second key invention of Drs. Powell and Danby is the Linear Synchronous
Motor (LSM). The guideway has a set of LSM windings that carry a
small alternating (AC) current, e.g., several thousand amp turns.
The AC current is supplied from a conventional electrical power
grid. When energized, they produce a traveling magnetic wave that
pushes on the vehicle, overcoming whatever drag forces are acting
on it. In effect the vehicle "rides" the AC LSM wave much as a surfer
rides a water wave. It moves at the same speed as the LSM wave.
The speed is determined only by the frequency of the AC power fed
into the guideway. The maglev vehicles maintain a constant separation
distance between them regardless of variations in head or tail winds,
up or down grades, etc. acting on individual vehicles. Constant
separation is very important for safety. The vehicle is easily controlled
by adjusting the frequency of the LSM AC power wave.
High energy efficiency of the LSM propulsion system is achieved
by not continuously energizing the entire guideway. Instead, only
the local block of guideway on which the vehicle travels is energized.
The length of the energized block is typically several hundred feet.
When the vehicle leaves the energized block, LSM power is electronically
switched to the next block and the old block is switched off. In
this mode, LSM efficiency (propulsion power/input electric power)
is over 80%.