U.S. patent application number 14/744754 was filed with the patent office on 2016-12-22 for method for in-situ magnetization or degaussing of generator rotor.
This patent application is currently assigned to General Electric Company. The applicant listed for this patent is General Electric Company. Invention is credited to Eric Steven Buskirk.
Application Number | 20160372245 14/744754 |
Document ID | / |
Family ID | 57351993 |
Filed Date | 2016-12-22 |
United States Patent
Application |
20160372245 |
Kind Code |
A1 |
Buskirk; Eric Steven |
December 22, 2016 |
METHOD FOR IN-SITU MAGNETIZATION OR DEGAUSSING OF GENERATOR
ROTOR
Abstract
A method for in-situ magnetization of a generator rotor is
provided. The generator has a stator and the rotor is located
inside the stator. An air gap is formed between an outer radial
portion of the rotor and an inner radial portion of the stator. The
rotor has a plurality of excitation windings and a plurality of
permanent magnets. The method includes the step of applying a
current to the excitation windings, and the current is greater than
a normal excitation current. A maintaining step maintains the
current for a time period sufficient to magnetize the permanent
magnets. The magnetization of the permanent magnets occurs on the
rotor in-situ and while the rotor is inside the stator.
Inventors: |
Buskirk; Eric Steven;
(Scotia, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Assignee: |
General Electric Company
|
Family ID: |
57351993 |
Appl. No.: |
14/744754 |
Filed: |
June 19, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01F 13/006 20130101;
H02K 15/03 20130101; H01F 13/003 20130101 |
International
Class: |
H01F 13/00 20060101
H01F013/00 |
Claims
1. A method for in-situ magnetization of a generator rotor, the
generator having a stator and the rotor is located inside the
stator, an air gap is formed between an outer radial portion of the
rotor and an inner radial portion of the stator, the rotor having a
plurality of excitation windings and a plurality of permanent
magnets, the method comprising: applying a current to the
excitation windings, the current is greater than a normal
excitation current; maintaining the current for a time period
sufficient to magnetize the permanent magnets; and wherein the
magnetization of the permanent magnets occurs on the rotor in-situ
and while the rotor is inside the stator.
2. The method of claim 1, the current being about two to about five
times greater than the normal excitation current.
3. The method of claim 2, the time period being about five to about
fifteen seconds.
4. The method of claim 3, further comprising: placing a plurality
of filler pieces in the air gap, the filler pieces configured to
reduce reluctance and to permit increased magnetic flux
densities.
5. The method of claim 4, wherein the filler pieces are comprised
of a cobalt-iron alloy or an iron-cobalt-vanadium alloy.
6. The method of claim 4, wherein the filler pieces include a low
friction material to facilitate insertion into the air gap and
removal from the air gap.
7. The method of claim 6, wherein the low friction material
comprises polytetrafluoroethylene (PTFE).
8. A method for in-situ magnetization of a generator rotor, the
generator having a stator and the rotor is located inside the
stator, the stator having a plurality of stator windings, an air
gap is formed between an outer radial portion of the rotor and an
inner radial portion of the stator, the rotor having a plurality of
excitation windings and a plurality of permanent magnets, the
method comprising: applying a current to the stator windings, the
current is greater than a normal stator winding current;
maintaining the current for a time period sufficient to magnetize
the permanent magnets in the rotor; and wherein the magnetization
of the permanent magnets occurs on the rotor in-situ and while the
rotor is inside the stator.
9. The method of claim 8, the current being about two to about five
times greater than the normal stator winding current.
10. The method of claim 9, the time period being about five to
about fifteen seconds.
11. The method of claim 10, further comprising: placing a plurality
of filler pieces in the air gap, the filler pieces configured to
reduce reluctance and to permit increased magnetic flux
densities.
12. The method of claim 11, wherein the filler pieces include a low
friction material to facilitate insertion into the air gap and
removal from the air gap.
13. The method of claim 12, wherein the low friction material
comprises at least one of: bearings or polytetrafluoroethylene
(PTFE).
