Homopolar motor for a flywheel energy storage system

A homopolar motor, comprising:

• a housing;
• a rotor, attached to a shaft of the motor, which rotates with the shaft around a vertical axis;
• a stator, encircling the rotor, comprising a plurality of pole pieces, wherein the space between each pole piece and the rotor forms a first, radial, air gap;
• a stationary field winding, attached to the housing, which generates magnetic flux that magnetically energizes the first air gap;
• and
• a field coupler comprising an interleaved top portion and a bottom portion, wherein the top portion is attached to and rotates with the rotor, and the bottom portion, attaches to the housing and is stationary, wherein the space between the interleaved top portion and the bottom portion of the field coupler forms a second, axial, air gap, and wherein the cross section of the interleaved top portion is at least one downward pointing trapezoid that narrows from top to bottom and the cross section of the interleaved bottom portion is at least one upward pointing trapezoid that narrows from bottom to top, and wherein each of the trapezoids are isosceles trapezoids with an angle, θ, between the base of the base of the trapezoids and each of its two equal length sides, wherein θ is substantially between 75 degrees and 85 degrees, thereby increasing the surface area of the second air gap and proportionately reducing the magnetic flux density across the second air gap.