US2401154A - Control system - Google Patents

Description

ummm KUUIW lu'lvl vuuu W. R. HARRIS CONTROL SYSTEM Filed Sept. 14, 1944 INVENTOR V0/fer@ Har/'115,

ATTORNEY Patented May 28, 1946 CONTROL SYSTEM YValter R. Harris. Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 14, 1944, Serial No. 554,079

9 Claims.

My invention relates to motor control systems and in particular to systems for maintaining a drive motor at a substantially constant speed of adjustable magnitude.

It is an object of my invention to provide a variable speed drive whose speed-controlled motor is energized directly from a direct-current line rather than through a motor generator setv but which nevertheless affords the essential advantages of a Ward-Leonard type drive by simple and reliable control means.

Another object of my invention is tol provide a variable voltage drive with a line-energized motor circuit that affords a speed control whose aecuracy and sensitivity are substantially unaffected by line voltage variations.

rlhese and other objects as well as the means provided by the invention` for achieving these objects will be apparent from the following description of the embodiment illustrated in the drawing.

The drawing shows diagrammatically a variable speed drive for a paper machine. The machine denoted as a whole by PM is provided with| a plurality of processing units which are traversed by the pump or Web material f to be converted into paper. One of these units is exemplified by acouch roll comprising a pair of rollers 2 which are driven through a gear 3 by a shaft 4. This shaft, ln turn, is driven by a belt 5 from a pulley 6 mounted on the line shaft LS ol' the machine. Another unit is represented by a pair of rolls 'l driven through a gear 8' and a shaft 9' by a belt I from another pulley II of the line shaft LS. A third unit I2 is similarly driven by gear i3, shaft I4, belt I and pulley l5 also from the line shaft LS which extends along the' machine and may serve to drive other units such as calenders, dandy rolls, coating or waxing devices and the like machinery. The line shaft LS is actuated by an electric motor LM whose armature Il is mounted on, or geared to, the shaft LS. The line shaft motor LM is equipped with a series fieid winding I8 and a separately-excited field winding I9. Armature I1 and series field winding I8 are conected, at terminals LI and L2, to a directcurrent line of substantially constant voltage and hence are directly energized from the line. The connection includes a series connected group of starting resistors 2|, 22 and 23 under control by the main contact 24 of a contacter M The contactor has a control coil 25 which actuates main contact 24 together with four auxiliary contacts 26, 21, 28 and 29. Resistor 2| is provided with a short-circuiting path which is controlled by the main contact 32 of a timing relay TI whose coil 33 controlsalso two interlock contacts 34 and 35. Similarly. the starting resistor 22 ls provided with a short-circuiting connection under control by the main contact 36 of a second timing relay T2 Whose coil 31 actuates also an interlock contact 38. The starting resistor 23 has a short-circuiting path which is controlled by the contact of a third timing relay T3. The coil 40 of relay T3 aetuates also twoy interlock contacts 4I and 42. The operation of the timing relays TI, T2, T3 and of contacter M is controlled by a start button BA and a stop button B0 in a manner to be described in a latter place and is energized from two direct-current mains X and Y which are connected to the line terminals LI and L2', respectively. The connection of main X to terminal Li is' controlled by they contact 43 of a control relay CRIl whose control coil is denoted by 44.

IThe field winding I'9 is connected across mains X and Y through an adjustable rheostat 45 and in seriesl with the armature 4G of a regulating generator RG. vConsequentlyy the voltage impressed on the field winding I9 depends on the adjustment of rheostat 45 and also on the voltage generated by the regulator armature 46. The adjustment of rheostat 45, as will be explained hereinafter, is selected by the operator in accordance with a desired operating speed of the line motor LM, while the voltage generated in the armature 46 changes in accordance with the departure of the line shaft speed from the desired value and in such a manner as to eliminate such departure. That is, the voltage of armature 46 is required to be zero when the motor speed is in accordance with the datum Value, or to boost and buck the voltage derived from mains X and Y depending upon whether the motor speed is above or below the datum value.

In order to control the armature voltage of generator RG in accordance with the just-mentioned requirements, three field windings 4l, 48 and 49 are provided. Winding 4l is a separately excited winding. Winding 48 is a self-energizing Winding which is designed so that its resistance line is coincident with the no-load saturation curve of the regulating generator RG. That is, this field Winding tends to maintain the armature voltage at any value determined by the other fields. The self-energizing Winding 48 may either be series connected to the armature or may be arranged as a shunt eld winding, the latter possibility being illustrated in the drawing. Field winding 49 is a separately excited eld winding which derives its energization from mains X and Y and is so rated and polarized as to act in opposition connected with to field winding 41. That is, windings 41 and 43 act diierentially with respect to each other and are capable of balancing their iields so that their resultant control effect is zero when the ampere turns of the two windings are equal.

