US2721299A

US2721299A - Device for maintaining at least substantially constant the speed of direct current shunt-motors

Info

Publication number: US2721299A
Application number: US253142A
Authority: US
Inventors: Wiel Alfred Van De
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1950-11-14
Filing date: 1951-10-25
Publication date: 1955-10-18
Anticipated expiration: 1972-10-18

Description

Oct. 18, 1955 A. VAN DE WIEL 2,721,299

DEVICE FOR MAINTAINING AT LEAST SUBSTANTIALLY CONSTANT THE SPEED OF DIRECT CURRENT SHUNT-MOTORS Filed Oct. 25, 1951 lNVE NTOR Alfred Van De iel yfihfl A enf United States Patent C) DEVICE FOR MAINTAINING AT LEAST SUBSTAN- TIALLY CONSTANT THE SPEED OF DIRECT CURRENT SHUNT-MGTORS Alfred Van De Wiel, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., trustee Application October 25, 1951, Serial No. 253,142

Claims priority, application Netherlands November 14, 1950 3 Claims. (Cl. 318 -331} ence between a constant direct voltage (comparison voltage) previously adjusted for the desired speed and a direct voltage proportional to the counter-electromotive force of the armature.

The direct voltage proportional to the counter-electromotive force of the armature is obtained as the difierence between a direct voltage proportional to the terminal voltage of the armature and a direct voltage which is proportional to the armature current and usually obtained through a current transformer.

In these systems, a constant speed is not always insured with a variable load current. A major reason for this is the characteristic of the so-called form factor, which is the quotient between effective and average current strength. This form factor of the alternating current, which is taken from the current transformer in the load circuit of the controlled discharge tubes in series with the armature of the motor is of greater percentages at lower current intensities, than at higher load currents. The relative difference may even go up to 30%. Consequently, there is overcompensation at lower motor speeds and undercompensation at higher motor speeds. Although proper adjustment of the control apparatus of controllable electron discharge tubes permits, for example, the maximum speed in revolutions per minute to be maintained substantially constant throughout the range of load currents, this. is not simultaneously feasible for the minimum speed. As a rule, this speed will increase with an increase in load current.

The armature reaction also adversely effects motor speed regulation. A magnetic field at right angles to the direction of the main field is produced in the armature by the armature current. The combination of the two magnetic fields produces a resultant field which at one side of a pole shoe is obtained by the sum and at the other side by the difference of the two fields. However, the field obtained by the summation of the two fields is not equal to the sum of the two separate fields, but is much smaller than said sum, because the saturation of the ferric circuit increases. At the other side of the pole shoe the resulting field is weak, due to the subtraction, with the result that the saturation of the ferric circuit decreases. It will be seen that the increase of the field at one side of the pole shoe is less than the decrease at the other side of the pole shoe. Thus, the main field is usually decreased due to the armature field. This decrease of the main field exerts a greater influence on the motor speed at higher speeds, in an absolute sense, than at lower speeds.

2,721,299 Patented Oct. 18, 1955 The aforesaid disadvantages are mitigated by the invention.

A motor control system, in accordance with the present invention, controls the operation of a direct current shunt motor by deriving a first control voltage from the counterelectromotive force of the motor, deriving a second control voltage composed of a compensation voltage derived from the current flow through the armature and a direct comparison voltage which is adjustable in value to effect a desired motor speed, said second control voltage being dependent on the difference between said compensation and comparison voltages, combining said first and second control voltages to produce an output voltage depending on said control voltages, and applying said output voltage to a gas discharge tube which determines the terminal voltage for the motor armature.

The construction of the shunt-motor used determines whether the compensation direct voltage should be connected in series with the comparison direct voltage in a positive or in a negative sense. This may easily be determined in practice. The correct value of compensation voltage may be determined experimentally by variation of said voltage by means of a variable resistor.