14. A method for in-situ magnetization or degaussing of a generator
rotor, the generator having a stator and the rotor is located
inside the stator, the stator having a plurality of stator
windings, an air gap is formed between an outer radial portion of
the rotor and an inner radial portion of the stator, the rotor
having a plurality of excitation windings and a plurality of
permanent magnets, the method comprising: applying a
voltage/current to a plurality of windings, an absolute value of
the voltage/current being greater than a normal winding
voltage/current; maintaining the voltage/current for a time period
sufficient to magnetize or degauss the permanent magnets in the
rotor; and wherein the magnetization or degaussing of the permanent
magnets occurs on the rotor in-situ and while the rotor is inside
the stator.
15. The method of claim 14, wherein the plurality of windings are
one of: the plurality of excitation windings or the stator
windings.
16. The method of claim 15, wherein the plurality of permanent
magnets are magnetized, and the applying step applies a positive
voltage/current to the plurality of windings.
17. The method of claim 15, wherein the plurality of permanent
magnets are degaussed, and the applying step applies a negative
voltage/current to the plurality of windings.
18. The method of claim 15, wherein the absolute value of the
voltage/current is about two to about five times greater than a
normal winding voltage/current.
19. The method of claim 18, the time period being about five to
about fifteen seconds.
20. The method of claim 15, further comprising: placing a plurality
of filler pieces in the air gap, the filler pieces configured to
reduce reluctance and to permit increased magnetic flux densities;
and wherein the filler pieces include a low friction material to
facilitate insertion into the air gap and removal from the air gap.
Description
BACKGROUND OF THE INVENTION
[0001] The method described herein relates generally to in-situ
magnetization and degaussing of generator rotors. More
specifically, the apparatus relates to in-situ magnetization and
degaussing of rotors incorporating permanent magnets.
[0002] Most synchronous machines include either permanent magnets
or excitation windings (fed by a regulated source) to provide MMF
(magnetomotive force) that provides the magnetic flux for machine
operation. The permanent magnets or excitation windings can be
situated on either the rotor or the stator. For rotor-based
embodiments, excitation power is brought through either a set of
collectors (slip rings) or a brushless system that uses a small
("inside-out") synchronous machine with a stator excitation
source.
[0003] Permanent magnet machines are not easily regulated.
Operation of permanent magnet machines in constant power mode can
be a problem because low power factor operation is forced through
flux weakening methods to reduce voltage at light loads or to
minimize inverter ratings. As a result, the machine terminal
voltage becomes load dependent, efficiency suffers, and, at partial
loads, the magnetic field source is underutilized.
[0004] Machines wound with excitation windings ("wound field
machines") can be regulated over a wide range of loads, but wound
field machines experience winding losses that decrease machine
efficiency. Additionally, windings and excitation sources for wound
field machines are sized to support the maximum requirements and
thus are often expensive and under-utilized.
[0005] Some known generators combine the advantages of the
excitation control of wound field machines and the advantages of
higher efficiency of permanent magnet machines. A hybrid machine
comprises a cylindrical element having slots, excitation windings
situated in at least some of the slots, and permanent magnets
situated at the poles. For example, U.S. Pat. No. 6,509,664
describes a hybrid synchronous machine with a field/rotor having
excitation windings is some of the slots and permanent magnets at
the poles, and this patent is incorporated herein by reference.
BRIEF DESCRIPTION OF THE INVENTION
[0006] In an aspect of the present invention, a method for in-situ
magnetization of a generator rotor is provided. The generator has a
stator and the rotor is located inside the stator. An air gap is
formed between an outer radial portion of the rotor and an inner
radial portion of the stator. The rotor has a plurality of
excitation windings and a plurality of permanent magnets. The
method includes the step of applying a current to the excitation
windings, and the current is greater than a normal excitation
current. A maintaining step maintains the current for a time period
sufficient to magnetize the permanent magnets. The maintaining step
may utilize a decaying step function to progressively lower the
current over time. The magnetization of the permanent magnets
occurs on the rotor in-situ and while the rotor is inside the
stator.
[0007] In another aspect of the present invention, a method for
in-situ magnetization of a generator rotor is provided. The
generator has a stator and the rotor is located inside the stator.
The stator has a plurality of stator windings, and an air gap is
formed between an outer radial portion of the rotor and an inner
radial portion of the stator. The rotor has a plurality of
excitation windings and a plurality of permanent magnets. The
method includes a step of applying a current to the stator
windings. The current is greater than a normal stator winding
current. A maintaining step maintains the current for a time period
sufficient to magnetize the permanent magnets in the rotor. The
maintaining step may utilize a decaying step function to
progressively lower the current over time. The magnetization of the
permanent magnets occurs on the rotor in-situ and while the rotor
is inside the stator.
