US2512378A - Control system for direct-current motors - Google Patents

Description

'JunQ Z O, K. P. PUCHLOWSKI 2,512,378

CONTROL SYSTEM FOR DIRECT-CURRENT IOTORS Filed June 22, 1946 INVENTOR Aow/an/yPMbwii ATTO IY Patented June 20, 1950 CONTROL SYSTEM FOR DIRECT-CURRENT MOTORS East Pittsburgh, Pa.

sylvania kl, Pittsburgh, Pa., asuse Electric Corporation, a corporation of Penn- Application June 22, 1946, Serial No. 678,574

10 Claims.

My invention relates to control apparatus and it has particular relation to apparatus for use in controlling an electric motor connected to drive a reel upon which a strip of material is to be wound.

In many reeling operations where a continuous strip of material such as yarn, fabric, wir or steel is drawn by a so-called windup reel and wound upon it, it is highly desirable to maintain the speed of the strip substantially constant. Moreover, it is desirable that the speed of the strip be adjustable over a rather wide range. When the speed of the strip is properly controlled the tension of the strip in many such operations tends to remain within an acceptable range so that it is often unnecessary to consider strip tension in controlling the drive for the strip.

A winding reel customarily has a core type drive. The reel is mounted on a central axle to which a rotational force is applied by a motor, and the reel forms a core upon which the strip of material is to be wound. Now as the strip of material is wound on the reel the effective radius of the reel increases. Consequently, the angular speed of the reel must be varied as the strip is wound upon it to maintain the linear speed of the strip substantially constant. This situation has given rise to considerable difficult in providing a satisfactory control for maintaining th strip speed constant at a given speed which may be selected over a. wide range.

It is an object of my invention to provide a novel apparatus for controlling an electric motor.

Another object of my invention is to provide a novel apparatus for controlling a motor which is connected to drive a windup reel.

Still another object of my invention is to provide a novel apparatus for controlling a motor arranged to drive a windup reel on which a strip of material is to be wound, the motor being controlled to provide a constant strip speed.

A further object of m invention is to provide a novel apparatus controlling a motor for driving a reel on which a. strip of material is to be wound, the motor being controlled to provide a substantially constant strip speed at a given speed which may be selected over a wide range.

During a reeling operation the radius of the reel as it 'aiTects the speed of the strip which is being wound upon the reel increases from a minimum radius R1 to a maximum radius R2 and the speed of the strip may be expressed by the following equation V=21rnR feet per min. 1)

where n represents the motor speed in R. P. M. and R represents the instantaneous radius of the reel in feet. The torqu developed by the motor is T=FR+T0 pound feet (2) P=21rnT or considering equation 2 the first term of Equation 3 obviously represents the useful power required to draw the strip and is the product of the strip speed 1) or 2 1r nR and the strip tension F. The second term which is the product of the motor angular speed and the friction torque of the reeling system at the motor shaft represents the loss of power of the reeling system.

It is apparent from Equation 1 that in order to maintain a constant strip speed c, the speed of the driving motor must change in inverse proportion to the diameter of the reel. On the other hand, if the friction torque To is neglected then the power required during the reeling operation for a given strip speed a and assuming constant strip tension F is P=vF=21rnRF=constant (4) Thus, if a constant strip speed is to be maintained during the reeling from an empty reel with a radius R1 to a full reel with a radius R2, the speed of the driving motor n corresponding to any intermediate radius R of the reel should be where m is the motor speed for an empty reel.

My invention arises from the realization that since the power requirement imposed on the motor drive during reeling for a given strip speed remains substantially constant, the most adequate and economical way of obtaining the desired control of motor speed to maintain constant strip speed, is by controlling the field excitation of a direct current shunt wound motor arranged to drive the reel. This is true since with field current control, the power obtainable at different speeds from the drive without overloading it remains substantially constant. However, the strip speed itself is to be adjustable over a very wide range. For example, it is often necessary to provide a speed range as wide as 10:1. Field excitation control cannot provide such a wide range and take care of the numerous factors tending to vary the motor speed in a practical arrangement. If an approximately constant tension is assumed, then for any given radius of the reel, the required torque is independent of the strip speed as may be seen from Equation 2. In addition, the required horsepower is directly'proportional to the strip speed if the friction torque is neglected. Consequently, insofar as the provision of a wide range of strip speed is concerned, a control of armature voltage of a direct current shunt wound motor is most suitable. This is true since with armature voltage control, the power obtainable at different speeds from the drive without overloading it varies in direct proportion to the speed.

On the other hand if control of the reel is to be accomplished through the control of the armature voltage of the motor, the required horsepower rating of the drive becomes greater in direct proportion to the increase in reel diameter to make the cost of the system too high for practical applications. Moreover, control of the field provides a much better overall amplification since a relatively small change in the field current results in a very large change in the speed of the motor.

