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United States Patent Ries Et Al United States Patent Ries et al. (15) 3,656,038 (45) Apr. 11, 1972 54 FIELD WEAKENING CIRCUIT FOR A Primary Examiner-Bernard A. Gilheany SERES FELD MOTOR Assistant Examiner-W. E. Duncanson, Jr. (72) Inventors: Robert G. Ries, Milwaukee; Robert C. Attorney-Harold J. Rathbun and William H. Schmeling Montross, Mequon, both of Wis. (73) Assignee: Square D Company, Park Ridge, Ill. (57) ABSTRACT In a control system for a direct current series motor, the con 22 Filed: Dec. 4, 1970 tacts of a single throw reed relay are responsive to motor cur (21 Appl. No.: 95,047 rent through a winding in a motor supply conductor to selec tively control the application of a motor field weakening shunt resistor by a switching circuit. The charging of a timing (52) U.S. Cl............................................... 318/139,318/251 capacitor delays action of the switching circuit to prevent 51) int.C. ......................................................... H02p 5/16 motor "hunting". The opening of the reed relay contacts 58) Field of Search.................. 3181246,251,332, 139,527, under low motor load conditions permits charging of the 318/528,536,537,359; 317/13, 16 capacitor and activating of the switching circuit to place the shunt resistor across the motor field for high speed operation. 56) References Cited Under increased motor load, the reed relay contacts close and the capacitor discharges to time the removal of the shunt re UNITED STATES PATENTS sistor from across the field. A by-pass transistor by-passes the 3,384,799 5/1968 Thiele.................................... 318/251 reed relay contacts preventing activation of the switching cir 3,382,409 511968 Assow et al.............................. 317/16 cuit until the motor has reached its maximum unweakened 3,360,705 12/1967 Morris.......... ...318/251 field speed and the relay is provided with shunt windings to 3,230,437 / 1966 Cappello................................ 318/332 prevent"chattering' due to wavering motor current. 13 Claims, 1 Drawing Figure 7c alt6 SCR 4. T 29 4. Patented April 11, 1972 3,656,038 22 SCR 4. t" 29 2 4. INVENTOR. ROBERT G. RES BY ROBERT C. MONTROSS 3,656,038 1. 2 FIELD WEAEKENING CERCUT FOR ASERIES FELD MOTOR by the current flow through the motor, is used to control the The present invention relates to speed control systems for selective connection of a resistive shunt across the series field. direct current series motors of the type useable as traction Another object of the present invention is to provide timing drive means for battery operated industrial trucks and the like, means to delay the connection and removal of the resistive and more particularly to a circuit which will control the appli shunt for a time sufficient to prevent motor "hunting'. cation and removal of a shunt resistor across a series field Further objects and features of the invention will be readily winding in accordance with the magnitude of the load on the apparent to those skilled in the art from the specification and motor. appended drawing illustrating a preferred embodiment, in To provide economical and efficient industrial trucks, the 10 which a schematic wiring diagram of a direct current motor size of the motor must be minimized and the power output of control circuit incorporating the features of the present inven the motor translated through a step down gear train to provide tion is shown. a relatively large torque at the driving wheels of the truck. In the drawing, a source of direct current such as a battery Thus industrial trucks are generally limited to slow speed 11 has its positive terminal connected to a conductor 12 and operation so that they can transport loads which are great in 15 its negative terminal connected to a conductor 14. The battery comparison with the size of the traction motor. However, it is 11 supplies direct current for a motor 15, preferably a series at times desirable to be able to propel a truck faster than at wound motor, having a series field 15F and an armature 15A. load carrying speeds; such an instance may occur when a A field weakening resistor 16 and normally open contacts 17c truck is required to move across a level surface of a warehouse of a field weakening relay 17 are serially connected with each in an unloaded condition to a load pick-up point. It is well 20 other in parallel with the field 15F. A conductor 19 connects known that the speed of a series motor increases when the the motor 15 through a winding or current loop 21 and strength of its field is reduced, a technique commonly known through normally open contacts 20c of a shorting relay 20 to as field weakening. Field weakening may be accomplished the conductor 14. The current