[0008] In yet another aspect of the present invention, a method for
in-situ magnetization or degaussing of a generator rotor is
provided. The generator has a stator and the rotor is located
inside the stator. The stator has a plurality of stator windings.
An air gap is formed between an outer radial portion of the rotor
and an inner radial portion of the stator. The rotor has a
plurality of excitation windings and a plurality of permanent
magnets. The method includes applying a voltage/current to a
plurality of windings, and an absolute value of the voltage/current
is greater than a normal winding voltage/current. A maintaining
step maintains the voltage/current for a time period sufficient to
magnetize or degauss the permanent magnets in the rotor. The
maintaining step may utilize a decaying step function to
progressively lower the current over time, and the current may
alternate between positive and negative values. The magnetization
or degaussing of the permanent magnets occurs on the rotor in-situ
and while the rotor is inside the stator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a sectional view of one known hybrid
synchronous machine, such as a generator.
[0010] FIG. 2 illustrates the magnetic flux patterns created during
a permanent magnet magnetization or degaussing process, according
to an aspect of the present invention.
[0011] FIG. 3 is a flowchart of a method for in-situ magnetization
or degaussing of permanent magnets in a machine's rotor, according
to an aspect of the present invention.
[0012] FIG. 4 illustrates a cross sectional view of a filler piece,
according to an aspect of the present invention.
[0013] FIG. 5 illustrates a simplified schematic of the rotor and
stator excitation systems.
DETAILED DESCRIPTION OF THE INVENTION
[0014] One or more specific aspects/embodiments of the present
invention will be described below. In an effort to provide a
concise description of these aspects/embodiments, all features of
an actual implementation may not be described in the specification.
It should be appreciated that in the development of any such actual
implementation, as in any engineering project, numerous
implementation-specific decisions must be made to achieve the
developers' specific goals, such as compliance with
machine-related, system-related and business-related constraints,
which may vary from one implementation to another. Moreover, it
should be appreciated that such a development effort might be
complex and time consuming, but would nevertheless be a routine
undertaking of fabrication and manufacture for those of ordinary
skill having the benefit of this disclosure.
[0015] When introducing elements of various embodiments of the
present invention, the articles "a", "an" and "the" are intended to
mean that there are one or more of the elements. The terms
"comprising," "including," and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements. Any examples of operating parameters and/or
environmental conditions are not exclusive of other
parameters/conditions of the disclosed embodiments. Additionally,
it should be understood that references to "one embodiment", "one
aspect" or "an embodiment" or "an aspect" of the present invention
are not intended to be interpreted as excluding the existence of
additional embodiments or aspects that also incorporate the recited
features.
[0016] FIG. 1 is a sectional view of one known hybrid synchronous
machine. The hybrid synchronous machine 10 includes a rotor 12 and
stator 14. The stator includes a stator core 18 and the rotor
includes a rotor core 16. The rotor 12 is shown inside the stator
14, or in the machine's in-situ state. An air gap 40 is formed
between an outer radial portion of rotor 12 and an inner radial
portion of stator 14. The rotor (or field) slots include
winding/excitation slots 20 and permanent magnets 28. Each
permanent magnet can be situated in a respective individual one of
the permanent magnet slots/location as shown by permanent magnet 30
or a plurality of permanent magnets can situated in a respective
one of the magnet slots/location as shown by permanent magnets 130.
Although a two pole machine is shown for purposes of simplicity,
the method of the present invention additionally extends to
machines having more than two poles.
[0017] The stator 14 comprises magnetic steel laminations, rotor 12
comprises a solid forging of high strength magnetic steel,
permanent magnets 30 or 130 comprise neodymium-iron-boron or
samarium-cobalt, for example, and the rotor excitation windings 24
and stator windings 26 comprise insulated copper conductors. The
depth and width of the rotor slots and stator slots 22 for the
windings and/or permanent magnets will vary according to
motor/generator technical constraints. Typically the permanent
magnets 30, 130 will extend along the entire or a substantial
portion of the axial length of the rotor. An advantage to having
the permanent magnets extend to a portion of the rotor that does
not extend to the entire length is that magnet flux is directed
straight from the rotor to the stator and does not extend along the
fringes in the same manner that occurs with rotor end windings.