The shaft 50 of armature 45 is driven by a constant speed motor RM which is energized, for instance from an alternating current line I, under control by the contacts 52, 53 and 54 of a line contactor LC whose coil 55 actuates also a self-holding contact 58 and an interlock contact 51.

A Calibrating resistor 53 is provided in the circuit of the self-energizing winding 43. Another calibrating resistor 53 is series connected in the l5 circuit of field winding 43. The separately excited pilot eld winding 41 is connected to the amature 50 of a pilot exciter PE whose field winding BI derives its excitation from mains X and Y through a Calibrating resistor S2. The circuits of field windings 48, 43 and 5I are c ontrolled by the contacts 53 and 64 of a control relay CB2 whose actuating coil is denoted by 55. The adjustment of resistors 53, 53 and 52, once properly selected, need not be changed during the normal operation of the system.

The energizing circuit of pilot winding 41 contains a calibrating rheostat 35. The slide contact 51 of this rheostat is mechanically connected with the slide contact 63 oi' the above-mentioned rheostat 45. A suitable transmission, here represented by a shaft 53, connects the sliders 51 and 68 with the armature 1l of an auxiliary motor AM. This motor has two oppositely acting neld windings 1I and 12 whose intermediate point is the amature 1l through a Calibrating rheostat 13 and whose terminals are l connected to main X through control buttons BS and FS, respectively. Depressing one of these buttons will cause the respective neld winding 1| or 12 to be energized so that motor AM runs in the corresponding direction and actuates the two sliders 61 and 53 as long as the button is kept depressed. Actuation of the other button will cause the motor AM to adjust the sliders 31 and 53 in the opposite direction. Limit switches (not shown) may be provided, if desired, in order to stop the motor AM when the sliders reach a limit position at either end of their path of travel.

When the paper machine is at rest, contact 24 of contactor M interrupts the energizing circuit of the motor. Since coil 25 of contactor M obtains its energization from the mains X and Y, and since its energizing circuit extends through contact 43 of relay CRI which is open when motor M is at rest, contactor M cannot close its contact 24 before relay CRI is energized through contact 51 of line contactor LI. Consequently, a starting of the line shaft motor LM is prevented as long. as the line contactor LC is deenergized and the regulating generator RG at rest. Consequently, in order to start the line shaft drive, it is rst necessary to bring the regulating generator RG up to speed. This is done by depressing the control button B2. As a result, coil 53 of contactor AC is energized and connects motor RM to line 5I, thereby placing the regulating generator RG in condition for operation. Contact 53 of contactor LC provides a self-holding circuit so that the regulating generator remains in operation when the control button B2 is subsequently released. In order to stop the generator RG, button BI must be actuated thereby interrupting the circuit of coil 55.

The actuation of line contactor LC has also the effect of closing the circuit of coil 44 ancontact 51. This causes control relay CRI toclose its contacts 43, thereby placing the remaining portion of the control system in operative condition.

It is now possible to start the line shaft motor LM by depressing the start button BA. This has the effect of energizing coil 33 of relay TI through the circuit elements X, BO, BA, 21, 33 and.

Y. Relay TI opens contact 32 and closes contacts 34 and 35. Coil 31 of relay T2 is now energized through contact 35. At the same time, coil 43 of,

relay B is energized through contacts 34 and 35. Relays T2 and T3 open their contacts 35 and 33, and close contacts 38 and 4I, respectively. Coil 25 of contactor M obtains energization through contacts 4I, 33, 34 and 35 and closes its contacts 24, 26, 23 while opening contacts 21 and 29. This sequential operation of relays TI, T2 and T3 and M occurs within a very short period because the timing relays have an unretarded pick-up operation and introduce a timing effect only when dropping oil'. Consequently, the contactor M responds almost immediately upon the actuation of button BA. When this stage is reached, contact 25 closes a self-holding circuit for coil 25 so that contactor M stays in when the button BA is subsequently released.

