The present invention relates to electrical rotating machinery and rotor bearings.
During motor operation, strong magnetic forces tend to pull the rotor against the stator. In general, these forces are quite well balanced across the circumference of the air gap, and suitable shaft bearings maintain the rotor in proper position. However, small misalignments in rotor position can cause an imbalance of forces on the bearings, which will cause increased bearing wear, and may lead to machine failure.
Magnetic bearings are well known to the field of rotating machinery. Their operation may be seen in
U.S. Pat. No. 6,559,567 discloses an electromagnetic rotary drive, designed as bearingless motor, which comprises a magnetically journalled rotor and a stator which comprises a drive winding for producing a magnetic rotary drive field which produces a torque on the rotor, and a control winding for producing a magnetic rotary control field by means of which the position of the rotor with respect to the stator can be regulated, with the stator having exactly six stator teeth. These two windings, which, in one embodiment are combined into a single winding, must each generate a magnetic field of a different number of poles from one another.
In a three-phase induction motor, the currents are controlled in each of the phase windings in such a way as to establish a magnetic field in the rotor and cause the rotor to align with the field flux. Then, by properly controlling the currents in the stator field, a vector is produced that leads to the shared magnetic field of the stator and rotor, which causes the rotor, and ultimately the shaft, to move. In a three-phase motor, the stator is an electromagnet made with a winding for each phase on a soft iron casting. In each winding, current may flow in a forward (positive) or reverse (negative) direction; this results in six unique steps or pole alignments. The amount of current that flows is controlled by either pulse width modulation (PWM) or analog means. The resolution of control depends on the resolution of the positioning feedback device, the current feedback, and the update rate.
At a fixed point in time, two currents are involved in the motion of the rotor. One current, id, is associated solely with the stator flux. This is the current that induces a magnetic field in the rotor of an induction motor and, held constant, causes the rotor to align with it. Use of that current alone gives a stepper motor, as its motion can be controlled by indexing the stator flux in a circular fashion. This produces very little torque, however. The only torque it does produce results from the motion of the flux to the next indexed step. The second current is 90 degrees out of phase with the first and is called the quadrature current, or iq. This current produces a flux that either leads or trails the stator flux. If it trails the stator motion, the motor is a generator. If it leads, there is torque, and thus, a motor. The size of iq determines the amount of torque.
To control or correct the operation of a motors, it is necessary to know the currents and position of the rotor. Generally the Clarke transform is used to change the reference of the three-phase currents, ia(t), ib(t), and ic(t) to currents in the two-phase orthogonal stator axis: ia and ib. This conversion is illustrated in
ia(t)+ib(t)+ic(t)=0
Va(t)+Vb(t)+Vc(t)=0
φa(t)+φb(t)+φ(t)=0
which denote currents, voltages, and flux linkages. The final relationship maintains the balance of currents, voltages, or flux linkages as explained by Kirchoff's Law, that is, their sum will be zero. Anytime there is a current, voltage, or flux in one phase there must be corresponding currents, voltages or fluxes in the other two to balance it. Both the forward and reverse Park and Clarke transforms may be applied to currents, voltages, or linkages in exactly the same way. Currents in the phase windings are used to compute new voltages for the drive waveforms (eg PWM). With this in mind, the following relationship exists between a homopolar and three-phase system:
U.S. Pat. No. 6,054,837 discloses polyphase induction machine operated by an inverter drive system. The machine is constructed with concentrated full span windings. Twelve or more phases are used to sufficiently cover the airgap region, in contrast to the conventional three phases using distributed and chorded windings. Substantial efficiency and starting torque benefits are thereby obtained
U.S. Pat. No. 6,570,361 discloses an electrical rotating apparatus comprising an inverter system that outputs more than three phases. The apparatus further includes a stator comprising a plurality of slots and full span concentrated windings, with the windings being electrically coupled to the inverter system, and a rotor electromagnetically coupled to a magnetic field generated by the stator. A signal generator generates a drive waveform signal, that has a fundamental frequency, and the drive waveform signal drives the inverter system. The drive waveform signal has a pulsing frequency and is in fixed phase relation to the fundamental frequency.