Thus the two-dimensional flux from the rotor to the stator can be
increased and the fringe end flux can be decreased.
[0018] The permanent magnets in FIG. 1 are situated on the pole or
the direct axis of machine 10. Machine 10 may be a generator or a
motor. In one embodiment, the permanent magnets 30, 130 are
attached to the rotor core with an adhesive such as an epoxy and
the windings are applied using a conventional winding technique.
Then the rotor is wrapped with a metallic or non-metallic,
non-magnetic shell 13 which may comprise carbon, for example. In
one embodiment, a non-magnetic shell is applied in a carbon
fiber-epoxy composite form.
[0019] The use of permanent magnets in generators or motors is not
new. However, using them makes handling, assembly and operation of
the machine more difficult. In a typical DC excited field/rotor
(containing no permanent magnets) when the field current is
removed, the magnetic field is removed. In a permanent magnet
machine, the magnetic flux cannot be turned off. This creates
handling difficulties for the rotor, as well as a potentially
hazardous work environment. The rotor will attract (very strongly)
magnetic tools and tooling. Trying to insert a permanent magnet
rotor into a stator is also very difficult, as the rotor will want
to be attracted to one side of the inner radius of the stator. The
same difficulties and dangers present themselves when trying to
remove a permanent magnet rotor from the stator.
[0020] According to the present invention a method is provided for
magnetizing and degaussing a permanent magnet containing
field/rotor in-situ to facilitate handling thereof. The permanent
magnets can be magnetized by using the excitation windings 24 or
the stator windings 26. A strong yet short duration current from
positive voltage is passed through the windings to create a very
large magnetic field. This magnetic field magnetizes the permanent
magnets, and the permanent magnets will retain this magnetization
until acted on by an opposite magnetic field or increased
temperature beyond a predetermined limit. In like manner, the
permanent magnets can be degaussed (un-magnetized) by using the
excitation windings 24 or the stator windings 26. Degaussing is the
process of decreasing or eliminating a remnant magnetic field. A
strong yet short duration current from negative voltage is passed
through the windings to create a very large magnetic field. This
degaussing process "reverses" the magnetizing process and greatly
reduces (or possibly eliminates) the magnetization of the permanent
magnets. The degaussed permanent magnets make the rotor much easier
and safer to handle. Both the magnetizing and degaussing methods
are performed in-situ or with the rotor inside of the stator, as
the rotor is safely secured in this state. Degaussing (or
magnetizing) may also be performed by using multiple decaying steps
that may switch from progressively lower positive to negative
current over time.
[0021] FIG. 2 illustrates the magnetic flux patterns created during
a permanent magnet 130 magnetization or degaussing process. The
machine 10 is similarly constructed as to the machine shown in FIG.
1, but the stator windings 26 are omitted for clarity. A current is
passed through the rotor's excitation windings 24. This current
creates a magnetic field illustrated by the lines of magnetic flux
200. As one example only, the machine is a model GE GEN-H53
generator manufactured by the General Electric Company,
Schenectady, N.Y. The exciter system for this generator can put out
a maximum of about 3,000 amps/1,000 volts (continuous) or about
6,000 amps/2,000 volts for short durations (e.g., up to about 10
seconds). The typical rated rotor winding current/voltage is about
2,000 amps/700 volts. At the maximum allowed voltage/current for
the excitation windings 24 it would take about 6 seconds to
magnetize (or degauss) the permanent magnets 130. A short burst of
high voltage and current will be tolerated by the machines
electrical systems and will provide a strong enough magnetic field
for in-situ magnetization or degaussing. To magnetize the permanent
magnets 130 a positive voltage/current may be used, and to degauss
the permanent magnets 130 a negative voltage/current would be used.
Alternatively, magnetizing may be accomplished with a negative
voltage/current, and degaussing with a positive voltage/current. An
important aspect is that the polarity of the voltage/current
passing through the windings is reversed for the magnetizing and
degaussing operations.
