HTS Motors and Generators
Office of Naval Research – 2G HTS Generator | Reliance Electric - HTS Motor Program
Office of Naval Research - 2G HTS Generator Program (2005–ongoing)
Large generators are utilized in both the electric utility and the shipbuilding industries to convert mechanical energy (i.e. rotational input energy from a steam or gas turbine) into electricity. The US Navy is developing many new technologies to support its integrated power system (formerly ‘all electric ship’) program. Superconducting motors and generators for ship propulsion and on-board utility service are being evaluated for their applicability in this environment.
SuperPower, Inc. (2G Wire manufacturer, Coil Manufacturer)
Baldor's Reliance Electric (Motor and Generator Manufacturer)
(Formerly Rockwell Automation)
General Dynamics – Electric Boat Division
Oak Ridge National Laboratory (ORNL)
Naval Research Laboratory (NRL)
Naval Surface Warfare Center (NSWC)
HTS generators offer improved efficiency thereby reducing machine losses by as much as 50 percent compared to conventional generators of comparable size. In addition, HTS generators are substantially smaller and lighter than copper based machines. These advantages are very attractive in a ship environment where space and weight are at a premium. Heat and thermal cycling of conventional rotating machines are one of the biggest detractors to reliability and life expectancy. HTS rotating machines virtually eliminate these failure modes by operating a near constant cryogenic temperature.
The project team commenced work in the fall of 2005 and has completed a feasibility study to examine the advantages of high temperature superconducting (HTS) generators for naval applications and has also been awarded funding for a conceptual design/risk assessment study.
A typical superconducting generator configuration consists of the rotor with the HTS field windings spinning inside the stationary windings (stator) that surround the rotor core. The rotor assembly is typically cooled to cryogenic temperatures while the stator windings are maintained at ambient conditions. The HTS field windings will initially operate in the 25 to 40 Kelvin temperature range with DC magnetic field of up to 4 Tesla. As 2G materials become available with improved performance characteristics, higher temperature operation becomes viable. The field coils located on the rotor are cooled by a commercially available cryocooler system using either cold gas or liquid as the heat transfer medium.
As the magnetic field produced by the rotor field windings cuts across the turns of wire in the stationary coils, an electric current is set up in the wire. In a three phase generator, there are usually three separate stator windings, each producing its own separate single-phase voltage. Since these windings are staggered around the generator circumference, each of the single-phase voltages is "out of phase" with one another.
The primary application of this technology is propulsion motors and generators for both military and commercial vessels. The technology developed under this program will be applicable to other uses such as large electric utility motors and generators.
Baldor's Reliance Electric (formerly Rockwell Automation)
- HTS Motor/Generator Program (2002 – 2005)
Large motors convert 30 percent of all U.S. electrical energy generated and 70 percent of these motors are well suited to utilize HTS technology. Converting the installed base of 1,000-hp and larger motors to HTS could save more than $600 million annually. The worldwide market for HTS motors greater than 1,000- hp is estimated at more than $300 million per year.
Reliance Electric (division of Baldor, formerly of Rockwell Automation)
Oak Ridge National Laboratory
High-temperature superconducting (HTS) motors may increase machine efficiency beyond 98 percent, reducing losses by as much as 50 percent compared to conventional motors, producing energy savings, reduced pollution per unit of energy produced, and lower life-cycle costs.
In March 1996, Rockwell Automation demonstrated a 200-hp motor prototype using 1G HTS coils, which exceeded specifications by 60 percent. In 2001, Rockwell Automation completed the assembly and testing of a 1,600-hp motor. This motor, using 1G HTS coils, has exceeded expectations for performance and reliability. The results of the tests have been used to develop specific design parameters for a 5,000-hp motor. In 2004, Rockwell Automation demonstrated a 1.2-hp motor using 2G HTS coils, supplied by SuperPower, in the rotor windings, the world’s first rotating machinery using this new type of HTS wire. Rotor coil performance and design has since improved with 7.5 hp (on a 5 hp frame) being demonstrated in 2005.
The HTS motor is a cryogenically cooled, “super” efficient synchronous motor with HTS field windings. In one configuration, the adjustable speed drive (ASD) used to power the motor is a conventional rectifier/ inverter system as used with conventional AC induction motor products, modified to operate with a synchronous motor. The field coils are cooled by a commercially available cryocooler system that feeds cold helium gas (25 to 40 K) to the rotor and receives warmed helium gas in return.
The rotating HTS field winding creates a magnetic field in the copper armature winding. In planned commercial models, the magnitude of this field is approximately twice that of a conventional motor. In one configuration, the HTS motor has an air core (i.e., nonmagnetic) construction, so that the air gap field can be increased without the core loss and saturation problems inherent in a laminated iron stator and rotor core. The copper armature winding lies just outside the air gap.
Under steady-state operation, the rotor spins in-sync with the rotating field created by the three-phase armature currents, and the superconducting field winding experiences only DC magnetic fields. Under load or source transients, however, the rotor will move with respect to the armature-created rotating field, and it will experience AC fields. The AC fields are shielded from the HTS field winding by warm and cold AC flux shields located between the HTS coils and the stator winding. Inside the warm outer AC flux shield will be a thermal insulation space (vacuum) that surrounds the rotor cryostat. The cold AC flux shield is on the inside surface of this vacuum space and is a high-conductivity shell that stays at about the temperature of the superconducting coils. The superconducting field coils are located within the inner shield on a non-magnetic support structure.
The primary applications of these devices will be large motors (greater than 1,000-hp) used for pump and fan drives for utility and industrial markets. Applications requiring continuous operation will provide the best operational advantages for those motors.
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