U.S. Pat. No. 6,351,095 discloses an electrical rotating machine comprising an inverter drive system wherein alternating current comprising more than three phases is produced from the inverter drive system. The machine further includes a stator comprising a plurality of slots and windings, wherein the windings are electrically coupled to the inverter drive system and a winding chording factor of the stator is approximately 1. Further, a winding distribution factor of the stator could also be approximately 1. A rotor in the machine is electromagnetically coupled to a rotating magnetic field generated by the windings and the rotating magnetic field has a flux density level that exhibits saturation effects.
U.S. Pat. No. 6,348,775 discloses a polyphase induction motor operated by an inverter drive system comprising a logic level controller. A number, preferably twelve or more, of independently driven phases causes harmonic fields, up to a number equal to the number of phases, to oscillate in synchrony with the fundamental oscillating field. A pulse-width modulation (“PWM”) carrier is used by the logic level controller to synthesize a desired drive alternating current, in which the pulsing distortion produced by the pulse width modulation produces a synchronous oscillating field in the driven polyphase induction motor.
In these high phase order motor, a plurality of stator windings are individually controlled by independent inverter half bridges. Normally in a multiple pole motor, there will be several windings located in different portions of the stator, each driven by a separate inverter half bridge, but operated at the same electrical phase angle. During balanced operation, these separate windings will be operated under conditions of the same voltage, frequency, and phase. In a large machine, numerous independently driven windings may be used. Provision is made to ensure that drive balance is achieved at all times by selecting driven winding ends in sets which have odd numbers and which are symmetrically distributed. Most commonly, in the case of motors wound with a multiple of three phases, driven winding ends are selected in sets of three, and in each set the windings are driven 120 electrical degrees apart.
Thus, for example, an eighteen phase machine having 18 windings in 36 slots may have winding ends at: 0°, 10°, 20°, 30°, 40°, 50°, 120°, 130°, 140°, 150°, 160°, 170°, 240°, 250°, 260°, 270°, 280°, and 290° be driven. As described above, this will result in a balanced drive. A better connection may include a winding connection which is not only balanced for the primary, or fundamental waveform, but which is also maximally balanced for harmonic waveforms. In the above example, the winding is not balanced for the third harmonic, and will thus exhibit uneven flow of the third harmonic. The general rule for selection of winding connections is that the winding connections are preferably maximally distributed. Thus, for this example with an 18 phase machine, with star connection, a possible connection might be: 0°, 10°, 40°, 50°, 80°, 90°, 120°, 130°, 160°, 170°, 200°, 210°, 240°, 250°, 280°, 290°, 320° and 330°. This winding is perfectly balanced for the fundamental, third, fifth, and seventh harmonic, and exhibits unbalanced drive at the ninth harmonic.
From the foregoing, it may be appreciated that a need has arisen for an electric motor in which deviations from a balanced operation, which places unwanted stress on the bearings, are corrected. Deviations from balanced operation may arise, for example, as a result of gravity, or as a result of the effect of the load on the rotor rotation.
The invention is directed to a motor having an actively alignable rotor comprising a rotor and a stator. The stator comprises a plurality of conductors supplied with electrical current for rotating said rotor, and some or all of the conductors, termed “a conductor set”, span less than 180 rotational degrees on the stator—these are the windings through which rotor alignment is applied. The motor also includes a rotor position sensor for determining rotor misalignment over time, and a control unit for controlling the current supplied to said stator conductors in the usual way. Specific to the invention is a processing means, connected to an output of said rotor position sensor, for calculating a magnetizing torque correction factor for the individual windings of the conductor set to substantially realign the rotor.
The motor should preferably be an induction motor, and the control unit should involve field oriented control or any other open or closed loop control system used in the art to control rotation.
