said first valve means for controlling the direction of rotation of said motor, a stop control device for controlling the opening and closing of said switching means, means responsive to the opening operation of said switching means for controlling said additional valve means to strengthen the field of said motor, and means for preventing the strengthening of said field when said switching means is open during a reversal of said motor by said reversing switching means.

5. A control system for an electric motor comprising in combination, a first electric valve means for controlling the supply of direct current to the armature of the motor, additional electric valve apparatus provided with a control grid for controlling the supply of direct current to the field of said motor, forward and reverse control devices and directional contactors in the output circuit of said first electric valve means controlled by said control devices for controlling the polarity of the voltage supplied to said armature to control the direction of rotation, a stop control device for controlling said contactors to interrupt the output circuit of said first electric valve means, an electromagnetic switching device responsive to the closing operation of either of said contactors and having a sealing in circuit independent of said contactors for controlling said additional valve means to maintain the field of said motor at its existing strength when said reversing contactors are open during a reversal operation, and means responsive to stopping of said motor following an operation of said stop control device for interrupting said sealing in circuit to cause said electromagnetic switching device to control said additional valve means to strengthen the field of said motor.

6. A control system for an electric motor comprising in combination, first electric valve means for supplying direct current to the armature of the motor, additional electric valve means for supplying direct current to the field of said motor, reversing switching means for connecting the armature of said motor to said first valve means for rotation in either direction, and means responsive to the armature voltage of said motor following a reversal of the connections of said armature to said first valve means for controlling said additional valve means to vary the excitation of said motor to limit the armature voltage of said motor to a predetermined value.

'7. A control system for an electric motor comprising in combination, first electric valve means for supplying direct current to the armature of the motor, additional electric valve means for supplying direct current to the field of said motor, means for controlling said additional valve means comprising a control valve provided with a control grid and means for supplying a variable control voltage to said grid, reversing switching means for connecting said armature to said valve means for rotation in either direction, and means for limiting the armature voltage of said motor to a predetermined value following operation of said switching means to reverse the connection of said armature to said first valve means comprising a second control valve having an anode, cathode, and control grid and connections from the cathode and grid of said second control valve to said armature and connections from the anodecathode circuit of said second valve to the grid of said first valve to control said first valve to limit the excitation of said motor field.

8. A control system for an electric motor comprising in combination, first electric valve means for supplying direct current to the armature of the motor, additional electric valve means for supplying direct current to the field of said motor, means for controlling said additional valve means comprising a control valve provided with a control grid and means for supplying a variable control voltage to said grid, reversing switching means for connecting said armature to said first valve means for rotation in either direction, and means for limiting the armature voltage of said motor to a predetermined value following operation of said switching means to reverse the connection of said armature to said first valve means comprising a second control valve having an anode, cathode, and control grid and connections from the cathode of said second control valve to the positive terminal of said armature and from the grid of said control valve to the negative terminal of said armature during normal running operation of said motor thereby to maintain said second control valve inactive, said reversing switching means serving to reverse said grid and cathode connections upon reversal of the connections from said armature to said first valve means thereby to render said second control valve conducting While the voltage of said armature is reversing and nonconducting when the reversal of said armature voltage is completed, and a connection from the anode of said second control valve to the grid of said first control valve to control said first control valve to limit the excitation of said motor during said reversal operation.

9. A control system for an electric motor comprising in combination, first electric valve means for supplying direct current to the armature of said motor, additional electric valve means for supplying direct current to the field of said motor, means responsive to armature current for controlling said additional electric valve means to strengthen the field of said motor to limit the armature current to a predetermined value, reversing switching means for connecting said armature to said first valve means for rotation in either direction, and means responsive to the armature voltage of said motor following a reversal of the connections of said armature for counteracting the action of field strengthening means thereby to control said additional valve means to vary the excitation of said field to limit the armature voltage to a predetermined value until the reversal of said armature voltage is completed.

10. A control system for an electric motor comprising in combination, electric valve apparatus for supplying direct current to the armature of said motor, additional electric valve apparatus for supplying direct current to the field of said motor, a speed control device for controlling said additional valve means to vary the excitation of said motor field, reversing switching mechanism for connecting said armature to said first electric valve means for rotation in either direction, and means responsive to the armature voltage of said motor during the period in which the armature is disconnected from said first valve means during a reversing operation for controlling said second Valve means to prevent strengthening the field of said motor by said speed control device in a reversal of said motor from a high speed to a relatively lower speed.

11. A control system for an electric motor comprising in combination, electric valve apparatus for supplying direct current to the armature of said motor, additional electric valve apparatus for supplying direct current to the field of said motor, a dynamic braking resistor, forward and reverse directional contactors for connecting said armature to said first electric valve means for rotation in either direction and provided with normally closed contacts for connecting said dynamic braking resistor across said armature when said armature is disconnected from said first valve means, a speed control device for controlling said additional valve means to vary the excitation of said motor field, and means for preventing strengthening the field of said motor when said armature is disconnected from said first valve means during a reversal of said motor from a high speed to a relatively lower speed comprising a full wave rectifier valve having a pair of anodes connected to points on said braking resistor of opposite polarity and having its cathode connected to an intermediate point on said resistor, a voltage drop device connected in the output circuit of said full wave rectifier, and means responsive to the voltage across said voltage drop device for controlling said second valve means.

12. Control apparatus for an electric motor comprising electric valve apparatus for supplying a rectified voltage to the armature of the motor, reversing switchingmeans for connecting said armature to said valve apparatus for rotation in either direction, a stop control device, means responsive to operation of said stop control device for controlling said reversing switching means to reverse the connections of said armature to said electric valve apparatus to provide for inverter operation of said valve apparatus to brake said motor to rest, and means responsive to a predetermined low speed of said motor to disconnect said motor from said valve apparatus.

13. Control apparatus for an electric motor comprising electric valve apparatus for supplying a rectified voltage to the armature of the motor, reversing switching means for connecting the armature of the motor to said electric valve apparatus for rotation in either direction, means for producing an adjustable reference voltage, means for producing a signal voltage having a predetermined relationship to the speed of said motor, means responsive to the difference of said voltages for controlling said electric valve apparatus to effect operation of said motor at a speed corresponding to said reference voltage, a stop control device, means responsive to operation of said stop control device for controlling said reversing switching means to reverse the connections of said armature to said valve apparatus, means responsive to operation of said switching means to adjust said reference voltage for zero speed of said motor, and means responsive to a predetermined low speed of said motor for controlling said reversing switching means to disconnect said armature from said valve apparatus.

14. Control apparatus for an electric motor comprising electric valve apparatus for supplying a rectified voltage to the armature of said motor, reversing switching means for connecting said armature to said valve apparatus for rotation in either direction, a source of adjustable reference voltage, means for deriving a voltage from the countervoltage of said motor, means responsive to the difference of said reference voltage and said derived voltage for controlling said valve apparatus to effect operation of said motor at a 24 speed corresponding to said reference voltage, a stopping control device, means responsive to operation of said stopping control device for controlling said reversing switching mechanism to reverse the connections of said armature to said electric valve apparatus, an electromagnetic switching device responsive to the reversal operation of said reversing switching means for adjusting said reference voltage for zero speed, an electric valve connected to be responsive to the countervoltage of said motor and an electromagnetic switching device controlled thereby for controlling said reversing switching means to disconnect .said armature from said electric valve apparatus at a predetermined low speed of said motor.

HENRY H. LEIGH.

GORDON E. WALTER.

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