Figure 1. A 1.2-megawatt generator equipped with magnetic bearings.
(Photo courtesy of Turbo Power Systems, Ltd. UK, www.turbopowersystems.com.)
Localized power generation is a growing trend around the globe and, of course, stands in contrast to centralized power generation, in which central generating facilities create electricity and distribute it throughout a country or region. Many times the local units serve as prime power units, as in remote regions or telecomm applications. Other times, the units augment utility power, providing backup and emergency solutions in data centers, hospitals, and other mission-critical facilities.
Figure 2. Cutaway of a 1.2-megawatt generator reveals magnetic bearings. (Photo courtesy of Turbo Power Systems, Ltd. UK, www.turbopowersystems.com.)
Magnetic Bearings
Generators outfitted with magnetic bearings often have advantages over generators with conventional bearings. To understand these advantages, one must first understand the fundamental features and characteristics of equipment outfitted with magnetic bearings.Magnetic bearings use electromagnetic coils to suspend or levitate a rotating shaft in whatever the surrounding medium might be - usually air, but the bearings will function in a vacuum or other medium. Sensors monitor the shaft’s position and supply those data to a digital controller. In turn, the controller can change the current in the coils and thereby change the electromagnetic forces on the shaft. In terms of control loop speed, these changes happen very quickly, allowing the shaft’s position to be precisely maintained at very high rotational speeds.
With the rotating shaft suspended in “space,” there is no metal-to-metal contact. Consequently, magnetic bearings require no lubrication. That means there are no regular bearing lubrication schedules for compressors with magnetic bearings, and it also means that no elaborate lubrication systems are required for them.
Generators with magnetic bearings enable the manufacture of smaller, faster units. The ability to run faster means these generators have greater power density. Consider, for example, a one-megawatt wind turbine. The generator’s rotor weighs several tons. In a typical design, the wind drives the turbine blades, which turn a low-speed shaft at 30 to 60 revolutions per minute (rpm). A gearbox converts this rotational speed to about 1200 to 1500 rpm. Contrast the wind turbine with a high-speed generator that operates at 20,000 rpm. A one-megawatt power generator running that fast would have a rotor weight of less than 100 kilogram. That’s the advantage that speed provides. Low-speed generators are very large; high-speed generators are small and compact.
Also, generator packages with magnetic bearings offer another way of reducing the weight and size of driver/generator packages. These systems can be designed without gearboxes because the generators can operate at the speed of the driver. This arrangement eliminates still another point of lubrication and maintenance - the gearbox.
Figure 3. A bearing cartridge used in 1.2-megawatt generator. (Photo courtesy of SKF USA Inc.)
Cost Comparison
The initial cost of a small backup generator using magnetic bearings could be an order of magnitude greater than the initial cost of system using conventional bearings. Most of the additional cost is due to the relatively sophisticated system required to control the bearings. The hardware associated with a magnetic-bearing system is rather inexpensive. For example, the cost of winding a magnetic-bearing coil is comparable to the cost of winding a motor stator. By contrast, a magnetic bearing’s control system includes such items as an advanced digital controller, sensors to monitor shaft position, cables to convey the shaft-position data to the controller, and other cables to carry the power from amplifiers in the controller to the bearing’s electromagnetic coils.So, these new compact machines presently cost more per installed kilowatt-hour than conventional low-speed generators. Compared to conventional generators, generators with magnetic bearings are less costly to operate and produce more electrical power per horsepower expended to run them. There are significantly fewer mechanical losses. If a driver puts energy into a generator and the bearings consume (via friction) a large fraction of that energy, less energy is available for generating electricity. The power consumption of a standard bearing can be ten times that of a magnetic bearing. Reduced energy consumption has a positive environmental impact, too. As noted, magnetic-bearing units also have fewer associated maintenance costs than conventional units. That fact does not mean, however, that magnetic-bearing systems are maintenance free. (See the box, “Magnetic-bearing System Maintenance and Reliability.”)
Today, there are several kinds of distributed power applications that favor the use of magnetic-bearing technology in compressors. These applications include 1) operations with power demands that can not be controlled or scheduled, 2) operations with excessive and presently unused potential energy sources, 3) activities in remote, off-the-grid locations and 4) recovery, rescue, and reconstruction operations at disaster sites.
Operations with power demands that cannot be controlled or scheduled can use a distributed power system to manage energy consumption and cut energy costs by peak shaving or reducing demand charges In both these cases, a facility also realizes the added security associated with having backup power that protects against rolling blackouts and grid outages.
But why, given the host of new and emerging technologies on the horizon for distributed power generation, would an energy manager select a generator with magnetic bearings? The answer lies in the inherent reliability and energy efficiency of these systems, but there are other considerations having to do with their compact designs.
In new facilities, the relatively small footprint offered by a system in which the generator has magnetic bearings will greatly reduce the foundation/building requirements, cutting construction costs. In existing facilities, if there are space constraints associated with the installation of a distributed power system, these systems’ compact size is an added bonus, perhaps allowing the new equipment to fit into existing mechanical rooms where conventional generators might not fit. Also, these compact designs allow double the power generation capacity in the same space conventional low speed designs might occupy.
Facilities in remote, off-the-grid locations could consider using generators with magnetic bearings because of the low-maintenance nature of these units and their portability.
Rescue, recovery and reconstruction operations at disaster sites can be supported by small, relatively light-weight generator systems using magnetic bearings. At both Ground Zero in New York and flood-ravaged New Orleans, the infrastructure was obliterated. Using relatively small and portable generator packages that can be set in place by helicopters and powered up to support recovery, rescue, and rebuilding efforts would have been useful in these two disasters.
Operations with excessive and presently unused potential energy sources have an opportunity to practice energy recovery, using energy that would otherwise be wasted. Many operations have pockets of high potential energy in their processes, e.g., a high pressure or a high temperature gas. When that energy can’t be used for anything else, a distributed power unit allows for the extraction of that energy to power, say, an expander turbine to drive a generator.
SIDEBAR: Magnetic-Bearing System Maintenance and Reliability
While magnetic-bearing systems require none of the traditional mechanical maintenance associated with traditional bearing systems, there are, nonetheless, some diagnostic assessments that should be performed regularly. These evaluations are easily done, since the control system provides constant feedback about the state of the system. Among the important diagnostic reviews is a check of internal clearances--the gaps between the shaft and the mechanical backup bearings that limit the shaft’s axial and radial movement.Technicians can keep records over time to verify that a bearing is performing as it was built to perform. If there has been a change, the techs can document the change and analyze the data to find the cause.
Another regular maintenance task required for a magnetic-bearing system relates to the fact that most controllers have cooling systems that circulate air to cool the power electronics. The filters on these cooling systems require periodic cleaning or replacement. Frequency of this maintenance is a function of the operating environment’s cleanliness.
Most maintenance tasks for magnetic-bearing systems are common-sense items. What maintenance personnel do not have to do is lubricate the bearings periodically and replace them every few years. In other words, magnetic bearings require none of the upkeep traditionally associated with bearings.
Controller power electronics have finite lives. The mean time between failures for controllers is roughly eight to 12 years depending on how hard the electronics have been stressed and if adequate cooling has been provided. End users can expect to make changes in the power and control systems for magnetic bearing after eight to 15 years of service. Like an electric motor, a magnetic bearing will last 20 to 30 years, depending upon the environment in which it operates and how fast the insulation breaks down in that environment.
