WO1995027326A1

WO1995027326A1 - Adjustable airgap motor/generator for flywheel system

Info

Publication number: WO1995027326A1
Application number: PCT/US1995/003619
Authority: WO
Inventors: Robin M. Miller
Original assignee: United Technologies Corporation
Priority date: 1994-03-31
Filing date: 1995-03-22
Publication date: 1995-10-12
Also published as: TW262608B

Abstract

A flywheel system having permanent magnets (20) embedded in a flywheel (10) rotating about a shaft (16) is provided with a stator (28) which is slidably mounted to the shaft (16) thereby allowing a gap g1 between the stator (28) and the rotor/flywheel (10) to be adjustable. When energy is being provided to (spin-up mode) or extracted from (spin-down mode) the flywheel (10), the stator (28) is positioned so the gap g1 is small to provide strong electromagnetic interaction between the stator (28) and the flywheel (10). Conversely, when the flywheel (10) is freewheeling, the stator (28) is positioned so the gap g1 is large enough to minimize electromagnetic drag on the flywheel (10).

Description

Adjustable Airgap Motor/Generator
For Flywheel System
Cross References to Related Applications
Copending US Patent Application, Serial No.

(UTC Docket No. R-3733), entitled"Self-Adjusting
Airgap Motor/Generator For Flywheel System", filed contemporaneously herewith, contains subject matter related to that disclosed herein.

Technical Field
This invention relates to flywheels and, more particularly, to efficient freewheeling using an adjustable airgap motor/generator.

Background Art
It is known in the art to use a spinning inertial mass called a"flywheel"as an energy storage/retrieval system. It is also known to dump energy into the flywheel using electromagnetic force to"spin-up"or accelerate the flywheel to a predetermined rotational speed, such that it "stores"kinetic energy. The higher the speed, the more energy the flywheel stores. To spin-up the flywheel, the system acts in a motor type operation with the flywheel being the rotor. This stored kinetic energy can then be extracted from the spinning flywheel by use in a generator-type action using electromagnetic fields to"spin-down"or reduce the rotational speed of the flywheel.

When no energy is being input into the flywheel nor extracted from the flywheel, the flywheel is said to be"freewheeling"or in a"freewheeling" mode. During freewheeling, it is desirable to minimize the electromagnetic losses on the flywheel due to the motor/generator configuration, thereby avoiding extracting wasteful energy from the spinning flywheel. If such losses are not minimized, they cause the flywheel rotational speed to be reduced, thereby reducing the amount of energy available to be extracted.

Numerous configurations exist in the art for this motor/generator flywheel system. For example, permanent magnets may be located on the spinning rotor portion of the flywheel and motor coils may be located on a stationary or stator portion of the system. In that case, when the rotor is freewheeling, the coils are typically open-circuited through some maximum impedance to minimize the current and to minimize the amount of electromagnetic force generated in the coils, thereby minimizing the amount of energy extracted from the rotor during freewheeling. However, even though the coils are open-circuited, eddy currents are still generated in the motor parts due to the rotating permanent magnets of the flywheel, thereby generating some magnetic fields which extract energy from the flywheel and reduce its speed.

Thus, it would be desirable to design a flywheel motor/generator system that minimizes electromagnetic drag on the flywheel during freewheeling.

Disclosure of Invention
Objects of the invention include provision of a flywheel system which minimizes electromagnetic drag on the flywheel during freewheeling operation.

According to the present invention a flywheel system comprises a shaft having a longitudinal axis; a flywheel capable of rotating about the shaft; at least one bearing which facilitates the rotating of the flywheel about the shaft; a plurality of permanent magnets embedded within the flywheel; and stator disposed on the shaft which couples electro magnetic energy to and from the flywheel and which moves axially along the longitudinal axis, thereby allowing for an adjustable gap between the stator and the flywheel.

According further to the present invention, the flywheel is housed within a chamber. According still further to the present invention, the chamber is an evacuated chamber and no electrical wires pass into the evacuated chamber.