In order that the invention may be more clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing wherein:

Fig. 1 is a schematic diagram of one embodiment of the motor control system of the present invention; and

Fig. 2 is a schematic diagram of another embodiment of the motor control system of the present invention.

In Fig. 1, an armature 1 of a direct current shuntmotor comprises a shunt winding 2 which is supplied in a known manner. The winding 2 may be supplied, for example, from a separate rectifier (not shown in the figures).

The terminal voltage of the armature 1 of the motor is supplied by a full wave rectifier comprising two controllable gas or vapor filled electron discharge tubes 3 and 4 supplied from a transformer 5.

An amplifier circuit 6 of a known type, by which an ignition alternating voltage displaceable in phase is supplied to the grid 7 of a discharge tube 3, is connected to two parallel-connected regulating circuits. Seen from the amplifier circuit 6, one of these circuits comprises a grid resistor R1, a portion of a variable potentiometer R2 for the constant direct voltage (comparison voltage), a part of the potentiometer R3 (provisionally left out of consideration) and a connection to the cathode of the tube 3.. The constant. comparison voltage is supplied by a source of direct voltage 8 through a stabilizing tube 9 and a connection through-R3 to the potentiometer R2. The second circuit comprises a grid resistor R4, a resistor R5 and a portion of a potentiometer R6 and a connection to the cathode of the tube 3. (In the known circuit arrangement, the resistor R3 fails and the upper end of the potentiometer and the lower end of the resistor R8 are connected directly to the cathode. In this known arrangement, the amplifier circuit 6 is consequently influenced by the difference between the constant comparison voltage set up at the potentiometer R2 and the voltage across the other circuit R4, R5, R6.) The resulting direct voltage across the circuit is of opposite polarity from the direct voltage across the other circuit, as may be seen from the polarity signs. Thus, with respect to the amplifier circuit 6, the difference between the two direct voltages may be applied.

The direct voltage across the circuit R4, R5, R6 is proportional to the counter-electromotive force of the armature at any given speed and is obtained from the difference between a direct voltage proportional to the terminal voltage set up at resistors R5 and Re, which are connected to the armature 1 by way of a resistor R7, and a direct voltage proportional to the armature current set up at resistor Re which is connected by way of R3 and Rs to a rectifier 10.

The rectifier 10 is supplied from a current transformer 11, whose primary windings 12 are included in the anode circuits of the discharge tubes 3 and 4, so that the supplied direct voltage is a measure of the value of the armature current. Consequently, the direct voltage set up at resistor Rs is also proportional to the value of the armature current taken, the armature voltage component from ReRR7 being considered hereafter. This voltage is subtracted, in the grid to cathode circuit R4, R5, R6 of tube 3 from the voltage set up at resistors R5 and Rs, which voltage is proportional to the terminal voltage of the armature 1. Thus, in the grid circuit of the tube 3 a resulting direct voltage remains which is proportional to the counter-electromotive force of the armature and is connected oppositely to the comparison voltage set up at the resistor R2. The desired speed of the motor is determined by adjustment of the contact 13 of the resistor R2. If the contact 13 is at the lower end of resistor R2 maximum motor speedis attained, and if the contact 13 is at the upper end of said resistor, theminimum motor speed is attained. Thetube 3 is controlled, for example, by phase displacement of a control alternating voltage (having a peaked wave form, if desired) supplied by the amplifier circuit 6, which, in turn, is influenced in known manner by the resulting direct voltage from the grid circuits. Proper adjustment of this bridge type control circuit permits the number of revolutions of the motor to be maintained constant at maximum speedwith a varying load current through the armature 1. This is, however, not feasible at minimum speed if resistor R3 has failed. In accordance with the present invention, by connecting a direct voltage (compensation voltage from the resistor R3) proportional to the armature current, in series with the comparison voltage at the resistor R2, and particularly at that side of the resistor R2 which corresponds to the low speeds (in this instance, the top of R2), compensation may be obtained with the result that the low speeds also remain practically constant with variable load currents. When the resistors R2 and R3 are connected as shown in Fig. 1, the direct voltage at R3 is subtracted from the direct voltage at R2. This is due to the fact that the 'direct voltage at low speeds, if the contact 13 is at the topof the resistor R2, already has a given negative value (the voltage at Rs) proportional to the armature current in the grid to cathode circuit R1, R2, R3.

