Magnetic
bearings have been utilized by a variety of industries for over a
decade with benefits that include non-contact rotor support, no
lubrication and no friction.
Conventional
mechanical bearings, the kind that physically interface with the shaft
and require some form of lubrication, can be replaced by a technology
that suspends a rotor in a magnetic field, which eliminates friction
losses.
There are two types of magnetic bearing technologies
in use today – passive and active. Passive bearings are similar to
mechanical bearings in that no active control is necessary for
operation. In active systems, non-contact position sensors continually
monitor shaft position and feed this information to a control system.
This in turn, based on the response commanded by the system, flows to
the actuator via current amplifiers. These currents are converted to
magnetic forces by the actuator and act on the rotor to adjust position
and provide damping.
Additional benefits of magnetic bearings include:
- No friction
- No lubrication
- No oil contamination
- Low energy consumption
- Capacity to operate within a wide temperature range
- No need for pumps, seals, filters, piping, coolers or tanks
- Environmentally friendly workplace
- Impressive cost savings
In
practice, these attractions are balanced in order to maintain a gap
between the shaft (rotor) and static parts (stator). The function of the
magnetic bearing is to locate the shaft’s rotation axis in the center,
reacting to any load variation (external disturbance forces),
Floating rotors could boost compressor efficiencies
Traditional
centrifugal compressors are based on low-speed drives, mechanical gears
and oil-film bearings, resulting in high running costs because of their
high losses, wear, and need for maintenance.
This
new compressor drive (above) uses a permanent magnet motor, operating
at an efficiency of around 97%, to drive a rotor "floating" on magnetic
bearings, which spins the compressor impeller at speeds of around 60,000
rpm. These drives experience almost no friction or wear, and need
little maintenance. They also minimize the risk of oil contamination,
and result in compressors that are about half the size of traditional
designs.
How they work
Magnetic bearings are basically a system of bearings which provide non-contact operation, virtually eliminating friction from rotating mechanical systems. Magnetic bearing systems have several components. The mechanical components consist of the electromagnets, position sensors and the rotor. The electronics consist of a set of power amplifiers that supply current to electromagnets. A controller works with the position sensors which provide feedback to control the position of the rotor within the gap.
The
position sensor registers a change in position of the shaft (rotor).
This change in position is communicated back to the processor where the
signal is processed and the controller decides what the necessary
response should be, then initiates a response to the amplifier. This
response should then increase the magnetic force in the corresponding
electromagnet in order to bring the shaft back to center. In a typical
system, the radial clearance can range from 0.5 to 1 mm.
This
process repeats itself over and over again. For most applications, the
sample rate is 10,000 times per second, or 10 kHz. The sample rate is
high because the loop is inherently unstable. As the rotor gets closer
to the magnet, the force increases. The system needs to continuously
adjust the magnetic strength coming from the electromagnets in order to
hold the rotor in the desired position.
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