The closure of contact 24 connects the motor -to the line with all contacts 33, 33 and 32 open so that the total resistance of resistors 2l, 22 and 23 is effective. However, as contact M picks up, its contact 21 interlocks the coil circuit of relay 21 so that this relay drops out with delay and closes contact 32, thereby short circuiting resistor 2I. Relay TI, at the end of its timing period, opens also its contacts 34 and 35. As a.

result, coil 31 is disconnected so that relay T2, at the end of its timing period, short circuits the resistor 22 while opening its contact 33. Coil 43 of relay T3 is now also deenergized so that eventually contact 33 is closed thereby starting resistor 23. The motor LM is now under full voltage and operates at full speed.

The magnitude of thisspeed depends upon the excitation of field winding I3. As mentioned before. this excitation is determined by the slider adjustment of rheostat 45. The voltage generated by the pilot generator PE is a measure of the actual motor speed so that the excitation of the pilot ileld winding 41 on generator RG is also indicative of the speed. If this excitation is in accordance with the oppositely acting excitation of the pattern ield winding 43 as adjusted by the rheostat 53, both field eiects cancel each other so that the voltage of amature 45 is zero and the motor speed merely determined by the setting of rheostat 45.

If due to an increase in load, the speed of motor LM drops below the intended value, the voltage of pilot generator PE drops accordingly and lowers thev excitation of pilot iield winding 41. The field of winding 43 is now predominant and causes the voltage of armature 43 to rise. As a result, the current in the totor neld winding I3 is decreased thus allowing the motor speed to rise. When the speed reaches the correct value. the pilot eld again balances the pattern eld so that no further action takes place. The selfenergizing winding 43 maintains the excitation of the regulating generator at any value reached because, as mentioned, the resistance line of the circuit of winding 43 is coincident with the n oload 'saturation curve of the generator. l

Ii the load on the line shaft motor decreases.

Ufff' v so that a rise in speed takes place, the speed responsive voltage impressed on the pilot field winding 4T increases accordingly and the pilot eld prevails over that of the oppositely acting pattern eld winding 49. Consequently, the voltage of armature 46 is now decreased thereby increasing the current in field winding I9 with the effect of reducing the motor speed.

The datum speed of the line shaft to be kept constant by the control system can be varied by actuating the motor driven rheostat 45. If the speed is to be reduced, button BS is depressed by the operator thereby causing the motor AM to displace the sliders 68 in the corresponding direction. In a similar manner, the actuation of button BF causes motor AM to adjust the slider 68 or rheostat 45 for a faster speed of the line shaft motor.

The pilot exciter PE operates preferably on the straight portion of its saturation curve. If this condition is complied with. the operating speed of the line shaft motor remains substantially unaffected by changes in the direct-current bus voltage, thus securing a high accuracy and constancy of operation. both the eld winding 6l of the pilot exciter and the pattern eld winding 48 are excited from the direct-current buses so that changes in bus voltage will aiTect the pattern field winding 4S of the regulating generator RG to the same extent as the pilot field winding 4'! is aiected by such voltage changes through the medium of the pilot exciter PE. Since the fields of windings 49 and 41 act in opposition, the changes in bus voltage are materially compensated within the limits of expectable voltage variations.

The above-described system has the further advantage that the current in the motor lic-ld winding I9 is changed substantially the same percentage for all positions of the speed adjusting rheostat 45. This is due to the presence of the correcting rheostat 66 in the circuit of the pilot eld winding and the fact that this rheostat is tied up with the speed adjusting rheostat so that the excitation circuit of the pilot eld winding is recalibrated for any changes in the setting of rheostat 45. When the line shaft motor is near full eld excitation and near saturation. the change in field current is correspondingly larger because of less resistance in the circuit. sequently, the sensitivity of the regulation remains approximately constant during the entire speed range. The above-described use of a calibrating rheostat in the pilot exciter circuit permits also maintaining the same number of ampere turns in the pilot and pattern elds of the regulating generator. This also insure: a good sensitivity of the control at all available operating speeds.

It will be evident to those skilled in the art upon studying this disclosure that the control systems accordin-Cr to the invention may be modied and altered as to details without departing from the gist and essential features of the invention as set forth in the claims appended hereto.

I claim as my invention:

l. A control system comprising a direct-current motor having an armature and a field winding, circuit means for supplying direct current of substantially constant voltage to said armature and eld winding, a regulating generator having an armature series connected with said motor field winding and being provided with a self-energizing eld winding rated for substantially self-sustaining the generated voltage and This is due to the fact that f two differentially acting field windings for separate excitation, means for variably exciting one of said differential windings in dependence upon the speed of said motor, and means for providing said other differential winding with substantially constant excitation, whereby said motor is caused to operate at a speed determined by said constant excitation.