In accordance with my invention a direct current shunt wound motor for driving a windup reel is controlled by two electronic controls acting jointly on the motor. A field control is provided to control the field current of the motor. An armature control is provided to control the armature voltage of the motor.

The field control is arranged to respond automatically to changes in the diameter of the reel. With increasing diameter the field current of the motor is increased automatically to slow the motor down just enough to prevent the speed of the strip from being increased. The field control of the motor does not respond, generally speaking, to any change in strip speed obtained through the armature control itself.

The armature control is arranged to maintain the speed of the motor constant for any given field excitation, even if the torque is varied between wide limits. It is adjustable to enable a selection of various strip speeds over a wide range. The armature control is not responsive to changes in the diameter of the reel.

By having the motor controlled jointly by the armature control and the field control in the manner described a quite practical and economical arrangement is provided. The advantages of both field control and armature control are incorporated in the system while the disadvantages of each type of control are largely eliminated. The features of my invention which are believed to be novel are set forth with more particularity in the accompanying claims. The invention itself, however, together with additional objects and advantages thereof may be better understood from the following description of a specific embodiment when read in connection with the accompanying drawing in which the single figure is a schematic diagram of a preferred embodiment of my invention.

The control apparatus as shown in the drawing is applied to a strip winding arrangement in which a strip 3 of material previously wound upon an unwinding reel 5 is to be rewound upon a windup reel 1 after passing over the idler rolls 55 the armature B5 of the motor l3.

9 and H. The axle of the windup reel 1 is mechanically connected to be rotated by a directcurrent motor I3 having an armature l5, a series field winding l1 and a separately excited main 5 field [9. To' control the motor there is provided a field control and an armature control. Of course, a motor which does not have a series field may be used in the system if desired for any reason.

10 The field control is, in effect, an automatic regulator or servo-system employed to vary the speed of the motor [3 in such a manner as to maintain the speed of the strip 3 constant with a varying effective diameter of the windup reel 1.

5 A tachometer generator 2i is connected to one of the idler rolls ii to be driven by the strip 3 and is employed as an indicator of the strip speed. The tachometer generator 2! is required to deliver only negligible power and therefore it may 29 be very small with correspondingly small losses.

Although other types of speed indicating tachometer generators may be used, I prefer to use a tachometer generator of the direct-current permanent-magnet type since the output voltage thereof represents a linear function of its speed,

particularly when the current output is negligible. Furthermore, the voltage output is relatively high and for all practical purposes independent of temperature changes or other external influences.

The output voltage of the tachometer generator M which is directly proportional to its speed is introduced into the field control where it is balanced or compared with a reference voltage provided through a resistor 23 and a manually adjustable potentiometer 25. The difierence between the reference voltage and the output voltage of the tachometer generator 2| is then fed into an amplifier tube 26, preferably a high vacuum tube, and the amplified voltage is used to control the output of a single-phase full-wave rectifier employing a pair of supply valves 21 and 29 of the arc-like type, preferably thyratrons. The output of the rectifier is supplied to the field winding 89 of the motor I3. As will be explained later, the field control is effective so that when the strip speed tends to increase because of the increase in the diameter of the windup reel, the field current of the motor is increased to slow the motor down and keep the strip speed substantially constant. 1

The armature control includes a single-phase full-wave rectifier employing a pairof supply valves 34 and 330i the arc-like type, preferably thyratrons, the output of which is supplied to Of course, in some cases a polyphase rectifier may be used if necessary. The voltage of the armature i5 is then controlled by controlling the supply valves 3! and 33 of the rectifier. Such control is pro- 5 vided by a system similar to that shown in my copending application, Serial No. 495,993, filed July 24, 1943, now Patent No. 2,422,567. The armature supply valves 3! and 33 are controlled by the operation of a master control valve 35, pref- 5 eraoly a high vacuum tube, which is effective to compare the armature voltage with a reference voltage obtained through a manually adjustable speed setting potentiometer 31 with compensation for IR drop in the armature circuit pro- 14 vided through a compensation valve 39, preferably a high vacuum tube. An armature current limiting feature is also incorporated in the system, in conjunction with the master valve 35 through a current limiting valve ll, also preferably a high vacuum tube, to take care of the starting and overload conditions of the system.

The armature control provides a constant speed-torque characteristic so that for a given excitation of the field winding l9, the speed of the motor I3 remains essentially constant for any variation of torque within the operating range of torque values. In addition, the armature control enables a selection of the speed of the motor for a given field current and therefore of the stri speed by manual adjustment of the speed setting potentiometer 31. Each setting of the potentiometer 31 corresponds to a difi'erent but strictly determined strip speed automatically regulated by the field control to stay constant and independent of changes in the diameter of the reel.

Three other problems of particular importance are solved in accordance with my invention. These problems relate to stabilization, acceleration and selection of the strip speed.

