Why the 50DN is Great

[davidbrady]

Attached is a document and a pic showing the internals of a 50DN. The brushless design piqued my interest so I dug a little deeper. In the pic below you can see the rotor (3) which is made of solid steel. The field coil (4) sits inside the rotor but it doesn't spin; it's stationary, the rotor spins around it. The magnetic flux of the field coil magnetizes the rotor. One end of the rotor is always the North magnetic pole while the other end is South. The rotor has fingers that interleave and which direct the magnetic flux lines out radially and into the stator (2). At one end of the rotor the flux lines are directed from the rotor into the stator; at the other end of the rotor the flux lines are directed from the stator into the rotor. This orientation of flux lines stays constant and hence the name of this style of alternator: "homopolar induction alternator". The odd shape of the rotor with its alternating and interleaving steel fingers modulates the magnetic reluctance that the stator sees which induces an alternating current in the stator windings. In a typical Lundell alternator (the usual automotive kind), the field loop is wound onto the rotor and it spins with the rotor requiring slip rings or brushes. In a homopolar induction alternator a given section of the stator sees only a North pole or a a South pole, but never alternating poles. The benefit of homopolar is that the rotor is brushless so it can be spun at extremely high rpms. Spinning at high rpms means the same power can be realized with less torque, less torque means a smaller form factor (smaller case). Other advantages are better cooling, fewer moving parts which leads to greater reliability and longevity, and higher efficiency. Interestingly, current Formula 1 teams use a kinetic energy recover/delivery system. The kinetic energy is stored in the rotor of a homopolar induction alternator and the output of the alternator charges Li-Ion batteries. The energy is stored during braking and the alternator acts as a motor during acceleration drawing from the Li-Ion batteries and the stored flywheel energy of the spinning rotor. Homopolar designs are also used in guided missiles and in spacecraft. The 50DN may be a legendary bus and coach workhorse but it's actually leading edge!