An electrical machine having: a variable reluctance rotor, and a stator formed as an annular array of stator segments, the reluctance of the rotor-to-stator magnetic flux path varying with rotor position whereby the stator segments are magnetically energizable to rotate the rotor; wherein: the stator segments are non-axisymmetrically distributed in the array such that, when energized to rotate the rotor, they produce an unbalanced force on the rotor; and characterised in that the machine further has a compensator including one or more balancing segments which are configured to be magnetically energizable to produce a balancing force on the rotor which balances the unbalanced force, the reluctance of the rotor-to-compensator magnetic flux path being substantially invariant with rotor position, wherein the, or each, balancing segment has a core structure and a conductor winding mounted to the core structure, which conductor winding is electrically excitable by a control system to magnetically energize the balancing segment, and wherein the core structure of the, or each, balancing segment has plural salient teeth projecting towards the rotor and arranged in a row which extends to either side of the conductor winding, and wherein, to produce the substantially invariant reluctance of the rotor-to-compensator magnetic flux path, the teeth are unequally spaced to either side of the conductor winding and/or the teeth are of unequal width to either side of the conductor winding. a variable reluctance rotor, and a stator formed as an annular array of stator segments, the reluctance of the rotor-to-stator magnetic flux path varying with rotor position whereby the stator segments are magnetically energizable to rotate the rotor; wherein: the stator segments are non-axisymmetrically distributed in the array such that, when energized to rotate the rotor, they produce an unbalanced force on the rotor; and characterised in that the machine further has a compensator including one or more balancing segments which are configured to be magnetically energizable to produce a balancing force on the rotor which balances the unbalanced force, the reluctance of the rotor-to-compensator magnetic flux path being substantially invariant with rotor position, wherein the, or each, balancing segment has a core structure and a conductor winding mounted to the core structure, which conductor winding is electrically excitable by a control system to magnetically energize the balancing segment, and wherein the core structure of the, or each, balancing segment has plural salient teeth projecting towards the rotor and arranged in a row which extends to either side of the conductor winding, and wherein, to produce the substantially invariant reluctance of the rotor-to-compensator magnetic flux path, the teeth are unequally spaced to either side of the conductor winding and/or the teeth are of unequal width to either side of the conductor winding. the stator segments are non-axisymmetrically distributed in the array such that, when energized to rotate the rotor, they produce an unbalanced force on the rotor; and characterised in that the machine further has a compensator including one or more balancing segments which are configured to be magnetically energizable to produce a balancing force on the rotor which balances the unbalanced force, the reluctance of the rotor-to-compensator magnetic flux path being substantially invariant with rotor position, wherein the, or each, balancing segment has a core structure and a conductor winding mounted to the core structure, which conductor winding is electrically excitable by a control system to magnetically energize the balancing segment, and wherein the core structure of the, or each, balancing segment has plural salient teeth projecting towards the rotor and arranged in a row which extends to either side of the conductor winding, and wherein, to produce the substantially invariant reluctance of the rotor-to-compensator magnetic flux path, the teeth are unequally spaced to either side of the conductor winding and/or the teeth are of unequal width to either side of the conductor winding.