The stabilization problem is solved for all practical purposes by an anti-hunt circuit associated with the amplifier tube 26 in the field control and including a potentiometer 43 and a capacitor 253 and a feed-back from the field current through the auxiliary transformer 204. In response to abrupt changes in shunt field current, the antihunt circuit operates through the amplifier tube 26 to oppose such changes.

With respect to the acceleration problem, it should be noted that at the instant of starting of the reel driving system, the voltage output of the tachometer generator 2| is zero. Furthermore, at the start of the reeling operation the windup reel 1 is empty and the motor and windup reel obviously must accelerate to a maximum operating speed corresponding to the lowest operating value of the motor field. It follows that on starting of the system when the strip speed is zero or very low, the field control responding to a low voltage output of the tachometer generator 2| tends to decrease the field current to a minimum. With a relatively low value of starting armature current which for safety may be limited to a value of the order of 200% of the rated armature current, it becomes apparent that a very low starting torque is developed by the motor so that it may not accelerate at all.

To avoid a very low starting torque a special electronic accelerating circuit is added to the field control and employs an accelerating tube 45, preferably a high vacuum tube. When the motor is not running the accelerating circuit is effective to maintain full field current. When the motor is started, the field current is still maintained during the initial portion of the accelerating period. Thereafter the field current is gradually decreased exponentially with time, but, owing to a special electrical coupling between the field control and the armature control, the accelerating circuit prevents the field current from decreasingbelow a certain minimum value so long as the armature current exceeds the rated value by more than a given amount, preferably of the order of 50%. As soon as the armature current drops to its normal operating value indicating that the acceleration of the motor is completed, the efiect of the accelerating circuit disappears and the field current is controlled by the voltage amplified by the amplifier tube 25, in accordance with the output voltage of the tachometer generator 2 I.

With respect to the problem of selection of the strip speed it is to be noted that the slightest change in the tachometer generator output voltage, indicating a change in strip speed, normally results in operation 01 the field control to change the field excitation to oppose the change in speed. However, if the change of the tachometer generator voltage is caused by a change in strip speed resulting from a readjustment of the speed setting potentiometer 31 in the armature control, it is obvious that a corresponding change in field excitation to oppose the change in the strip speed must be avoided. In other words, when the change in the tachometer generator voltage is a result of the change in windup reel diameter the field control should respond to maintain a constant strip speed, but for any given reel diameter the field current should remain essentially the same for different strip speeds.

The foregoing requirement is met by mounting the two potentiometers 31 and 25 in tandem so that when the speed setting potentiometer 31 is adjusted to select the speed of the motor by armature voltage control to thereby determine the strip speed, the reference voltage potentiometer 25 properly changes the reference voltage which is compared with the tachometer generator output voltage. Thus when the speed setting potentiometer 31 is readjusted to call for a higher speed, the output voltage of the tachometer generator 2| increases, but the simultaneous read- Justment of the reference voltage potentiometer 25 increases the reference voltage so that the difference between the reference voltage and the output voltage of the tachometer generator remains the same. Consequently, the field current is not changed.

Considering the circuits of the system in detail, it is to be noted that the control apparatus is energized from a pair of alternating voltage supply lines 41 and 49. Starting apparatus is provided to initiate operation of the motor I 3 and includes a normally open starting pushbutton 5|. When the starting pushbutton 5! is closed, a circuit is completed from one of the supply lines 41 through a normally closed stop pushbutton 53, the starting pushbutton 5| and the coil 55 of a relay 51. The relay 51 is then operated and its first contact 59 is closed to complete a holding circuit about the starting pushbutton 5|. The second contact 6! of the relay 51 is closed to complete a biasing circuit for the accelerating tube 45. The third contact 63 is closed to connect the coil 55 of a second relay 61 across the supply lines 41 and 49. The fourth contact 59 of the relay 51 is opened and the fifth contact 1| is closed to effect changes in the control circuit of the current limiting valve 4i.

Immediately after relay 51 operates, relay 61 is operated. The first and second contacts 13 and 15 of relay 61 are closed to complete the circuit through the armature l5 while the third contact 11 in the biasing circuit for the accelerating tube 45 opens and the fourth contact 19 in the biasing circuit for the current limiting valve 41 closes.

A supply transformer 8| has its primary winding 83 energized from the supply lines 41 and 49, and one of its secondary windings 85 is used to supply voltage to the armature l5 through supply valves 3! and 33. The anodes 81 and 89 of the supply valves 3i and 33, respectively, are connected to opposite ends of the secondary winding 85 while the cathodes 9| and 93 are connected to one side of the armature 15 through contact 15 of relay 61. The other side of the armature I5 is connected through the first contact 13 of the relay 51 and the series field H to an inter- 7 mediate tap 95 on the secondary winding 85 of its transformer 8|. An auxiliary transformer 91 is provided for purposes to be explained hereinafter and has two primary windings 99 and IN, one of which is connected between the anode 81 of the supply valve 3| and secondary winding 85 while the other transformer IIII is connected between the anode 89 of the supply valve 33 and the secondary winding 85.

