Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-10-15
2004-08-03
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06770992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic bearing apparatus and to a magnetic bearing apparatus for reducing vibration caused by run-out in a radial direction of a rotor.
2. Description of the Related Art
A magnetic bearing is constituted by the arrangement of, for example, a plurality of coils (electromagnets) around both end portions of a rotor (rotary member). In an ordinary bearing, a rotor is pivotally supported by means of ball bearings or the like. However, in the magnetic bearing, a magnetic field generated by coils is applied to the rotor and an attractive force due to this magnetic field is balanced so that the rotor is supported (or levitated) in a non-contact manner in a constant position in space.
A run-out of the rotor (a shift in the radial direction from the constant position, i.e., a shift in the radial direction of the rotor) is detected by means of a radial direction sensor arranged in the vicinity of the magnetic bearing portion. In order to adjust the attractive force of the coils so that the run-out always falls within a constant range, a current of the coils is fed back and controlled.
In such a system where the rotor is supported by the magnetic bearing and rotated, there are some cases where a gravitational center (or inertia center) of the rotor and a rotary axis of the rotor are not identical to each other. When the rotor is rotated in such a condition, in the rotor, a run-out rotation in synchronism with a rotary cycle of the rotor caused by the misalignment between the gravitational center and the rotary axis is generated. In order to suppress the run-out rotation, the magnetic bearing generates a brake force in synchronism with the rpm of the rotor. Due to this cyclic brake force, the run-out in synchronism with the rpm of the rotor on the stator side where the magnetic bearing coils are arranged is generated in accordance with the law of action and reaction.
For instance, in the case where a turbo molecular pump carrying a magnetic bearing is used in an electronic microscope and so on, one of the more serious problems to be solved is how to control the vibration generated in the turbo molecular pump.
Attempts for suppressing the vibration caused by the above-described misalignment between the rotary axis of the rotor and the gravitational center thereof have been made. For instance, Japanese Patent Laid-open No. 259854/1995 discloses a magnetic bearing apparatus as such a magnetic bearing.
This magnetic bearing apparatus is shown in
FIG. 11. A
rotor
110
is pivotally supported at an air gap X
0
from coils
107
and
108
by the balance of the attractive force of the coils. The vibration &Dgr;X of the rotor
110
is detected by means of a radial direction sensor
109
and a shift detector circuit
102
. A magnetic bearing control circuit
101
feeds to power amplifiers
103
and
104
a signal of current to flow through the coils
107
and
108
for offsetting &Dgr;X.
In accordance with a signal of the magnetic bearing control circuit
011
, the power amplifier
103
feeds a current I
0
+&Dgr;I to the coil
107
and the power amplifier
104
feeds a current I
0
−&Dgr;I to the coil
108
. The rotor
110
is returned back to a constant position X
0
by means of the attractive force of the coils
107
and
108
receiving this current feed. Here, I
0
is a bias current to flow through the coils
107
and
108
, and &Dgr;I is the shift component of coils by a positional control signal of the rotor generated in the magnetic bearing control circuit
101
when the air gap is generated by &Dgr;X.
The magnetic bearing apparatus disclosed in this Japanese Patent Laid-open No. 259854/1995 is to separately control &Dgr;I for a frequency (expressed as fr) equal to the rpm of the rotor
109
and the frequency components other than that out of power spectrum of the shift signal outputted from the radial direction sensor
109
.
The explanation will be given with reference to formulae. In general, the attractive force F generated by the coils used in the magnetic bearing is given by the following formula.
F=K
{(
I
0
+&Dgr;I
)/(
X
0
+&Dgr;X
)}
2
(1)
Where, K is the constant determined by the number of turns or the shape of the coils.
A band-pass filter for passinc a signal of the frequency fr and a band-pass filter for passing signals other than the frequency fr are connected in parallel with each other within the magnetic bearing control circuit
101
The shift signal &Dgr;X of the rotor
110
is inputted into these filters and this signal is separated into the component having the frequency fr and the others (The components due to the run-out by the misalignment between the rotary center of the rotor
110
and the inertia center thereof is included in the signal of the frequency component fr).
For the shift signal &Dgr;X having the components other than the frequency fr obtained from the band-pass filters, the current of the coils are fed back and controlled in the same manner as in the conventional magnetic bearing apparatus.
The component &Dgr;X having the frequency fr obtained through the band-pass filters is controlled to become the value expressed by the following Formula (2). Then, the attractive force of the coils is kept constant as indicated by Formula (3).
&Dgr;
I
=(
I
0
/X
0
)&Dgr;
X
(2)
F=K
(
I
0
/X
0
)
2
(3)
As shown in Formula (3), since for the vibration having the frequency component fr, the attractive force for affecting the coils
107
and
108
is always kept constant, there is no vibration of the frequency fr in themagnetic bearing portion. Namely, the vibration (having the frequency fr) caused by the misalignment between the rotary axis and the inertia axis of the rotor
110
(in other words, it is safe to say that the gravitational center of the rotor
110
is not on the rotary axis) would not generate.
Note that a magnetic pole is provided in the central portion of the rotor supported at both ends by the magnetic bearings. The stator provided with coils (motor windings) is arranged around its periphery. A motor portion is formed in the rotor. In this case, when the rotor is vibrated in the radial direction, there is an unbalance in the magnetic force affecting the rotor by the magnetic field generated by the stator. This unbalance of the magnetic force is one of causes for generating the vibration upon the rotation of the rotor.
However, in the magnetic bearing apparatus disclosed in this Japanese Patent Application Laid-open, the vibration caused by the magnetic bearing per se may be suppressed but the vibration caused by the unbalance of the magnetic force in the above-described motor portion could not be suppressed although the existence thereof is clear.
Note that, brush-less DC motors carrying strong permanent magnets have been extensively used in accordance with the recent demands for miniaturization and high efficiency of the motors. In the case where the motor portions are formed by these motors, the vibration is likely to generate due to the magnetic unbalance concomitant with the phase of the permanent magnets in addition to the simple unbalance of the magnetic force caused by the difference in air gap intervals.
Further, there would be some cases where the rotor sticks to the stator due to the attractive force of the magnetic field so that the rotor could not be lifted by the magnetic bearings. In some cases, for this reason, the magnetic bearings must be enlarged more than necessary and the capacitance of the current amplifier must be increased.
Accordingly, in view of this point, an object of the present invention is to provide a magnetic bearing apparatus for reducing vibration in a bearing portion and a motor portion.
SUMMARY OF THE INVENTION
In order to attain the above-described object, according to the present invention, there is provided a magnetic bearing apparatus characterized by comprising a rotor, a motor portion provided in the rotor for rotating the rotor by a ma
Yamamoto Masayuki
Yamauchi Akira
Adams & Wilks
BOC Edwards Japan Limited
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