US1916415A - Crane hoist control - Google Patents

Description

W. G. COOK CRANE HOIST CONTROL July 4, 1933.

Filed May 20, 1950 2 Sheets-Sheet l INVENTOR IV/flard 6. 600K.

ATTORNEY July 4, 1933.

w. G-. COOK 1,916,415

CRANE HOIST CONTROL Filed May 20, 1930 2 Sheets- Sheet 2 76M HOIIS) Lower WITNESSES: INVENTOR /3% M/fflara 600K BY 7 f W ATTORNEY Patented July 4, 1933 warren STATES WILLARD G. COOK, on

PATENT OFFICE WILKINSBURG, PENNSYLVANIA, ASSIGI-TOR TO VIESTI'NGHOUSE CRANE HOIST CONTROL Application filed May 20,

My invention relates to motor-control systems and particularly to such control systems as are employed in connection with the driving motors for cranes, hoists, and similar 6 devices. v

The object of my invention is to provide for cooperating manual and automatic timelimit speed control for a motor under all conditions of load and for accomplishing the 10 desired result smoothly and without imposing abnormal strains on the motor or other parts of the system.

Other objects of my invention are to provide for inductive time-limit acceleration of the motor during starting, regardless of the load on the mot-or, and for dynamically braking the motor during stopping by a gradual, though comparatively rapid, change in the field current and a final stopping of the armature by connection through a low-resistance dynamic-braking circuit.

On lowering a crane carrying a heavy load, the operator may rapidly move the handle of the master switch from the full-speed hoisting position to the off position or, possibly, to the lowering position. In the control systems available at present an improper operation of the master switch may cause the dropping of the load in some instances and, in others, may cause a. very strong dynamic-braking operation which subjects the hoisting machinery to abnormal stresses.

According to my invention, a control system is provided with a series motor which is controlled by a master switch in such manner that, in one direction of rotation, when the load is to be hoisted, the motor is started very slowly by reason of the inclusion of an armature shunt and a starting resistor in the motor circuit. The speed is readily controlled by opening the circuit of the armature shunt and by successively shunting portions or sections of the starting resistor.

Nhen it is desired to reverse the motor,

. as in lowering a load that is notoverhauling, the armature connections are reversed, and the field winding is connected in shunt relation to the armature. The accelerating resistor may be connected in series withthe 1930. Serial No. 453,928.

field winding. A dynamic-braking resistor is connected in series with the armature Circuit to limit the initial rush of current. A comparatively heavy, current traverses the shunt field winding, since the starting resistor is short-circuited, thus raising the brake-shoe of the hoist brake and providing ample field strengthto start the motor in the lowering direction. The lowering speed may be controlled through the master switch or by means of the automatic control relays, depending on the speed with which the controller is moved in the lowering direction.

In bringing the load to rest, the operator may, with the present system, return the controller'to the off position or any one of the hoist positionsand rely on the automatic means provided to gradually shunt the resister from the field circuit to procure graduated dynamic braking prior to establishing the final dynamic-braking circuit for stopping and holding the motor. The motor speed is, therefore, first reduced by increased field excitation can be applied. 7

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific em bodiment, when readin connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view showing the circuits and apparatus, embodying my invention, in operative relation;

Fig. 2 is a sequence chart v indicating the order of operation ofthe several cont actors;

Figs. 3, Land 5 are circuit diagrams showthe successive dynamic braking circuits ing before final dynamic braking '7 that are established during the stopping operation of the load; and

Fig. 6 shows the final low resistance dynamic braking circuit for stopping of the hoisting as well as the lowering operation.

7 Referring more particularly to the drawings, wherein the same reference characters lowering of an overhauling designatecorresponding elements, 1 and 2 designate positive and negative conductors respectively leading from a suitable source of power not shown. A motor 190 is disposed to be connected to the source of power through the agency of a plurality of line contactors 71, 145, 167, and 88. The line contactor 88 is disposed intermediate the negative line conductor'2 and the starting resistor sections, while the other line contactors are disposed adjacent the positive line conductor 1. Actuation of line contactor 88 and 71 operates to connect the motor to the power lines as a series motor, whereas actuation of line contactor 88 and one or both of the line co ntactors 145 and 167 operates to connect the motor to the power lines as a shunt motor.

