Title of Invention : Apparatus and method for modular stabilization of hubless rotors.

[1 ] Apparatus and method for modular stabilization of hubless rotors.

Technical Field

[2] The invention pertains to rotor stabilization in motors and generators.

Specifically the active magnetic stabilization of rotors in motors and generators where the rotor has no central shaft or hub.

Background

[3] In motors and generators, magnetic bearings may be utilized in order to support, levitate or stabilize the rotor inside the stator by exerting magnetic force on the rotor. The magnetic force may be generated with magnetic bearings involving permanent magnets, electromagnets, or combinations thereof. Magnetic bearings involving electromagnets may be referred to as active magnetic bearings (AMBs). Conventional in-plane (radial) stabilization of the rotor by AMBs is typically achieved with multiple electromagnets magnetically connected by a common core surrounding the rotor. The electromagnets may then be utilized in a four quadrant xy plane so to control the radial position of rotor.

Summary of Invention

[4] The objective of this invention is to overcome the shortcomings of traditional active magnetic stabilization apparatuses when scaling up the size and speed of motor and generator, particularly in rotating kinetic energy storage devices.

[5] This is achieved with an electromagnetic actuator apparatus for rotors in motors and generators comprising: an actuator target mounted on a hubless ring shaped rotor having a rotational axis, and; a multitude of modular radially opposed groups of electromagnetic bearing assemblies not connected by a common core and acting in groups of radially opposed groups for the stabilization of the corresponding arc sections of the rotor for radial support of the rotor, the assemblies thereof being distributed along the circumference of the rotor at fixed distances from each other, the assemblies thereof further being both oriented toward the actuator target mounted on the rotor, and separated from the rotor by radial gaps, the electromagnetic bearing assemblies thereof comprising: a multitude of spaced radial electromagnetic poles facing the rotor and configured to induce electromagnetic flux with the actuator target mounted on the rotor, and; a bias winding coil both wound around each electromagnetic poles of a single electromagnetic bearing assembly and connecting each electromagnetic bearing assemblies together, and; a control winding coil wound around each electromagnetic poles of a single electromagnetic bearing assembly, the winding being in the opposite direction of its radially opposed electromagnetic bearing assembly.

[6] The focus of this invention is also the aforementioned electromagnetic actuator apparatus for rotors in motors and generators wherein the electromagnetic bearing assemblies further comprise a linear movement apparatus being moveable individually in either a radial direction, an axial direction or in any other possible direction.

[7] The focus of this invention is also a method of electromagnetic radial support of rotors in motors and generators characterized in that a multitude of modular electromagnetic bearing assemblies not connected by a common core and acting in groups of radially opposed groups for the stabilization of the corresponding arc sections of the rotor are actuated through a linear movement apparatus being moveable individually in either a radial direction, an axial direction or in any other possible direction. Brief Description of Drawings

[8] Turning to the drawings wherein like reference characters designate identical or corresponding parts:

[9] [Fig.1 ] Illustrates a plan view of the general apparatus arrangement of a typical active magnetic bearing apparatus. In this figure, the active magnetic bearing cores(1 ) are connected by a yoke(2) so to form a continuous structure. Further, the actuator cores(1 ) are oriented inwards towards the actuator target on the rotor(3).

[10] [Fig.2] Illustrates a plan view of an example embodiment of the electromagnetic actuator apparatus disclosed in claims 1 and 2. Without limiting the scope of this invention, the example configuration of this figure illustrates the apparatus consisting electromagnetic bearing actuator assemblies(4) which for the single force axis in the Y-direction are divided into two radially opposing groups(5) and(6), of four active electromagnetic bearing assemblies each. Those modular active electromagnetic bearing assemblies could then operate in radially opposed groups where an addition in the electromagnetic force on one group results in the deduction of the same force from its opposing counterpart. The force axis in the X-direction consists of actuator groups (7) and (8). For this exemplary configuration, the total number of electromagnets for stabilization in the XY plane would be sixteen. Without limiting the scope of this invention, for the purpose of this example, the active magnetic bearing apparatus is implemented in heteropolar configuration. In this figure, the actuator target is mounted on the ring shaped rotor(9). The multitude of modular radially opposed groups of electromagnetic actuator assemblies do not form a common core, the cores are instead segmented. Those radially opposed groups act in groups for the stabilization of the corresponding arc sections of the rotor for radial support of the rotor. The electromagnetic bearing assemblies are distributed along the circumference of the rotor at fixed distances from each other, they are further both oriented towards the actuator target mounted on the rotor, and separated from the rotor by radial gaps. Each electromagnetic bearing assemblies comprise a multitude of spaced radial electromagnetic poles facing the rotor and configured to induce electromagnetic flux with the actuator target mounted on the rotor, and; a base winding coil(10) both wound around each electromagnetic poles of a single electromagnetic bearing assembly and connecting each electromagnetic bearing assemblies together, and; a control winding coil(11 ) wound around each electromagnetic poles of a single electromagnetic bearing assembly, the winding being in the opposite direction of its radially opposed electromagnetic bearing assembly. Note that although this is not exemplified in the drawing, the modular electromagnetic bearing assemblies can be alternatively situated inside, outside, above or below the rotor or in any combination of the aforementioned configurations, so long as they are facing the rotor.

