ISO 14839-4:2012
(Main)Mechanical vibration — Vibration of rotating machinery equipped with active magnetic bearings — Part 4: Technical guidelines
Mechanical vibration — Vibration of rotating machinery equipped with active magnetic bearings — Part 4: Technical guidelines
ISO 14839-4:2012: a) indicates a typical architecture of an active magnetic bearing (AMB) system so that users can understand which components are likely to comprise such systems and which functions these components provide; b) identifies the primary similarities and differences between AMB systems and conventional mechanical bearings; c) identifies the environmental factors that have significant impact on AMB system performance; d) identifies the operating limitations that are unique to AMB systems and defines standardized methods of assessing these limitations; e) identifies typical mechanisms for managing these limitations, especially rotor unbalance; f) provides considerations for the design and performance of touchdown bearing systems; g) defines a typical signal set for provision in an AMB system for proper system/process interface as well as condition and diagnostic monitoring; h) details current best practices for monitoring, operation and maintenance to achieve highest operational system reliability; i) identifies typical fault-handling practices; j) recommends inspection and preventive maintenance processes for AMB systems.
Vibrations mécaniques — Vibrations de machines rotatives équipées de paliers magnétiques actifs — Partie 4: Lignes directrices techniques
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 14839-4
First edition
2012-03-15
Mechanical vibration — Vibration of
rotating machinery equipped with active
magnetic bearings —
Part 4:
Technical guidelines
Vibrations mécaniques — Vibrations de machines rotatives équipées de
paliers magnétiques actifs — Partie 4: Lignes directrices techniques
Reference number
ISO 14839-4:2012(E)
©
ISO 2012
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ISO 14839-4:2012(E)
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ISO 14839-4:2012(E)
Contents Page
Foreword . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Active magnetic bearing system architecture . 2
5 Important differences between magnetic bearings and conventional bearings . 3
5.1 Some advantages of active magnetic bearings . 3
5.2 Some disadvantages of active magnetic bearings . 5
5.3 Comparison among rolling, fluid film and magnetic bearings. 6
6 System condition monitoring . 6
6.1 General . 6
6.2 Excess rotor shaft displacement (radial x, y, and axial z) . 6
6.3 Excess of rotor expansion . 8
6.4 Overload of bearing (over current of bearing coil) . 8
6.5 Bearing temperature high . 8
6.6 Overspeed of rotor . 8
6.7 Power supply defect . 8
6.8 Battery power defect . 8
6.9 Controller temperature high . 8
6.10 Cooling . 9
7 Environmental factors . 9
7.1 Introduction . 9
7.2 Environmental category tables .10
7.3 Explosive atmosphere types .13
8 System requirements .13
8.1 Estimation of bearing load .13
8.2 Limitation of dI/dt for laminated bearings .14
8.3 Balancing .16
8.4 Location of bearings and transducers .17
8.5 Fault recovery and fault handling .17
8.6 Signal processing .17
8.7 Monitoring system .17
9 Touchdown bearings .18
9.1 Touchdown bearing requirements .18
9.2 Design of touchdown bearings.18
9.3 Touchdown bearing monitoring .20
9.4 Touchdown test methods .20
10 Preventive inspection .22
10.1 Introduction.22
10.2 Regular inspection and maintenance .22
10.3 Condition monitoring (recommendation) .22
10.4 Inspection checklist .23
Annex A (informative) Sizing of magnetic bearings .24
Annex B (informative) Example of a design specification check list .27
Annex C (informative) Example conditions for acceptance tests .29
Annex D (informative) Touchdown test method example .30
Annex E (informative) Example of system limitations (current/voltage saturation) .32
Annex F (informative) Unbalance control .35
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ISO 14839-4:2012(E)
Bibliography .40
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ISO 14839-4:2012(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 14839-4 was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and condition
monitoring, Subcommittee SC 2, Measurement and evaluation of mechanical vibration and shock as applied to
machines, vehicles and structures.
