Patent GB-663159-A (GB)
Title: Improvements in or relating to automatically controlled high frequency heating apparatus
IPC: H05B6/04
Filed: 1949-12-21
Abstract: 663,159. Automatic temperature control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Dec. 21, 1949 [Dec. 22, 1948], No. 32732/49. Class 38 (iv). Apparatus for regulating the temperature of work being conveyed at a variable rate through the workreceiving space of high-frequency heating apparatus comprises a highfrequency oscillation generator, the output of which is controlled in dependence on the difference between a voltage proportional to the output of the generator and a voltage proportional to a function of the heat energy required to effect heating of the work to a predetermined temperature, e.g. speed of travel of work, heat energy or light-energy radiated from the work. As shown in Fig. 2 a high-frequency oscillator 21 supplies current to a heating coil 3 through which the work to be treated passes. The oscillator 21 is supplied from a series of gridcontrolled thyratrons 53, 55, 57 in series with fixed grid thyratrons 59, 61, 63 respectively. The grids of the thyratrons 53, 55, 57 are supplied from transformers 91, 93, 95 in turn supplied from the secondary 109 of a phase shifter 111. The primary 113 of the phase shifter is connected mechanically to a reversible motor 15 which is controlled by normally cut-off valves 133, 135 when switches 145, 147 are in the automatic position. Manual push-button control of the motor 15 is provided, if desired. The motor 15 is supplied from line 119 which also supplies a transformer 185 feeding either a resistance 169 or 171, depending upon the position of a differential relay 175. These resistors control the bias of valves 135, 133, respectively. The differential relay 175 has two coils 189, 191, the coil 189 being fed with the rectified output of a tachometer generator 17 driven in accordance with the speed of the work to be treated. The other coil 191 is fed with a fixed voltage derived from a rectifier 211, a predetermined amount being tapped off the resistor 209 by a slide 207, and with a voltage, developed across resistor 203, depending on the generator 21 output. When the apparatus is running normally with the temperature of the work and the speed of the work at the predetermined values the currents flowing through the coils.189, 191 of the differential relay balance each other, the valves 133, 135 remain cut off and motor 15 is de-energized. If the speed of the work increases, its temperature therefore falling, the output of the tachometer generator 1 7 increases so that there is a predominance of current through the coil 189 causing the relay contact 181 to engage stationary contact 179 energizing resistor 171. The valve 133 therefore conducts on positive half-cycles of voltage and current flows through the valve energizing a relay 121 in turn energizing the motor 15 to adjust the phase shifter 111 to increase the power supplied by the thyratrons 53, 55 ... 63 to the H.F. generator 21. The temperature of the work is thereby restored to its predetermined value and the increased output of the generator 21 causes an increased voltage across resistor 203 with a resultant increased current flow through the coil 191 until a balance is again restored. If the speed of the work decreases with a consequent rise in temperature the relay 175 causes energization of the resistor 169 and the motor 15 is operated in the reverse direction by a relay 123, the output of the generator 21 being decreased. In a further embodiment (Fig. 3), the temperature of the work is maintained constant by detecting the temperature directly by means of a thermocouple 328. The thermocouple output is applied to a first amplifier 252 and second amplifier 264, the output of which supplies the differential relay coil 189. A voltage stabilizer 308 is provided and the supply is derived from a rectifier unit 274. The apparatus of Fig. 3 replaces the tachometer generator of Fig. 2, the remainder of the apparatus remaining unchanged except that the connections to the motor 15 are reversed, since an increase of temperature calls for a decrease of generator 21 output whereas on increase of speed calls for an increase of output. In another embodiment the temperature is directly detected by means of a photo-electric cell, Fig. 4 (not shown).