To provide for the appropriate acceleration of the motor, whether operating as a series motor or as a shunt motor. the starting resistor sections are disposed to be shunted by acceleratingcontactors 110, 127, 133, and 136. A plurality of time- limit contactors 23, 41, 47, and 122 are provided to automatically control the time and sequence of operation of theaccelerating contactors, thereby providingautomatic timecontrolled acceleration for the motor. The time controlled acceleration for the motor be comes effective only when the controller 197 is operated too rapidly. By a slow movement ofthe controller 197, the motor 190 may be subjected to manual control, as will be pointed out more in detail hereinafter. F or the present, itissufiicient to state that the time-limitv contactors prevent abusing the motor or the control circuits, by careless, in- .adverte'nt-v or malicious manipulation of the controller and provide a definite maximum rate of acceleration extending over a predetermined interval of time, regardless of the load on the motor. These facts will be more apparent after a study of Fig. 1, in connection with the following discussion.

A magnetic brake 191 is associated with the motor, and the actuation of the brake is controlled bythe motor current. A plurality of control contactors 148, 94, 99,153, 106, and 129 are also associated with the motor to control, among other operations, the establishment of a dynamic braking circuit for the motor when the voltage fails or the controller 197 is actuated to the off position or the motor is plugged.

The position of the elements shown in Fig. lis the position they occupy when the conductors 1 and. 2 are energized and the con troller stands in the off position, and the various coils energized by the energization of the conductors 1 and 2 have actuatedllllell respective armatures.

When the conductors l and 2 are energized a circuit is established from the positive conductor 1,

through the conductors 3 and 4, actuating coil 5 of contactor 153 and conductor 6 to the negative line conductor 2. The contactor 153 is provided with two coils 5 and 104, but the coil 5 is so designed that it alone is not capable of operating the contactor. A second circuit is established from the positive line conductor 1, conductor 3, through conductor 8, controller segments 26, conductor 9, actuating coil 10 of contactor 23 to the negative conductor 2. A resistor 12 is disposed in parallel-circuit relation to the actuating coil 10.

The time-limit contactor 23 is also provided with a neutralizing coil 56, and, as

shown, the armature having an associated contact-bridging member is biased to its contacomaking position by a spring, the'tension of whichmay be adjusted. By means of the adjustable spring and the resistor 12, the time constant of the time-limit contactor 23 may be accurately adjusted. The timelimit contactors 41, 47, and 122 are constructed like the time-limit cont-actor 23 except that, in connection with these contact-ors, no discharge resistors are required, since the magnetizing coilsof the respective contactors are connected in parallel with sections of the starting resistor and thusdischarge through these resistor sections.

1 Attention is'called to the fact that the controller 197 appears to be provided with fifteen contact segments. Since the upper fourteen segments are all connected by heavy bars, there is, electrically, only one terminal segment and this terminal segment will hereinafter be referred to as controller segment 26. I

Since the controller segment 26 is energized as abovepointed out, a circuit is also established from contact finger 27 through conductor 14, actuating coil: 16 of the accelcrating contactor 110, conductor 17, contact fingers 18 and 20 bridged by the controller segment 19, and conductor 21 to the negative line 2. Connected in parallel circuit relation to the contactor 23 are the conductors 17 and 21 and contact fingers 18 and 20 bridged by the controller segment 19. Interruption of the contactor 23, therefore, doesnot interrupt the circuit for coil 16. A circuit is also established from the energized controller segment 26 through contact finger 38, through conductors 39 and 40, time-limit con'tactor 41, actuating coil 44 of the accelerating contractor 127, conductor 17, contact fingers 18 and 20bridged by the controller segment 19 and conductor 21, to the conductor 2. Energizing circuits for the actuating coils 16 and 44-are also established through the contact finger 29 and conduc tors 30, 30 and contact members to the conductor 40, and conductor 31 and contact members 32 tothe conductor 14, respectively.

I The neutralizing coils ofthe time-limit, contactors are energized from conductor '1,

through conductor 3, resistor sections 51 and '54, and neutralizing coils 52, 53, 55, and 56 to the negative conductor 2. The resi'stor 51 has'a resistance value of about 2400 ohms, Whereas the resistancevalue of the resistor 54 is only about 800' ohms. The actualresistance values used for these resistors may vary over a great range but the ratio variations of the resistance values are kept very small for a purpose discussed more in detail hereinafter.

