Patent GB-625565-A (GB)
Title: Improvements in and relating to the speed control of direct current motors by varying the energising of the shunt field
IPC: H02P7/298
Filed: 1945-07-06
Abstract: 625,565. Control of D.C. motors. MOZZANINI, G. G., and LEBERTRE, M. July 6, 1945, No. 17317. Convention date, Dec. 15, 1941. [Class 38(iii)] In a system of field control for a reversible D.C. motor which may drive a reciprocating device such as the platen of a planing machine, or an electric lift or hoist, starting is initiated with a resistance included temporarily in the shunt field circuit, in series with the rheostat to give a sub-maximum flux to obtain rapid acceleration, and at the same time full excitation is applied to the series field which is subsequently reduced either to zero to give a complete shunt effect, or to maintain a slight compound effect, the subsequent braking at the end of a stroke being regenerative until the motor reaches a speed corresponding to that for the predetermined sub-maximum field, whereupon the armature is disconnected from the supply and dynamic braking effected. Preparation for braking is effected in two stages, and the speed controlled by switches actuated by adjustable stops on the driven member, so motor stops a constant distance from the point at which the armature circuit is broken, thereby maintaining a constant stroke of the driven member, e.g. the platen of a planing machine. As applied to a planing machine, the speed during the cutting and idle strokes is determined by two independently adjustable contacts on the shunt field rheostat, and adjustable stops or tappets on the underside of the platen may be arranged to provide at pre-determined points on the stroke any of the following three speeds: (a) the speed corresponding to the direction being considered, as determined by the relative rheostat contact, (b) the selection, during part of a forward stroke, of the rapid speed corresponding to the reverse stroke, (c) the speed corresponding to full shunt field. Fig. 5 illustrates the basic principle of the control of the shunt field 10, which is in series with a safety resistance 9 and has a rheostat 13 associating with a sliding contact 7a. The starting torque is obtained with the aid of a series field (not shown), and with a contact 8a closed to short-circuit part of the rheostat, the part 7 thereof in the circuit resulting in a "submaximum" field, i.e., less than the maximum. Immediately starting is initiated a relay coil 8 is energized to open contact 8a and a relay contact 12a is closed to put a resistance 11 in parallel with the rheostat. The result, shown in Fig. 4a, is that a speed V, is reached rapidly on a curve E relevant to a speed V2, in less time than on. a normal curve F, the distance travelled being C1 as compared with C2, whereupon the insertion of resistance 11 transfers the relevant characteristic from E towards F, as indicated by the unbroken line. In the case of a planing machine, deceleration is obtained automatically by tappets located in the path of a stop on the underside of the platen, the first tappet closing the circuit of a relay 17 whereby a resistance 15 is inserted in series with the part 16-13 of the rheostat, and in parallel with 14-13, contact 8a being open still. This produces an initial regenerative braking effect which is a function of the operating speed. The next tappet to be operated by the stop closes the relay circuit of contact 8a which produces the second regenerative braking effect until a further tappet opens the armature circuit and effects rheostatic braking to stop the motor. In the complete system, Fig. 15, the motor armature 2 is associated with two series fields 1, 1<SP>1</SP> provided with control resistances 3, 3', 3" with short-circuiting and interconnecting contacts 4a, 4, 7<SP>1</SP>a, 7"a and 4 associated with corresponding relay coils. The shunt field winding 10 with a discharging resistance 39 in parallel is connected across the supply when double-pole contacts 27a are closed for motoring, or alternatively connected directly across the armature when contacts 27'a are closed for rheostatic braking. This latter circuit includes double-pole contacts 30a and 31a closed alternatively in accordance with the direction of armature current. The shunt field control comprises two rocker arms 33a, 33'a respectively, controlled by a coil 33 which closes arm 33a, for a cutting-stroke and thereby connects a rheostat slide 7<SP>1</SP>a in the shunt field circuit. During a return stroke coil 33 is de-energized, so that arm 33<SP>1</SP>a connects a rheostat slide 7<SP>11</SP>a to increase the resistance in the shunt circuit. However, at the commencement of a stroke the slides 7<SP>1</SP>a and 7"a and adjacent parts of the rheostat are short-circuited through closed contact 8a when re-starting the motor in either direction from rest. Contact 8a is opened when the motor has accelerated to a predetermined speed, the opening of this contact being consequent on the energization of relay coil 4 to short-circuit the first part 3 of the series field resistance. The shunt field current is now taken from contact 7'a or 7<SP>11</SP>a, dependent on whether the stroke is a cutting or reverse one, and the motor takes up a speed corresponding therewith. As a first preparation for braking the resistance 15 is connected across the ends of the rheostat by closure of contact 17a to lower the speed, and subsequently, as already described, the second preparation for braking is effected by closure of contact 8a to restore the field to its sub-maximum value, these operations being effected by the actuation of the tappets below the platen or work table. Controlling devices. The control system, Fig. 15a, comprises a push-button switch 3 in a circuit including a relay coil 27 with holding contact 27b, controlling a contact 27c forming one of a series of contacts including a rocker-switch 11, 12 operated by stroke-limiting stops on the underside of the platen. For automatic control a pushbutton 116 is operated to energize coil 42 and close contact 42e in series with the closed arm 11 of the rocker switch, thereby energizing a coil 25, and also opening a contact 42b in the circuit of push-buttons I12, I13, thereby neutralizing the manual control. Contacts 42c, 42d are also opened. Coil 25 is energized for a cutting stroke, with I, closed by the platen, and coil 26 is energized for the return stroke, with I2 closed. Locking-contacts prevent simultaneous energization of coils 25 and 26. When coil 25 is energized it closes associated contacts to excite the relay coils 4, 4', 4" and short-circuit progressively the series field resistances, the action being to start with the series field windings I, 11 cumulative, and finally in opposition. In regard to the shunt field, the closure of contacts 42d in series with a switch I', closed by the work table, energizes contact 33 to connect rheostat slide 7'a, Fig. 15, in the shunt field circuit. Transfer of speed control from slide 7<SP>1</SP>a to 7<SP>11</SP>a is effected by a stop on the work table opening a contact 15 in series with coil 33, which is thereby de-energized so that the switch arms 33'a and 33a take up the positions shown in Fig. 15. Similarly the closing of contact 16 by a mechanical stop re-energizes coil 33 and returns the control to slide 7<SP>1</SP>a. When a speed lower than that provided by side 7<SP>1</SP>a is required a contact 17 is closed by a stop suitably disposed on the platen and energizes a relay 34, 34a to short-circuit the shunt rheostat, thereby effecting a deceleration to minimum speed. In this manner a surface of varying contour, e.g., one traversed by channels of varying width, can be planed by operating the platen at speeds varying between V1 and V3, Fig. 14, by placing stops 19a-19e in appropriate positions on the underside of the platen to co-operate with tappets I5-I8. Once the tool has engaged the work the platen may be accelerated but, as shown, speed is reduced before the tool leaves the work. The platen is adapted for short reciprocating strokes by providing on its underside stops which are contoured to co-operate with reversing switches of the rocker type mounted on a pivot located higher than normally, as described with respect to Fig. 8a (not shown).