As the pioneers of superconducting maglev, Powell and Danby made
two basic inventions that have been key to its development:
- The Null Flux Suspension
- The Linear Synchronous Motor (LSM)
The Null Flux suspension makes the power losses in the guideway
from the induced currents in normal metal loops very low. As a result
the magnetic drag force on the vehicle is small. In fact, it is
much smaller than the air drag force. This is important because
maglev vehicles then need much less energy per passenger mile and
ton mile than other modes of transport. Moreover, the Null Flux
suspension also is inherently and passively stable, and strongly
counteracts all external forces (winds, up and down grades, curves,
etc.) that try to push the vehicle away from its equilibrium point.
Because the levitated vehicle does not contact the guideway, conventional
propulsion cannot be used. Instead, maglev vehicles are magnetically
propelled by the highly efficient Linear Synchronous Motor (LSM).
In the LSM, a small alternating current in a second set of guideway
loops (the LSM propulsion loops are distinct from the loops that
levitate and stabilize the maglev vehicle) magnetically push on
the superconducting magnets, propelling the vehicle along the guideway.
The superconducting magnets on the vehicle are DC (Direct Current)
magnets - that is, their magnetic fields do not vary with time.
However, the magnetic polarity -that is, the direction of the magnetic
field- of the magnets alternates along the vehicle. Accordingly,
the guideway loops experience an alternating wave of magnetic flux
as the vehicle moves past. A downwards magnetic flux is followed
by an upwards flux, then by a downwards flux, and so on.