The supply valves 3| and 33 are controlled by a circuit which extends from the cathodes 9| and 93 through a resistor I03, 9. portion of the speed setting potentiometer 31, three resistors I05, I01 and I09 to an intermediate tap III on the secondary winding II3 of an auxiliary grid transformer IE5. The opposite ends of the secondary winding N3 of the grid transformer II5 are connected through corresponding grid resistors H1 and I I9 to the control grids I2! and I23 of supply valves 38 and 33.

The primary winding I25 of the grid transformer I65 is energized from another secondary winding N1 of the supply transformer 8| through a phase-shifting circuit 529. The phase-shifting circuit i253 is arranged so that an alternating voltage is applied in circuit between the control grid and cathode of each of the supply valves 3| and 33 which lags behind the anode voltage of the corresponding supply valve by approximately 90".

In addition to the alternating current voltage provided through the grid transformer iI5, a direct current voltage is provided in the control circuit for the supply valves 3| and 33. The resultant voltage is arranged to become more positive than the critical voltage of the corresponding supply valve to render that valve conductive in each positive half-period of its anode voltage during operation of the motor. Then if the direct current voltage component becomes more positive, the instant in a positive halfperiod at which each supply valve becomes conductive is advanced. On the other hand, if the direct current voltage component becomes more negative, the instant in a positive half-period in which each supply valve becomes conductive is delayed. The instant in a positive half-period in which a supply valve becomes conductive is referred to hereinafter as the firing point.

The direct current voltage component for controlling the supply valves 3| and 33 is provided through resistors I03, I05, I01 and I09 and potentiometer 31. in series with a resistor I3I across the armature it of the motor I3. Thus a direct current voltage appears across resistor I03 which is proportional to the armature voltage. However, a voltage of opposite polarity and of the same order of magnitude appears across the portion of the potentiometer 31 in series therewith. The difference between the voltages on resistor I03 and the portion of potentiometer 31 is then rather small and the changes in that difference are too small to play an efiective part in the control of the supply valves 3| and 33.

The voltage on potentiometer 31 is obtained from a substantially constant direct current voltage source such as a battery I31 connected in series with the potentiometer 31 and the resistor 105. It follows that resistor I05 has a substantially constant direct current voltage thereacross Resistor I09 also has a substantially constant direct current voltage thereacross as it is in series with a resistor I33 across a substantially constant The resistor I03 is connected direct current voltage source, such as a battery I35.

The resistor I01 is connected in series with the anode I39 and cathode I of the master valve 35 across resistor I05. The voltage developed across resistor I01 then varies with the current through the master valve 35. Since the direct current voltages across resistors i03, I05, I09 and potentiometer 31 are essentially constant as far as the control circuit of the supply valves is concerned, the variable voltage across resistor I01 is efiective to control the firing points of supply valves 3| and 33. This variable voltage in turn depends upon the conductivity of the master valve 35.

It is to be noted that the voltage across resistor W1 has a negative polarity with respect to the control grids I2I and I23 of the supply valves 3| and 33. Thus when the master valve 35 is highly conductive,'the firing points are greatly delayed and the supply valves 3| and 33 may be maintained non-conductive. Conversely, when the conductivity of the master valve 35 decreases the firing points are advanced causing an increase in the armature voltage.

The control circuit of the master valve 35 may be traced from the cathode MI through a portion of the speed setting potentiometer 31 to the adjustable tap A43 thereof and thence through the resistors M3, I45, I41, potentiometer I49 to the adjustable tap I5I thereon which is connected through resistor I53 to the control grid 55 of the valve. As previously indicated a voltage is developed across resistor I03 which is proportional to the armature voltage. This voltage on resistor I03 is opposed in polarity with respect to the control grid I55 of the master. valve 35 to that across the potentiometer 31. Then if the effects of the current limiting valve BI and the compensating valve 39in developing voltages on resistors I45, I41 and potentiometer I49 are m0- mentarily disregarded, it is apparent that the voltage across resistor I03 being proportional to the armature voltage is compared with the adjustable portion of the potentiometer 31 which represents a reference voltage, in controlling the master valve 35. Thus with the adjustable tap I43 of the speed setting potentiometer 31 at a selected setting the conductivity of the master valve 35 varies primarily with the armature voltage tending to maintain that armature voltage constant.

It is well known that if the armature voltage remains constant, the speed of the motor drops with increasing torque with a constant field current. This drop in speed is mainly caused by the IR drop in the armature circuit. To correct this objectionable drop in speed, the IR, drop compensating circuit is provided which employs the compensating valve 39. The anode I51 of the compensating valve 39 is connected to the positive terminal of a substantially constant direct current voltage supply such as a, battery I59, the negative terminal of which is connected to the cathode IIiI through the potentiometer I49.

