199903518
Description
CA 02381148 2002-02-O1
Demagnetization protected permanent-magnet ship
propulsion system
The invention relates to an electrical propulsion
system for high power levels, for example for power
levels above 500 kW, with high availability and long
life, in particular for an ocean-going ship, with the
propulsion system having a permanent-magnet electric
motor, which interacts with at least one rotating power
load in particular a ship's propeller, and at least one
power converter for supplying power to the electric
motor, as well as a control, regulating and monitoring
device for the system.
Permanent-magnet electric motors have been known in
widely differing physical forms for a very long time.
various physical forms for permanent-magnet electric
motors are described, for example, in the article in
the Siemens Journal 49, 1975, Issue 6, Pages 368 to
374. The power levels which could be achieved at that
time were up to 500 kW. For a long time, this was the
power limit for permanent-magnet electrical machines,
and even larger machines, for example up to 30 MW, have
been developed only recently. For machines such as
these, which represent a considerable investment, a
long life is required with high availability, in order
to justify the investment costs. In general, ships have
a life of 25 to 30 years, so that the same life is
therefore required for their propulsion systems. Until
now, it has not been possible to guarantee such a life
for large permanent-magnet electric motors. One object
of the invention is to specify a system by means of
which such a long life is reliably achieved by large
permanent-magnet electric motors.
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The object is achieved in that the electric motor is
designed to operate reliably over a long time period
and, in particular, is designed to be protected against
total or partial demagnetization, with this being
achieved by supplementary design and operating
measures, for example circuitry and control measures,
with respect to the motor and the power converter. The
life of the permanent magnets is the major criterion
for the life of a [lacuna] designed in accordance with
the design rules for long life. The critical factor
governing the life of a propulsion system, which
operates with permanent magnets, for ships or for other
large electrically operated systems - a ship may also
be regarded as an industrial system - is the life of
the magnetic components. Demagnetization can occur, for
example, due to overheating or due to large internal
magnetic fields. Furthermore, the magnets may corrode,
and demagnetization due to aging is also possible. It
is also possible for the magnet to migrate, or the
like, on the rotor of the electric motor, for example
if large circumferential accelerations occur in the
event of a defect. The propulsion system according to
the invention takes account, in a way which is novel to
date, of life-reducing factors described above.
Analyses relating to the demagnetization of permanent-
magnet electric motors can also be found in the Diploma
Thesis by Ville Nahkuri entitled "Large Power Permanent
Magnet Propulsion Motors", 24.02.1998, Helsinki
University of Technology, Faculty of Electrical and
Communications Engineering. The Diploma thesis shows
that the temperatures within the electric motor are
below the critical values, but that the magnetic field
can form a risk for the permanent magnets in the event
of malfunctions in the electrical system. The Diploma
Thesis
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does not indicate any solution to this problem which
can not be achieved without adversely affecting the
other system components.
The refinement of the invention provides that the
propulsion system has permanent magnets composed of a
magnetically aging-resistant magnetic alloy, for
example based on sintered and heat-treated neodymium-
iron-boron, which is attached, in particular in an
interlocking manner, to the rotor of the electric
motor. Appropriate magnetic alloys have been known for
some time from small electrical drive systems, for
example for actuating drives. Their long-term behavior
has been sufficiently investigated in practice and
theoretically. The investigations have indicated a life
which, when they are used correctly, is considerably
greater than the required time of 25 to 30 years. This
is dependent on the magnets being in a constant
position and orientation, and they must not migrate.
A further refinement of the invention provides for the
electric motor to be designed without cooling by means
of a coolant with closed-circuit cooling and,
especially when used has a steering propeller motor, in
a motor pod, to have external wall cooling. This
advantageous refinement provides the greatest possible
confidence with respect to overheating. If there is no
coolant circulation system, it cannot fail, either.
External wall cooling operates satisfactorily in all
conditions, in particular for a ship's pod propulsion
system. When the ship is in motion, cooling is ensured
by the movement of the propulsion system through water.
The cooling effect increases as the vessel speed
increases, that is to say with the amount of power
consumed in the propulsion system. This automatically
results in cooling which reacts as a function of the
power.
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A further refinement of the invention provides for the
magnetic circuit of the propulsion system to be
designed such that any short-circuit current at the
rating point is automatically limited to non-critical
values. The short-circuit current - for example caused
by short-circuiting the terminals - results in 1.7
times the rated current, in a typical application. This
value is not critical since the magnet system according
to the invention is designed, for example, for 2.2
times the current without the magnetic field produced
by the overcurrent damaging the permanent magnets.
Since this interacts with current limiting in the power
converter, for example to 110 ~ to 120 of the design
current, which is configurable, this results in the
current flowing through the motor being reliably
limited to non-critical values at all points. To
achieve this, the invention provides for the power
converter to have maximum current limiting, which is
configurable, for its individual branches, for example
to a value which reliably prevents demagnetization due
to overcurrent. Since the individual branches of the
power converter, and not just this power converter as a
unit, are included in the monitoring, this results in
the current flowing in the motor being limited to non-
critical values at all points, even in improbable
situations.
