Design Considerations

Design Loads The specification of a magnetic bearing requires a different approach to that required for a conventional bearing system. This section identifies the key areas where these differences occur.

The rotor can move considerably within the magnetic bearing and catcher bearing clearances, typically up to 10 mil (0.25 mm), before any contact is involved. Thus the clearances G3 in the internal ring-seal system (see Figure 7) become the key controlling parameter in setting the bearing clearance design limits and in the degree of motion permitted during transients. Thus, very early in the design process, the magnetic bearing clearances, catcher bearing clearances, and sealing-ring clearances must be optimized with due consideration for manufacturing tolerances and assembly concerns.

Synchronous filters or open-loop control methods can be used to handle imbalance loads so that the degree of imbalance acceptable (based on allowable bearing loads and shaft motion within the catcher bearing clearances) can often be significantly greater than in conventional bearings.

There is an area where careful analysis is needed for each new pump configuration. A conventional bearing, if overloaded, will accept the load with a higher wear rate, but a magnetic bearing has a sharp cut-off at the point where the flux saturation level is reached. At that point, any additional load will be transferred to the catcher bearing. Thus it is important to include all loads in the design specification, with the appropriate margins. All applications to date have generally shown a) that there are loads which were mistakenly considered to be insignificant, or b) that the values of the loads were underestimated due to lack of knowledge.

Magnetic bearings also must be carefully designed to handle transient loads, some of which may occur only once in a lifetime. Two approaches can be taken. One is to overdesign the bearing to handle the load without contacting the catcher bearing; the other is to allow momentary contact with the catcher bearing. Examples of these types of loads are hydraulic loads, seal touchdown loads, system loads such as water hammer, valve operation, pump switchover, pump to driver alignment loads, and seismic loading.

Rotordynamics Considerations Magnetic bearings have the capability to control the rotor dynamics of a pump very effectively. If the pump is running below its first flexible mode, this is usually straightforward. If the pump has to traverse a flexible mode, the position of the bearings and the position sensor must be such that the modes can be recognized by the position sensor, and the bearings can exert a positive restoring force to control the mode. That is, the position sensors and bearings must not be positioned at or close to a node and certainly not positioned on opposite sides of a mode. Thus, rotordynamics considerations should be taken care of very early in the configuration of the system.

The two typical rigid body modes are shown in Figure 9. The location of the bearing and position sensor is not usually an issue for these modes, but the flexible modes require that the position sensor and bearing be positioned in such a way that the rotor deflection can be measured and the bearing can exert the necessary restoring force and damping to control the mode.

Magnetic bearing controllers are programmed with a transfer function designed specifically for the pump. This transfer function, or control algorithm, provides the necessary stiffness and damping at all operating speeds to control the rotor as previously described.

The key requirements of the transfer function are that it

• Provides correct damping and stiffness to handle the rotor rigid body modes

• Provides sufficient force with the appropriate control bandwidth to handle the rotor flexible modes below the maximum operating speed

FIGURE 9 Flexible mode design considerations

• Does not excite any rotor modes above the maximum operating speed

• Does not excite any stator vibration modes at any frequency

• Takes into account the stiffness and damping contribution from wear rings and interstage annular seals

Auxiliary or Catcher Bearings However reliable magnetic bearings become, a landing surface for maintenance will be required. Further, designing a bearing that will take all transient loads without any possibility of overload will usually result in over-sized and costly bearings. Hence catcher, bearings are expected to be required. The design limits for catcher bearings are usually established by calculating the forces that will result from a drop at operating speed.

For applications running at significant speeds, a non-linear analysis is required to determine the motion and loading on the bearing during such a drop. The key design considerations are impact loads, heat generation during the rundown, and the response to imbalance if, when running on the catcher bearings, any critical speeds have to be traversed.

Rolling element bearings have typically been used as catcher bearings; however, sleeve bearings and bushings have been used in several applications, and are better suited to a submerged application.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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