The control circuit for the compensating valve 39 may be traced from the cathode ISI through resistors I41 and I65, another potentiometer M1 to the adjustable tap I69 thereof which is connected through the grid resistor 51! to the. control grid I13. The potentiometer I61 is connected across a substantially constant direct current voltage source, such as a, battery I15 and provides a negative bias for the compensating valve 39.

A positive armature current indicating voltage is developed across the resistor I65, which is supplied from the secondary winding I11 of the auxiliary transformer 91 through a full wave rectifier I19. As previously mentioned, the primary windings 99 and MI of the auxiliary transformer 91 are in the supply circuit for the armature I so that the voltage across resistor I55 is proportional to the armature current. The polarity of voltage across resistor I65 is opposite to that of the biasing voltage across potentiometer I 61 so that an increase in armature current, as would result from an increase in motor torque, causes the compensating valve 39 to conduct more current. The voltage across potentiometer I49 is thus varied in accordance with variations in armature current. As described hereinbefore a portion of potentiometer I49 is connected in the control circuit of the master valve 35. Then, with increasing armature current, the control grid I55 of the master valve 35 becomes more negative, decreasing the current through the master valve to advance firing points of the supply valve 3| and 33 just enough to compensate for the increasing IR dlop of the motor. Conversely, with a decreasing armature current, the firing points of the supply valves 3| and 33 are delayed.

The resistor I41 and a capacitor I63 are connected in series across the potentiometer I49 to provide an anti-hunting circuit in the armature control. As hereinbefore set forth, the voltage across potentiometer I49 varies in accordance with variations in the armature current. Now the capacitor I93 effectively blocks direct current from the potentiometer I49 through the resistor I41. However, if the armature current changes at a rapid rate to make an abrupt change in the voltage across potentiometer I49, a transient current flows through resistor I41 to develop a, transient voltage thereacross. Since the resistor I41 is also in the control circuit of the master valve 35, the transient voltage acts through the master valve to change the firing points of the supply valves 3I and 33 in a manner to oppose the abrupt change in armature current.

The current limiting feature of the armature control is provided through the action of the current limiting valve 4 I. The current limiting valve 4| has its anode I8I connected to the positive terminal of the battery I31. The cathode I83 of the current limiting valve M is connected to the negative terminal of the battery I31 through reresistors I45 and I03 and a portion of the potentiometer 31. Thus, when the current limiting valve M is conductive, a voltage is developed across the resistor I45 which is also connected in the control circuit of the master valve 35. Consequently, an increase in voltage across resistor I45 causes the firing point of the supply valves 3| and 33 to be delayed.

The control circuit of the current limiting valve 4| may be traced from the cathode I93 thereof through resistor I 65, a portion of a potentiometer I85 to the adjustable tap I81 thereof and thence through grid resistor I89 to the control grid I9I. A negative biasing voltage with respect to the control grid I9I appears across the potentiometer I85 which is connected in circuit across the battery I15 through a resistor I93, the fourth contact 19 on relay 61 and the fifth contact H on relay 51. The biasing voltage provided through the potentiometer I85 is sufficient to maintain the current limiting valve 4I non-conductive for all of the normal loads of the motor. However, when the load current of the motor exceeds a certain critical value, the voltage across resistor I55 becomes so high that the valve 4I begins to conduct current. As the valve 4| conducts current, the voltage across resistor I45 builds up to efiect a delay in the firing points of the supply valves 3| and 33 through the action of the master valve 35. If the motor current continues to rise, the delay of the firing points soon becomes so great and the armature voltage so low that the motor stalls with armature current reaching its limit value. This limit is preferably of the order of 200% of the motor rated current and may be adjusted by means of the potentiometer I85.

The armature current limit, which for a given excitation of the motor may be called the torque limit, plays an obviously important part during the acceleration and overload of the motor. On starting the motor I5 the supply valves 3I and 33 are non-conductive since initially there is no negative biasing voltage across the potentiometer I and the current limiting valve M is therefore highly conductive. After closure of the starting pushbutton 5I followed by operation of the relays 51 and 61, the biasing voltage across I85 appears gradually since it has to follow the voltage across a capacitor I in parallel therewith, which is charged from the source I15 through the resistor I93. Consequently, the conductivity of the current limiting valve 4| is decreased and therefore the firing points of the supply valves 3I and 33 are advanced, but gradually because of the capacitor I95, until the armature current builds u sumciently to take control of the acceleration. Thus at the beginning of the operation, no objectionable current surges may occur in the supply valves.

