Control of electrical machines

An electrical machine for converting electrical energy into mechanical energy and/or mechanical energy into electrical energy, the machine comprising:

• a rotor having a plurality of rotor teeth;
• a stator having a plurality of stator teeth for rotatably receiving said rotor and having (i) at least one field magnet device for generating a first magnetomotive force between said rotor and said stator and including at least one first electrical winding, and (ii) at least one armature magnet device including at least one second electrical winding adapted to carry electrical current varying in synchronism with rotation of said rotor relative to said stator to generate a second magnetomotive force having a component transverse to said first magnetomotive force;
• at least one control device for controlling supply of electrical current to or from the or each said second electrical winding by applying a sequence of switching cycles wherein the electrical current in the or each said electrical winding has positive and negative conduction blocks, each of the said conduction blocks has at least two regions, including a first region and an end region;
• and
• at least one rotor position sensor device for detecting at least one electrical signal related to the rotational position of the rotor relative to the stator and induced in a respective said first or second electrical winding as a result of a current passing through a respective second or first electrical winding, wherein at least one said rotor position sensor device is configured to undertake the steps of measuring the said electrical signal during two different states of a switching cycle in the end region, determining a difference between the measurements taken in each of the two different states and comparing the difference with at least one similar measured difference in at least one previous switching cycle to determine if a known rotor position has been reached.