.A circuit for the actuating coil 50 of the accelerating contactor 133 is established from the energized controller segment 26 through contact finger 45, conductors 46 and 49, timelimit contactor 47, actuating coil 50, conductor 1?, contact fingers 18 and 20-bridged by controller segment 19-and conductor 21 to the line conductor 2.

The circuits just discussed cause the energization of coils 10, 16, 44 and and, as a consequence, contactor 23 opens, and accelerating contactors 110, 127 and 133 are closed. The control circuits are, therefore, so energized that the controller may be moved either to a hoisting or to a lowering position. It the controller is moved to the first hoisting position, coils 16, 44 and 50 are deenergized, and contactors 110, 127 and 133 are opened, and other circuits are established.

Movement of the controller to the first hoist position causes operation of line contactors 71', and 88, and armature shunt cont actor 94. The circuits for these contactors may be traced from the positive line conductor 1, through conductors 3 and 8, contact fingers 25 and 57-bridged by the controller segment 26-conductor58, actuating coil 59 for the line contactor 71 and conductor 60 to the line 2. A parallel circuit to the coil 59 is also established from conductor 58 through actuating coil 62 and conductor 60 to the line 2. Another circuit extends from controller segment 26 through contact finger63, conductor 64, actuating coil 65 of line contactor 88 to the line 2. Connected in parallel.- circuit relation to coil 65 is operating coil 66 of contactor 99. A third circuit is established from controller segment 26 through contact finger 68, actuating coil 70 of the armature shunt contactor 94 and conductor 60 to the negative line 2.

Operation of the controller to the first hoist position also causes deenergisation of all the energized operating coils ot the accelerating contactors and, in consequence, the contacts of the accelerating contactors open, thusinserting all the sections of the starting resistor in the circuit of the motor. While the controller is still in the first hoist position and by reason of the circuits thus established, contactors 88, 71, 94 and 106 close and contact members 99 and 32 open. I

A circuitfor the motor is thus established from the positive line conductor, contactor T1, conductor 73, the armature of the motor raised connected in series with the motor, the motor with the armature.

190, field magnet Winding 7 6, actuating coil 78 of the hoist brake 191, conductor 7 9, the

resistor sections 80, 81, 82, and 83 of the starting resistor 192, contactors 85 and 88 to the negative line 2. An a'rmature'shunt is established'whichextends from conductor 73 through bralring resistor 91, auxiliary resister 93, contact r 94, conductor 96,-auxiliary starting resistor sections 97 and :98 to the lower armature terminal. This armature shunt permits avery slow initial hoisting operation, so that chains and hooks may be adjusted before the load is actually being A circuit in parallel with thearmature is i also established from one armature terminal through conductor 7 3, conductor 101, coils 102 and 104 in. parallel to the other armature terminal. In this instance, the coil 104 opposes coil 5, and the contactor 153 is not 0perated, Whereas the coil 102 aids coil 62 and the contactor 106 is operated. The relative strength of coils 62 and 102 is such that coil 102 alone will cause operation of contactor 106 when the motor has attained a predetermined speed. This is important only in' lowering when the load is non-overhauling, as Will be made clearer hereinafter.

Since all the starting-resistor sections are starts at a slow speed and, at the same time, the coils otthe time-limit relays 41, 47 and 122 are energized by circuits from conductor 113, magnetizing coil 114 oftime-limit relay 122; conductor 118 through magnetizing coil. 116 oftime-limit relay 47; and conductor 119 through magnetizing coil 117 of the-time limit relay 41 to conductor 115, contactor 88 to the negative line conductor2.

Energization of the time-limitrelays causes contactors 41, 47, and 122 to open and contact members 170 to close. This operation and the closing of contac-tor 106 does not cause any immediate effect, since contact fingers 29, 38, and 45 are not in engagement with con troller segment 26. The opening of the contactor 99 prevents the shunting of the starting-resistor sections, and the opening of contact members 32 permits the continuance of the manual control of the energization of coil 44 When the controller is moved to the third position, tor,in the third position, contact fingers 25 and 27 are again bridged by controller segments, as in the off position.