One particular advantage in this case is the use of
current limiters which switch off very quickly, for
example in less than one millisecond, on the basis of
the physical effect, for example HTS current limiters.
HTS (High Temperature Superconducting) current limiters
operate at about 77 K, that is to say cooled by liquid
nitrogen. If the critical current density is exceeded,
this immediately results in a relatively large finite
resistance, that is to say it results in primary
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disconnection. Secondary disconnection, for example by
means of a circuit breaker, is then required a short
time later.
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Appropriate current limiters have already been proposed
in the public domain, for example for a size of 1 MVA
by the applicant. The switching element is a YBCO layer
on a ceramic plate conductor.
Limiting is possible in specific form for the various
power semiconductors used, depending on the
configuration of the power converter, that is to say,
for example, for GTOs, for IBGTs or for thyristors. The
corresponding power semiconductors are advantageously
also monitored individually, in order to detect short-
circuited semiconductors immediately, in order that
they can be replaced. Even the short-circuiting of the
power semiconductors that are used, which can never be
completely precluded, can thus not damage the magnet
system.
In order to monitor the propulsion system, the
invention furthermore provides for the system to have
measurement devices between the power converter and the
electric motor, as well as between the power converter
and the transformer, both overall and for individual
power branches . Faults in the power converter can thus
be identified and rectified quickly. Appropriate, known
measurement devices are provided for identification.
Current limiting as a function of the motor rotation
speed is of particular importance. This takes account
of the fact that the internal magnetic field depends on
the motor rotation speed.
As further safety measures, the system has a ground
fault indication and protection device, for ships and
the like, a cable discontinuity monitoring device, a
phase balance monitoring device and further monitoring
and protection circuit components, in particular for
overcurrents and excess heat. It is thus possible to
take account of all conceivable faults in the
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entire system which could lead to overheating of the
motor.
A further refinement of the invention provides for the
system according to the invention to have a multiple
winding system in the motor with two power converters,
which each feed the windings, and for operation with
two electric propulsion motors each having a power
converter, and with three winding sections of the power
converter in each case being interconnected to form a
three-phase system. Overall, this results in relatively
small sub units, which can each be monitored and
switched off individually. Once again, this ensures
that no damaging overcurrents can occur in the
propulsion motor.
A remote diagnosis device is also provided in this
case, which covers, in particular, the ship operating
system and the power converter with its components.
Such a remote diagnosis device which operates, for
example, for ships with satellite communication allows
the manufacturer's specialists to report to the ship
engineer which components he should replace, and which
components could fail in the near future. This also
further improves the safety with regard to
demagnetization and operational safety.
In order to ensure the life of the permanent magnets,
the invention provides for them to be composed of a
magnetically aging-resistant and, in particular,
corrosion-resistant magnetic alloy, for example based
on sintered and heat-treated neodymium-cobalt-copper-
iron-boron, for example the Vac-quality Vacodynm 677HR.
The invention furthermore provides for the permanent
magnets to be long-term coated or varnished, and to
have a smooth
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surface. The shape is, for example, cuboid. This thus
results not only in the corrosion-resistant base
material, but also in a corrosion-protected overall
design. In the conditions that occur in the electric
motor, there is no loss of material in the permanent
magnets, and they do not lose their magnetic force
either, over the required life.
The magnet blocks are advantageously fixed on their
base by means of adhesive bonding, in particular by
means of a fully crosslinked silicone adhesive in the
form of a single-component adhesive. The magnet blocks
must be fixed on their base for installation. The use
of a fully crosslinked silicone adhesive in the form of
a single-component adhesive in this case advantageously
prevents the possibility of corrosion centers forming
at the junction between the magnet and the pole shoe.
In order to monitor the electric motor, the invention
provides for the stator windings to have temperature
sensors, in particular temperature sensors with
measurement evaluation and/or initiation of a warning
function. In this case, it is also advantageously
possible to provide for the temperature sensors to be
connected to the regulating part of the system, in
order to reliably prevent overheating of the windings
and of the magnets. This results in an additional
monitoring capability, which allows, in particular,
damaging trends relating to the heating of the electric
motor to be counteracted.
The attraction forces of the individual magnet blocks
are very high. One advantageous refinement of the motor
provides for the diameter and the maximum operating
rotation speed of the electric motor to be designed
such that, even at the maximum rotation speed, a resi-
dual force remains between the magnet cuboid and its
contact surface (automatic permanent magnet adhesion).
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the magnet blocks are also fixed in an interlocking
manner and with a force fit on their base by means of a
binding in conjunction with the geometric configuration
of the pole shoes, this interlocking and force-fitting
adhesion is further assisted by automatic permanent
magnet adhesion. The magnet cuboids are positioned
reliably without adversely affecting the life, and even
if extreme forces occur, for example in the event of a
defect. The binding may be composed of either fiber-
reinforced plastic or else of a non-magnetic material.
Fiber-reinforced plastic allows the binding to be
designed to be particularly thin, however, so that the
motor can be designed with a particularly narrow air
gap.