[0022] The stator windings 26 may also be used to magnetize or
degauss the permanent magnets in the rotor. The process is the same
as described above, except that the high intensity and short
duration voltage/current is applied to the stator windings 26. The
current passing through the stator windings 26 will create a
similar magnetic field (compared with flux lines 200) that will
either magnetize or degauss the permanent magnets (depending on
polarity). In addition, the voltage/current passing through either
the rotor or stator windings may be configured as an increasing or
decreasing step function. In this embodiment the voltage/current is
ramped up or down in a stepped fashion. If the temperature of the
permanent magnets is raised to a predetermined level, then the
magnetization/degaussing operations may be facilitated.
[0023] FIG. 3 is a flowchart of a method 300 for in-situ
magnetization or degaussing of permanent magnets in a machine's
rotor. As described previously, the machine may be a generator or a
motor. As an option, step 310 may be used to place a plurality of
filler pieces 50 in the air gap 40 between the rotor 12 and stator
14. Only a few filler pieces 50 are shown for clarity, and it is to
be understood that one or more filler pieces could be used. FIG. 4
illustrates a cross sectional view of a filler piece 50, according
to an aspect of the present invention. The filler pieces 50 are
configured to reduce reluctance and to permit increased magnetic
flux densities. The filler pieces 50 may be comprised of a high
magnetically permeable material with a high saturation level. As
non-limiting examples only, a cobalt-iron alloy or an
iron-cobalt-vanadium alloy may be used, and these alloys have a
magnetic saturation of about 24 kilogauss. In addition, the filler
pieces 50 may include a core 51 of a high magnetic permeability
material, and a low friction material 52 on one or both sides of
the core 51. The low friction material 52 facilitates insertion of
the filler pieces 50 into the air gap 40, as well as facilitating
removal of the filler pieces from the air gap. For example, the low
friction material 52 may be comprised of polytetrafluoroethylene
(PTFE), or even roller or spherical bearings, or any other suitable
low or reduced friction material. The filler pieces 50 may extend
the axial length of the rotor, the axial length of the permanent
magnets 130, or be of any suitable length as desired in the
specific application.
[0024] In step 320, a voltage/current is applied to the rotor's
excitation windings 24 or the stator windings 26. FIG. 5
illustrates a simplified schematic of the rotor and stator
excitation systems. A rotor excitation system 60 applies a
voltage/current to the rotor excitation windings 24. Typically,
this voltage/current is of a sufficient magnitude (i.e., the normal
excitation voltage and/or normal excitation current) to generate a
suitable magnetic field for power generation. However, in the
method of the present invention the magnetizing/degaussing
voltage/current is preferably about two to about five times greater
than the normal excitation voltage/current. The
magnetizing/degaussing voltage/current could be below this range,
but the time required to obtain the magnetization or degaussing
will be increased. In addition, the magnetizing/degaussing
voltage/current could be more than three times the normal
excitation voltage/current, but the machine components may be
undesirably affected unless they are capable of handling these
increased levels of voltage and current.
[0025] In step 330, the voltage/current applied to the rotor's
excitation windings 24 or the stator windings 26 is maintained for
a time period sufficient to magnetize or degauss the permanent
magnets 130. The time period may be limited to about five to about
fifteen seconds, or any suitable short duration time period. The
time period is limited because the voltage/current is greater than
the normal excitation voltage or stator winding voltage. If the
magnetizing/degaussing voltage/current was maintained for extended
periods of time, then damage to the machine could occur. Most
generators or motors can handle the increased levels of
voltage/current for brief periods without experiencing any damage,
and a time period of about two to about fifteen seconds, or any
subranges therebetween should be tolerated by most generators and
motors.
[0026] In step 340, the permanent magnets 130 in rotor 12 may be
magnetized. For example, a positive voltage/current may be used to
magnetize the permanent magnets 130, and a negative voltage/current
may be used to degauss the permanent magnets 130, or vice-versa. An
advantage provided by the present invention is that the rotor's
permanent magnets 30, 130 may be magnetized and/or degaussed with
the rotor 12 in-situ, or with the rotor 12 located inside the
stator 14. This makes handling the rotor 12 much safer and easier
as it can be transported and handled in a degaussed or
un-magnetized state. In addition, the cost for manufacturing and
handling the rotor is greatly reduced due to the fact that special
rooms (without any magnetic materials) and equipment are no longer
required.
[0027] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they have structural elements that do not differ
from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
* * * * *