A three-phase permanent magnet-type outer rotor synchronous motor, comprising: a rotor with a plurality of permanent magnets that define a number N = 20 magnetic poles arranged circumferentially; and a stator with an arrangement of teeth separated by a number P = 24 slots, a fraction of 2N/3P thereby not being an integer, wherein each tooth faces a surface defined by the plurality of magnets via an airgap, characterised in that a tooth width (L 3) of the tooth in a circumferential direction is 1/2 of a slot pitch (L 4) of the stator; each magnet is embedded in a surface of the rotor and facing the stator, the magnet being shaped such that both an inner surface facing the teeth of the stator and an outer circumferential surface define respective outwardly curved arcs, an angle subtended by the outer circumferential arc at a radial centre being larger than an angle defined by respective ends of the inner surface arc and the centre; the rotor is provided with a plurality of recesses, located between adjacent magnets and facing the stator; and each magnet has a width (L 1) and is distributed about the rotor in accordance with a pole pitch (L 2), wherein a ratio of the width (L 1) to the pole pitch (L 2) is 2/3. a rotor with a plurality of permanent magnets that define a number N = 20 magnetic poles arranged circumferentially; and a stator with an arrangement of teeth separated by a number P = 24 slots, a fraction of 2N/3P thereby not being an integer, wherein each tooth faces a surface defined by the plurality of magnets via an airgap, characterised in that a tooth width (L 3) of the tooth in a circumferential direction is 1/2 of a slot pitch (L 4) of the stator; each magnet is embedded in a surface of the rotor and facing the stator, the magnet being shaped such that both an inner surface facing the teeth of the stator and an outer circumferential surface define respective outwardly curved arcs, an angle subtended by the outer circumferential arc at a radial centre being larger than an angle defined by respective ends of the inner surface arc and the centre; the rotor is provided with a plurality of recesses, located between adjacent magnets and facing the stator; and each magnet has a width (L 1) and is distributed about the rotor in accordance with a pole pitch (L 2), wherein a ratio of the width (L 1) to the pole pitch (L 2) is 2/3. each tooth faces a surface defined by the plurality of magnets via an airgap, characterised in that a tooth width (L 3) of the tooth in a circumferential direction is 1/2 of a slot pitch (L 4) of the stator; each magnet is embedded in a surface of the rotor and facing the stator, the magnet being shaped such that both an inner surface facing the teeth of the stator and an outer circumferential surface define respective outwardly curved arcs, an angle subtended by the outer circumferential arc at a radial centre being larger than an angle defined by respective ends of the inner surface arc and the centre; the rotor is provided with a plurality of recesses, located between adjacent magnets and facing the stator; and each magnet has a width (L 1) and is distributed about the rotor in accordance with a pole pitch (L 2), wherein a ratio of the width (L 1) to the pole pitch (L 2) is 2/3.