In still further accord to the present invention, the adjustable gap is set to place minimal electro-magnetic drag on the flywheel when the flywheel is freewheeling. Further according to the present invention, stator positioning means are provided for setting the axial position of the stator, thereby setting the gap.

The invention represents a significant improvement over prior motor/generator techniques by providing a pancake brushless DC motor design with an adjustable airgap between the rotor and the stator. The rotor rotates about a stationary rod within a sealed chamber and the stator slides along the same axis as the rod outside the chamber. This configuration allows the flywheel rotor, having permanent magnets embedded therein, to be in a vacuum chamber, and allows the stator to be outside the vacuum chamber. This configuration allows for no physical connections between the flywheel inside a vacuum chamber and the coils outside the vacuum chamber. Thus, the only connection between the stator and the rotor is the electromagnetic forces.

This eliminates the need for any electrical wires (needed to drive the motor/generator) to enter the vacuum chamber where the flywheel is located, thereby avoiding leaky seals.

Also, permanent magnets are embedded in the flywheel near the rotational axis, thereby minimizing the centrifugal force on the magnets and reducing the possibility of the permanent magnets flying off at high rotational speeds. Also, the invention provides for a very simple mechanical linkage to engage ana disengage the motor/generator during spin-up (or energizing) and during spin-down or de-energizing, respectively. The invention may be used on any flywheel application such as any land, air, sea, or space vehicle, as well as appliances such as air conditioners, and moving carriers such as elevators.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.

Brief Description of Drawings
Fig. 1 is a cut-away side view of a variable airgap pancake motor/generator flywheel system, in accordance with present invention.

Fig. 2 is a detailed cut-away side view of the pancake motor/generator flywheel interface, in accordance with present invention.

Best Mode for Carrying out the Invention
Referring to Fig. 1, the hardware below the line 9 is known in the art. A variable gap pancake brushless DC motor flywheel system comprises an inertial wheel 10 (or flywheel) disposed inside a housing 12, which provides an evacuated chamber 14.

The flywheel 10, rotates about a shaft 16 by use of bearings 18 in a direction as indicated by the arrow 19. The bearings 18 may be sleeve, air, magnetic, rolling element or superconducting bearings or any other type of bearing that allows the flywheel 10 to spin about the shaft 16. The shaft 16 is affixed to and does not rotate relative to the flywheel housing
12.

A plurality of permanent magnets 20, are embedded in a region 22 of the flywheel 10 and are siibstantially flush with an outer surface 24 of the flywheel 10. The permanent magnets 20 in the
flywheel 10, comprise a rotor portion of a pancake brushless DC motor. Also, a shoulder 25 is provided to keep the flywheel from moving in an axial direction along the shaft 16. Other types of
support maybe used to prohibit axial movement if desired. Also, the magnets 20 may protrude from the surface 24 of the flywheel 10 if desired. In that
case, the edges of the magnets 20 should be beveled to cause the stress vector due to centrifugal force to be partially along the longitudinal axis of the
shaft 16.

A stator 28 portion of the pancake motor slides
axially along the shaft 16 as indicated by the
arrows 30. An airgap gl exists between the stator
28 and the flywheel housing 12 which varies based on
the position of the stator 28 along the shaft 16.

The stator 28 slides on a sleeve bearing 32;
however, other bearings may be used if desired. A
circular stop 33 is attached to the outside surface
26 of the flywheel housing 12 to stop the stator
from contacting the flywheel housing, and to set a
minimum gap gl spacing.

The stator 28 has a stator housing 34 which
contains laminations 36 and coils 38 which define
three phases of operation for the pancake motor. A
cap 40 is disposed on the backside of the stator 28
to allow three sets of wires 42, for the three
phases of the pancake motor, to interface with
external components. Also, an axial spring 43 is
wrapped around the shaft 16 to put a right to left
force on the stator 28.