The motor should have at least some of the windings spanning less than 180 degrees, eg a 2 pole short span motor, or a motor with four or more poles. Each phase of the “conductor set” should have N/2 individually driven windings, where N equals the number of stator poles. These windings are wound between adjacent poles.
In one embodiment, a control unit determines a phase current for each phase to cause a required rotor rotation, and then the processor distributes this unevenly amongst the windings of each phase of the “conductor set” amongst the individually driven windings of that phase, according to the effect of the position of each winding on the rotor, to realign the rotor.
In another embodiment, the processor allows a certain amount of imbalance and varies the magnetizing current for a winding of a phase without balancing it out by varying the magnetizing current in the other windings of the phase to an equal and opposite degree.
For a more complete explanation of the present invention and the technical advantages thereof, reference is now made to the following description and the accompanying drawings, in which:
a (prior art) is a schematic of magnetic bearings;
b (prior art) shows how the magnetic flux caused by the stator can influence rotor position;
c (prior art) shows a diagrammatic representation of field oriented control;
d (prior art) is a diagrammatic representation of field oriented control;
Embodiments of the present invention and their technical advantages may be better understood by referring to
Referring now to
Rotor 150 is, in operation, located substantially co-axially with the stator, along a stator axis Z (not shown). Radial sensors 160-165, of which only 164 and 160 are labeled in
In operation, the method of the present invention is now described with reference to
According to an alternative embodiment, a pair of sensors is provided for each orthogonal direction, and the differential used as the measurement. Thus, referring to
According to a related embodiment, shown in
According to a further embodiment, as shown in
Processor 180 provides drive information to the inverter. Typically, the information is based on upon mathematical calculations such as Field Oriented Control, combining a required Torque Producing Current (id) with a required Quadrature Current (iq). Field Oriented Control is a preferred control method, but other equally suitable methods known in the art for controlling the waveform current may be used, for example and without limiting the scope of the present invention, Classical Direct Torque Control. Using Field Oriented Control (FOC), the static X-Y stator frame is transformed into a rotational equivalent in the rotor's d-q frame. The quadrature current component of the rotating d-q frame serves to induce current in the rotor, which produces for the rotor a magnetic field. This rotor magnetic field rotates together with the sinusoidal cycling of the waveform current in the stator. At the same time, the stator waveform current also includes a direct current component. This is usually 90 electrical degrees away from the quadrature current component, and serves to provide a magnetic field to intersect the magnetic field of the rotor. The effect of these two components of the current in the stator windings is the interaction of the two magnetic fields, which causes movement of the rotor. A plurality of phases is usually set up in the stator, to enable the magnetic field of the rotor to be continuously intersected and maintain the steady rotation of the rotor.
In order to control the rotor's position within the stator, each phase needs to be offset in amplitude, not time, from the value predicted for it by the field oriented control algorithms. This means that after the current measurement on each phase, its offset value is subtracted from the measurement prior to running the FOC algorithms. The offset value is added back and the output sent to the amplifier stage, and thence to the motor.
In the present invention, rotor position is corrected by adjusting the magnetizing current component of the AC waveform current to the stator windings. As mentioned, using FOC, current for the windings is first calculated using the rotor's d-q frame, and then transformed for application to the windings in the stationary X-Y frame of the stator. In the present invention, it is recognized that slightly altering the magnetizing current component after the d-q frame has been transformed, of the waveform applied to any one winding will serve to increase the strength of the magnetic force applied by that winding, to the rotor, with the effect of subtly moving the rotor closer to, or further away from that winding. The principle may be applied to various windings at the same time, serving to position the rotor appropriately within the stator X-Y frame.
In this embodiment, the extra magnetizing current, causing magnetic attraction of the rotor according to the required correction factor, is added to a d-q frame, and stator currents are then calculated. It will be noted, however, that instead of having one standard d-q frame, from which all the stator winding currents are calculated, in this embodiment there are different d-q frames for the individual windings involved in the correction. The AC current for each stator winding will then be calculated according to that frame.
