05-04-2011, 11:26 AM
ACTIVE MAGNETIC BEARING
Presented by
PRANEETH MAXIM NORONHA
[attachment=11727]
INTRODUCTION
Active magnetic bearings (AMB) have been designed to overcome the deficiencies of conventional journal or ball bearings. They have the ability to work in vacuum with no lubrication and no contamination, or to run at high speed. Today, magnetic bearings have been introduced into the industrial world as a very valuable machine element with quite a number of novel features, and with a vast range of diverse applications. Let us discuss key features such as load, size, stiffness, temperature, precision, speed, losses and dynamics.
PRINCIPLE
An Active Magnetic Bearing (AMB) consists of an electromagnet assembly, a set of power amplifiers which supply current to the electromagnets, a controller, and gap sensors with associated electronics to provide the feedback required to control the position of the rotor within the gap. The power amplifiers supply equal bias current to two pairs of electromagnets on opposite sides of a rotor. This constant tug-of-war is mediated by the controller due to control current as the rotor deviates by a small amount from its center position.
Schematic Diagram of AMB
LOAD
The term load touches upon basic properties of magnetic bearings. The load capacity depends on the arrangement and geometry of the electromagnets, the magnetic properties of the material, and of the control laws.
The load carrying capacity of an AMB depends on the magnetomotive force i.e., the product of the maximum current imax and winding number n.
The load that can be carried depends on possible heat dissipation. Therefore, one limitation for a high static load is the adequate dissipation of the heat generated by the coil current due to the Ohm resistance of the windings.
This is called soft limitation on load.
When the current imax reaches a value where the flux generated will cause saturation then the carrying force has reached its maximal value. Any overload beyond that will cause the rotor to break away from its centre position and touch down on its retainer bearings.
This is hard limitation
It has been found that the maximum specific carrying force of 32 N/cm2 (or 0.32 MPa) can be generated which is considerably lower than that for oil lubricated bearings, which is about four times as high.
Using expensive cobalt-alloys with a saturation flux density of over 2 Tesla, from which a specific carrying force of up to 60 N/cm2 will result
Geometry of a radial bearing
d Inner diameter (bearing diameter)
da Outer diameter
dr Rotor diameter
c Leg width
di Shaft diameter
l Bearing length
h Winding head height
b Bearing width (magnetically active part)
An Slot cross section (winding space)
p Pole shoe width
s0 Nominal air gap
STIFFNESS
The stiffness of a bearing is the ratio of the supported load with respect to the resulting displacement of that load
High stiffness can be obtained by using PID controller
The PID controller brings the position the shaft to the same position after the load and thus the rotor shows a behavior that cannot be obtained with classical bearings
SPEED
It can be discussed under 3 categories
Rotational speed
Circumferential speed
Supercritical speed
Rotational speed
In today’s industrial applications rotational speed range of about 3kHz to 5kHz has reached. Problems arise from eddy current and hysteresis losses in the magnetic material, air losses, and the related requirements for power generation and adequate heat dissipation if the rotor runs in vacuum.
Circumferential Speed
The circumferential speed is a measure for the centrifugal load and leads to specific requirements on design and material. The centrifugal load leads to tangential and radial stresses in the rotor.
The tangential stress is the most critical one. Highest stress values occur at the inner boundaries of a rotor disc
Rotor speeds of up to 340 m/s in the bearing area can be reached with iron sheets from amorphous metal (metallic glass), having good magnetic and mechanical properties.
Supercritical Speed
A rotor may well have to pass one or more critical bending speeds in order to reach its operational rotation speed. In classical rotor dynamics this task is difficult to achieve. In AMB technology it is the controller that has to be designed carefully to enable a stable and well-damped rotor behaviour.
SIZE
In principle, there appears to be no upper limit for the bearing size, it can be adapted to any load. Small bearings are of special interest to micro-techniques. Potential applications are video heads, medical instruments, hard disk drives, and optical scanners. The challenge lies in simplifying the design and in the manufacturing process.
HIGH TEMPERATURE AMB
In order to utilize the full advantages of active magnetic bearings, operation in gas turbine and aircraft engines requires that the magnetic bearing should work properly at high temperatures
Challenges in designing such bearings consist in material evaluation, manufacturing process and high temperature displacement sensor development.
Operating temperatures of up to 550º C have been reached, at rotor speeds of 30,000 rpm. Such a performance cannot be reached by any other kind of bearing.
Experimental tests were quite successful, but the long-term exposure to high temperature needs further research, as the actual results are not yet convincing
Test Rig For High Temperature AMB
LOSSES
With contact-free rotors there is no friction in the magnetic bearings. The operation of active magnetic bearings causes much less losses than operating conventional ball or journal bearings
Losses can be grouped into losses in
Stator part
Rotor part
Stator Loss
Stator losses are mainly from copper losses in the windings of the stator and from losses in the amplifiers. The copper losses are a heat source, and, if no sufficient cooling is provided, can limit the control current and hence the maximal achievable carrying force
Rotor Loss
These losses comprise iron losses caused by hysteresis and eddy currents, and air drag losses. The losses heat up the rotor, cause a breaking torque on the rotor, and have to be compensated by the drive power of the motor
In general, the eddy current losses are the largest ones.
The iron losses depend on the rotor speed, the material used for the bearing bushes
The iron losses in the rotor can limit operations, as in particular in vacuum applications it can be difficult to dissipate the generated heat.
The hysteresis losses arise if the B-H-curve travels along a hysteresis loop
The eddy-current losses arise when the flux density within the iron core changes. A compact core acts like a short circuit winding and generates large eddy currents. The eddy-current losses can be reduced by dividing the iron core in insulated sheets or in particles (sinter cores). The smaller these divisions, the smaller the eddy-current losses.
a) Iron Core b) Sheet
PRECISION
Precision in rotating machinery means most often how precise can the position of the rotor axis be guaranteed
Active magnetic bearings levitate an object, with feedback control of measured displacement sensor signal. The performance of AMB systems is therefore directly affected by the quality of a sensor signal
MATERIALS
Common AMB materials are
Silicon alloys
Cobalt alloys such as Hiperco
ADVANTAGES OF AMB
Lubrication free
Reliability
Operation in vacuum
Reduced energy consumption
Condition monitoring
High speed capability
No oil contamination
Precision
DISADVANTAGES
Requires auxiliary bearing
Load carrying capacity is limited
Requires continuous power supply
APPLICATIONS
Refineries and petrochemical plants
Offshore and under sea gas extraction
Cryogenic pumps, compressor and expanders
Industrial refrigeration and air conditioning
Turbo blowers, Turbo booster pumps, Turbo molecular pumps etc
CONCLUSIONS
The maximal load depends on design
The specific load depends on the available ferromagnetic material and its saturation properties, and is therefore limited to 32 to 60 N/cm2
Circumferential speeds, causing centrifugal loads, are limited by the strength of material. Values of about 250 to 300 m/s have been realized with actual design
Supercritical speed means that one or more critical speeds can be passed by the elastic rotor. It appears to be difficult to pass more than two or three
High temperature bearings have been realized, running in experiments at an operating temperature of 550º C.
The losses of magnetic bearings at operating speed are much smaller than that of classical bearings. Eddy current losses will limit the rotation frequency of massive rotors, the air drag will be crucial at high circumferential speeds
A high precision of the position of the rotor axis (in the range of mm) requires high resolution sensors and adequate signal processing to separate disturbance signals from the desired ones