When the motor is to be stopped, the stopping pushbutton 53 is opened causing the relay 51 to be deenergized. As relay 51 is deenergized, the fifth contact II thereof removes the voltage from the potentiometer I85 immediately. This occurs even before the opening of the third contact 63 of relay 51 can act through relay 61 to open the first and second contacts 13 and 15 of relay 61 so that the firing points of the supply valves 3I and 33 are delayed sufficiently to avoid arcing at the contacts 13 and 15.

As previously indicated, the field control of the motor includes a pair of field supply valves 21 and 29 arranged to supply current through the shunt field I9. The anodes MI and 203 of supply valves 21 and 29 are connected to opposite ends of another secondary winding 205 of the supply transformer 8|. Another auxiliary transformer 204 is provided and has two primary windings 206 and 208, one connected between the anode 20I of valve 21 and the secondary winding 205 and the other connected between the anode 203 of valve 29 and the secondary winding 205. The cathodes 201 and 209 of the field supply valves 21 and 29 are connected together, and through the shunt field I9 to an intermediate tap 2I I on the secondary winding 205.

The control circuit for the field supply valves 21 and 29 may be traced from their cathodes 201 and 209 through resistors 2I3 and 2I5 to the center tap 2I1 of another secondary winding 2I9 of the grid transformer II5, the end terminals of which are connected through individual grid resistors HI and 223 to the control grids 225 and 221 of the valves 21 and 29, respectively. As has been explained in connection with the armature control, an alternating voltage is supplied through the grid-transformer II5 which lags behind the anode voltages of the supply valves 21 and 29 by approximately 90. In addition to this alternating voltage, a variable direct current voltage component is supplied in the control circuit of the thyratrons. As the direct current voltage increases and decreases, the firing points of the field supply valves 21 and 29 are advanced and delayed.

Resistor 2I5 is connected in circuit with resistors 229 and 23 and reference voltage potentiometer 25 across a substantially constant direct current voltage source such as battery 23 I Thus a substantially constant negative biasing voltage for field supply valves 21 and 29 appears across resistor 2 I5.

Resistor 2|3 is connected in circuit with the battery 23 I through the anode 233 and cathode 235 of the amplifier tube 26, resistor 23 and potentiometer 25. Consequently, when the amplifier tube 26 is conductive, a variable direct current positive voltage appears across resistor 2|3 having a magnitude of the amplifier tube the firing points of the field supply 29.

The control circuit of the amplifier tube 26 may be traced from the cathode 235 through the resistor 23, a portion of the reference voltage potentiometer 25 to the adjustable tap 231 thereon, a

26 and effective to control valves 21 and dependent upon the conductivity Q justable tap I43 on the speed setting potentiometer 31 in the armature control, the output voltage of the tachometer generator 2| across resistor 239 increases with the increase in speed. However, at the same time, the adjustment of the potentiometer 31 has caused a similar adjustment of'the tap 231 of potentiometer 25 in the field control so that the reference voltage across resistor 239, a portion of the potentiometer 43 to the adjustable tap 24I thereof and grid resistor 23 to the control grid 245 of the amplifier tube. Because of their connection through resistors 2 I 5 and 229 across the battery 23 I, the resistor 23 and the potentiometer 25 provide a substantially constant adjustable negative reference voltage which is to be compared with the output voltage of the tachometer generator 2|. Although the plate current of the amplifier tube 26 flows through resistor 23 and potentiometer 25, such a simplified arrangement is permissible since the resistance of resistor 23 and potentiometer 25 is relatively low so that the voltage drop thereacross'as caused by the plate current is quite small. In addition, the plate current of the amplifier tube changes only slightly during the operation of the field control because of the general amplification of the system. Consequently the actual change in reference voltage caused by the change in the amplifier tube is negligible. For example, in a typical system with a total reference voltage of 25 volts and a resistance of resistor 23 and potentiometer 25 equal to 1,000 ohms and the change in the operating plate current of the amplifier tube 26 of 0.1 milliampere for an entire reel diameter buildup of 2:1, the resulting change in reference voltage is equal to 0.1 volt. Thus the error caused by the change in thereference voltage amounts to only 0.4 of 1%.

The tachometer generator output voltage appears across resistor 239 which is connected in series the field supply valves 21 and 29 to effect a reduction in the speed of the strip. If the strip speed is increased by a clockwise turn of the adplate current of p with the tachometer generator 2| through a resistor 241. Thus the difference between the resistor 23 and potentiometer 25 is likewise increased. As a result, the difference in thereference voltage and the output voltage of the tachometer generator 2| remains substantially constant, so that the field current is not changed.

The potentiometer 43, a portion of which is included in the control circuit of the amplifier tube 26, ,is connected to a secondary winding 249 of the second auxiliary transformer 204 through a full wave rectifier 25| and a capacitor 253. The capacitor 253'efiectively blocks the direct current component of the rectified voltage. However; if the field current tends to change abruptly, the resulting abrupt change across the secondary winding 249 of the second auxiliary transformer 204 causes a transient current which is effected by the charging or discharging, as the case might be, of the capacitor 253. Thus, at such times a transient voltage appears across potentiometer 43 of such polarity with respect to the control grid 245 of the amplifier tube 26 as to effect a change in the firing points of field supply valves 21 and 29 opposing the change in field current. It should be noted that this action takes place in response to the rate of change of the average field current and not to the change itself.

