Active magnetic bearings for high-speed motors
This post presents some basic information about active magnetic bearing technology in modern variable speed drive systems.
Two principal solutions for electrically driven turbomachinery
Electric drives for turbomachinery can be realized in two ways:
- Conventional drives with medium-speed motor and a speed-increasing gear
- High-speed direct drives (gearless)
Each of the two has specific pros and cons.
The later category involves special high-speed motors. One of the enablers of the high-speed motor design are the active magnetic bearings. Those bearings have practically no maximum speed.
Motors in medium and semi high-speed segment mostly use fluid film bearings where mineral oil serves as fluid for lubrication. Above certain speed, the conventional bearings reach their mechanical limits and the magnetic bearing technology is preferred.
Principle of magnetic bearings
Active magnetic bearings work on the principle of magnetic levitation. The electromagnetic forces hold the shaft in the desired position without a mechanical contact. It is the same principle of magnetic suspension as known from the Maglev trains, just applied on a rotating motion instead of linear.
The components of active magnetic bearings include electromagnets, position sensors, signal amplifiers and a fast closed-loop controller.
Advantages of magnetic bearings
Oil free technology
The absence of oil allows use in environmentally sensitive areas, e.g. installations close to water reservoirs. It eliminates any risk of oil leakages at high circumferential speeds. Since there is no oil in the motor and compressor bearings and no gear requiring lube oil, the entire system can be labelled as oil-free (especially when combined with a dry type transformer).
No mechanical contact
Levitation principle of magnetic bearings is contactless between the stationary and rotating part. That results in no wear and no friction losses. The user benefits from extended lifetime and less maintenance.
Precise control
The technology enables extremely precise control of rotor shaft. According to [2], rotors weighing several tons are held in a desired position with a tolerance as tiny as a human hair.
Adjustable
The active control of magnetic bearings allows an adjustment of bearing properties in real time, a feature that can’t be achieved with fluid film bearings. The stiffness of the bearings can be adjusted in multiple directions. It allows an implementation of active damping algorithms. Consequently, the drive system can operate in very wide speed range without any critical speed issues.
Diagnostic information
The active magnetic bearings inherently provide valuable information about the drive conditions. Such information is partly required for the closed loop control and partly utilized for extensive monitoring and diagnostics.
What if the control system fails?
Although the control of active magnetic bearings is very reliable, a failure can never be completely excluded. What happens in such a case?
The design of machines with active magnetic bearings thinks of such scenario. High reliability can be achieved by certain redundancy in the control and using an uninterruptible power supply. In case a failure would still happen, there is a solution: backup bearings. These bearings are supposed to catch the rotor and to minimize the consequences. The mechanical backup bearings are not suitable for continuous operation but can support the rotor in the rare case of magnetic bearing failure.
Applications of active magnetic bearings
In turbomachinery, the active magnetic bearings are used for high-speed motor-compressors to enable a gearless direct drive with motor spinning the same speed as the compressor. In the megawatt range we talk about rotational speeds of up to 15’000 rpm and the limit is constantly pushed higher. Other application that may emerge in the near future are high-speed flywheels as energy storage. High rated speed makes the flywheel compact and active magnetic bearings help to increase the overall efficiency.
References
[1] High-speed drives: Introduction, MB Drive Services, April 2023, available online, https://mb-drive-services.com/high-speed-drives-introduction/
[2] Everllence – Hermetically sealed compressors, https://www.everllence.com/industries/products/compressors/hermetically-sealed-compressors
[3] Simotics active magnetic bearing technology, Siemens AG
[4] Magnetic bearing, Wikipedia – The free encyclopedia, https://en.wikipedia.org/wiki/Magnetic_bearing
