1,219,836. Control of A.C. motors. BROWN, BOVERI & CO. Ltd. 8 March, 1968 [10 March, 1967], No. 11498/68. Heading H2J. [Also in Divisions G1 and G3] A polyphase motor 2 is energized from an inverter or frequency changer 1, the control signals for which are produced by an A.C. tachogenerator 6 which has two outputs in quadrature per phase, of constant amplitude for all frequencies (i.e. speeds), the two outputs being individually adjustable by " potentiometers " 4, 5 so that the phase angle of the resultant, at 48, can be varied. The " potentiometers " comprise electrical circuits whereby the ratio of their output to their input is adjusted by a D.C. control signal. As shown, the motor 2 is a synchronous machine and the control signals for 4 and 5 are derived from further potentiometers 7, 8 which are supplied with a motor current signal J1 and are controlled by signals proportional to cos α and sin α, where α is the angle between the rotor flux and the flux due to armature reaction, and is derived from a function generator 11 according to the motor current J1. The combined signal at 48 is compared with the motor current measured at 47 and the difference amplified at 3 to control the frequency and current output of the inverter or frequency changer 1. In an alternative embodiment the motor is an asynchronous machine and control is effected according to a desired slip frequency, which is produced by a function generator (equivalent to 11) according to motor current J1 and controls the " potentiometers " 7, 8. The object of the invention is to provide " four-quadrant operation" of the motor with a uniform torque at all speeds. The tachogenerator 6 may comprise pairs of Hall probes on a stator associated with a permanent magnet rotor or Hall probes built into the stator of the motor. Alternatively, in Figs. 6 ... 8 (not shown), it comprises aluminium tabs (34) carried by the rotor which pass through air gaps in iron cores 36 associated with transistor oscillators. When this happens the circuit stops oscillating and a pulse is produced. There are a larger number of these iron/oscillator circuits and their output pulses are fed through resistors (42) which are graded so that the combined outputs, when amplified (43) approximate to a sine wave.
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