ISO 14839 consists of the following parts, under the general title Mechanical vibration — Vibration of rotating
machinery equipped with active magnetic bearings:
— Part 1: Vocabulary
— Part 2: Evaluation of vibration
— Part 3: Evaluation of stability margin
— Part 4: Technical guidelines
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INTERNATIONAL STANDARD ISO 14839-4:2012(E)
Mechanical vibration — Vibration of rotating machinery
equipped with active magnetic bearings —
Part 4:
Technical guidelines
1 Scope
This part of ISO 14839:
a) indicates a typical architecture of an active magnetic bearing (AMB) system so that users can understand
which components are likely to comprise such systems and which functions these components provide;
b) identifies the primary similarities and differences between AMB systems and conventional mechanical bearings;
NOTE This information helps AMB system users better to understand the selection process and implications of
transition to AMB technology.
c) identifies the environmental factors that have significant impact on AMB system performance;
d) identifies the operating limitations that are unique to AMB systems and defines standardized methods of
assessing these limitations;
e) identifies typical mechanisms for managing these limitations, especially rotor unbalance;
f) provides considerations for the design and performance of touchdown bearing systems;
g) defines a typical signal set for provision in an AMB system for proper system/process interface as well as
condition and diagnostic monitoring;
h) details current best practices for monitoring, operation and maintenance to achieve highest operational
system reliability;
i) identifies typical fault-handling practices;
j) recommends inspection and preventive maintenance processes for AMB systems.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 1940-1, Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state — Part 1:
Specification and verification of balance tolerances
ISO 14839-1:2002 + Amd.1:2010, Mechanical vibration — Vibration of rotating machinery equipped with active
magnetic bearings — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14839-1 apply.
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ISO 14839-4:2012(E)
4 Active magnetic bearing system architecture
Active magnetic bearings (AMBs) can be used as suspension elements in rotating machines in lieu of
conventional types of bearings such as rolling element bearings and sleeve/journal bearings. AMBs support or
levitate a shaft using an electromagnetic force controlled by a position feedback loop. A typical radial magnetic
bearing actuator consists of electromagnets arranged at four directions around a rotating shaft as shown in
Figure 1. In this case, there are two orthogonal control axes.
Key
1 controller
2 power amplifier
3 magnetic coil
4 displacement sensor
5 rotor with rotational angular frequency ω
Figure 1 — Schematic drawing of a magnetic bearing system
Key elements of the AMB are:
a) a displacement transducer that detects the displacement of the shaft from a reference position or setpoint;
b) a processor or controller that produces a control command signal based on the position error;
c) a power amplifier to convert the low level command signal to a control current;
d) an electromagnetic actuator that applies a control force to the shaft based on the use of a magnetic field.
Rotational drag losses are quite low in an AMB because the shaft is supported by a magnetic field without
mechanical contact. The only drag losses are from eddy currents generated in the rotor and from windage.
These losses are small compared with the friction drag of rolling element bearings and very small compared
to the losses in sliding bearings. On the other hand, control of shaft position is not trivial. The magnetic force
acting on the shaft from each electromagnet is an attractive force that becomes larger as the shaft gets closer
to the actuator (see Figure 2). Thus it is passively unstable since a displacement from the equilibrium position
results in a force pulling the shaft further from its equilibrium position. This force/displacement relationship is
characterized by a negative stiffness.
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ISO 14839-4:2012(E)
Key
F attractive force F rotor weight
g, r
C
clearance C levitation point
lev
I = c current is constant
Figure 2 — Relationship between attractive force and clearance when the current is constant
AMBs are operated with a bias flux produced either by the electromagnet or by a permanent magnet. This
bias flux linearizes the force/control current relationship of the magnetic bearing making position control
easier. Historically, magnetic bearings were controlled by analogue control hardware executing single input
single output (SISO) proportional, integral and differential actions (PID) control or simple multi-input multi-
output (MIMO) PID schemes. Digital controllers are used almost exclusively in new installations at the time
of publication. Digital control provides all functionality available with the analogue control along with easier
implementation and calibration.
