US2456950A - Electric hoist control system - Google Patents

Description

Dec. 21, 1948.

Filed Dec. 27, 1947 G. E. KING ELECTRIC HOIST CONTROL SYSTEM Q Torque WITNESSES: INVENTQR 24 4 eory'ei'. A409,

CJWM 7%M A CRNEY 2 Sheets-Sheet l Dec. 21, 1948. 5 K N 2,456,950

ELECTRIC HOIST CONTROL SYSTEM Filed Dec. 27, 1947 2 Sheets-Sheet 2 WITNESSES: lNVENTOR 5Q Georg-e5 Axhg ATTORNEY Patented Dec. 21, 1948 ELECTRIC HOIST CONTROL SYSTEM George E. King, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa... a corporation of Pennsylvania Application December 27, 1947, Serial No. 794,165

6 Claims. 1

My invention relates to electric control systems for hoisting equipment, such as used on cranes, elevators, lift-bridges, lifting or winch machinery for marine purposes, mine hoists and the like.

Such hoisting equipment must satisfy various operating conditions which, for special applications, may involve an appreciable number of diversified requirements. A hoist drive for cranes, for instance, may have to satisfy the following requirements.

i When hoisting at lowest speed, the speed should be very low at minimum or no load and the stalled torque should be relatively high. The maximum hoisting speed at no load should not exceed a predetermined value; and in the lastpoint hoisting position of the master controller, the stalled torque should remain below an adjusted overloa limit. 2

When lowering, a proper positioning of the master controller should cause the motor either to produce drive-down torque or to retard the load by up-torque, depending upon whether or not the load is overhauling. The maximum lowering speed at no load should be a desired predetermined value, and the'speed when lowering under load for any selected positioning of the master controller, should be the smaller the higher the load.- It is also desired that the control system, when hoisting maximum load, remains capable of providing suflicient braking torque for securing a reliable lowering of the same load. On the other hand, the control system should not cause stalling during lowering performance. I

' The system should secure a reasonably safe perforniance even if the adjustment of the master controller is changed more rapidly thandesired, and should brake and stop the motor in the event of power failure.

In order to satisfy, within a single hoist coritrol system, many or all of these requirements, the known systems must be given a rather complicated design composed of a large number of circuit elements or circuit sections and involving a large number of contactors and relays. Another known system provides the desired characteristics with the aid of an exciter generator of special design which has two individually excited magnetic flux paths with crossed flux axes. The output voltage of such a cross-flux exciter has always the same polarity regardless of the polarity of excitation applied to the control field wind- In contrast thereto, it is an object of my invention to devise, hoist control systems that permit achieving many or even all above-mentioned desiderata with the aid of a comparatively very simple control system which involves a minimum of relay and other auxiliary equipment, and which does not require using cross-flux excited or other special dynamos thus affording a less costly design and simple maintenance requirements.

According to one feature of my invention, I provide a Ward-Leonard type drive whose generator field'and motor field are varied in inverse relation to each other by an exciter generator of normal voltage characteristic, i. e. whose flux has symmetrical distribution and varies in the same manner in all field poles and whose output voltage has a polarity determined by that of the exciter control field as is the case in conven tionally excited direct-current generators; and I interconnect the motor, generator and exciter in the manner apparent from the followin description of the embodiments shown in the drawing.

Figure 1 represents the circuit diagram of a crane hoist drive capable of satisfying all abovementioned requirements;

Fig. 2 is an explanatory coordinate diagram of speed-torque characteristics obtainable with a system according to Fig. 1; and I Figs. 3 and 4 show circuit diagrams of two other embodiments, respectively.

According to Figure 1, the hoist drum I of a crane is connected to the armature 2 of a hoist motor M whose field winding is denoted by 3.

Armature 2 is connected in a loop circuit with the stant-speed motor for driving the exciter E in-.

dependent of the generator G. The exciter E has two field windings I I and I2 which operate in differential relation to each other. The field winding ll (pilot field winding) is connected across a tapped-01f portion of the rheostat I and hence is excited in proportion to the load current flowing in the loop circuit. A valve or rectifier may be provided at 20. The field Winding l2 (pattern field winding) receives excitation in series with an adjusting rheostat l3 from the buses X and Y of a suitable source of substantially constant direct-current voltage.

Field winding of the main generator G is connected across a resistor [4 which is associated with resistors l5 and H5. The resistance ar rangement is controlled by a hoist contactor H and a lower contactor L. Contactor H has two contacts I1 and I8 controlled by a coil 19, and contactor L has two contacts 2i and 22 under control by a coil 23.