2. A control system comprising a direct-current motor having an armature and a field Winding, circuit means for supplying direct current of substantially constant Voltage to said armature and eld winding, a regulating generator having an armature series connected with said motor held winding and being provided with a selfenergizing field winding rated for substantially self-sustaining the generated voltage and two differentially acting field windings for separate excitation, circuit means connected to one of said differential windings and controlled by said motor for energizing said latter winding in accordance with the motor speed. and adjustable potentiometric means connected between said circuit means, and said other differential winding for energizing it by substantially constant excitation of selected magnitude in order to cause said motor to operate at a speed determined by said selected magnitude.

3. A control system comprising a direct-current motor having an armature and a eld winding, circuit means for supplying direct current of substantially constant voltage to said armature and eld winding, a regulating generator having an armature series connected with said motor eld winding and being provided with a self-energizing eld winding rated for substantially selfsustaining the generated Voltage and two differentially acting eld windings for separate excitation, a pilot exciter having a field winding energized from said circuit means and an armature circuit connected to one of said differential windings to excite the latter in accordance with the speed of said motor, and an energizing connection between said circuit means, and said other differential winding for energizing it by substantially constant excitation of selected magnitude.

4. A control system comprising a direct-current motor having an armature and a field Winding, circuit means for supplying direct current of substantially constant voltage to said armature and eld winding, a regulating generator having an armature series connected with said motor eld winding and being provided with a self-energizing eld winding rated for substantially self-sustaining the generated voltage and two differentially acting field windings for separate excitation, means for variably exciting one of said differential windings in dependence upon the speed of said motor, means for providing said other differential winding with substantially constant excitation, a rheostat series connected with said motor field winding, another rheostat series connected with said rst differential winding, and actuating means for adjusting said two rheostats in a xed relation to each other for selecting the motor speed to be kept constant by the system.

5. A control system comprising a direct-current motor having an armature and a field winding, circuit means for supplying direct current of substantially constant voltage to said armature and iield winding. a regulating generator having an armature series connected with said motor field winding and being provided with a selfenergizing eld winding rated for substantially self-sustaining the generated voltage and two differentially acting neld windings ior separate excitation, means for variably exciting one of said diilerential windings in dependence upon the speed oi said motor, means for providing said other diiierentiai winding with substantially constant excitation of selected magnitude, starting means connected in series with said motor armature. drive means for operating said generator at substantially constant speed, and control means disposed for energizing said drive means and interlocked with said starting means so as to permit starting said motor only when said generator is in driven condition.

6. A control system comprising a direct-current motor having an armature and a. field winding, circuit means for supplying direct current o! substantially constant voltage to said armature and ileld winding, an amplifying generator having two oppositely acting and balanceable held windings, means for variably exciting one of said diii'erential windings in dependence upon the speed of said motor, and means for providing said other differential winding with substantially constant excitation, whereby said motor is caused to operate at a speed determined by said constant excitation.

'1. A control system comprising a direct-current motor having an armature and a field winding, circuit means for supplying direct current of substantially constant voltage to said armature and ileld winding, a regulating generator having an amature series connected with said motor field winding and being provided with a self-energizing field winding rated tor substantially self-sustaining the generated voltage and two dinerentially acting eld windings for separate excitation, circuit means connected with one of said diil'erential windings for supplying it with excitation dependence upon the speed of said motor and containing an adjusting member for varying the relation of said excitation to said speed. and circuit means connected to said other differential winding for providing it with constant excitation, whereby said motor is caused to operate at a speed adjustable by means of said circuit member.

8. A control system comprising a direct-current motor having an armature and a field winding, circuit means for supplying direct current of substantially constant voltage to said armature and field winding, an amplifying generator lhaving an armature series connected with said motor tleld winding and being provided with' two differentially acting ileld windings for separate excitation, a pilot exciter controlled by said motor and having an armature circuit connected to one of said diierential windings to excite the latter in accordance with the speed o! said motor, an adjustable impedance member disposed in said armature circuit, and circuit means connected with said other differential winding for providing it with constant excitation.

9. A control system comprising a direct-current motor having an armature and a field windy ing, circuit means for supplying direct current

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