The effect of individual windings upon the rotor is determined according to the X and Y components of both sides of that winding within the X-Y stator frame.
The correction required is ideally updated in real time, according to any ongoing change in rotor position. The magnetizing current component through the windings will likely affect many windings at once, and throughout operation. The need for and methods for damping or removing high frequency components of the signals are well known to the art.
If complementary phases are used (eg a 6 phase machine set up as 180 degree opposed dual 3 phase machine), the FOC is performed on complementary pairs of phases; by running both phase lines through the current sensors, and doing all of the FOC algorithms. The total current going through the pairs of phases would remain correct for FOC, but after the FOC algorithm rotor positioning algorithms would be applied to set up the difference between the complementary halves. The rotor positioning algorithms could be applied before or after or as a part of the FOC.
In a further embodiment, input AC current is modified for only two or three of the windings in order to re-position the rotor, whilst the other windings have AC current whose magnetizing current portion is independent of rotor position. Referring now to
In another embodiment, all of the windings of the motor are used to control rotor position, as well as for their normal usage, of providing torque to the rotor. In order to control rotor position, as mentioned above, the magnetizing current portion of the electrical current fed to the windings must be controlled according to a continuing sensor output string.
In a further embodiment, an additional magnetic thrust bearing will be needed. In many of the embodiments described above and below, the magnetic reluctance will tend to draw the rotor into the center of the stator, but in this embodiment, an additional magnetic thrust bearing is used. The direction of this bearing is shown with lines z1-z2, in
With reference to
In a further embodiment, the application of this approach to polyphase motors is contemplated. As described in my previous inventions (U.S. Pat. Nos. 6,054,837; 6,570,361; 6,351,095 and 6,348,775), stators with many different phases can deal more effectively with temporal harmonics, as the harmonics below the phase count are not aliased to become spatial harmonics. As a result inverters with lower grade output can be successfully used without substantial effect on the rotor rotation smoothness. In the present invention either all or some of the phases may be used to control the rotor alignment. In a many-phased machine, it may be economical to use the minimum number of phases to control rotor position, which is usually three, as described below. In this way, phases that are not involved in the control of the rotor position do not need, for each pole, to be fed by a separate inverter half-bridge, but instead, may have a common half-bridge inverter output feeding the same phase in each of the poles. This reduces the number of half-bridges needed. In a preferred embodiment, one phase is selected as a base phase. Two adjacent poles of this phase are connected together, to provide control from a first direction, and the other two poles in a four pole machine, are connected together to provide control from the opposite direction. Then a pair of phases, to provide control in a direction 90 physical degrees away from the first direction, is chosen. The two phases must be chosen so that when these two phases are each wound with a single winding to the same phase in the adjacent pole, the sum of the angular difference between each pair of joined phases should equal 90 degrees from the base phase. For example, when using a four pole seven phase motor, the motor may be divided up into four quadrants, 1, 2, 3 and 4, as shown in
In a related embodiment, three phase windings are chosen, each to provide rotor positioning effects, while the remaining phase windings are simply used to provide the regular current for production of magnetic flux and torque.
In a further related embodiment, a six pole motor is used, with any number of phases. Being that there are six poles, there are three windings for each phase. According to this embodiment of present invention these three windings of any one phase would each be separately driven by dedicated inverter phases to control the rotor position. Particularly in a concentrated winding machine, the three windings of any one particular phase would be equally spaced around the stator, and would be well suited to being used as the windings that position the rotor.