A master controller C with an "of!" position, four sequential hoist positions, and four sequential lower positions, is equipped with con- 7 tact segments 24 and 25 for controlling the respective circuits of contactor coils l9 and 23. Two other segments 26 and 21 serve to control a group of series-connected resistors denoted by 28, 29, 30 and 3|. The master controller is shown in developed form, as customary. A number of leads appertaining to the motor control system are denoted by 32 through 36 and will be referred to in the description of the operation given s in a later place. Safety limit switches and other auxiliary devices, including a magnetically released brake, may be used as customary but are not shown in the drawing because they do not form part of the invention proper.

In the above-described system, the motor M forms part of the variable voltage or Ward- Leonard system in which the controlling field excitation ior the main generator G is provided by the exciter generator E in a circuit which includes the operator controllable resistors 28, 29, 30 and 3| in series connection. The voltage generated by the exciter depends on the differential value of the control excitation provided by the field windings H and I2. The excitation of the field winding l2 remains constant at an adjusted value during the operation of the system. Since the field winding II is excited proportionally to the armature current of the Ward-Leonard system, the pattern field of the winding I2 is unopposed under no-load conditions of the Ward- Leonard system. The rheostat i3 is so adjusted that under these conditions the voltage generated.

in the exciter armature 8 has the desired maximum value. The rheostat 1 is adjusted so that,

at maximum desirable overload of the hoist motorM, the ampere turns of the pilot field winding H equal the ampere turns of the pattern field winding l2 so that the armature voltage of the exciter E is zero.

Since under no-load conditions the field excitation of the exciter E is constant, the voltage effective across the main generator field winding 5 then depends only upon. the total resistance value oi the resistor group 28 to 3|. speed of the hoist motor M under no-load conditions is determined by the selected position of the master controller C.

The motor field winding 3 is connected across the constant voltage mains X and Y in seriesopposed connection with the exciter armature 8 so that the exciting voltage for field winding 3 is equal to the difference between the constant voltage of mains X and Y and the variable exciter voltage. The field circuit is designed so that with maximum exciter voltage the motor M wil operate at twice full field speed. At the exciter voltage obtained with full load on the hoist motor M, the motor field winding 3 is at full field value, and at maximum overload the motor field is stronger than full field to provide maximum torque and slowest operating speed.

When hoisting a load, the exciter voltage is a function of the load, and the circuit of the Ward-Leonard system is adjusted so that the Hence, the

motor M will stall when the desired maximum load is exceeded. When lowering a load, the load will cause the motor M to regenerate so that the load current is in the same direction as when hoisting. The exciter voltage is then again a function of the hoist motor load.

A family of speed torque characteristics for hoisting operation in the four hoist positions of the master controller is shown in Fig. 2 and denoted by Hl, H2, H3 and H4, respectively. A corresponding family of characteristics for lowering operations is represented by curves Ll, L2, L3 and L4 in Fig. 2. The shape of these curves can be altered within limits by changing the design of the circuit elements, for instance by using an exciter generator E with a difierently shaped magnetic saturation characteristic.

A detailed description of hoisting and lowering operations follow. In the off position of the master controller C, with mains X and Y energized and the motor generator set G-E running at the proper speed, motor M remains at rut because the armature voltage of main generator G is zero since the main generator field windin 5 is not excited.

When the master controller is placed in first hoist position, contactor H picks up and closes contacts 11 and i8. Resistor l4 andv the parallel connected field winding 5 are now energized in the circuit:

30' and 3|. Motor field winding 3 is now con-- nected in the circuit:

The motor now operates in the hoisting direction. at low speed, for instance in accordance with r the characteristic HI in Fig. 2.

-When the master controller C is moved. to subsequent hoist positions,- the field circuit (t) of the main generator remains the same except that the resistors 29, 30 and 3i are progressively shorted out of the circuit. Hence, the voltage across resistor Hi and field winding 5. is progressively increased for producing a higher voltage of the main generator G. The field circuit (2) for motor M remains the same. The motor now operates substantially in: accordance with the hoist characteristics H2, H3 or H4 (Fig. 2) depending upon the selected position of the man-- ter controller.