Further, many features became more practical with digital control, including robust control techniques, unbalance
response control, as well as monitoring and diagnostic functions. Generally, a digital controller for a magnetic
bearing has a software control program running in a digital signal processor (DSP) that is essentially the same
for all machine applications. Additionally, for a given machine application, there are parameters that define
the control law and other application-specific characteristics. Magnetic bearings are typically accompanied
by touchdown bearings that support the shaft when power is turned off, in the event of an equipment failure
during operation, or in case an overload is applied to the bearing. The touchdown bearings are also commonly
referred to as back-up bearings, auxiliary bearings, catcher bearings, and retainer bearings.
The clearance between a touchdown bearing and the shaft is commonly set to less than or equal to half of
the clearance between a magnetic bearing and the shaft. The magnetic bearing that controls shaft position
in the radial direction is called a radial magnetic bearing. A common arrangement of a magnetic bearing with
displacement transducers and touchdown bearings is shown in Figure 3.
On the other hand, a magnetic bearing that controls shaft position in the axial direction is called a thrust
magnetic bearing, and a common configuration of this bearing, displacement transducer, and touchdown
bearing is shown in Figure 4.
5 Important differences between magnetic bearings and conventional bearings
5.1 Some advantages of active magnetic bearings
5.1.1 A magnetic bearing system has many special features that differ from conventional bearings because it
functions by supporting or levitating a shaft in a magnetic field controlled by position feedback.
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ISO 14839-4:2012(E)
Key
1 shaft C ≈ 0,5δ
r r
2 radial touchdown bearing C radial clearance
r
3 displacement sensor δ radial magnetic gap
r
4 radial magnetic bearing
Figure 3 — Typical arrangement of radial magnetic bearings, displacement transducers and
touchdown bearings (ISO 14839-1:2002, Figure 6)
Key
1 thrust touchdown bearing C ≈ 0,5δ
a a
2 thrust displacement sensor C axial clearance
a
3 thrust magnetic bearing δ axial magnetic gap
a
4 thrust disc
Figure 4 — Typical arrangement of thrust magnetic bearings,
thrust displacement transducers and thrust touchdown bearings
5.1.2 The following functions arise because the AMB uses an active control system:
a) AMBs have high static stiffness and lower dynamic stiffness;
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ISO 14839-4:2012(E)
b) AMBs typically use unbalance control techniques which can:
1) minimize unbalance loads and transmitted vibration (using inertial axis rotation) or,
2) minimize harmonic displacement;
c) AMB control can be used to increase damping when passing a critical speed;
d) AMBs can be used for monitoring and diagnostic purposes due to built-in instrumentation.
5.1.3 The following advantages of AMBs relative to conventional bearings arise because of the non-contact
nature of the AMB.
a) There are no mechanical friction losses and only small electrical losses due to eddy currents, allowing
AMB machines to have higher efficiency.
b) Higher peripheral speeds are possible, typically limited only by rotor lamination stresses.
c) There is no wear on the machine components (actuator and transducer), therefore there is no maintenance
required for these components.
5.1.4 There are the following advantages on the grounds that AMBs are used without lubrication.
a) AMBs eliminate oil contamination problems.
b) AMBs can be used in a vacuum.
c) AMBs can be used in a cryogenic environment.
d) Auxiliary lubricating systems, such as a hydraulic pump, an oil cooler, an oil filter, and piping of a hydraulic
system, are unnecessary.
e) The system can be made simpler and installation space can be saved since the magnetic bearing control
hardware is smaller and more easily placed than an auxiliary lubrication system.
f) Maintenance is reduced substantially.
