A motor includes a rotor having a plurality of pole magnets angularly spaced about the rotor's rotational axis through an angle of 360°, and a stator including a plurality of windings angularly spaced about the rotational axis through an angle less than 360° to provide an angular section free of windings, wherein at least one of the plurality of pole magnets is not radially opposite any of the plurality of windings at any given rotational position of the rotor. The motor is designed for use in powering a horizontal blade reel of a cylindrical lawn mower, and includes a housing having a cross-sectional shape in the form of a chord-truncated circle, such that the housing has a flat portion corresponding to the angular section free of windings. The truncated motor housing avoids unattractive “layover” of grass during mowing.
1. A motor comprising:
a rotor rotatable about a rotational axis, the rotor including a plurality of pole magnets angularly spaced about the rotational axis through an angle of 360°, wherein the rotor includes exactly twenty pole magnets;
a stator including of a plurality of windings angularly spaced about the rotational axis through an angle less than 360° to provide an angular section free of windings, wherein the stator includes a laminated core having a plurality of teeth for supporting the plurality of windings, wherein the plurality of teeth are angularly spaced about the rotational axis at regular angular intervals through an angle of approximately 270°, wherein the laminated core includes exactly eighteen teeth, and wherein the angular interval between adjacent teeth is 15°;
a Hall effect sensor for detecting rotational position of the rotor relative to the stator; and
a back EMF zero-crossing detection circuit;
wherein at least one of the plurality of pole magnets is not radially opposite any of the plurality of windings at any given rotational position of the rotor; and
wherein the motor is commutated based on a signal from the Hall effect sensor when the motor is operating at or below a predetermined threshold rpm, and wherein the motor is commutated based on a signal from the back EMF zero-crossing detection circuit when the motor is operating above the threshold rpm.
2. The motor according to claim 1 , further comprising a housing, wherein the housing has a flat portion corresponding to the angular section free of windings.
3. The motor according to claim 2 , wherein the motor housing is elongated along the rotational axis and has a cross-sectional shape in the form of a chord-truncated circle.
4. The motor according to claim 1 , wherein one-quarter of the plurality of pole magnets are not radially opposite any of the plurality of windings at any given rotational position of the rotor.
5. The motor according to claim 1 , wherein the motor is commutated based on a signal from the Hall effect sensor as a failsafe when the motor is operating above the threshold rpm and the back EMF zero-crossing detection circuit fails to detect back EMF zero-crossing.