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Abstract

A drive arrangement comprises an a.c. induction motor (M1) having a rotary output shaft, and a drive control (FIG. 2), by which the speed of rotation of the shaft is controlled. The drive control comprises a signalling arrangement (e.g. a transducer (T) supplying an analogue signal converted by an analogue-to-digital converter (ADC) or a direct digital signalling arrangement) for supplying a digital signal having a value corresponding to the desired speed of rotation of the motor shaft, together with a processor (CPU) which receives the digital signal and at least one other input and processes the digital signal in accordance therewith. The processor supplies a corresponding, but modified, digital output signal to a digital-to-analogue converter (DAC) which converts the digital output signal to an analogue control signal and supplies it to an inverter which, in response thereto, drives the a.c. induction motor. An encoder (E) is operatively connected to the output shaft of the motor and supplies monitoring signals to the processor which processes them to provide a comparison value (representing actual rotation speed of the shaft) for comparison with a value corresponding to the signal from the signalling arrangement (representing the desired rotation speed). In the event of a discrepency between the two values, the digital output signal to the DAC is modified accordingly. A further input is provided by setting means (SS7) for setting a maximum speed of rotation of the shaft as a proportion of the 'basic speed' of the motor, said setting means also supplying a signal to the processor (CPU) whereby the output digital signal is modified by a corresponding reduction factor.

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