Magnetic bearings 18-6
Most of us have seen an application of magnetic bearings in watt-hour meters used by electric utilities to monitor home power consumption. This same principal can be used to support the rotating shaft in a centrifugal pump application.
Designs using permanent magnets have not been proven practical. The only successful magnetic bearings use a continuous power input and an active control system to hold the load stable.
In a typical application, two radial magnetic bearings support and position the shaft in the radial directions, and one supports and positions the shaft along the axial direction (thrust)
An active magnetic bearing (AMB) consists of:
- An electromagnetic assembly in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases.
- A set of solid state power amplifiers that supply current to two pairs of electromagnets on opposite sides of a rotor.
- A microprocessor controller that provides equal but opposite varying amounts of current as the rotor deviates by a small amount from its center position.
- Gap sensors, with associated electronics, to provide the feedback required to control the position of the rotor within the gap. These sensors must be protected from contact with the process liquid and must not compromise pressure containment integrity.
Advantages of magnetic bearings:
- Since there is no friction, no lubrication is necessary.
- They can run in a vacuum .
- They have no known relative speed limit.
- It’s possible for them to be free from contamination.
Disadvantages of magnetic bearings:
- The envelope dimensions and the high initial cost of the bearings, along with the associated hardware needed to make them function are the biggest problems to date.
- A backup bearing is needed for start-up, shut down and in case of power or control failure.
- Magnets give an unstable static forces that decreases at longer distances and increases at shorter distances.
- Magnetic force provides very little, if any damping, to control oscillations in ever present driving forces.
- Be sure you’ve done a good job in calculating the maximum loads on the rotating shaft because active magnetic bearings have a lower inherent transient overload capacity than conventional hydrodynamic bearings,
- Cooling is necessary to remove heat from electrical losses in windings and heat generated by windage losses.
Current magnetic bearings are heavy and costly. Companies like Avcon, Waukeshaw and SKF are working on smaller and less costly designs to tap into the huge rotating equipment field.
With the pumping industry as competitive as it is, It’s going to be some time before we see them in conventional pumping applications.
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