5.2 Some disadvantages of active magnetic bearings
AMB has many features and advantages specified in 5.1. Nevertheless, there are also the following
disadvantages.
a) AMBs require electrical power.
b) The maximum load capacity of AMBs mainly depends on the maximum magnetic flux capacity of the
actuator materials preventing the AMB from having an overload capacity.
c) The specific load limit imposed by the magnetic saturation limits of available materials results in a specific
load (load per unit area) considerably lower than oil film and rolling element bearings.
d) Since the control circuit can be complex, sufficient verification to establish reliability is required.
e) Time and cost are needed to establish the control system reliability when the system is out of order.
f) Control of many modes is required, even beyond the operating speed range.
g) Advanced knowledge that fuses concept of mechanical engineering and electrical engineering is needed
for designing the magnetic bearing/rotor system.
h) Touchdown bearings have to be installed near the magnetic bearing to avoid unexpected contact between
the rotor and stator of the magnetic bearing in cases of overload, failure of the magnetic bearing controller
or power supply.
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ISO 14839-4:2012(E)
5.3 Comparison among rolling, fluid film and magnetic bearings
Table 1 summarizes and shows the differences among rolling bearing, fluid film bearing and magnetic bearing types.
The development of a dynamic model for an AMB system requires techniques beyond those used for a
conventional bearing system. The dynamic coefficients concept known for conventional bearings cannot
generally be applied directly due to the inherent characteristics of AMB systems. Examples include actuator
and transducer non-collocation, high-order control characteristics, MIMO control, dynamics of power and
transducer electronics. Thus, AMB vendors and their customers should agree on suitable analysis models
covering all required system dynamics.
6 System condition monitoring
6.1 General
Since an AMB relies on transducers for control, the position signals can be applicable for monitoring the
working condition. For this reason, it is possible to perform condition monitoring of the rotor more delicately,
and the function of a failure diagnosis can be easily given. Since rotation of a rotor without levitation is harmful,
a rotation request, e.g. of the motor inverter, is denied by the AMB system as long as the rotor is not levitated.
For AMB-equipped machines, it is common practice to establish operational condition limits. These limits
take the form of ALARMS and TRIPS. An ALARM is set to provide a warning that a defined value of condition
has been reached or that a significant change has occurred, at which remedial action may be necessary. In
general, if an ALARM situation occurs, operation can continue for a period while investigations are carried
out to identify the reason for the change and to define any remedial action. A TRIP is set to specify the value
of condition beyond which further operation of the machine can cause damage. If the TRIP limit is exceeded,
immediate action should be taken to reduce the change or the machine should be shut down.
Other commonly used names for ALARM are WARNING and ALARM1. Other commonly used names for TRIP
are ALARM2, FAULT, EMERGENCY STOP and EMERGENCY SHUTDOWN (ESD) (this ESD should not be
confused with PLANT ESD for petrochemical applications).
What can be considered as TRIP or ALARM items which detect abnormalities in the diagnostic equipment
during operation is explained in 6.2 to 6.10.
6.2 Excess rotor shaft displacement (radial x, y, and axial z)
In ISO 14839-2, typical evaluation zones are defined to permit a qualitative assessment of the shaft displacement.
ALARM limits may vary considerably for individual machines. The values chosen are normally set relative
to a baseline value determined from experience for the measurement position or direction for that particular
machine. It is recommended that the ALARM limit be set higher than the baseline by an amount equal to
25 % of the zone boundary B/C. If the baseline is low, the ALARM may be below zone C. Where there is no
established baseline (e.g. with a new machine) the initial ALARM setting should be based either on experience
with other similar machines or relative to agreed acceptance values. After a period of time, the steady-state
baseline value is established and the ALARM setting should be adjusted accordingly. If the steady-state
baseline changes, e.g. after machine overhaul, the ALARM setting should be revised accordingly.
