Patent GB-884800-A (GB)
Title: Arrangement for the automatic operation of a three-phase motor fed with variable frequency currents
IPC: H02P1/16
Filed: 1959-06-18
Abstract: 884,800. Control of A.C. motors. SANDOZ Ltd. June 18, 1959 [June 19, 1958], No. 20978/59. Class 38 (3). A centrifuge is driven, at a preselected speed and for a predetermined period, by a threephase motor which is supplied from two valve oscillators, the tuning of one being fixed and the other variable, the variable-frequency oscillator being associated with a capacitor C6 adjusted by servomotor M1 Three parallelconnected frequency-mixer triode-hexodes provide a manually-adjustable three-phase output, at a frequency which is the difference between the oscillator frequencies, to an amplifier each phase of which comprises a double-triode phasesplitting stage and a push-pull pentode stage coupled through a transformer and power tetrodes to a step-down transformer. The secondary winding of the latter, together with the secondaries of the other two phases, are connected to the stator windings of the motor. A field winding 55 of the servomotor M1 is supplied from a stable valve oscillator associated with capacitor C2 driven by the servomotor M1 and coupled through two amplifier stages to a discriminator comprising a triode-pentode and a double diode-triode.  A field winding 57 of a servomotor M2 is energized to afford a continuous indication of the temperature of the rotor of the three-phase motor, a temperature-responsive resistor being connected with potentiometers 59, 60, and the slider of the potentiometer 60 being adjusted by the motor M2 to a point of balance. A control set for the generating circuit comprises an indicating tube 61 associated with the field windings 55 or 57 through a selector switch 62, whilst an indicating tube 63 compares the phase relationships of the three-phase supply. When the installation is first switched on, a pilot lamp L1 is lit and the speed of the three-phase motor is pre-set by capacitor C1 through reversing switch W1. The time of operation is determined by a disc clutched to synchronous motor M3 which returns the disc to its original position after the pre-selected speed has been reached. Operation of switch S1 energizes relay 65 which is kept operated by coil 67 until the discriminator circuit de-energizes the servomotor M1 when the desired speed of the threephase motor has been attained. The motor M1 also drives a limit switch 68 to prevent the generator frequency from becoming excessive, whilst switches S4, S5 prevent starting under unsafe conditions. When the preselected speed is reached and the relay 65 returns to its central position, the speed is maintained constant by the discriminator circuit and the motor M3 is started together with a seconds-counting mech-  anism driven by synchronous motor M4. After the preselected time has elapsed, the relay 65 drops and returns the motor M1, whilst the motors M3, M4 are stopped. The seconds-  counting mechanism is returned to zero by a motor M5 whereupon pilot light L4 is lit. When zero speed is reached, limit switch 68 deenergizes the relay 65 to stop the motor M1, disconnect motor M5 and de-energize the lamp L4. The angular positions of the capacitor C1 and motors M1-M3 are indicated by sliders on a scale.