The end windings of the stator windings are
advantageously designed to be encapsulated, and are
connected to the external wall via fixed thermally
conductive links. This results in a failure-resistant
design, which is undoubtedly far superior to forced
circulating cooling, with closed-circuit cooling.
Designing the electric motor with an encapsulated outer
housing without any openings also has the same aim.
This ensures that no foreign bodies can enter, or can
be introduced, into the electric motor from the
outside, for example during repairs or the like.
Therefore, this results in the electric motor requiring
no maintenance, being in an encapsulated form, and
having a very long life. There is therefore
advantageously no need for any accessible shaft between
the ship and the pod, as is known for pod propulsion
systems using permanent magnets and with closed-circuit
cooling. There is thus no need for motor repairs during
the routine docking of ships every five years, and all
that need be done is to inspect, and if necessary to
replace, parts which are subject to wear such as sails
and bearings.
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Finally, the invention also provides for magnetic field
sensors to be arranged in the motor in order to improve
reliability, which can be switched on, in particular,
periodically or under event control. Furthermore, a
computation unit is provided, which determines the
electrical and magnetic state of the motor continuously
from measurement data, for example from individual
currents in the electric motor, from the motor
temperature, from the emitted power and from the
rotation speed, and possibly from other characterizing
influencing variables, and emits a warning when
critical magnitudes are approached, preferably also
initiating countermeasures. A permanent [lacuna] is
advantageously reached, in order, in particular for the
remote diagnosis, to provide continuous monitoring of
the propulsion motor taking account of the interactions
between the individual influencing variables.
The invention will be explained in more detail with
reference to drawings, from which further details,
which are also significant to the invention, will
become evident, in the same way as from the dependent
claims.
In detail, in the figures:
Figures l, 2, 3 and 4 show the individual circuits
provided for the system
Figure 5 shows a typical magnetic flow
pattern for the permanent
magnets,
Figure 6 shows a temperature curve over
the length of the rotor,
Figure 7 shows the profile of the
magnetic force, and
Figure 8 shows the magnetic force profile
of a neodymium-iron-boron magnet
plotted against time
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The drawings, in conjunction with the teaching in the
patent claims, together with the detailed statement
contained in them and the description, are self-
explanatory to those skilled in the art.
Figures 1 to 4 use the conventional symbols from
electrical engineering for transformers, power
converters and electric motors. As can be seen, the
three-phase machine shown in Figure 1 and having a 6-
pulse converter has the minimum number of phases to be
monitored and of half-power semiconductors. Individual
monitoring of the phase and power semiconductors is
thus advantageously chosen in this case.
In Figure 2, which shows the circuit diagram of a
three-phase machine with a 12-pulse power converter,
the number of power branches in the power converter is
doubled, and the monitoring can be simplified here, to
the extent that it relates to the power converter. In
Figure 3, the monitoring in the electric motor can also
be reduced. The circuit of a 6-phase machine shown in
Figure 4 with two 12-pulse power converters, results in
the optimum configuration with regard to reliability.
The magnetic profiles which are shown in Figure 5 and
are illustrated in an idealized form are distinguished
in particular by their symmetry. It is thus possible to
avoid particular flux concentrations.
Figure 6 shows the stator temperature as a function of
the length of the electric motor. The rotor temperature
is approximately 10° below the stator temperature, so
that there is advantageously no need to monitor the
rotor temperature separately. It is evident that the
air gap temperature is the highest on the field side of
the electric motor, this is where the thermocouples for
monitoring are thus advantageously concentrated.
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The motor temperature can be monitored easily and
reliably by means of densely concentrated thermocouples
on the magnet side of the electric motor, with the
thermocouples advantageously being firmly connected to
the stator winding.
Finally, Figure 7 shows the magnetic force on no load,
at the rated load and in the event of a short-circuit.
As can be seen, the magnetic force is in each case
within the reversible part of the characteristic
profile, even for the short-circuit case. Protection
against demagnetization is thus provided, even in the
event of a short-circuit.
Finally, Figure 8 uses a logarithmic representation to
show the irreversible polarization losses found in
detailed investigations for various coefficients B/~,OH
at 130°c. 130°c is considerably greater than the
highest temperatures, as shown in Figure 6, in the
electric motor according to the invention, that is to
say, with regard to over temperature, there is a safety
margin against demagnetization, which is so high that
it can be assumed that the magnets will have a long-
term life even beyond the required 25 to 30 years.
The required life of 25 to 30 years for permanent-
magnet propulsion systems for pod motors for ships can
be ensured on the basis of the transformer resins that
are used, which have been proven over many years, with
a long-term temperature resistance of more than 150°
(more than 20 years of operational experience is
likewise already available here), relating to the long-
term magnetic force of 130° and the maximum temperature
that occurs of around 90 ° in the area of the permanent
magnets, and since overcurrents are reliably avoided in
the windings of the electric motor. The encapsulated
configuration with direct external wall cooling, which
is fail-safe, makes a not inconsiderable contribution
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