When setting the master controller to first position lowering, contactor L picks up and connects the resistor [4 with field winding 5 across the exciter armature B in the circuit:

This generator field circuit is similar to circuit (I) except that the polarity of connection. for field winding. 5 is reversed and that the resistor I5 is replaced by the resistor it which is differently rated to adjust a reduced. field excitation for generator G as desirable for lowering operation. As mentioned above, the resistance calibration is now such that the motor operates regeneratively under underhauling load that the load responsive control of the exciter generator E leads to the desired lowering characteristics, such as those exemplified by curves LI, L2, L3 and L4 in Fig. 2.

The embodiments illustrated in Figs. 3 and 4 are represented by simplified diagrams in which a master controller C is merely shown as a stepwise adjustable rheostat while the contactors H and L are indicated only by the appertaining contacts, it being assumed that the two contactors are controlled by the master controller in substantially the same manner as in the system shown in Fig. 1 and described in the foregoing. In Fig. 3, the armature of the motor to be controlled is denoted by 40. The motor has two mutually difierential field windings 4| and 42. Field winding 42 is connected across the positive bus X and the negative bus Y of a suitable source of direct current of substantially constant voltage. Field winding 42 provides main excitation for the motor and its voltage is adjusted by a resistor 43' to provide the full rated motor field when the excitation of field winding 4| is zero. Hence, field winding 4| merely provides a subtractive efiect which, when in full force, reduces the resultant field excitation to a predetermined minimum value. The armature of the generator G energizing the motor armature is denoted by 44, and the appertaining main field winding by 4'5. The common armature circuit of motor and generator includes a series resistor 41. The exciter E whose armature is denoted by 48 is equipped with a single field winding 49. Exciter armature 48 is connected to the main generator field winding 45in series with the adjustable resistor 50 of the master controller C and in. parallel with a resistor5l. The polarity of connection is controlled by the hoist and lowering contactors H and L. The control field winding 49 of exciter E is connected in the output branch of a differential control circuit which has two pairs of input terminals. One pair of terminals is attached to the buses X and Y in order to provide a normally constant control voltage across the tapped-off portion of a voltage-dividing rheostat 52. The other pair of terminals is attached across the resistor 41 in order to provide a variable control voltage upon the current flowing in the common armature circuit of motor and generator. The differential value of the two control voltages is effective across the control field winding 49. A valve or rectifier 53 is inserted between the resistor 41 and the control field winding 49.

When the armature current of motor M is zero, the constant control voltage from rheostat 52 is alone effective to excite the control field winding 49. Consequently, the exciter now receives maximum field excitation and generates maximum voltage across its armature 48. This voltage is applied across the field winding 4| of motor M. Consequently, field winding 4| has maximum opposing efiect relative to the main field winding 42 of the motor, and the resultant motor field has a minimum value. At the same time, the exciter voltage is applied to the field winding 45 of generator G'so that the generator field excitation for any selected adjustment of the master controller C is a maximum. On the other hand, with a high current flowing in the rheostat 52 so that the excitation Of field wind ing 49 is reduced. The voltage generated by the exciter armature 48 is also reduced so, that the motor field winding 4| receives less excitation. As a result, the resultant field excitation of motor M is increased. At the same time, the excitation of generator field winding 45 is reduced. Consequently, themotor field and generator field are varied by the exciter E in inverse relation to one another. In this respect, the performance of the systemresembles that of Fig. 1 so that the same functions and advantages are obtained.

In the embodiment of Fig. 4, the motor M has a field winding 43 excited from the constant voltage buses X and Y and also from the armature 48 of the exciter E. Similar to the embodiment oi! Fig. l, the exciter armature 48 is series-connected between the motor field windin 43 and the buses X, Y. However, in contrast to Fig. l, the polarity of connection of the exciter in Fig. 4 is such that the exciter voltage acts in a cumulative or boosting sense relative to the constant voltage across the buses. That is, the voltage applied to the motor field winding 43 is the sum of the exciter and bus voltages. The armature 40 of motor M is energized from the armature 44 0f the generator G in series with a resistor 41 across which the control field winding 49 of the exciter E-is attached through a valve or rectifier 53. Generator G has two field windings 45 and 46 which act in opposition to each other. Field winding 45 is connected across a resistor 5| and in series with the resistor 50 of the master controller C from the exciter armature 48 under control .by contactors H and L; Field windin 46 receives constant excitation from buses X and Y through a rheostat 54.