In a multiphase motor, such as a seven phase motor, or with even a much higher phase count, temporal harmonics below the phase count may be added, without becoming spatial harmonics. For example, in a six or seven phase motor, third and fifth harmonic may be added to produce extra torque synchronized in speed and direction with the fundamental torque. In order to control the rotor position, according to the method of the present invention, harmonics, such as third and fifth harmonic in a six or seven phase machine, may be added to the waveform. According to this embodiment, the magnetizing current component of the fundamental AC waveform is not modified to control the rotor position. Instead, the magnetizing current component of the extra, injected harmonics, is modified to control rotor position. Similar to the first embodiment mentioned above, in order for this embodiment to be effective, windings cannot span 180 physical degrees on the stator, therefore the machine should be wound with a four pole or higher pole count configuration. Each phase of each pole is connected with one winding to the equivalent phase in the adjacent pole, and each winding is connected to its own inverter output. Third harmonic (and/or other harmonics) are synthesized with a magnetizing current component sized according to the required correction factor. In a further embodiment, a harmonic, for example, the third harmonic may be synthesized purely with a magnetizing current component equivalent only to that required for correction of rotor position and with no direct current component at all. Using the third or other harmonic in this way may be a good way to separate out the rotor positioning algorithm from the FOC algorithm. Again, some or all of the phase windings can be used for rotor positioning, in a preferred embodiment, all of the phase windings also include rotor positioning ability, whereas in another embodiment, only some of the windings are used with this extra capability.
In a further embodiment, a motor having four or more poles is proposed, in which the motor is designed to be operated horizontally, that is, the rotor 150 rotates around an axis parallel to the ground. The stator slots are arranged so that there are two stator slots for the same electrical phase located vertically higher than the rotor and equidistant in a horizontal direction from the rotor. While this embodiment can be used in conjunction with the first embodiment, described above, this second embodiment does not require the use of rotor position detectors. Referring now to
In a further embodiment, the windings above the rotor do not have extra turns but they are provided current having a modified magnetizing current component to act against gravitational effects. The modified magnetizing current component of the current waveform may be pre-calculated, or subject to look-up tables, or the result of sensor output, etc. If it is pre-calculated, it should take into account the rotor weight and additional forces caused by the environment and the load.
In some embodiments, stator windings, and thus electrical phase angle, are not necessarily evenly distributed. In other embodiments, an increase in stator windings in two poles of one phase, is compensated by a decrease in stator windings in the other two poles of the same phase, so that the total phase current amplitude of that phase is equal to the phase current amplitudes of the other phases.
With reference now to
In
While this invention has been described with reference to numerous embodiments, these are not to be construed as limiting the scope of the invention. For example, the processor 180, may have the additional capability of determining the effect that various windings have on the rotor displacement, and may be able to use only one winding to correct rotor displacement, or a combination of windings. Furthermore, the individual inverter outputs may be connected each to individual processors, for the calculation of their waveform current, instead of there being one centralized processor. In the course of this specification, the processor has been used as a generic term, and may contain the FOC and inverter, or these may be separate units. These features are known in the art and while the processing and algorithms are new according to the present invention, the units themselves are known in the art, and the means are related to those used for active magnetic bearings.
The present invention may be used in combination with passive bearings, such as ball-bearings. Slight changes in rotor position could be accurately measured by the rotor position detectors, and compensated for by altering the magnetizing current component applied to one or more windings, to re-align the rotor. In this way, bearing wear and tear is minimized. The use of passive bearings can greatly enhance the usage of the present invention. Also, magnetic bearings may be used in combination with the present invention. Alternatively, the method of the present invention could be used in place of passive bearings, serving to completely align the motor.
