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The most advanced vacuum cleaners are now using brushless drives, with operating speeds in excess of 100,000 revs/min. The drives are very sophisticated, but must also maintain low-cost. Sensorless control is desirable, but most sensorless methods involve extensive computation, which is prohibitively expensive at such high speeds. This paper looks at a 100,000 r/min, 1,600 W switched reluctance machine and drive system using the current gradient sensorless scheme and analyses parameters that affect the stability of the system. Commercial products require very robust control schemes: effects such as motor magnetic saturation, speed variations, changes in advance angle and changes in DC link voltage are examined to determine the boundaries within which the current gradient sensorless scheme is stable. Both simulation and measurements are used, demonstrating measures which ensure stable operation over the entire operating range.