When the controller'is moved to the second hoist position, coil70 is deenergized to cause the opening of contactor94. The opening of contactor 94 opens thearmature shunt circuit from 91 to 98. The motor is thus connected as a series motor with all starting resistor sections 80 to 83' inclusive, in series The fi er section 160, shunted bycontactor 85, is n'o'ta portionof the starting resistor, and is shuntedduring the hoisting operations. During the lowermg operation, the resistor section 160 is in the circuit ofthe field magnet winding and causes I a predetermined interval of time, depending upon the setting and design of the time-limit relay. After the lapse of the required time, a circuit is established from the positive line conductor 1, through conductor 8, contact fingers 25 and 27, conductor 14, actuating coil 16 of accelerating contactor 110, conductor 17 and contactor 23 to the line conductor 2.

The operation of accelerating contactor 110 shunts resistor sections 83 and 160. The shunting of resistor section 83 causes the de energization of time-limit relay 41, and this relay, after a predetermined interval of time, closes its contact members. The controller may now be moved to the fourth controller position.

Movement of the controller to the fourth position establishes a circuit from the positive line conductor 1, through conductor 8, contact fingers 25 and 38 bridged by'scgment 26- conductors 39 and 40, time-limit contactor 41, actuating coil 44 of accelerating contactor 127, contactor 23 to the conductor 2. A circuit is also established from conductor 39, contact members 35, conductors 30' and 105, contactor 106, coil 108 of contactor 129, conductor 17 and contactor 23 to the conductor 2. /Vhether contactor 129 operates by reason of the energization of coil 108 is immaterial at this stage of the operation, since no circuits are thereby established but the design is such that contactor 129 does not operate, or, if it does operate, it operates slowly.

The coil 44 is energized, and the contactor .127 stands closed; therefore, when the controller is actuatedto its fourth position, resistor sections 82, 83 and 160 and magnetizing coils116 and'117 are shunted. After a definite time interval, time-limit contactor 47 closes and the controller may be moved to the fifth hoist position.

When the controller stands in the fifth position, contact fingers 25 and are bridged, and the accelerating contactor 133 closes to shunt another sectionof the starting resistor and to shunt the magnet-izing coil 114 of the time-relay"-122. After a definite time interval, relay 122 closes to energize the last, accelerating contactor .136, and the controller may be -moved to the sixth hoist position. Movement of the controller to the sixth hoist position causes the shunting or" the last starting resistor section 80,-'and the motor operates at full hoisting speed.

If the attendant desires to stop the hoisting operation, the controller is moved to the off position. By this operation, the line 'contactors 71 and 88 are deenergized, and the motor is disconnected from the source of power. The coil 66 is also deenergized, and the control relay 99 immediately closes its contacts. This relay is provided with .a spring 198 which insures the establishment of a firm contact between the contactv members of the relay 99. A, low-resistance dynamic-braking circuit is established from one motor armature terminal, through field magnet winding! 6, the contactor 99, resistor 91 to the other motor armature terminal, and the motor is stopped quickly.

When it is desired to move the hoist hook other circuit may be traced from the enerv gized conductor 8, through contact fingers 25 and 63.bridged by segment 26conductor 64, actuating coil of line contactor 88 to the conductor 2. The actuating coil 66 of contactor 99 is connected in parallel to the coil 65. Energization of coil 65 operates to close the line contactor 88 and; the-simultaneous energization of coil 66 causes opening of contactor 99. A further circuit is established from segment 26, through contact finger 120, conductor 121, time-limit contactor 122,,actuating coil 125 of accelerating contactor 1.36 to the negatively energized conductor 17.

A circuit for the motor is now established from conductor 1 through contactor 145, auxiliary starting resistor sections 97 and 98, field magnet winding 76, operating coil 78 of the hoist brake 191, conduct0r79, accelerating contactors 136, 133, 127 and 110, line contactor 88 to the conductor 2.

From the right hand junction of resistor section 98, a motor circuit; is established through the motor armature, conductor 73,

braking resistor 91, conductor 147, contactor of the actuating coil 78 of the hoist brake 191.

The heavy current thus traversing the field winding and the actuating coil 78 insures that the hoist brake is raised and that the motor starts. Sincethe resistor sections in series with the field winding are shunted, the

voltage drop across coil 151 does not cause operation of relay 129. Attention is also called to the fact, for the firstlowering posit-ion, the shunts for the starting resistor sections were established by the energization of coils 16, 14:, 50, and 125 by circuits ezitend ing from the energized contact fingers 27 29, 38,45, and 120, respectively, and the c011- tact fingers 18 and 20 bridged by controller segment 19. Portions of the control circuit are maintained throughout the lowering positions by reason of the fact that contact fingers 18 and 20 are always bridged during the lowering operation.