Other modifications are considered within the scope of the present invention. In a further embodiment, there may be more sensors (or other rotor position detectors), with more complex responses. One embodiment uses more than two rotor position sensor elements within one X-Y plane, and the combination of signal outputs is computed by the processor to produce a composite mapping describing rotor position relative to a desired position, from which mapping, appropriate magnetizing current and other currents are calculated for each inverter output individually. Sensors may instead be located inside a hollow in the rotor core, or between the rotor bearings and the housing. Sensors are not limited to any particular type, and may take the form of any sensor or measurement technique that can determine, for example, the rotor misalignment or detect movement of the rotor from an aligned position, or determine the proximity of the rotor to the end bells. Sensors may use optical interferometry, ultrasonic, radio frequency (RF) or be pressure sensitive. They may also alternatively measure the wear or the pressure on the bearings. Additionally sensors may be placed at both ends of the stator so that the processor may determine whether a mal-positioned rotor has simply moved to one side along the whole of its length, or only at one end. If the rotor is tending towards a wrong position along the whole of its length, this may be corrected by the varying the magnetizing current of the stator conductors according to the present invention. However, if the rotor is tending to a wrong position only at one end, it would be inappropriate for the processor to apply magnetizing current to the stator coils, for they would act to move the rotor towards the opposite direction, along the whole of the rotor length, resulting in correction where none had been needed. Sensors are arranged to measure two orthogonal directions at one end of the stator, while further sensors are arranged against the same two orthogonal directions, at the other end of the stator. The output signals from the sensors are sent to the processor and used to calculate any errors in rotor positioning.
In a further embodiment, the stator windings provide control over the rotor's positioning while active magnetic bearings separately feature at one or both ends of the stator to further help in the rotor positioning, and to compensate for tilting, twisting and drag of the rotor.
In a further embodiment, there may be many more slots, such as thirty, and a single phase in a single pole may cover more than one stator slot. However for the sake of clarity, these have been reduced to a single stator slot in
In the foregoing, for a four pole motor, four stator slots are all filled with windings of essentially the same phase; nevertheless, since the magnetizing current component of the current to one of the windings is modified in order to reorient the motor, this will cause the AC waveform current in the one winding to be slightly out of phase with the AC waveform current of the other winding. Similarly, in a three phase, six pole motor, although the three phases may be arranged physically on the stator with equal physical angle difference between each phase and the next, nevertheless the current will not be exactly in phase—with the differences being suited to positioning the rotor correctly while maintaining drive balance.
The stator is shown as having three phases, and four poles, however, the number of different phases may be increased (or there may even be just two different phases) and there may be six or more poles. In addition, with short pitch windings, only two poles may be used. A motor has been described as having four or six poles, but it could equally contain more or fewer poles.
In a further embodiment, incorporating a high phase order motor or generator, the standard magnetic bearing coils might be added to the main body of the stator. The magnetic bearing coils would be a high frequency (high pole count) winding, superimposed on then main traction winding. This supplementary winding would have a problem of having “end turns in the center” of the rotor, but it would be a small winding, with very little in the way or end turns, so very little iron would be lost.
The industry standard induction machine is the squirrel cage induction motor. In this motor, the region of interaction between the stator and the rotor may be considered the surface of a cylinder. Rotation is about the axis of the cylinder, lines of magnetic flux pass through the cylinder normal to the cylinder, and current flow in both the stator conductors and the rotor conductors is parallel to the axis of the cylinder.
The present invention is applicable to any geometry in which the region of interaction between stator and rotor has circular symmetry about the axis of rotation, magnetic flux is generally normal to the region of interaction, and current flow is generally perpendicular both to flux and the direction of motion.
The present invention is applicable to all geometries of the AC induction machine. It is further applicable to both squirrel cage and wound rotor machines. The present invention is also applicable to many different inverter topologies used for the operation of three phase machines. These include voltage mode pulse width modulation inverters, which provide an alternating current regulated to a specified RMS voltage, current mode pulse width modulation inverters, etc.
While this invention has been described with reference to numerous embodiments, it is to be understood that this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments will be apparent to persons skilled in the art upon reference to this description. It is to be further understood, therefore, that changes or modifications in the details of the embodiments of the present invention and additional embodiments of the present invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.
The present invention describes an approach for reducing bearing wear in electric motors.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US05/13748 | 4/22/2005 | WO | 10/24/2006 |
Number | Date | Country | |
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60565802 | Apr 2004 | US |