Magnetic Bearing Principles

How Magnetic Bearings Work In an active magnetic bearing system, a stator composed of an array of stationary magnets, or electromagnetic coils, interacts with a ferrous rotor (or a ferrous sleeve on a non-ferrous rotor) so as to suspend the shaft in a magnetic field (see Figure 3).

The position of the shaft is maintained dynamically through a continuous feedback system which comprises a position sensor, a controller, and an amplifier system (see Figure 4). Typically there are two radial bearings and one thrust bearing for a complete sys-

Rotor Laminations

Control Coil

FIGURE 3 Typical radial electromagnetic bearing. (Axial thrust bearings have the stator coils arranged in a disk configuration, a ferrous rotating disk being supported in the resulting magnetic circuit.)

FIGURE 4 Typical system control loop

tem. This system is tuned to the required characteristics of the pump, through a digital or analog controller (Figure 5), with the capability of adjusting the bearing stiffness and damping as a function of pump speed. Alarms and trips can be set at any required rotor offset or bearing load to provide the operator with warnings or to trip the drive unit as necessary. Figure 6 illustrates a typical bearing transfer function, showing a statically stiff bearing, with a dynamic stiffness over the operating speed range designed to meet the rotor dynamics requirements of the unit. The stiffness is then rolled off above the operating range to avoid excitation of higher modes in the rotor or stator.

Controller redundancy can be provided with the control loops switching to backup units upon sensing a failure. Also, backup power supplies should be provided, either through alternate sources or a battery system. Typically the power required is only one or two kW or less.

A catcher (also known as auxiliary, backup, or touchdown) bearing (indicated in Figure 1) is required to protect the rotor stator interface during maintenance and in the event of

FIGURE 5 Controllers for magnetic bearings, containing rectifier and amplifiers: a) digital controller; b) analog controller

10' 102 Frequency (Hz) FIGURE 6 Bearing transfer function loss of power or a severe transient beyond the force capability of the bearing. Typically, the catcher bearing is designed for 5 to 20 lifetime drops from full speed, and will be a readily replaceable rolling element or sleeve bearing. The pump of Figures 1 and 2 has rolling element catcher bearings. The radial clearances G1 (see Figure 7) between the magnetic

FIGURE 7 Clearance arrangement: Seal ring clearance G3 is greater than catcher-bearing clearance G2, but is less than magnetic-bearing clearance G1.

bearing stator and rotor are of the order of 20 to 40 mil (0.5 to 1 mm), and those in the catcher bearings (G2) are about half of that value.

Reasons for Using Magnetic Bearings There are several reasons to use magnetic bearings in pumps. While any one of these reasons may not be sufficient justification on its own, together they can provide a strong justification.

Reliability is a key incentive. The components of a magnetic bearing are essentially the same components as are found in an electric motor: laminations and coils. Because no wear is involved due to the lack of contact, these components will generally last the life of the equipment involved. Thus maintenance of a magnetic bearing system is transferred from mechanical components inside the pump to the external controller, which has plugin card replacement maintenance. Pump reliability is therefore improved, whereas repair times and costs are reduced.

Reduced power consumption is a second advantage, with the elimination of all losses associated with fluid film bearings and oil pumping equipment. This is replaced by the smaller power requirements of the bearing controller. Further, if the lifting force is supplied by a permanent magnet, supplemented by an active control circuit, this power requirement can be even smaller.

The ability to submerge the bearing in the pump fluid is a major advantage that allows the outboard mechanical seal to be eliminated, thereby eliminating maintenance and replacement of this seal8. [This was not done for the pumps of Table 1 (and Figures 1 and 2), because in both cases magnetic bearings were retrofitted to existing machines.]

More indirect savings are also possible in two other areas. Rotor dynamics can be controlled through the ability to adjust stiffness and damping as a function of pump speed, allowing higher imbalance without the need for shutdown. The diagnostic output inherent in the information provided in the controller can be fed into the overall plant operating system and the short-term and long-term health of the pump and the system can be monitored. This is done by inferring seal wear, transient hydraulic loads, and so on.

The actual figures for the savings possible due to the previous advantages are very pump- and system-specific, and general numbers are not very useful. Reference 9 has developed methodology for considering the economic effect of the types of advantages given.

Main Types of Magnetic Bearings and Their Selection There are two main types of magnetic bearings: passive and active. Passive bearings rely only on permanent magnets in repulsion and provide low stiffness, low damping, and no ability to control either of these parameters. Passive bearings are not applicable to pumps for this reason.

Elecromagnetic Bias

Permanent Magnet Bias

Permanent Magnet Bias

Homopolar Design Heteropoiar Design

FIGURE 8 Heteropolar and homopolar bearings

Active bearings using the feedback system previously described are essential for pump applications. Within the active bearing systems are the options of heteropolar and homopolar and of electromagnetic and permanent magnet bias. The principles of the heteropolar and homopolar approaches are shown in the Figure 8.

The main difference between the two types is that in the heteropolar design, the bias and control flux flow in the same magnetic circuit radially through the rotor, whereas in the homopolar design the bias flux flows axially along the rotor and only the control flux flows radially through the rotor.

The homopolar design has two options for providing the bias flux for the bearing sys-tem10. One is to use an electromagnetic effect, and the other is to use a permanent magnet. The permanent magnet generation in the homopolar configuration results in a more linear relationship between force and distance. In a simple magnetic circuit, the attraction force of a magnet on a ferromagnetic target decreases as the square of distance (the target cuts ^ as many flux lines at twice the distance). With the permanent magnet in a homopolar circuit, the effect of the air gap is therefore reduced.

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