Because of the fact that the starting resistor is shunted during initial stages of the lowering operation, none of the time-limit relays l1, 17 or 122 have operated to open circuit position. By completion of the motor circuit. control contactor 153 is caused to operate by reason of the circuit established from the one armature terminal, coils 102 and 104 connected in parallel and conductors 101 and 7 3 to the other armature terminal. Energization of coil 104: operates the contactor 153, whereas energization of coil 102, acting in opposition to coil 62, has no effect on contactor 106.

Since it is very desirable thatthe lowering speed be as high as possible when the load is not overhauling, means are provided for changing the time constant of the timelimit relays during the lowering operation. Closing of control. contactor 153 establishes a circuit from the energized segment 26 through contact finger 29, conductor 30, contactor 153, resistor 54, neutralizing coils 52, 53, 55, and 56 of the time-limit relays 122, 4:7, 11, and 23, respectively. The relative resistance values of resistors 51 and 5 1 is about three to one, or, it desirable, may be as high as two to one. The resistor 51 is shunted by the circuit just traced, and such shunting increases the demagnetizing effect of the coils 52. 53, 55, and 56, so that these time-limit relays, after'having been energized, operate much more rapidly than during the hoisting operation. As was previously suggested, the resistance value of the resistor 51, for this particular application, may be somewhere near 2400 ohms, whereas the resistance value of resistor154 may be somewhere near 800, ohms. However, I do not wish to limit .my-

self to any particular resistance value herein mentioned.

Movement of the controller to the second lowering position causes thedeenergization of coil 125 and the consequent insertionof resistor sec ion 80 in series with the field winding, thereby causing acceleration of the motor in the lowering direction.

The movement of the controller to the third lowering position causes deenergization or" the coil 50 of the accelerating contac- I tor 133 with the consequentinsertion of-resister section 81 in the field c1rcu1t. 'Inser tion ofthe' resistor section 81 causes ener gization of magnetizing coil 11 i and thus the opening of contactor 122. When the con-.

troller is moved to the fourthlposition, coil 44 of accelerating contact-or 127 1s deenergized and resistor 82 is inserted in the field circuit. By this operation, magnetizing coil 116 of time-limit relay or contactor el'Zis energized and the contactor opens. Movement of the controllerto the fifth lowering position causes the 1nterrupt1 on of the circuit through contact fingers 25 and 27: however, this operation does not, of it self, interrupt the circuit for the actuating coil 16 of the accelerating 'contactor 110 because a circuit still exists to this coil from. the positively energized conductor 8, through contact fingers 25 and 29-bridged by segment 26-conductors 30 and 31', contact members 32, conductor 14, coil 16, contactor 23, or through segment 19 to conductor 2. After the motor has'reached a predeterminedspeed, the cont-rolcontactor106 will operate to open contact members 32, thereby deenergizingthe coil 16, whereupon the resistor section 83 is placed in series withthe field winding 76. By this-latter operation; magnetizing coil 117 is energized'and, in consequence, contactor l1 is opened. Attention is called to the 'fact that contactor 85" is still closed, so that the resistorsection 160 is still shunted. How

ever, with the insertion of resistor sections to 83, inclusive, the IR-drop across actu- -ating coil 151 is suiiicient to'cause control contacto-r 129 to close; however, closure of contactor 129 does not establish actuating circuits for control contactors and 167.

lovement of the controller to the sixth lowering position causes the bridging of contact fingers 25 and 131 by segment 26, and the establishment of a circuit from contact finger 131, conductor 161, control contactor 129, conductor 162, actuating coil 164:, conductor 163 to conductor 2. .The actuating coil 166 for the auxiliary line contactor 167 is connected in parallel to boil 164. Energization of these circuits causes the shunting of resistor section 97 of the auxiliarystarting resistor and the shunting of resistor connected in series with the. field winding 76. In consequence, a very high-speed lowering of the hook is provided;

