Abstract: A magnetic field sensor coil wherein the coil is formed into a shape comprising two generally C-shaped limbs (2, 3) one within the other, one of these limbs being covered by a magnetic screening material (4). In operation an e.m.f. is induced in each turn of the unscreened part of the coil (3) while no e.m.f. is induced in the screened part (4) and so the total e.m.f. induced is approximately equal to the e.m.f. that would be induced in a simple concentrically wound coil of the same number of turns and of the same dimensions as the unscreened portion of the coil. The coil according to the invention is especially suitable for use with a system for monitoring an electric motor where the coil may be placed adjacent the motor end plate concentrically around the motor shaft without having to uncouple the motor from its load.

Abstract: A magnetic field motor monitor comprises an analogue-to-digital converter arranged to provide digital representations of instantaneous values of a motor magnetic field to a digital processor. A source of clock pulses provides clock pulses for the processor. The source is synchronized to a cyclical external signal, e.g. line (mains), as follows. A timing pulse is generated after a fixed number of clock pulses and the frequency of the clock pulses provided by the source is controlled in dependence on the time difference between the occurrence of the timing pulse and a fixed phase of the cycle of the external signal.

Abstract: A signal analysis system for a magnetic field motor monitor utilizes an analog-to-digital converter for sampling the motor magnetic field at accurately determined and equally spaced times synchronized with the line frequency driving the motor. A plurality of separate memories are provided to accumulate the respective samples which are added to or subtracted from the contents of respective memories, depending on which of the samples is being processed as indicated by control bits in a status register. Signals indicative of the amplitudes of respective harmonically related frequency components of the motor magnetic field are provided from the contents of the memories by combining the respective in phase and quadrature values. A monitor provides an indication of when there has been a change in these combined signals.