The accelerating tube 45 has its anode 255 and cathode 251 connected in series circuit with the resistor 2|3 across the battery 23I. Thus the voltage across resistor 2|3 which controls the firing points of the field supply valves 21 and 29 may be determined by the combined effect of both the amplifier tube 26 and the accelerating tube 95.

The control circuit of the accelerating tube 45 may be traced from the cathode 251 thereof through resistors 259, I45, I65 and grid resistor 26I to the control grid 263. Under normal operating conditions, a biasing voltage appears across resistor 259 of sufiicient magnitude to maintain the accelerating tube 45 non-conductive under such conditions, the position of. the firing points are determined solely by the operation of the amplifier tube 26. The biasing voltage across resistor 259 is obtained from a substantially constant direct current voltage source such as a battery 265 connected through resistor 261 and the second contact 6| of relay 51 across the resistor 259. A capacitor 269 is connected in parallel with resistor 259 through the second contact M of relay 51. p

It will be noticed that in addition to the resistor 259 the control circuit of the accelerating tube 45 includes the resistors I45 and I65 in the armature control. As previously indicated, the voltage across resistor I65 is proportional to the armature current, while the voltage across resistor I 45 is also proportional to the armature current but appears only when that current is above the normal operating value.

Before the system is started, the relay 51 is deenergized so that there is no voltage across resistor 259. Consequently, the accelerating tube 45 conducts full current, resulting in full voltage across resistor 2 I3 and therefore full exciting current for the field winding I9. At the same time. the capacitor 269 is maintained in a discharged state by the short-circuit thereacross through the normally closed third contact 11 to relay 61.

After the starting pushbutton is depressed, both relays 51 and 6'! are energized so that the negative biasing voltage appears across the resistor 259 but builds up gradually since that voltage has to follow the voltage across capacitor 269 which is gradually charged from the battery 265 through the resistor 261. As a result, the plate current of the accelerating tube 45 decreases gradually, as does the excitation current of the field winding iii.

In the meantime, the motor l3 and the tachometer generator 2| are caused to accelerate so that at a certain moment when the tachometer generator voltage becomes high enough, the ampl ifier tube 26 starts to conduct current and takes over the control of the shunt field current in the manner previously described. If, however, the

motor does not have enough time to accelerateto its operating speed before the capacitor 269 is completely charged, the presence of the positive voltage across resistors I45 and H55 caused by the high armature current prevents the plate current of the accelerating tube 45 and therefore the field current of the motor from decreasing below a certain minimum value. Thus, an adequate torque is maintained during acceleration of the motor and system. As soon as the acceleration of the reel system is completed, the armature current drops to a normal operating value and, after charging of the capacitor 269 has been completed, the accelerating tube 45 becomes non-conductive.

With the accelerating tube 45 non-conductive, the amplifier tube 26 is efiective to control the shunt field current in accordance with the speed of the strip 3 while the armature control is effective to control the armature voltage tending to maintain a constant speed for a given field current.

It is to be understood that for purposes of simplification all grid controlled tubes have been represented as triodes although in many cases tetrodes and pentodes are preferable in a practical device. Heating circuits for the tubes have likewise been omitted to avoid confusion in thedrawing. In an actual circuit batteries will be normally replaced by auxiliary rectifiers and filters.

While I have shown and described a specific embodiment of my invention, I am aware that many modifications may be made without departing from the spirit of the invention. I do not intend, therefore, to limit my invention to the specific arrangement shown.

I claim as my invention:

1. Control apparatus for a direct-current motor having an armature and a field winding, comprising direct-current supply means disposed for energizing the armature and having current adjusting means for setting a desired base value of motor speed, direct-current supply means disposed for energizing said winding and having voltage control means for providing said winding with variable excitation for varying the speed from said base value in response to a motor operatin condition, said control means having a condition-responsive control member and having an adjusting member for adjusting the effect of said control member on said excitation, said adjusting member being connected with said current adjusting means so that a change in setting of said adjusting means is accompanied by adjustment of said adjusting member to set said control means for the changed speed base value.

2. A drive comprising a direct-current motor having an armature and a field winding, directcurrent supply means connected to said armature and having adjustable regulating means for maintaining the armature current substantially at an adjusted value independent of variations in excitation of said winding, direct-current supply means connected to said winding and having voltage control means for providing variable excitation for said winding in order to change the motor speed from a base value determined by said adjusted current value, said,

control means having a condition-responsive control member and having an adjusting member for adjusting the effect of said control member on said excitation, said adjusting member being connected with said regulating means so that a change in adjustment of said current is accompanied by an adjustment of said adjusting member to set said control means for the changed speed base value.

