Patent GB-661668-A (GB)
Title: Motor speed regulator
IPC: H02P7/34
Filed: 1948-09-29
Abstract: 661,668. Control of D.C. motors. CARNEGIE-ILLINOIS STEEL CORPORATION. Sept. 29, 1948. [July 12, 1948] No. 25408/48. Class 38 (iii). [Also in Group XXXV] A speed regulator for motor 4 driven from a generator 8 having a field winding 10 comprises a valve 48 having a reference potential applied to the grid 48G and a potential derived from a circuit supplied with current from a tachometer generator 62 driven at a speed proportional to the motor speed applied to the cathode 48C the flow of current through the valve controlling the amount of current flowing in the field winding 10. The motor 4 may drive a pair of rollers 2 for pulling a strip S through a processing line. The motor 4 is provided with a field winding 6 excited from a constant D.C. source P, N, whilst power for driving the motor is obtained from the generator 8, the generator field winding 10 also being excited from the source P, N, through a rectifier 12 and shunting resistor 14 to the line P and through the arm 16 of a motor operated potentiometer rheostat 18 to the line N. The potentiometer motor 20 is connected directly to the line P and through normally open contact 22 of a relay 104 or normally closed contact 24 of a relay 106 to the line N, the motor 20 being reversible by means of two field windings 26 and 28 energised through the contacts 22 and 24 respectively. A constant D.C. reference voltage is derived from A.C. lines L1, L2, across potentiometers 38, 40 via a transformer 30 and rectifier 32 ; potentiometer 38, adjustable contact 44 and switch 50 being designed for low operating speeds and potentiometer 40, adjustable contact 46 and switch 52 for high operating speeds. The reference voltage so derived is impressed upon the grid 48G of the valve 48 via switch 54 and resistances 58, 60. The voltage derived from the tachometer generator 62, developed across a variable resistor 64 is impressed on the cathode of the valve 48. The valve 48 controls the flow of current through the D.C. winding 70 of a saturable core reactor 72, thus controlling the power output of a pair of thyratrons 84, 86 by means of a phase shifting network including the reactor 72 and a further reactor 76, power to the thyratrons being supplied through a transformer 90. A transformer 96 is provided to isolate the thyratron control circuit from the rectifier 12 supply circuit. A. saturable core reactor 98 having a D.C. winding 100 under the control of the thyratrons 84, 86 controls the flow of current to the primary 96P of transformer 96. Connected across the D.C. terminals of the rectifier 12 through adjustable resistors 108, 110, are the two relays 104, 106 respectively, resistors 110 and 108 being adjusted so as to cause operation of relays 106 and 104 when the voltage output from rectifier 12 is approximately 40 per cent and 60 per cent of its rated output respectively. A current limiting resistor 114 and antihunting transformer primary 116P are provided in parallel with the shunt field winding 10. The secondary winding 116S is connected in parallel with the adjustable resistor 58. A rectifier 118 is also provided supplied through a transformer 120, the positive terminal being connected through resistance 124 and winding 70 to anode 48P of valve 48, and the negative terminal being connected through resistor 122 to the grid 48G of valve 48. Potentiometer 130 and resistance 126 are connected across the rectifier. When it is desired to start the line at a low speed switches 50 and 54 are closed thus impressing the reference voltage upon the grid 48G of valve 48. Anti-hunting transformer secondary 116S provides a potential having directional polarity and magnitude dependent upon the acceleration or deceleration of generator 8 due to changes in the voltage across the field winding 10. Owing to the voltage impressed upon grid 48G current flows in the valve 48 and reactor winding 70 which increases the current flow through thyratrons 84, 86 and reactor winding 100 which in turn increases the impressed voltage on rectifier 12, the current across field winding 10, the speed of generator 8, and hence the speed of the motor 4. As the rectifier output exceeds 60 per cent of its rated value relay 104 is energised thereby causing motor 20 to adjust rheostat 18 in a sense to further increase the motor speed. As the speed of the motor further increases the output from tachometer generator 62 applied to the cathode 48C of the valve 48 also increases. When the potentials of the cathode 48C and grid 48G become equal the current flow through the valve is reduced then reducing the voltage output from rectifier 12 until it is lowered to a value below 60 per cent of its rated value when relay 104 becomes de-energised. The motor 4 speed is then kept constant solely by the voltage difference between cathode 48C and grid 48G and consequent variation in a restoring sense of the rectifier 12 output. If the motor speed falls to such an extent that the rectifier output 12 is increased to above 60 per cent of its rated value, relay 104 is de-energised to operate motor 20 in a sense to increase the speed, whereas if the speed increases to such an extent that the rectifier output falls below 40 per cent of its rated value relay 106 is energised to operate motor 20 in a sense to decrease the speed. Rectifier 12 is arranged to operate normally at 50 per cent of its rated output. If high operating speeds are required switch 52 and potentiometer 40 are employed. To stop the motor switch 54 is opened which causes grid 48G to become negative with respect to the cathode owing to the voltage impressed from resistor 122, and to cease conducting. Rectifier 12 output falls and the motor is brought to rest. The conductivity of valve 48 may be varied when the system is at rest by closing a switch 134 in the potentiometer circuit 130 and moving the contact 132. The output from rectifier 12 for various grid volts mav thus be checked with the aid of a volt-meter 112.