An electrical machine having: a stator with windings; a first rotor rotatably fixed relative to a drive mechanism, the first rotor being radially inwards of the stator, the first rotor carrying alternating polarity first field magnets arranged in a circumferential direction of the first rotor, such that, on rotation of the drive mechanism, the windings interact with the magnetic flux produced by the first magnets to create an EMF across the windings; and a second rotor inwards of the stator, the second rotor carrying alternating polarity second field magnets arranged in a circumferential direction of the second rotor, the second rotor having a first rotational position relative to the first rotor in which the second magnets of the second rotor are aligned relative to the first magnets of the first rotor to reduce the magnetic flux energy of the combined magnets of the first and second rotors, the second rotor having a second rotational position relative to the first rotor in which the second magnets of the second rotor are aligned relative to the first magnets of the first rotor to increase the magnetic flux energy of the combined magnets of the first and second rotors, and the second rotor being freely rotatable relative to the drive mechanism at least between the first and second rotational positions; wherein the field magnets are permanent magnets; characterised in that the electrical machine further has an electrical output regulator configured to regulate an electrical current drawn from the windings to produce a torque on the first and second rotors (31, 32) such that, as the drive mechanism increases from zero rotational speed, the torque rises above a threshold level that moves the second rotor from the first rotational position to the second rotational position, and, as the drive mechanism further increases the rotational speed, the torque peaks and then drops below the threshold level to move the second rotor back to the first rotational position. a stator with windings; a first rotor rotatably fixed relative to a drive mechanism, the first rotor being radially inwards of the stator, the first rotor carrying alternating polarity first field magnets arranged in a circumferential direction of the first rotor, such that, on rotation of the drive mechanism, the windings interact with the magnetic flux produced by the first magnets to create an EMF across the windings; and a second rotor inwards of the stator, the second rotor carrying alternating polarity second field magnets arranged in a circumferential direction of the second rotor, the second rotor having a first rotational position relative to the first rotor in which the second magnets of the second rotor are aligned relative to the first magnets of the first rotor to reduce the magnetic flux energy of the combined magnets of the first and second rotors, the second rotor having a second rotational position relative to the first rotor in which the second magnets of the second rotor are aligned relative to the first magnets of the first rotor to increase the magnetic flux energy of the combined magnets of the first and second rotors, and the second rotor being freely rotatable relative to the drive mechanism at least between the first and second rotational positions; wherein the field magnets are permanent magnets; characterised in that the electrical machine further has an electrical output regulator configured to regulate an electrical current drawn from the windings to produce a torque on the first and second rotors (31, 32) such that, as the drive mechanism increases from zero rotational speed, the torque rises above a threshold level that moves the second rotor from the first rotational position to the second rotational position, and, as the drive mechanism further increases the rotational speed, the torque peaks and then drops below the threshold level to move the second rotor back to the first rotational position.