The TRIP limits generally relate to the mechanical integrity of the machine and are dependent on any specific
design features which have been introduced to enable the machine to withstand abnormal dynamic forces. The
values used are therefore generally the same for all machines of similar design and are not normally related to
the steady-state baseline value used for setting ALARMS. There can, however, be differences for machines of
different design and it is not possible to give more precise guidelines for absolute TRIP limits. In general, the
TRIP limit is within zone C or zone D.
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ISO 14839-4:2012(E)
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Table 1 — Comparison of rolling element bearing, fluid-film bearing and magnetic bearing
Parameter Rolling bearing Fluid-film bearing Magnetic bearing
Specific bearing load Radial bearing: 1 MPa to 3 MPa Radial bearing: 2 MPa to 5 MPa Radial bearing: 0,5 MPa to 0,7 MPa
Thrust bearing: 0.2 MPa to 2,0 MPa Thrust bearing: <7 MPa Thrust bearing: <0,8 MPa
Friction Medium High Low
General speed limit Depending upon component and lubrication, ~120 m/s ~200 m/s (radial AMB)
90 m/s to 150 m/s
~400 m/s (axial AMB)
Stiffness High Medium Low to medium, controllable and frequency
dependent
Damping Low High High, controllable, and frequency dependent
Lubricant Required Required Not required
Operation life Short Long Long
Maintenance Periodic replacement or periodic greasing or Oil filter replacement and oil replacement/ Electromagnets and transducers are
oil system maintenance are required disposal are required maintenance free. Controller hardware needs
periodic maintenance
Interchange standardization Yes In progress (in ISO/TC 123) No
Temperature range Wide Narrow Wide
Monitoring By another instrument By another instrument Built-in
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ISO 14839-4:2012(E)
6.3 Excess of rotor expansion
If there is a possibility that the rotor can come into contact with the stator parts, influenced by the amount
of thermal expansion in the axial direction of the rotor, the TRIP value should be set by using an additional
installation of the displacement transducer which detects the elongation.
6.4 Overload of bearing (over current of bearing coil)
Bearing load can be evaluated by monitoring the electric current of the bearing. The electric current value
restricted with the capacity of amplifier is set as the TRIP value.
6.5 Bearing temperature high
ALARM and TRIP values should be determined from the coil wire specifications.
EXAMPLE A temperature of approximately 155 °C is set as the TRIP value for the F class of wire insulation
classification.
6.6 Overspeed of rotor
Overspeed TRIP value should be established based on rotor design.
6.7 Power supply defect
6.7.1 General
If the mains power supply is cut off or fails, as a matter of course, an AMB loses functionality. First the batteries
are used and at low speed, then the levitation of the rotor is stopped, and the rotor falls on the touchdown bearing.
Usually, the methods specified in 6.7.2 and 6.7.3 are adopted as measures against electrical power failure of
the magnetic bearing control device.
6.7.2 Uninterruptible power supply
Electrical power is supplied to the magnetic bearing control device automatically from an uninterruptible power
supply (UPS), inserted in the first input side of the magnetic bearing control device at the time of electrical
power failure.
6.7.3 Regenerative power generation by the (a.c. or d.c.) motor equipped on the rotor
This method can be adopted when the built-in type motor is assembled on the rotor. At the time of detection of
electrical power failure, the control system drives the motor as an electric generator automatically and obtains
electrical power from rotational energy. To detect an electric power failure, a transducer can, for example,
monitor the supply voltage for voltage drops.
6.8 Battery power defect
When the normal power supply is cut off, the power supply generally switches to that from the battery. TRIP
action for stopping the shaft rotation is carried out immediately corresponding to the battery capacity.
6.9 Controller temperature high
Cooling of the control panel is performed by attaching a heat sink of enough cooling capacity for the power
amplifier, and making it discharge to the control-device exterior by passing surrounding air using a cooling fan.
TRIP or ALARM values should be managed by taking account the thermal examination result of the controller.
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ISO 14839-4:2012(E)
6.10 Cooling
The rotor and stator cooling can be achieved by a flow of gas (or air). A liquid cooling system can also be
located close to
...
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