When in the control system according to Fig. 4 the current in the armature circuit of motor and generator is zero, the field excitation of'the exciter E and the output voltage of the exciter are also zero. Then, the constant excitation from buses X and Y is alone effective in the field winding 43 of motor M in order to provide a minimum field of a desired value. At the same time, the excitation of generator field winding 45 is zero so that the field winding 46 isalone effective to provide the generator with maximum field excitationof a desired value adjusted by the rheostat 54. On the other hand. when the current in the armature circuit of the motor has a high value, the exciter receives high field excitation and generates a correspondingly high exciter voltage. This voltages increases the excitation of motor field winding 43. At the same time, the excitation of generator field winding 45 has a finite value in opposition to field winding 46 so that the resultant generator field is reduced. It will again be recognized that despite difierences in design, the motor field and generator field are controlled by the exciter in inverse relation to each other and in substantially the same manner as in the embodiment of Fig. 1 so that a similar performance is achieved, i

Referring to all three illustrated embodiments, it should be noted that the rectifier or valve in the-control field circuit of the exciter may be omitted if the invention is used for hoists that operate only under overhauling load conditions when lowering. As mentioned above, the direction of current fiow in the motor armature circuit during overhauling load, i. e. when motor M is regenerating, is the same as during hoisting performance so that the control field winding of the exciter is excited by voltage of the same polarity even" it no valve or rectifier is inserted. It the guano hokt is called upon to lower a load under drivedown torque, and it it is desired to control the speed torque characteristic of the drive-down perlormanoe in a manner similar to that desired for hoisting operations, a rectifier must be inserved in order to prevent the control field of the oiuclter from reversing its polarity.

Speed-torque characteristics of the type exemplified by Fig. 2 and obtained in systems according to the invention, are predicated upon operating the exciter E within a properly chosen rangeot its saturation characteristic. When the resultant control excitation of the einciter has its rated maximum value, the operating point oi the waiter should lie within the saturated portion :beyond the substantially linear section of the magm1 characteristic, the particular point to be selected in accordance with a desired shape or curvature of the speed-torque characteristics. The proper selection of the operating point at foil control field excitation is made by accordingly tilting the ampere-turns of the field windings or the applied field voltages or the resistance rating the field rheostats.

It desired, the inverse field regulation of motor and generator can be made dependent upon the occurrence of a motor armature current above a predetermined minimum value. To this end, amltnge limiting device may be inserted in the control 'field circuit of the excitcr. As a matter 0! fact, the above-mentioned valves or rectifiers (20 in Fig. 1, 53 in Figs. 8 and 4,) function also 18811211 limiting devices. That is. since these recflflers conduct current only when the voltage across their respective electrodes exceeds a threshold value, an appreciable excitation of the exciter control field occurs only when the current in the motor armature circuit exceeds a finite minimum value. Another possibility alforded by my invention of modifying the control effect at armature currents below a given minimum value, consists in the use of an exciter whose field poles are provided with :saturable magnetic shunt mths as disclosed, for instance, in the copending ape pliuatlon of J. G. Ivy, Serial No. 532,370, assigned to the assignee of the present invention. Such exciters may be used in any of the illustrated ems; bodiments of my invention instead of the excite: E without change in the circuit connections shown in the drawing and described in the foregoing. Exciters with saturably shunted field pole-s suppress the generation of an exciter volt.- ue at low control field excitation until the excitntion, and the current which causes the excitation, exceed a predetermined value.

The above-mentioned modifications will sufilce to show that my invention is capable of being altered in various respects, and it will be obvious to those skilled in the art that the invention can be unlbodied by circuit designs other than those specifically disclosed, without departing from the essential features of the invention corset forth in the claims annexed hereto.

I claim as my invention:

1. A hoist control system, comprising a. direct-' current hoist motor having an armature and field means, a main generator having an armature cireuit connected with said motor armature to pro-1 vide variable voltage therefor and having fieldmeans for controlling said voltage, on enciter generator of symmetrical field distribution and field-controlled output polarity having an emitter armature connected to said main generator field means and to said motor field means, resistance loans and appertaining selective master control means connected between said exciter armature :andsnid main generator field means for selective- 1y varying the proportion of the exciter voltage applied to said field means of said main generator and motor respectively, .a resistor series-connected in said armature circuit, a. control circuit connecting said exciter field means across said resistor for controlling said exciter generator in dependence upon the current flowing in said armature circuit, a rectifier disposed in said control circuit for maintaining the exciter output voitage at a given polarity irrespective of the flow direction of said current, direct-current supply means of substantially constant voltage connected to said motor field means and having the polarity of connection required to cause an increase in resultant excitation of said motor field means with increase of said current, circuit means connecting said current supply means with the field means of one of said generators and having the polarity of connection required for causing a decrease in resultant excitation of said main generatorfield means with increase of said current.

