Magnetic Bearings

With the advent of magnetic bearings, the dream of an all-dry compressor can now be realized. This is to say that no external lube system is needed. Not all compressor applications at this point can qualify, because control oil is generally required for steam and gas turbine drivers. Gear bearing loads at present are higher than can be carried by current magnetic bearing designs.

The magnetic bearing is made up of a series of electromagnets located circumferentially around the shaft to form the radial bearing. The electromagnets (Figure 5-42) are laminated to limit the eddy current losses. The shaft must be fitted with a laminated sleeve (see Figure 5-43) for the

Figure 5-42, Radial magnetic bearing with a view of the circumferential electro magnets. (Courtesy of Mafi- Trench Corp.)

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Figure 5-43. Radial magnetic bearing rotor sleeve. (Courtesy of Mafi-Trench Corp.)

same reason. The thrust is carried by a single-acting or dual-acting set of electromagnets (see Figure 5-44) depending on the need for a unidirectional or bidirectional thrust load. The magnetic bearing operates with a fixed air gap so there is no contact under operating conditions.

Figure 5-44. Magnetic bearing thrust electro-magnets. {Courtesy of Mafi-Trench Corp.)

Sensors are incorporated in the bearing assemblies to sense position of the rotor relative to the bearing. A servo control system uses the position information provided by the sensors to increase or decrease the bearing force on the rotor as needed to keep the rotor properly positioned. Magnetic bearings react differently than hydrodynamic or rolling element bearings in that the mechanical bearings react immediately to a load change, while the electromagnet in line with the load change must increase its force at the same rate to maintain rotor position. The actual rate at which the servo amplifier can increase the force is a function of volt ampere product of the amplifier. If the rate at which the load is applied exceeds the capability of the servo control, a temporary perturbation will be experienced before the shaft is brought back to its normal position.

The magnetic bearing load capacity on a per-unit basis is less than that available from hydrodynamic bearings. The specific load limit is at approximately 80 psi with typical design values of 60 psi. Higher values can be achieved with special magnetic materials, but these are not normally used in compressor applications. The load carried by bearing may be compensated by increasing the physical size of the bearing. The heaviest compressor rotor weight has been approximately 4,000 pounds.

Speed is not limited by a surface speed as in the hydrodynamic bearing. Unfortunately, however, the stresses in the thrust collar pose one limit and the other is caused by need for an auxiliary bearing, which docs have a limitation based on the type of bearing being used.

The auxiliary bearing may be of the rolling element type, which is currently most common, or the dry lubricated bushing. The auxiliary bearing, which normally does not contact the shaft, is used to protect the rotating components from loss of the servo amplifiers (see Figure 5-45). The aux-

H 4a

Active Magnetic Bearing

Auxiliary Bearing

Figure 5-45. Schematic illustration of magnetic bearings and auxiliary rolling element bearings.

H 4a

Auxiliary Bearing

Active Magnetic Bearing

Figure 5-45. Schematic illustration of magnetic bearings and auxiliary rolling element bearings.

iliary bearing gap is approximately one half the air gap. Displacement due to momentary overload would also cause the auxiliary bearings to be pressed into service on a transient basis. It is critical that the compressor be tripped off line should the power to the magnetic bearings fail, because the auxiliary bearings have a limited life and are primarily intended for coastdown use. The life of the auxiliary bearings in general is considered to be five coastdowns from full speed. In some cases, the life has proven to be somewhat longer, particularly with the dry bushing design. Continued development in this area will no doubt increase this value in time.

An interesting aspect of the magnetic bearing is that in pre-startup the bearing servos are energized and the rotor levitated. It remains suspended as the startup begins. There is no minimum oil film type phenomena to pass through. On shutdown, the rotor is allowed to cease rotating and to remain in the levitated position until the power is removed should a full shutdown be required.

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