M. Bradford’s scientific contributions

The overall machine analysis technique described has been based on an equivalent circuit representation of the induction motor. Methods of determining each additional component of loss have been outlined and the most useful approach has been found to be that based upon the solution of a series of harmonic equivalent circuits. In some cases simplified finite-element models have been found to be particularly useful in the determination of equivalent circuit parameters. The major sources of additional loss in inverter-fed machines have been identified as: additional stator winding loss arising from skin and proximity effects at high frequency; additional rotor winding loss arising from deep bar effects at high frequency; additional core loss due to the existence of harmonic fluxes; and additional stray loss

Cage induction motors can be supplied from various types of solid-state adjustable frequency controllers with a wide range of operating characteristics. However, the inverter/machine combination results in the necessity to derate the motor due to harmonic effects arising from the nonsinusoidal nature of the applied voltage. The overall magnitude and distribution of the additional loss and consequent reduction in motor rating depends on the harmonic content of the applied waveform, and on certain aspects of the machine design. Calculation of this additional loss requires a detailed knowledge of the current and flux redistributions which occur at high frequencies. A comprehensive study of the performance and design of inverter-fed machines is being undertaken at ERA as part of a major collaborative research programme. The authors summarise the general approach adopted in the study and outline suitable techniques for evaluating the major sources of additional loss

An explosion occurred within an electric motor on an off-shore platform in late 1985. It is considered that this ignition could have been the result of electrical sparking at the joints of the motor enclosure, due to circulating currents. The authors show that where circulating currents are interrupted by discontinuities at joints in the current path, arcing or sparking can occur which may be incendive, despite the very low voltages involved. The energy available for ignition depends upon the inductance of the current path and the magnitude of the circulating current. Test results indicate that in machines of interest, the effective inductance of the current path is typically 1 μH to 2 μH

The use of variable speed drive motors fed from static frequency converters is increasing generally by approximately 30% per annum. To facilitate their use in hazardous areas, a thorough study is required of the appropriate safety requirements so that relevant safety standards can be updated and acceptable and convenient procedures developed. The authors present a certification of invertor-fed induction motors for use in hazardous areas