All'of the circuits just traced, and the sequence of operation for the lowering direction take place only when the load is nonve hauling, and it is desired to drive the hook down very rapidly. l/Vhen the load is overhauling, themotor,being driven by the overhauling load, operates as a series generator and prevents excessive lowering speeds for the load. If it isdesired to stop the 10W, ering' operation, the controller may be moved to the oil? position, or any one of the hoist positions, and the motor will be stopped by reason of the establishment of a dynamicbraking circuit through the armature and the starting resistor sections. Assuming that the controller is moved to the off position, circuits are established for the time-limit relay 23, actuating coil 16 of accelerating contactor 110, and since contact members 170 are closed by the previous energization of timelimit contactor 47, circuits are also established for the actuating coils 172 and 173 of control contactors 148 and 99, respectively. The energization of coil 172 merely aids the action of coil 142, since this control relay 148 is already in closed position, and energization of coil 173 insures that, for. the time being, contactor 99'remains open. The dynamicbraking circuit established for the motor may be traced from the armature, through conductor 73, braking resistor91, conductor 147, contactor 148, actuating coil 151 of control contactor 129, conductor 11.5, contactor 85, resistor sections 83, 82, 81, and 80, conductor 79, brake-operating coil 78, field winding 7 6 to the other armature terminal. Fig. 3 illustratesthis. dynamic-braking circuit. Coils 102 and 104 are also energized during dynamic braking. Energization of coil 104 insures that contactor 153 remains closed to connect resistor 54, having only about one third theresistance value of resistor 51, in circuit relation with the neutralizing coils 52, 53, 55, and 56. The time constants of the time-limit relays are, therefore, of a relatively lower value during dynamic braking.

Since magnetizing coil 10 is energized, the time-limit contactor 23 is open and the control circuit is maintained only through the contact fingers 18 and 20, segment 19. and conductors 17 and 21.

Since all of the magnetizing coils 10, 117, 116, and 114 are energized, the time- limit contactors 23, 41, 47, and 122 stand open, but movement of the controller to the oil position causes energization of coil 16 of the accelerating contactor 110. In consequence, magnetizing coil 117 is shunted by the operation of the contactor 110 to shunt resistor section 83. The time-limit relay or contactor 41, resistor 51 being shunted, closes after a very short predetermined interval of time. It should also be noted that the operation of contactor 110 establishes thedy1ia1nic bral*-- ing circuit shown in Fig. 4.

Closing of the time-limit contactor' 41 causes the energization of coil 44 of accelerating contactor 127 with the consequent shunt ing of resistor section 82 and the shunting of the magnetizing coil 116. This operation establishes the dynamic-braking circuit shown in Fig. 5. After a very short predetermined interval of time, time-limit relay 47 closes to thus effect the operation of accelerating con tactor 133, which shunts resistor section 81 and magnetizing coil 114 Time-limit relay 122 in turn ellects the shunting of resistor section 80. Two dynamic-braking circuits, similar to those shown in Figs. 4 and 5, having successively lower resistance values, may

thus be established.

From the foregoing, it is apparent that the resistance value of the dynamic-braking circuit gradually decreases and, since the field winding76 is in series with the dynamic bra-king circuit the excitation of the motor is correspondingly increased. The resistance value of the dynamic-braking circuit'thu's varies in inverse order to the field excitation, thus providing a graduated dynamic-braking effect which starts at a low value and rises, in a cumulative manner, to a very high value.

v T he dynamic-braking effect is, however, further increased, independent of thechanges in field excitation, by the provision of a lowresistance dynamic-braking circuit. After the operation of time-limit contact 47, con

tact members 17 O are interrupted to cause deenergization of coils 172 and 173, with the consequent opening of'contact 148 and the closing of contactor 99. r

The low-resistance dynamic-brahing circuit is shown in Fig. 6, and may be traced from one terminal of the motor, through conductor 73, braking resistor 91,0ontactor 99, and field winding 76 to the other armature terminal. By reason of this low-resistance dynamic-braking circuit and because of the fact that the hoist brake is also applied after the interruptionoi contact-or 148, the motor is brought to rest almost immediately, and accurate positioning of the load is thus accomplished. This last-stage low-resistance dynamic-braking circuit maybe established any time after opening of contact members 170, but the preferable design is such that this last circuit is established immediately after the closing of accelerating contactor 136. i