A thrust plate 44, is attached to the cap 40.

A linkage 46 pivots about a hinge point 48 and moves as indicated by the arrow 50 to convert vertical motion to horizontal axial motion of the thrust plate 44. A brake pedal 52, is connected through a lever arm 54 to a cable 56 which drives the linkage 46. Similarly an accelerator pedal 60, is connected by a lever arm 62 to a cable 64 which also drives the lever arm 46.

Position sensors 70,72 are located near the break pedal 52 and the accelerator pedal 60, respectively, to detect the amount of displacement of each pedal. The brake position sensor 70 provides an electrical signal on a line 74 to a known flywheel motor/generator control circuit 76 and to a known drive wheel motor/generator control circuit 77. Similarly, the accelerator pedal position sensor 72, provides an electrical signal on a line 78 to the flywheel motor/generator control circuit 76 and to the drive wheel motor/generator control circuit 77. The flywheel control circuit 76 and drive wheel control circuit 77 contain known electronic components, e. g., op-amps, transistors, etc., capable of performing the functions described herein. The actual detailed implementation of such functions is well known in the art and is not critical to the present invention.

The operation of the system is as follows.

When either the brake pedal 52 or the accelerator pedal 60 are depressed beyond a predetermined threshold, the stator portion 28 of the pancake motor is slid to the right against the stops 33, thereby reducing the gap g,. In this position, the airgap g, is small enough to allow optimal electromagnetic interaction between the stator 28 and the rotor 10, thereby causing the stator 28 to be"engaged"with the flywheel/rotor portion of the pancake motor to feed energy into (i. e., spin-up) the flywheel 10 (when the brake 52 is depressed) and to extract energy from the flywheel 10 (when the accelerator 60 is depressed).

Similarly, when neither the brake pedal 52 nor the accelerator 60 are depressed the spring forces the stator 28 to the left, thereby increasing the gap g. In this mode, the flywheel 10 is in "freewheeling"mode and the stator 28 of the pancake motor is"disengaged"from the spinning flywheel 10 or rotor portion of the pancake motor. The gap gl during disengagement is set to be large enough to minimize the amount of electro-magnetic interaction between the flywheel and the stator.

More specifically, the amount of electromagnetic force exerted on the flywheel 10, is determined by the total airgap between the stator 28 and the flywheel 10 which is equal to gaps gl plus g2 plus the thickness of the side wall 26 of the flywheel housing 12, which is about. 1 inches. The gap gl is about 3/8 inch minimum when the flywheel is disengaged and about. 020 inches when the flywheel is engaged. The gap g2 is fixed at about . 005 inches. The thickness of the wall 26 of the flywheel housing 12 is about. 095 inches. It should be understood that the wall 26 may bend inward toward the flywheel 10 due to the differential pressure across the wall; however, the total gap includes this deformation. Other gap values and wall thicknesses may be used if desired and will depend on the size and strength of the magnets.

The mechanical connection between the brake and accelerator pedals and the stator position along the shaft 16 is such that only a small movement of either pedal will cause the stator to move from engagement to disengagement, and visa versa.

The operation and interaction with the flywheel control circuit 76 and the drive wheel control circuit 77 is as follows. When the brake pedal 52 is depressed, the stator 28 is pushed to the right and engaged with the flywheel and the system enters spin-up mode. In this mode, a drive wheel motor/generator 80 is electro-magnetically and/or mechanically engaged as indicated by the dash line 82 to a rotating wheel 84. The drive wheel motor/generator 80, extracts energy from the rotating wheel 84 to provide voltage on lines 86 to the drive wheel motor/generator control circuit 77.

The drive wheel motor/generator engagement may be initiated by the drive wheel motor/generator control 77. The electrical signals from the drive wheel motor/generator 80 are signal conditioned by the control circuit 77 and appropriate electrical source signals are provided to the flywheel motor/generator control circuit 76 on lines 88. In this mode of operation, the flywheel control circuit 76 provides output drive signals on lines 90 to the coils 38 of the stator 28 of the pancake motor. The drive signals on the lines 90 spin-up the flywheel 10 in a motor-driving fashion.

