Superconducting Electromagnetic Suspension (EMS) System for Grumman Maglev Concept

Maglev vehicles normally employ one of the two types of Maglev suspension systems: Electrodynamic Suspension (EDS) and Electromagnetic Suspension (EMS). The EDS system provides suspension by a repulsive force between the magnets on the vehicle and the reaction magnets (or conductive metal sheets) on the guideway. On the other hand, the EMS system is based on an attractive force between electromagnets on the vehicle and iron rails on the track. Grumman has adopted the EMS for its Maglev. The Grumman Maglev system consists of superconducting C-iron cored magnets on the vehicle. They are attracted to iron rails mounted on the underside of the guideway. The magnets and rails are oriented in an inverted 'V' configuration in such a manner that the attractive force vectors between the magnets and the rails act through the center of gravity of the vehicle. These magnets simultaneously perform functions of vehicle levitation and propulsion. They are powered by NbTi superconducting oils operating at 4.2K. An electromagnet consists of a C-core, a superconducting (SC) coil on the back leg of the C-core and a normal control coil on each leg of the C-core. The SC coil provides the nominal lifting capability and the normal coils handle rapid variations in load with respect to the nominal value. The EMS system provides levitation at all speeds but the EDS only starts to levitate the vehicle at speeds above - 60 m.p.h. Below this speed, the vehicle must be supported by wheels. In an EDS system, SC magnets are shielded from the harmonics of propulsion winding by an aluminum shield. The thickness of this shield is inversely proportional to the square-root of the frequency. At very low speeds, the frequency of harmonic fields generated by the traction winding is low and it is, therefore, difficult to attenuate them. These low frequency harmonic fields generate losses in the SC coils and often force them to go normal. However, in the EMS system, the whole flux mostly remains confined inside the iron-core at all speeds and therefore does not have the inherent shielding problem of the EDS system. Because of the iron in the EMS system, stray field in the passenger compartments is also well below the acceptable levels whereas in the EDS systems this field is usually too high in the passenger compartment and it must be reduced down to acceptable levels by introduction of shielding materials at additional cost/weight penalty. The EMS can use conventional rebar in the guideway but the EDS system must use non-magnetic non-conductive rebar.