When the motor is'plugged from highspeed lowering, contact finger 28 is engaged long enough to cause energization of mag netizing coil 10 to operate contactor 23. The

motor thus comes to rest by dynami o braking,

- inasmuch as hoist direction. Obviously,

136 have time to open, thus placing t ing resistor sections in the motor circuit. The time-limit contactors 23, 41, L7, d 122, since the voltage on their magnetizing coils passes through Zero and then builds up in the opposite direction, do not operate, i. e, remain closed. During the transition, the contactor is obviously open for a predetermined interval of time, anc since contact fingers 18 and 20 are not bridged, the accelerating relays do not operate before the lapse of a predetermined interval of time, depending on the design and adjustment of time-limit contactor 23.

During movement of the comroller to plugposition, the main coils 66 and 142 of control contactors 99 and 1 18, respectively, are deenergized, and when the time-limit relays operate successively to open their re spective contact members and to close contact members 170, the control cont-actors M8 and 99 are not caused to operate, because coils 172 and 178 are comparatively weak, i. e., are merely holding coils once the main coils 66 and 142 have caused the contactors operate.

Assuming the controller is thrown to the sixth hoist position, contact fingers {33, t7, o8, 27, 45 and 120 are connected to the positive conductor 1. motor stops by th dynamic-braking effect set up, and, after a predetermined interval of time, contactor closes, coil 16 is energized and accelerating contactor 110 closes. This operation deenergizes coil 11', and, after definite time, contactor 41 closes. The closure of th s latter contactor effects energization of coil le and accelerating contactor 1 7 closes. By successive steps, like these discussed, accelerating contactor 136 is eventrilly operated to shunt the last starting re. stor section 80. The motor is now moving at full speed in the these same steps may be used to accelerate the motor from standstill to full hoisting speed by manual control as hereinbe'forc discussed.

By moving the controller to the sixth? hoist position initially, the manual control of acceleration is not used. From these considerations, it is apparent that the motor may not be forced to accelerate faster than a predetermined maximum rate, as determined by the design and adjustment of the several timelimit relays. The motor may, however, be accelerated at a slower rate. than that for which the time-limit relays are set. The circuits and sequence of operation above discussed, when a slow movement of the controller in the hoist direction was considered, show just such comparatively slow manual control.

It'will be recognized that a dynamic-braking resistor of low ohmic value is desirable, braking is usually accomplished However, an operator will the accelerating contactors 110. 127, 133, e s

from slow speed.

frequently attempt to stop a heavy lowering load more quickly than should be attempted,

thereby causing theload and control system to handle excessive currents and produce de-"' structive arcing of the motor commutator and the control contactors. Undue mechanical stresses are also induced. In the present system, the equipment is automatically protected, as described, by the provision of a graduated dynamic-braking efi'ect and by the provision of a control system wherein any false movement of the controller cannot injure the equipment. The manual and automatic control systems further protects the equipment by the provision of the same successive dynamiobraking circuits, regardless. of whether a false movement of the controller is made or voltage on the system fails.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention there'- tore, is not to be restrictedexcept insofar as is necessitated by the prior art and by the spirit of the appended claims.

. I claim as my invention:

1. The combination with an electric motor having a field-magnet winding, a dynamic-braking circuit cooperating with said motor, for producing a dynamic-braking eitfect in the motor, means for gradually and proportionally decreasing the ohmic resistance in the dynamic-braking circuit and the field-magnet winding, respectively.

2. The combination with an electric motor having a field-magnet winding, a dynamicbraking circuit for connection to the motor and including the field-magnet winding, means for progressively varying the field eX- citation during dynamic-braking, a' second dynamic-braking circuit having a resistance value lower than said first-named dynamicbraking circuit, and means for establishing the low-resistance dynamic-braking circuit for the motor during variations of the field excitation.

3. The combination with an electric motor having a field-magnet winding, a variable resister, dynamic-braking circuits for'connection in series with the field-inagnetwinding, the motor and the variable resistor, means for successively varying the resistance value of the resistor whereby the field excitation is gradually increased and whereby the dy namic-braking circuits are resolved into'a plurality of successively established dynamic-braking circuits having progressively lower resistance values, and a dynamic-hra king circuit, having a lower resistance value than the respective resistance-values of the dynamic-braking circuits includingthe vari able resistor for connection to the motor durin the establishment of the plurality o p dynamic-braking circuits.

