What are magnetic bearings?
Magnetic bearings replace traditional mechanical bearings like ball bearings. With magnetic bearings, the rotating part of an electric motor is floating in the air. This is done by using a magnetic field. The magnetic field is generated by magnetic bearings. There is no physical contact between the rotating and the stationary parts. The operating principle is called also magnetic levitation.
Figure 1 Tailored active magnetic bearing system parts
How does a magnetic bearing levitate the rotating shaft?
In a magnetic bearing system, the rotating part of the motor is also called a shaft or a rotor. The shaft is kept in a magnetic field. The magnetic field is created using electromagnets. Electromagnets are different from whiteboard magnets. In an electromagnet, the magnetic field is produced by an electric current.
There are a set of electromagnets, which are in pairs on opposite sides of the rotating part. Typically, there are two pairs of opposite magnets in vertical and horizontal directions. The magnets pull the shaft in their own direction by magnetic force. The magnetic forces keep the shaft in the center between the magnets. The set of these magnets is called magnetic bearings.
Typically, the magnetic bearing system is active. Active means that the pair of magnets are interconnected, and the magnetic field is controlled. Active control is intelligent. The control requires some electronics and control software. In active magnetic bearing (AMB) systems, there are also position sensors. The position sensors continuously measure the rotor position. With the data from the sensors, the magnetic bearing controller (MBC) monitors the rotor position and controls that the rotor stays in the middle. The MBC knows how much magnetic force is needed for each magnet to keep the rotor centered. The controller adjusts the magnetic force in each magnet by adjusting the current in the coils of the electromagnets.
What is a magnetic bearing used for?
There are several different industrial applications for magnetic bearings. Some of the most common applications are turbo blowers and turbocompressors, turbomolecular pumps, turbo expanders, Organic Rankine Cycle (ORC) systems, and flywheels.
Turbo blowers and compressors are used in factories and buildings. They are used for example in chemical, food processing, wastewater treatment, air-conditioning, heating & ventilation applications. For example, turbocompressors provide compressed air for bottling beer in the brewery.
Turbomolecular pumps are used in factories. They create a vacuum or in other words suction. Suction can be used to grip lightweight components in automated manufacturing and assembling lines to move and rotate pieces such as circuit boards. Turbomolecular pumps are used for example in the semiconductor industry.
Turboexpanders are used in gas pipelines. The expander lowers the pressure in the gas pipeline, for example in a gas station. It can harvest energy when the pressure of the gas is released.
An Organic Rankine Cycle (ORC) system is a type of Waste Heat Recovery System. It generates electricity from waste heat. It can use heat, for example from the exhaust gas of factories or lukewarm cooling water.
Flywheels are energy storage systems. They remind electrical batteries, but they are mechanical energy storages. Compared to batteries, flywheels release power fast and they do not have a lifetime limit for charging and re-charging as normal batteries.
The common thing for the applications is that there is a high-speed rotor involved.
Please check out more applications
Are magnetic bearings good?
Magnetic bearings are especially suitable for high-speed electrical machine applications. Higher rotational speeds enable achieving the same process outcome with lower energy. The main benefits of magnetic bearings are:
- Oil or lubricant freedom: Magnetic bearings run without any lubricants. Then there is no risk of oil contamination in the process. Also, the lubrication arrangements are avoided, which reduces the system size, complexity, and maintenance need.
- Higher system efficiency: The magnetic bearing itself consumes very little energy. Low energy usage together with higher process efficiency ensure very economical system operation.
- Various speed operation: The rotating speed has no effect on the magnetic bearing system functionality. The AMB system can also run at extremely high speeds like 150 000 rpm. And the same magnetic bearings can levitate a motor that does not rotate at all when it is not in operation.
- The rotating condition monitoring system is integrated into the AMB system. In conventional bearing systems, the monitoring needs separate sensors and other devices that are expensive.
Are magnetic bearings Frictionless?
Yes, magnetic bearings are frictionless. Active magnetic bearings remove friction losses, resulting in higher mechanical and energy efficiency. This will also extend the lifetime of the bearings since there are no wear-out parts.
Figure 2 Example of active magnetic bearings
How long do magnetic bearings last?
With the advantage of frictionless rotating, magnetic bearings can last much longer than conventional bearings. Usually, the lifetime of a magnetic bearing is over 20 years. Electronics do not last forever, but the external control box is easy to replace without disassembling anything from the machine.
Do magnetic bearings wear out?
Due to the contactless feature, magnetic bearings do not wear out easily. Usually, magnetic bearings last for a very long time compared with conventional bearings. Thus, it reduces the maintenance cost of the machines significantly.
What happens if there is a power blackout?
Magnetic bearings need the power to levitate the system. There are several protection layers for a safe stop if there is a power blackout. If the motor runs and there is a blackout, the motor must be shut down. When a motor shuts down, it can generate electricity from rotating motion. This electricity can be used for levitation during the shutdown and safe landing. Magnetic bearings can also use an uninterrupted power supply (UPS) that is like a battery that survives the blackout and gives the needed power. The final layer of security is the backup bearings that catch the rotating part if it drops. Backup bearings are typically ball bearings. The backup bearings do not rotate during normal magnetic bearing work.
If you want to read about more technical details, please check out our blog, what is active magnetic bearing.
Author: Dr. Nikita Uzhegov
Click the links to learn more about additional topics: