Magnetic Bearing

Magnetic Bearing

1. INTRODUCTION Conventional bearings provide support to rotating machinery by allowing relative movement in a plane of rotation. They allow rotation and provide support in either radial or axial planes of rotation. The most common types of bearings are rolling element bearings, and oil film or journal bearings. Rolling element bearings consist of a stationary outer race and a rotating inner race; in between them are the rolling elements – most common are spherical balls, but cylinders or tapered pins are also used.

During

rotation, these 3 items are in contact with each other and the weight being supported is transferred through the rolling elements between the inner and outer races. Oil film bearings have no rolling elements, but make use of pressurized oil to provide a film of support, and prevent galling between the shaft and the bearing journal. The oil is circulated so that fresh, cool oil is constantly entering the space between the stationary and rotating pieces. The shaft rotation shears the oil in this gap, causing it to heat up. The oil then exits the journal for cooling, filtering, and recirculation. Magnetic bearings allow contact – free levitation. This offers a number of interesting advantages.

Magnetic bearings do not require

lubrication, they allow high circumferential speeds at high loads, they do not

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Magnetic Bearing

suffer friction nor wear, the therefore they offer a virtually unlimited lifetime while no maintenance is needed. Furthermore, the bearing force can be modulated, either for compensating unbalance forces, or for deliberately exciting vibrations. Because of these advantages, they are used in an increasing number of commercial high-performance applications in the domain of rotating machinery.

These include ultra-high vacuum pumps, canned pipeline

compressors and expanders, high-speed milling and grinding spindles, flywheels for energy storage, gyroscopes for space navigation, spinning spindles, and others.

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Magnetic Bearing

2. WORKING PRINCIPLE AND TYPES

The application of magnetic bearings is based upon the principle that an electromagnet will attract ferromagnetic material.

A ferromagnetic

rotor can thus be supported in a magnetic field generated in the bearing electromagnet stator, as shown in the figure on the left.

Since the nature tendency of the stator is to attract the rotor until it makes contact, some control action is required to modulate the magnetic field and maintain the rotor in the desired position. The most common type of control involves the feedback of shaft position. This information is then used by the control system to modulate the magnetic field through power amplifiers, so that the desired rotor position is maintained even under changing shaft load conditions.

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Magnetic Bearing

An active magnetic bearing system consists of electromagnet bearing actuators, position sensors, a control system and power amplifiers, as shown in the figure below. The bearing actuators and sensors are located in the machine, while the control system and amplifiers are generally located remotely.

Magnetic bearings use an electromagnet to provide noncontacting, friction-free motion in rotary applications. They are constructed of a rotating and a stationary part (rotor and stator, respectively) separated by an air gap.

Magnetic bearings operate by applying an electric current to

ferromagnetic materials used in both the rotor and stator, creating a magnetic 4

Magnetic Bearing

flux path that includes the rotor, stator, and air gap. Magnetic bearings do not require lubrication, and are virtually maintenance-free. They are available in two different functional types : 1. Active magnetic bearings (AMB) and 2. Passive magnetic bearings (PMB).

1)

Active magnetic bearings (AMB) : Require a specialized control and software. They are comprised

of three distinct parts : the bearing itself, the electronic control system, and the auxiliary bearings. The purpose of the electronic control system is to control the position of the rotor by varying the current in the electro magnets. The electronic control system can be adjusted and adapted to the disturbance frequency of the machine itself. This information can be used to adjust and optimize performance due to process changes.

Active magnetic bearings

have higher stiffness and damping characteristics than similar size passive bearings. The control current for active magnetic bearings can be calculated by taking the square of the rotor-stator gap. The auxiliary bearings are used to support the rotor when the machine is stationary with the electronic control system switched off, or in the event of a failure in the magnetic suspension system to allow the rotor to run down without damage to the rotor itself or the stator of the magnetic bearings.

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Magnetic Bearing

2)

Passive magnetic bearings (PMB) Passive magnetic bearings (PMB) do not require control

hardware or software for operation. They have lower stiffness and damping characteristics than similar size active bearings. Passive bearings can be constructed with permanent magnets, diamagnetic materials, electrodynamic effects, superconductors, or ferrofluids. Magnetic bearings are available in three common configurations: radial, axial, and conical; although custom and specialized version can be manufactured according to need.

a)

Radial magnetic bearings : Redial magnetic bearings are used for rotary motion where the

primary external loads are in the radial direction, or perpendicular to the axis of rotation. They function via four electromagnets, each driven by an amplifier, which are arranged around the rotor to form the bearing.

In horizontal

applications, the magnet centerlines are orientated at 45 0 to the perpendicular so that gravity can act upon the upper two adjoining magnets. This increases the stability of the system, and adds to the stability of the load.

b)

Axial or thrust magnetic bearings : Axial or thrust magnetic bearings are used for rotary motion

where the primary external loads are parallel to the axis of rotation. They use 6

Magnetic Bearing

a flat, solid ferromagnetic disc, secured to the rotor as the collar, the the axial thrust bearing. Solid disc electromagnets are situated either side of the collar and operate in a similar manner to the radial bearing above but in one dimension only.

c)

Conical magnetic bearings : Conical magnetic bearings are used when the bearing may be

subjected to both radial and axial loads. These bearings are designed with a conical shaped stator and rotor.

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Magnetic Bearing

3. CONTROL SYSTEMS The control system allows the current in the bearing to be controlled by feeding back information on the position of the shaft. This is called closed loop feedback control and is necessary for the shaft to be held in a stable position. In simple terms, the control system reduces the upper bearing current when the shaft is above the center position and increases the current when the shaft is below the center position. Typically, magnetic bearing control is performed in a single inputsingle output (SISO) manner. This means that the position information form one sensor causes only the control current in the corresponding axis to be varied.

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Magnetic Bearing

Control systems can also be multi-input and multi-output (MIMO). MIMO is used when higher levels of control is required or when significant cross-coupling between axes is expected. The components of the control system include a position sensor and

accompanying

electronics,

a

controller,

and

amplifiers.

These

components are described below.

SENSORS : The sensors feed information about the position of the shaft to the controller in the form of an electrical voltage. Normally, the sensors are calibrated so that the when the shaft is in the desired position, the sensor produces a null voltage. When the shaft is moved above this desired position, a positive voltage is produced and when it is moved below, a negative voltage results.

CONTROLLER The function of the controller is to receive the voltage signal from the position sensors, process this information and send current requests to the amplifiers.

The controller consists of anti-aliasing filters, analog-to-digital

signal processor and Pulse-Width Modulation (PMM) generators. The voltage from the position sensors is passed through the antialiasing filters to eliminate high frequency noise from the signal. This noise 9

Magnetic Bearing

can cause the signal to inaccurately represent the position of the shaft. In addition, because the controller periodically samples the signal, some of the high-frequency information can “fold over” into false low frequency information, thus aliasing the information received by the controller. After the high frequency content is removed, the position signal is sampled by the Analog to Digital (A/D) converter. This converts the voltage signal to a form that can be processed by the digital signal processor. The digital information is then passed through a digital filter by the digital signal processor.

This produces an output proportional to the

amount of current required to correct the position error in the shaft. The requested current is compared to the actual current in t he bearing, which is also sensed, filtered and sampled with an analog-to-digital converter. The error between the actual and requested current is used to characterize the PWM signal sent to the amplifiers. This information is sent to the pulse-width modulation generators which create the PWM wave form sent to the amplifiers. The delivery of the control current request must occur well before the next sample of the shaft position is taken. The sampling and control delivery process is repeated at a frequency of 10 kHz.

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Magnetic Bearing

AMPLIFIERS : Each bearing axis has a pair of amplifiers to provide current to the bearing coils and provide an attractive force to correct the position of the rotor along that particular axis.

The amplifiers are simply high voltage

switches that are turned on the off at a high frequency, as commanded by the PWM signal from the controller.

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Magnetic Bearing

4. APPLICATIONS

Following are the various applications of magnetic bearings : 1.

100,000 rpm milling spindle :

100,000 rpm machine tool spindle for high speed three axis machining center. This spindle is a prototype demonstration. Magnetic bearings were necessary to achieve the extremely high speed. However, the primary concern is consistent, high quality surface finish. Magnetic bearings will allow a much longer service interval than the rolling element bearings normally used.

2.

Refrigeration compressor : This is an hermetic, oil-free compressor for refrigeration and

chiller applications using new CFC-free refrigerants. Oil-free operation is essential to maintain high cycle efficiency with this new class of refrigerant. In addition, there is no oil in the system to decrease the heat transfer efficiency of heat exchangers. Magnetic bearings were chosen as an enabling technology.

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Magnetic Bearing

3.

Grinding spindle : This high speed motorized spindle is designed for operation in a

computer controlled grinder used in rolling bearing production. Magnetic bearings give this spindle a much wider operating range, allowing one spindle to be used where two conventional spindles are used. This improves productivity by reducing resetting time. It is hoped that the magnetic bearings can be used to actively improve grinding performance using process feedback concepts.

4.

High speed spindle : This prototype spindle was constructed for use in a high speed

machining center. Although not yet in production, the spindle demonstrates many benefits of magnetic bearings in this application. Magnetic bearings allow a high speed, high power spindle to deliver consistent performance. Performance is not affected by thermal issues or by wear. Service intervals are greatly increased over conventional bearings. The magnetic bearings automatically detect overload situations and instantly shut down the machining operation.

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Magnetic Bearing

5.

Pulp refiner : This pulp refiner represents a new product direction for this well-

established OEM. The 220 kW, 900 rpm unit is the first of three base pulp refiner sizes planned by the manufacturer.

The current innovation, which

includes magnetic bearings, provides the OEM with the competitive advantages (listed below) necessary to secure a dominant position in this competitive market.

Magnetic bearings were selected for several reasons, among them : The magnetic bearings are used as part of a process control system that ensures the highest quality pulp. Coated magnetic bearings operate within the process control system that ensures the highest quality pulp. Coated magnetic bearings operate within the process-on seals are required, eliminating a common cause of downtime. The integrated design resulted in a solution with minimum part count and maximum performance.

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Magnetic Bearing

ADVANTAGES AND DISADVANTAGES

ADVANTAGES : 1. Reliability of these bearings is more compared to conventional bearings. No wear takes place as there is no contact between stationary and rotating parts. 2. Practical generators due to wear and the need for

lubrication are

eliminated. Hence the process can’t contaminate with oil, grease or solid particles. 3. The fact that the rotor spins in space without contact with the sensor means drag on the rotor is minimum. This allows the bearing to run at extremely high speeds. 4. Magnetic bearings use advanced control algorithms to influence the motion of the shaft and therefore have the inherent capability to precisely control the position of the shaft within microns and to virtually eliminate vibration. 5. These bearings are capable of operating through an extremely wide range such as – 2500 deg C to 2200 deg C, thus allowing operation where conventional bearings cannot function. 6. These can operate in corrosive environments by means of canning both stationary and rotating parts.

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Magnetic Bearing

7. A magnetic bearing functions by determining rotor position rotor vibration and bearing load. This information which is processed in an electronic control cabinet, can be used to detect incipient faults, plan maintenance and optimize performance.

DISADVATAGES : 1.

Magnetic bearings have a specific load capacity (maximum load per unit area of application) lower than other bearings. These results in bearings, which will be physically larger than other similarly, specified bearings.

2.

The higher complexity of magnetic bearings is higher tan competing technologies. However, magnetic bearing life cycle cost can often by less than the traditional bearing.

3.

Magnetic bearings require power to drive the control system, sensors and electromagnets. Magnetic bearing system represents a completely different approach to the support of rotating equipments.

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Magnetic Bearing

CONCLUSION As there is no friction between rotating and stationary part, wear phenomenon is eliminated in case of magnetic bearings and thus less maintenance is needed. Along with this the life of stator windings is about 25 years hence long life can be expected from magnetic bearings. The taking into consideration the above advantages, the adoption of magnetic bearings in industry will be more economical, efficient and environment friendly compared to the conventional bearings.

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Magnetic Bearing

REFERENCES

1.

www.globalspec.com

2.

www.google.com

3.

www.machindesing.com

4.

www.skfmegenitcbearing.com

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Magnetic Bearing

CONTENTS

S No. Particulars

Pg. No.

1.

INTRODUCTION

1

2.

WORKING PRINCIPLE & TYPES

3

3.

CONTROL SYSTEMS

9

4.

APPLICATIONS

13

5.

ADVANTAGES & DISADVANTAGES

17

6.

CONCLUSION

20

7.

REFERENCES

21

19