1. The combination with an electric motor ino having a field winding, a resistor having a plurality of sections for connection in series with the field winding, dynamic-braking cir- 'cuits including the resistor, the motor, and

' having a field-magnet winding, a starting resistor, dynamic-braking circuits for connection tothe motor and including the starting resistor, and time-limit relays for efiecting the shunting of the starting resistor sections at a predetermined rate during starting of the motor means for changing the characteristics of the time-limit relays for short-circuiting the starting-resistor sections at a more rapid predetermined rate during dynamic-braking.

6. The combination with an electric motor having field windings and dynamic-braking circuits the resistance value or" which may be Varied, means for simultaneously and proportionallyiiicreasing the field excitation of the motor and decreasingtlie resistance value of the dynamic-braking circuits over a predetermined range during a predetermined interval of time.

7. The combination. with an electric motor having field windings and an accelerating resistor, of dynamic-braking circuits for said motor, means associated with said resistor for controlling the fieldexcitation of said motor during dynamic braking and means including time-limit relays operable after the lapse ofa predetermined interval of time and a manual controller for controlling dynamic braking in accordance with the operation of'the time-limit relays andin accord ance with the electrical connections set up by the manual controller.

8. The combination with an electric motor.

having a field-magnet winding, a variable resistor disposed to be connected in series circuit relation. with said winding, a dynamicbraking circuit cooperating with said motor for producing a dynamic braking effect in the motor, a variable resistor disposed to be con nected in series circuit relation with said dynamic-braking circuit, and means for simultaneously varying the resistance values of said variable resistors.

9. The combination or" an electric motor having a field-magnet winding, a variable impedance, a dynamic-brakingcircuit including thevariable impedance and the field magnet winding, means for decreasing the impedance during dynamic braking, means for interrupting the dynamic-braking circuit through said variable impedance, and means for establishing another dynamic-braking circuit, having a lower impedance than the lowest impedance Value of said variable impedance before the circuit through the variable impedance of the first dynamica motor field winding circuit the resistance value of which may be varied in predetermined steps over a predetermined range, a dynamic-braking circuit the resistance value of which may be varied in predetermined steps over a predetermined range and means for simultaneously and step-by-step varying the respective resistance values of the field winding circuit and the dynamic-braking circuit. I

12. The combination of an electric motor,

a motor field winding circuit the resistance value of which may be varied in predetermined steps over'a predetermined range, a dynamic braking circuit the resistance value of which may bevaried in predetermined steps over a predetermined range, resistance varying means for simultaneously and step: by-step varying the-respective resistance values of the field winding circuit and dynamic braking circuit, an auxiliary dynamic-braking circuit having a resistance value lower than the resistance value of the first named dynamic-braking circuit, and means for connecting the auxiliary dynamic-braking circuit to said motor at any selected time during the operation oi. said resistance varying means.

18. The combination of anelectric motor having a field winding and a. dynamic braking circuit the respective resistance values of which may be varied, means to initiate the lynamic braking operation, time-limit means operable'at predetermined time intervals after the initiation of the dynamic braking operation for proportionally varying the resistance values 01 the field windin and d namic-.

braking circuit, respectively, and means for disproportiona-lly varying the resistance value of the dynamic braking circuit.

14. In a control system for an electric motor, in combination, a motor, a field. winding for the motor, a dynamic braking circuit, including the field winding, disposed to be connected to cooperate with the motor to produce a dynamic-braking effect in the motor, and means for gradually and proportionally decreasing the ohmic resistance in the dynamic-braking circuit whereby the excitation in the field Winding increases during dynamic braking.

15. The combination with an electric motor having a field winding, a variable resistor disposed to be connected in series circuit rela tion with said Winding, a dynamic-braking circuit cooperating With said motor for pro ducing a dynamic braking effect in the motor, a variable resistor disposed to be connected in series circuit relation With said dynamic-braking circuit, and means operable on both of said variable resistors for simultaneously varying the resistance values of said variable resistors in the same direction.

16. The combination of an electric motor, a motor field Winding circuit the resistance value of Which may be varied in predetermined'steps over a predetermined range, a

ing resistor and time-limit relays for controlling these contactors.

In testimony whereof, I havehereunto subscribed my name this 15th day of May 1930,

WILLARD G; ooojK,

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