by Mark D. Hinkley, SKF USA Inc.

Magnetic bearing systems represent a different approach from rolling bearings to support rotating machinery, and in recent years, their benefits have attracted attention for more applications.

As a non-contacting technology, magnetic bearings will exhibit negligible friction loss and no wear. They can attain high speeds with undetectable vibration and are valued for their energy-efficient performance and savings in applications ranging from vacuum pumps to gas and air compressors.

For example, a 12,000 rpm, 12 MW centrifugal compressor at a natural gas pipeline facility in upstate New York was fitted with magnetic bearings instead of traditional, hydrodynamic bearings. This switch to a system that consumes a fraction of the energy (because it rotates without contact) yielded documented annual energy savings of 700,000 kWh and an overall 88 percent energy saving for the compressor system (encompassing compressor and motor). In addition, an auxiliary oil lubrication system, cooling system, gearbox (variable, high-speed motor directly coupled to the compressor), and condition monitoring equipment were eliminated, which reduced the footprint of the machinery and the number of potential failure points.

Centrifugal compressor fitted with magnetic bearings

Centrifugal compressor fitted with magnetic bearings

While, depending on the application, the advantages of magnetic bearing technology compared with the oil film technology it replaces will vary in importance, the following features and benefits will often be cited:

  • Reduced wear. In normal operation, the rotating portion of the machinery is not in contact with any parts. Reduced wear decreases maintenance requirements and operating costs.
  • Increased efficiency. Virtually no shaft energy is consumed by bearing friction. More power goes directly into the process and enhanced efficiency follows.
  • “Green” operation. Without lubrication oil, concerns about potential leakage, accidental loss and disposal become irrelevant.
  • Programmable characteristics. Depending on the application or process variables, the physical response of the bearing can be adjusted “on the fly.” In some cases, this means that a shaft can safely pass through critical vibration speeds and operate at speeds that were previously unattainable.

For all the advantages, the technology is not without some limitations. Magnetic bearings tend to be physically larger than similarly specified bearing systems. Also, by necessity, magnetic bearings require electric power to drive the control systems, sensors and electromagnets.

Incorporating Distinct Technologies

An active magnetic bearing system consists of several distinct technologies: electromagnet bearing actuators, position sensors, control system and power amplifiers. The bearing actuators and sensors will be located in the machine, and the control system and amplifiers usually will be located remotely.

Magnetic bearings provide attractive electromagnetic suspension between the rotor and stator by applying electric current to ferromagnetic materials used in the stationary parts (the stator) of the magnetic bearing. This creates a flux path through both components and levitates the rotor, creating the air gap separating them. (The air gap between the stator and the rotor will usually be 0.5 mm to 2 mm and makes the non-contact operation possible.)

As the air gap between these two parts decreases, the attractive forces from the magnets increase. Since electromagnets are, in this way, inherently unstable, a control system is necessary to constantly adjust the strength of the magnets by changing the current and provide stability of the position of the rotor.

The control process begins by measuring the rotor position with a position sensor. The signal from this device is received by the control electronics, which compares it to the desired position established during machine start-up. Any difference between these two signals results in a calculation of the force necessary to pull the rotor back to the desired position. This is translated into a command to the power amplifier connected to the magnetic bearing stator. The current is increased, causing an increase in magnetic flux, an increase in the forces between the rotating and stationary components, and movement of the rotor toward the stator along the axis of control.

The entire process is repeated thousands of times per second, enabling precise control of machinery rotating with peripheral speeds of up to 200 m/s. A closer look at each of the system components follows.

Radial and Thrust Bearings

A typical system incorporates two radial bearings and a thrust (or axial) bearing. Each radial bearing has a stator and sensor system mounted over a ferromagnetic rotor installed on the shaft. The rotor consists of a stack of lamination rings mounted on a sleeve that fits onto the shaft. (Laminations are designed to reduce eddy current losses and improve the response of the bearing.) The stator includes a stack of lamination rings with poles on the internal diameter. Coils are wound around each pole to divide the bearing into four quadrants. The coils in each quadrant are wound in series to make each quadrant function as one electromagnet. Typically on horizontal machines, the quadrants are aligned 45 deg from vertical. Opposing quadrants constitute an axis (each radial bearing, then, can be described by two axes). A set of sensors to measure shaft position is mounted as close to the bearing as possible.