[11 ] [Fig.3] Illustrates a plan view of an example embodiment of the

electromagnetic actuator apparatus disclosed in claims 3 and 4. In this figure, the same electromagnetic bearing assemblies described in figure 2 further comprise a linear movement apparatus (12) being moveable individually in either a radial direction, an axial direction or in any other possible direction.

Description of Embodiments

[12] The following description is not intended to limit the scope or applicability of the invention in any way. Rather, the following description is intended to provide convenient illustrations for implementing various embodiments of the invention. As will become apparent, various changes can be made to the methods, structures, devices, apparatuses, components, and compositions described in these embodiments without departing from the spirit and scope of the invention.

[13] It is disclosed in this patent a novel active magnetic bearing apparatus and method for the position control of ring shaped rotors in motors and generators.

[14] For the purpose of this invention, the stabilization apparatus is implemented as a multitude of modular radially opposed groups of electromagnetic bearing assemblies(4), which are not connected by a common core. Those active magnetic bearing assemblies(4) act in groups of radially opposed groups for the stabilization of the corresponding arc sections of the rotor for radial support of the rotor (9). Those active magnetic bearing assemblies(4) are distributed along the circumference of the rotor(9) at fixed distances from each other, while being both oriented toward the actuator target mounted on the rotor(9), and separated from the rotor(9) by radial gaps.

[15] For the purpose of this invention,“active magnetic bearing assembly” are understood to comprise first a multitude of spaced radial electromagnetic poles facing the rotor and configured to induce electromagnetic flux with the actuator target mounted on the rotor(9), and secondly a base winding coil(7) both wound around each electromagnetic pole of a single electromagnetic bearing assembly(4) and connecting each electromagnetic bearing assemblies(4) together, and thirdly a control winding coil(10) wound around each electromagnetic poles of a single electromagnetic bearing assembly(4), the winding(10) being in the opposite direction of its radially opposed electromagnetic bearing assembly(4).

[16] For the purpose of this invention, although the stabilization apparatus is implemented for radial support of the rotor(9), the apparatus can also be used for stabilization in the Z-axis (into the page of Fig 1 ) with electromagnets(4) on top or bottom of the rotor(9). [17] For the purpose of this invention, it may be preferable to design the apparatus for stabilisation in either one or more planes depending on the size and shape of the rotor(9).

[18] For the purpose of this invention, the active electromagnetic

assemblies(4), not connected with a common core, act in modular groups of oppositely facing groups stabilizing an arc section of the rotor(9). This topology enables the freedom to choose the desired number of electromagnets depending on the size of the machine. The modularity of the system eliminates the need to define complex maintenance procedures which are required in case of system failures. For example in case of an inter lamination fault in the actuator apparatus or conduction insulation failures, only the particular faulty active magnetic assembly is replaced and not the whole apparatus.

The apparatus can be transported in parts and assembled on site. The modularity also reduces material wastage and size of the tools thus directly reducing the manufacturing costs.

[19] Without limiting the scope of this invention, an example configuration for a stabilization apparatus could consist of eight active electromagnetic bearing assemblies(4) for a single axis which are divided into two radially opposing groups of four active electromagnetic bearing assemblies(4) each. Those modular active electromagnetic bearing assemblies(4) could then operate in radially opposed groups where an addition in the electromagnetic force on one group results in the deduction of the same force from its opposing counterpart. For this exemplary configuration, the total number of electromagnetic assemblies(4) for stabilization in the XY plane would be sixteen.

[20] For the purpose of this invention, the active magnetic bearing apparatus can be implemented in either homopolar or heteropolar configuration. In a heteropolar configuration the electromagnetic assemblies(4) are placed such that the polarity seen by the rotor(9) is N-S-S-N-N-S-S-N and so on. However, in this configuration the rotor(9) experiences change in magnetic field polarity as it rotates which results in eddy current and hysteresis losses. The homopolar active magnetic bearing apparatus has low rotational losses because only a single polarity is seen by the rotor during rotation.

[21 ] For the purpose of this invention, the active magnetic bearing apparatus should be designed so that it can handle the forces which can destabilize the motion of the rotor(9). These forces may be generated for example from the misalignment in the levitation and the attraction between the motor cores and the permanent magnets in the rotor(9). The active magnetic bearing apparatus must have enough force generating capability after overcoming the disturbance forces to keep the rotor(9) stable.