loop 2 preferably comprises a either by connecting a resistive shunt across the field winding, single winding formed in the conductor 19 or may be formed by the use of a tapped field winding, or by splitting the field 25 by passing the conductor 19 through a clip formed of mag winding of the motor. The circuit according to the present in netic material, as is disclosed in the concurrently filed applica vention may be used successfully with any of these arrange tion No. 95,163 in the name of Robert G. Ries and assigned to ments. the assignee of the present invention. The current loop 21 Should the load on the motor be increased while the motor serves as the primary winding for a relay means which is is operating under conditions wherein the field of the motor is 30 preferably a single pole single-throw reed relay comprising the weakened, e.g., the unloaded truck is driven up an incline, an current loop 21, normally open contacts 21c and a pair of aux increase in motor current will accompany the increased load iliary windings 21a and 21b. An SCR control 22 is connected and cause the motor to operate inefficiently, overheating and serially with the motor 15 and in parallel with the shorting using excess battery current. The current may even rise to a contacts 20c and current loop 21. A preferred SCR control 22 value which will severely damage the motor or other electrical 35 is described in U.S. Pat. No. 3,361,921, issued to Robert C. components in its power circuit. Montross and John P. Cooper on Jan. 2, 1968. Therefore, it is necessary to provide a circuit which is capa A winding 20w of the shorting relay 20 and a silicon con ble of automatically applying a shunt resistor across the motor trolled rectifier (SCR) 23 are serially connected between the field during high-speed, light-load operation and removing the conductors 12 and 14, and a variable resistor 24, the auxiliary shunt resistor upon application of a large load. Such a circuit 40 winding 21a of the reed relay and reed relay contacts 21c are is disclosed in application Ser. No. 819,925, filed Apr. 28, also serially connected between the conductors 12 and 14. A 1969, by Robert G. Ries and Kenneth W. Retzer and now U.S. switch 29 is interposed in the conductor 14 between the junc Pat. No. 3,590,352, issued June 29, 1971. The prior circuit, tion with the contacts 20c and the junction with the SCR 23. A however, requires the use of a double-throw reed relay which 45 transistor 30 has its base connected through a biasing resistor is expensive and not readily available commercially. Also, 31 to the anode of the SCR 23. The transistor 30 is connected double-throw reed relays are susceptible to hysteresis in that across the reed relay contacts 21c with the collector of the they are capable of becoming "hung in the middle' during transistor 30 connected to the auxiliary winding 21a side of transfers from one contact to the other. the reed relay contacts 21c and the emitter connected to the In prior art field weakening circuits, a change in motor cur 50 conductor 14 side of the reed relay contacts 2ic. rent beyond a predetermined value causes the immediate ap A timing capacitor 32 is connected between the conductor plication or removal of a field weakening resistor. Further, if 14 and a junction 33. The junction 33 is connected through a the truck is being accelerated without a load, the field rectifier 34 and a resistor 35 to the conductor 12, through a weakening resistor in such prior circuits is applied immediate resistor 36 to the conductor 14, and through a biasing resistor ly upon termination of normal acceleration thereby not 55 37 to the base of a transistor 38. A rectifier 39 is connected providing sequencing time between acceleration stages. between the anode of the rectifier 34 and the auxiliary coil When the shunt resistor has been applied in response to a 2ia side of the reed relay contacts 21c. decrease in motor current, the weakened field reduces the The collector of the transistor 38 is connected to the con generated counter-voltage of the motor (CEMF) and this ductor 12 through a resistor 40 and, by a parallel path, results in an immediate increase in motor current. This cur 60 through the auxiliary winding 21b of the reed relay and a vari rent surge results in a torque increase that accelerates the able resistor 42. The emitter of the transistor 38 is connected motor to a new higher speed. In prior art devices, this current to a threshold voltage responsive means such as a silicon uni surge generally activates the circuit for removing the shunt re lateral type switch (SUS) 44 which is in turn connected to the sistor from the field as if it had been caused by an increased base of a transistor 45.
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