Learn About Magnetic Bearings
A non-contact magnetic bearing system (MBS) for rotor support offers many advantages when used with permanent magnet (PM) motor generators in designing advanced systems. These include:
- Eliminating physical contact between rotating and stationary components
- Eliminating lubrication system contamination
- Reducing operating costs
- Simplifying maintenance
- Improving reliability
- Opening up machine operating parameters, such as speed and temperature
- Making applications environmentally friendly
- Offering real time monitoring of system performance for predictive maintenance
- Optimizing installations
Basics of How Magnetic Bearings Work
In the mid-twentieth century, a successful magnetic levitation bearing was demonstrated. This first successful magnetic bearing utilized electromagnets to provide attractive forces in the five degrees of freedom (with rotation being the sixth). Active servo control stabilized the system by using feedback signals from position sensors in each axis of control to vary the currents flowing through the various electromagnets.
Several individual electromagnets, usually from 8 to 12, were arranged in a north-south-north-south configuration around each end of a levitated shaft to provide radial support.
This design approach, which results in a multiplicity of magnetic flux reversals around the circumference of the shaft, is known as heteropolar. Most commercially available magnetic bearing systems utilize this technology. A typical heteropolar magnetic bearing system is shown in the figure below. The stator, composed of an array of stationary electromagnets, generates powerful attraction forces that suspend the ferrous rotor shaft in the center of the magnetic field (with the help of an active servo-control unit).
Fig. 1Typical Heteropolar Magnetic Bearing
A magnetic flux bias field is used to linearize the relationship of force to control current. The bias is usually about 50% of the full magnetic field capability of the bearing. The position of the rotor is maintained dynamically through a continuous feedback system, consisting of a position sensor, a control loop and a current amplifier for each axis of control. This servo control system continuously varies the current flow through the control coils and thus the intensity of control flux, which modulates the air gap magnetic field so as to keep the rotor in the correct geometric position. Typically, there are two radial bearings and one thrust bearing for a complete 5-axis magnetic bearing system. Rotor laminations are needed to allow for high bandwidth response and to reduce eddy current loss and consequential rotor heating.
A typical magnetic bearing control system (see Figure 2 below) is tuned to the required dynamic characteristics of the rotating equipment, usually through the implementation of a proportional-integral derivative (PID) control algorithm that can vary the bearing stiffness and damping as a function of rotor disturbance frequencies. The algorithm is based on the bearing transfer function (bearing stiffness and damping factors). A typical transfer function is illustrated in Figure 3, where the gain required to achieve the desired bearing stiffness is shown as a function of frequency, and the phase-lead required to provide the desired level of damping is shown as a function of frequency.
Figure 2. Typical System Control Loop
Figure 3. Typical Bearing Transfer Function
The transfer function in Figure 3 provides a typical profile of increasing stiffness at low frequency, with the stiffness often approaching 108 lb/in at zero frequency. The notch shown at about 800 Hz is a filter introduced to make the bearing very compliant, either to avoid exciting a flexible mode at that specific speed or to allow the rotor to spin at the notched frequency without generating large imbalance loads. There will be some rotor eccentricity while operating at the notched frequency, with the orbit of motion determined by the amount of imbalance in the rotor. Note that the bearing stiffness drops off rapidly above the operating speed (about 20,000 rpm). This drop-off in gain is intentional to avoid exciting resonant frequencies that exist above the operating range.
Alarms and trips can be set for a pre-determined bearing load or a rotor deflection that exceeds a predetermined limit. An alarm will provide the operator with audio and visual warnings, while a trip can de-energize the drive unit.
To improve product reliability, an uninterruptible back-up power supply can be provided to improve system reliability. Typically, the capacity of a battery back-up system capable of providing up to 30 minutes of uninterrupted operation is less than one kilowatt-hour.
A catcher bearing (also known as an auxiliary, back-up or touchdown bearing) is required to protect the rotor/stator interface when the equipment is de-energized during assembly or a shutdown, and in the event of a power loss or a severe transient beyond the force capacity of the bearing. Typically the catcher bearing is designed to be a readily replaceable rolling element or sleeve type bearing, capable of surviving 5 to 100 lifetime drops from full speed.
The radial clearance between the magnetic bearing stator and the rotor shaft are usually on the order of 20 to 50 mil (G1 in Figure 4), and those of the catcher bearings are about half of that value (G2). The clearances in any rotating seals (G3) must be designed with the first two clearances in mind. Alternatively the catcher bearing clearance can be reduced somewhat if the machine process requires tighter seal clearances.