[22] For the purpose of this invention, the force density in the air gaps has been derived from the basic model of a U shaped core(11 ) with two windings(10) and (11 ) facing a section of the rotor(9). It forms a magnetic circuit with the section of the rotor(9) it is facing in which magnetic flux flows from one coil into a first air gap, passes through the rotor(9) into a second air gap and then into the second coil. It is assumed that the reluctance of the core path is negligible when compared to the reluctance of the air gap. The electromagnetic force density in a single air gap is given by the following equation, where,

Fm = force density in newton per square meter

Bg = magnetic flux density in the air gap

= magnetic permeability of free space

[23] The electromagnetic force in the air gap between the rotor(9) and the electromagnetic assemblies(4) is varied by controlling the current in the windings(10) and (11 ). The controller ensures that the net electromagnetic force in the air gaps between the opposing electromagnetic assemblies(4) and the rotor(9) is zero. This is realized with the help of a cascaded control apparatus which controls the position of the rotor by controlling the current flowing through the actuator coils(10) and (11 ).

[24] The cascaded controller has two controllers.

1. Position controller (PID controller)

2. Current controller (PI controller)

The position controller based on the difference between the desired and measured position generates a reference signal for the current controller. This signal is then compared with the measured current in the control coil. Based on the difference between reference and the measured current the current controller decides the duty cycle of the pulse width modulated (PWM) current waveform which is used to drive the gates of the active magnetic bearing power electronics inverters. The active magnetic bearing power electronics has two full H bridge inverters (IGBT based) one for each axis (X&Y). The output is the control current that is fed to the actuator coils(10) and (11 ) which then generates the electromagnetic force to stabilize the rotor(9). The control apparatus model is based on the non linear relationship between electromagnetic force and control current. The controller also takes into account the disturbance forces that destabilize the rotor(9) and the sensor measurement noise.

[25] The active magnetic bearing power electronics should have enough bandwidth to vary the maximum control current. Mathematically,

where,

w _BY = bandwidth of the power electronics in rad/s

U _DC = maximum dc-link voltage in volts

J _axis ⁼ inductance of control windings for a single axis (8 U-cores in Fig 1 )

maximum control current

[26] The apparatus bandwidth can be increased by increasing the dc-link voltage. But a higher dc-link voltage increases the overall cost of the apparatus because better insulation and safety measures are needed. One other solution can be the limitation of control current. It increases the apparatus bandwidth but limits the generated electromagnetic force.

[27] Controlling the inductance of the active magnetic bearing actuator coils(10) and (11 ) is a good option to have sufficient power electronics bandwidth. Ideally, the apparatus should have a constant inductance around the circumference of the rotor(9). One method by which this can achieved is by mounting the active magnetic bearing cores(11 ) on motor operated linear slides(12) to vary the air gap because of the inverse relationship between the airgap and inductance.

[28] In order to cope with rotor(9) expansion and to maintain the net change in inductance in a single axis at zero the stabilization apparatus can have U-cores(11 ) on the outside of the rotor(9). In case of ring expansion the gap on the inside would increase but at the same time it would decrease on the outside thus keeping the net change in inductance minimum.

Industrial Applicability

[29] The disclosed invention is key to the deployment of scalable electric motor or generators utilizing hubless ring shaped rotors such as flywheel energy storage devices : when the diameter and speed of the device reaches certain thresholds, especially in cases where the rotor is not located on a central shaft or hub.

Patent Literature [30] The most relevant technologies and literature to the disclosed invention are listed below:

[31 ] US 5300843 A (General Electric Company), 5 Apr. 1994.

[32] US 5578880 A (General Electric Company), 26 Nov. 1996.

[33] US 9559565 B2 (Calnetix Technologies Inc.), 31 Jan. 2017.

[34] US 20100090556 A1 , (Calnetix Technologies Inc.), 15 Apr. 2010.

[35] US 6727617 B2 (Calnetix Technologies Inc.), 27 Apr. 2004.

[36] US 6700258 B2 (Calnetix Technologies Inc.), 2 Mar. 2004.

[37] US 9531236 B2 (Calnetix Technologies Inc.), 27 Dec. 2016.

[38] US 20110101905 A1 (Calnetix Technologies Inc), 5 May 2011.

[39] Wide air gap and large scale bearingless segment motor with six stator elements, W. Gruber, T. Nussbaumer, H. Grabner, and W. Amrhein.

[40] Modelling of control system for an active magnetic bearing, Bronislaw

TOMCZUK, Dawid WAJNERT, Jan ZIMON.

[41 ] Direct Drive Wind Turbine Generator with Magnetic Bearing, G.

Shrestha, H. Polinder, D.J. Bang, J.A. Ferreira.