3. A drive comprising a direct-current motor having an armature and a field winding, directcurrent supply means connected to said armature and having adjustable regulating means for maintaining the armature current substantially at an adjusted value independent of variations in excitation of said winding, direct-current supply means connected to said winding to provide it with variable excitation and having adjustable voltage means to provide anormally constant reference voltage and condition-responsive voltage means to provide a variable voltage, said two voltage means bein connected in opposing relation to each other so that said excitation depends upon the diiferential value of said voltages, and connecting means joining said adjustable voltage means with said regulating means so that a change in adjustment of said regulating means is accompanied by an adjustment change of said adjustable voltage means so as to maintain said differential voltage value substantially independent of said current value.

4. Armature-current supply means for the motor having a current adjusting member for selecting a desired base value of motor speed, field-current supply means for the motor having control means for varying the field excitation, said control means having a variable voltage source responsive to the material speed for normally controlling said excitation to vary the motor speed from said base value in accordance with diameter changes of the wound material. and said control means having currentresponsive coupling means connected to said armature-current supply means for controlling said field-current supply means to provide a given minimum of field excitation when the armature current is above a given value so that the motor torque is temporarily increased during starting periods.

5. A control systemfor a direct-current motor. comprising an armature energizing circuit having first control means for adjusting the armature voltage of the motor, a field energizing circuit havin second control means for varyin the motor field excitation so that the motor speed is jointly controlled bv said first and second con gizing circuit for to maintain said excitation above a given value during a starting period timed by said timing means and as long thereafter as the armature current is above a given value.

6. A speed control system for a direct-current motor, comprising alternating-current supply means, an energizing circuit having a rectifier connected to said supply meansior providing armature current for the motor, a field rectifier connected to said supply means for providing field excitation for the motor and having a control circuit for varying said excitation, impedance means in said control circuit for imposing variable control voltage thereon, a first electronic control tube having a plate circuit connected with said impedance means and having a grid circuit with speed-responsive grid voltage supply means for normally controlling said excitation to maintain constant the speed responded to, a second electronic tube having a plate circuit connected with said impedance means and having a grid circuit coupled with said energizing circuit for maintaining said excitation above a given value when said armature current exceeds a given limit during accelerating periods.

7. A speed control system for a direct-current motor, comprising alternating-current supply means, an energizing circuit having a rectifier connected to said supply means for providing armature current for the motor, a field rectifier connected to said supply means for providing field excitation for the motor and having a control circuit for varying said excitation, impedance means in said control circuit for imposing variable control voltage thereon, a first electronic control tube having a plate circuit connected with said impedance means and having a first grid circuit with speed-responsive grid voltage supply.

means for normally controlling said excitation to maintain constant the speed responded to, a second electronic tube having a plate circuit connected with said impedance means and having a second grid circuit, said second grid circuit having current-responsive coupling means connected with said energizing circuit and having grid bias means for biasing said second tube to cause an increase of said excitation when said armature current exceeds a given limit above its normal value.

8. A control system according to claim 7, comprising contact means disposed in said energizing circuit for opening and closing said energizing circuit, and circuit means controlled by said contact means and connected with said second grid circuit ,ror biasing said second tube to cause an increased excitation also when said energizing circuitisopen.

9. A control system according to claim 8, comprising a capacitive circuit controlled by said contact means and having an impedance member so as to impose on said member a voltage drop of capacitively timed rate of change when said contact means close said energizing circuit,

said member being connected with said second grid circuit for having said voltage drop bias said second tube to cause increased excitation during a timed starting period of the motor.

10. A control system for a direct-current motor, comprising alternating-current supply means, an armature energizing circuit having a controllable electronic rectifier connected to said supply means, and having a control circuit, a current-responsive voltage source connected with said armature energizin circuit to provide a control voltage variable in dependence upon the current flowing in said energizing circuit, first voltage supply means of constant voltage having first adjusting means for selecting the magnitude of said constant voltage, said voltage source and said voltage supply means being connected in mutually opposed relation to said control circuit for controlling said rectifier to maintain saidcurrent substantially constant at a value dependent upon said selected magnitude, a field excitation circuit connected to said supply means and having another controllable electronic rectifier with a control circuit, speedresponsive voltage supply means and second voltage supply means of constant voltage connected in mutually opposed relation to said latter control circuit to provide it with differential voltage, said second voltage supply mean-shaving adjusting means for adjusting said latter constant voltage, and saidtwo adjusting means being joined with each other and adjustable together with each other to maintain said difierential voltage substantially at a given value independent of said selected magnitude.

KONSTANTY P. PUCHLOWSKI.

REFERENCES CITED The following references are of record in the file of this patent:

. UNITED STATES PATENTS Number

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