2. A hoist control system. comprising a directcurrent hoist motor having an armature and field means, a main generator having an armature circuit connected with said motor armature to provide variable voltage therefor and having a main field winding for controlling said voltage, an excite: connected to said main field winding to provide excitation therefor and having a pattern field winding and a pilot field winding disposed in differential relation to each other to produce I. single resultant flux, a resistor series-connected in said armature circuit, said pilot field winding being connected across said resistor to be controlled in. accordance with variations of the current flowing in said armature circuit, current supply means of substantially constant voltage connected to said pattern field winding for providing normally constant excitation therefor, resistance means disposed between said exciter and said generatorfield Winding, selective master control means connected with said resistance means for controlling them to selectively vary the excitation'oi said generator field winding, said excitcr and said current supply means being connected in mutually diflerential relation to said motor field means.

3. A hoist control system, comprising a directcurrent hoist motor having an armature and field means, a main generator having an armature circuit connected with said motor armature to provide variable voltage therefor and having a field winding for controlling said voltage, a regulating exciter connected to said generator field winding to provide excitation therefor and having a pattern fieldwinding and a pilot field winding 10pposing each other, current supply means of substantially constant voltage connected to said pattern field winding for providing normally constant excitation therefor, resistance means series-connected in said armature circuit and attached across said pilot field winding to control said pilot field winding in accordance with the current-flowing in said armature circuit, said resistance means having a resistance value rated so that said pilot field winding and said pattern field winding balance each other at a desired minimum overload value o fsa id current, resistance means series-connected between :said exciter and said generator field winding, selective master control means-connected with said resistance means for control-ling them to selectively vary the excitation of said generator field winding, said exciter and said current supply means being connected in mutually difierential relation to said motor field means.

4. A motor control system, comprising a directcurrent motor having an armature and a separately excited field winding, a main generator having an armature circuit connected with said motor armature to provide variable voltage therefor and having a field winding for controlling said voltage, a regulating exciter connected to said generator field winding to provide excitation therefor and having a pattern field winding and a pilot field winding opposing each other, current supply means of substantially constant voltage connected to said pattern field winding for providing normally constant excitation therefor, a resistor series-connected in said armature circuit and attached across said pilot field winding to control said pilot field winding in accordance with the current flowing in said armature circuit, said resistor having a resistance value rated so that said pilot field winding and said pattern field winding balance each other at a desired minimum overload value Of said current, resistance means series-connected between said exciter and said generator field winding, selective master control means connected with said resistance means for controlling them to selectively vary the excitation of said generator field winding, said exciter and said current supply means being connected in series-opposition to said motor field winding for differentially exciting said latter winding, and said motor field winding having ampere-turns rated to provide full motor field excitation when the voltage of said exciter substantially corresponds to rated full-load current in said armature circuit.

5. A motor control system, comprising a directcurrent motor having an armature and field means, a main generator having an armature cir cuit connected with said motor armature to provide variable voltage therefor and having a main field winding for controlling said voltage, an exciter connected to said main field winding to provide excitation therefor and having a control field winding, current supply means 01 substantially constant voltage, a differential control circuit having two pairs of input terminals and a diflerential output branch, one of said pairs of terminals being attached to said armature circuit to provide a variable control voltage in dependence upon the current flowing in said armature circuit, said other pair of terminals being attached to said current supply means to provide a normally constant control voltage, and said output branch including said control field winding for exciting it in accordance with the differential value of said two control voltages, resistance means disposed between said exciter and said generator field winding, selective master control means connected with said resistance means for controlling them to selectively vary the excitation of said generator field winding, said exciter and said current supply means being connected in mutually differential relation to said motor field means so that said exciter varies the excitation of said motor field means and of said main field winding in inverse relation to each other.

6. A motor control system, comprising a directcurrent hoist motor having an armature and field means, a main generator having an armature circuit connected with said motor armature to provide variable voltage therefor and having two mutually difi'erential field windings for jointly controlling said voltage, an exciter having an armature connected to one of said generator field windings to provide excitation therefor and having a control field windin connected to said armature circuit for varying said excitation in dependence upon the current flowing in said armature circuit, current supply means of substantially constant voltage connected to said other generator field winding, said exciter armature and said current supply means being connected to said motor field means in cumulative relation to each other relative to the resultant excitation of said motor field means.

GEORGE E. KING.

No references cited.

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