Patent GB-647244-A (GB)
Title: Improvements in and relating to high frequency heating apparatus
IPC: H05B6/06
Filed: 1946-07-22
Abstract: 647,244. Automatic control system for highfrequency heating apparatus. BRITISH THOMSON-HOUSTON CO., Ltd. July 22, 1946, No. 21845. Convention date, July 24, 1945. [Class 38 (iv)] High-frequency heating-apparatus for heating magnetic materials comprises driving means including an electric motor for feeding the workpiece into a heating coil, and means responsive to the current in the coil for varying the speed of the motor so that a predetermined amount of magnetic material is maintained in the coil, the material being heated to a temperature at least as high as the decalescent temperature. In the Fig. 1, a piece of magnetic material 1 is fed under control of electric motor 2 into a heating coil 3 supplied with energy from oscillator 4. Current for the armature of the motor is supplied through thyratron valves 8 and 9 connected to the ends of secondary winding 12, via lead 15, switch 17 (or reversing switch 18), the armature and lead 16, and voltages are applied to the grids of valves 8 and 9 to control the motor current. The grid voltages are derived in the first instance from a phase-shifting circuit consisting of resistance 21, condenser 23, condenser 24 and resistance 22, connected across secondary winding 19, so that the grid voltage differs by 90 degrees from the anode voltage in each case. This grid voltage is further modified by also connecting grid-controlled valve 31 and diode 32 across the winding 19, the circuit from the cathode of valve 31 consisting of resistance 7, part of resistance 36, lead 16, switch 17 (or reversing switch 18), the motor armature and conductor 15. The arrangement is such that diode 32 tends to provide a negative bias on the cathodes of the thyratrons but an increase in the current through valve 31 opposes this effect so that the thyratron current is reduced, thereby reducing the speed of the motor. The conductivity of valve 31 is jointly controlled by a voltage which is supplied to its grid, and a voltage supplied to its cathode. The grid control voltage tends to maintain a constant predetermined voltage across the motor armatures plus a small additional voltage to compensate for the resistance voltage drop in the armature, and reference is made to Specification 617,324. The armature voltage is impressed across a potentiometer consisting of resistances 47, 48, and a part of 50. The grid of valve 31 is connected to a tapping on resistance 48 so that as the armature voltage rises, the grid of valve 31 goes positive and eventually valve 31 conducts, thereby reducing the current supplied to the motor. Resistance 50 is also supplied with a voltage derived from rectifier 75, which is supplied from a transformer, the primary coils 77, 78 of which are fed with current from valves 8 and 9, so that the potential of the tapping on resistance 50 decreases with increase of current to the motor, thereby compensating for the voltage drop in the armature. The cathode potential of valve 31 includes the voltage across resistance 7 in the oscillator circuit which is superimposed on the positive voltage obtained from potentiometer 36. Potentiometer 36 is connected between the main negative lead 16 and lead 41 (via switch 17 or 18), which is supplied with a positive voltage from rectifier 40 connected to secondary winding 12. Thus, as the voltage drop across resistance 7 increases, corresponding to increased current in heating coil 3, the cathode of value 31 goes negative, tending to increase the current through valve 31 and therefore decrease the current supplied to motor 2. Valve 80 is provided to limit the armature current to a predetermined maximum value, and is connected, in series with resistances 81 and 82 between one end of winding 19 and the negative lead 16. The grid of valve 80 is connected to the cathode of rectifier 75, and the cathode is supplied with a positive potential due to current flowing through resistances 81 and 82 from positive lead 41, so that valve 80 is normally non-conducting. If the voltage across resistance 74 rises sufficiently due to excessive motor current, valve 80 conducts and since this valve is effectively in parallel with valve 31, this reduces the motor current supplied by valves 8 and 9. Valve 83 is provided to increase the speed of response of valve 80, and acts as a limiter to prevent the potential of the grid of valve 80 falling below that of the junction point of resistors 81 and 82. Starting switch 60 controls the operation of switch 17 through coil 61, and the operation of oscillator 4 through coil 90 and switch 87, and reverse switch 64a controls switch 18 to reverse the motor. Limit switches 63 and 73 are operated by the workpiece to stop the motor. Quenching means are also provided. In operation, the insertion of a magnetic substance into coil 3 causes the oscillator current through resistance 7 to increase so that valve 31 conducts to reduce the speed of motor 2. As the temperature of the work-piece rises above the decalescence point where it becomes non-magnetic, the current in coil 3 falls, and results in an increase in the rate of feed, the final result being such that a predetermined amount of magnetic material is maintained within coil 3, and the work-piece is heated to the decalescent temperature..