In accordance with the construction and arrangement of the shunt-motor under control it may, however, be necessary to take from R3 a voltage relatively positive relatively to the voltage at R2. In this event the schematic diagram of Fig. 1 should be modified as shown in Fig. 2.

Fig. 2 is a schematic diagram of another embodiment of the motor control system of the present invention. The corresponding components of Fig. 2 have the same reference numerals as in Fig. 1. Since the resistor R3 is now connected between the positive terminal of the rectifier and the cathode of the tube 3, the voltage at R3 in the cathode to grid circuit R3, R2, R1, is added to the constant comparison voltage at R2. served from the polarity signs of these resistors.

- The selection of the system of Fig. 1 or Fig. 2 may be determined experimentally, insofar as the connection of the resistor R3 is concerned.

' Since R3 is a potentiometer, the value of the necessary compensation voltage taken therefrom is adjustable in a simple manner.

From the foregoing it seems that there may be shunt This may be obmotors of such construction and arrangement that the voltage at R3 need neither be added to nor subtracted from another voltage; that is, the potentiometer Ra may be eliminated.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

The grid tube 4 is connected to a similar circuit as the grid 7.

What I claim is:

1. In a circuit for energizing a direct current shunt motor having a field and an armature, wherein the terminal voltage supplied to the armature is controlled by at least one gaseous discharge tube, apparatus for regulating the speed of said motor under varying load conditions comprising means coupled to the armature to derive from the counter electromotive force generated by said motor a first direct control voltage proportional thereto, means coupled to the armature to derive from the current fiow therethrough a compensation voltage dependent thereon, a direct comparison voltage source adjustable in value to effect a desired motor speed, means serially combining said comparison voltage and said compensation voltage to produce a second direct control voltage depending on the difference therebetween, means to combine said first and second control voltages to produce an output voltage propertional to the summation thereof, and means to apply said summation voltage to said gaseous discharge tube to control the conduction period thereof accordingly, thereby to regulate the motor speed.

2. In a circuit for energizing a direct current shunt motor having a field and an armature, wherein the terminal voltage supplied to the armature is controlled by at least one gaseous discharge tube, apparatus for regulating the speed of said motor under varying load conditions comprising means coupled to the armature to derive from the counter electromotive force generated by said motor a first direct control voltage proportional thereto, means coupled to the armature to derive from the current flow therethrough a compensation voltage dependent thereon, a direct comparison voltage source adjustable in value to effect a desired motor speed, said source including a potentiometer across which a voltage of such magnitude is produced that the voltage appearing at one end of said potentiometer represents the maximum adjustable speed of said motor While the voltage appearing at the other end of said potentiometer represent the minimum adjustable speed, means serially connected to said other end of the potentiometer and combining said comparison voltage and said compensation voltage to produce a second direct control voltage depending on the difierence therebetween, means to combine said first and second control voltages to produce an output voltage proportional to the summation thereof, and means to apply said summation voltage to said gaseous discharge tube to control the conduction period thereof accordingly, thereby to regulate the motor speed.

3. Apparatus as set forth in claim 2 wherein said first control voltage means and said second control voltage means are in parallel connection.

References Cited in the file of this patent UNITED STATES PATENTS 2,504,123 Haneiko et al Apr. 18, 1950 2,516,568 Haneiko July 25, 1950 2,546,014 Puchlowski et al. Mar. 20, 1951 2,554,267 Roman et al. May 27, 1951 2,629,850 McLane Feb. 24, 1953