Conversely, when the brake pedal is no longer depressed, the stator moves to the left and disengages from the flywheel, the system reverts to "freewheeling"operation, and the brake signal on the line 74 changes to indicate to the flywheel control circuit 76 to stop dumping energy into the flywheel 10. In that mode, the drive wheel control circuit 77 disengages the drive wheel motor 80 and the flywheel control circuit 76 stops providing the drive signals on the lines 90 and open-circuits the lines 90 to provide a maximum impedance path across the coils, thereby reducing current generation in the coils. In addition, the stator 28 is pushed to the left by the spring 43, thereby increasing the gap gl and minimizing any electromagnetic drag on the flywheel 10, as discussed hereinbefore.

When the accelerator pedal 60 is depressed, the stator 28 is pushed to the right, thereby casing the gap g, to decrease and the system enters a spindown mode where power is extracted from the flywheel 10. In that mode, the accelerator signal on the line 78, indicates to the flywheel control circuit 76 to extract energy from the flywheel 10. In particular, the flywheel control circuit 76 monitors the lines 92 for the back EMF voltage in the coils 36 of the stator 28 created by the rotating permanent magnets 20 in the flywheel 10.

The back EMF voltage signals on the lines 92 are converted by the flywheel control circuit 76 to source voltage signals on lines 94 to the drive wheel control circuit 77. The drive wheel control circuit 77 signal conditions the source voltage from the flywheel control circuit 76 and provides drive signals on lines 96 to the drive wheel motor/ generator 80 which causes the wheel 84 to increase in rotational speed.

Similarly, when the accelerator pedal 60 is released, the spring 43 pushes the stator 28 to the left, which increases the gap gl, thereby disengaging the stator 28 from the flywheel 10 and the system enters"freewheeling"operation. In this mode, the acceleration signal on the line 78 indicates to the flywheel control circuit 76 to discontinue monitoring the signals on the lines 92 and providing the output source signal on a line 94 to the drive wheel control circuit 77.

Because the shaft 16 does not rotate, the housing 12 may be affixed to the shaft 16, thereby allowing the chamber 14 to be completely evacuated without requiring any electrical connections to pass through the housing 12 which would degrade the vacuum in the chamber 14. Furthermore, this design allows for very efficient freewheeling by the increased gap gl, thereby minimizing the amount of electromagnetic drag on the flywheel 10 when it is in freewheeling state.

The material of the flywheel housing 12 is constructed of steel or plastic or any other material that can hold a vacuum. However, the wall 26 in the region of the permanent magnets must be non-magnetically conductive and non-electrically conductive (to prevent eddy currents).

It should be understood that when the flywheel control circuit 76 is driving the coils 38 in a motor-driving fashion, the type of commutation used is not critical to the invention. For example, the lines 92 may be monitored for back EMF to sense the position of the rotor using well known commutatorless brushless dc motor drive techniques.

Alternatively, well known position sensors (not shown), e. g., Hall effect, optical, capacitive, or inductive sensors, may be employed to provide signals on lines (not shown) to the flywheel control circuit 76. Where position sensors are used, they may be located, e. g., on the shaft 16 and on the rotor 10 where it is convenient. Alternatively, the sensors may detect the rotational position of the outer surfaces of the rotor 10, instead of the inner diameter region.

For example, when an optical sensor is used predetermined sections of the rotor may be painted with reflective paint and an optical source and sensor mounted to a stationary element (e. g., the stator 28) and pointed toward the spinning rotor 10.

To allow the light from the source to pass into the evacuated chamber, a window may be created in the wall 26 of the flywheel housing 12. When the light is incident on the reflective region it reflects the light to the sensor and the sensor produces a signal indicative thereof; otherwise, the signal is not present. Alternatively, when Hall effect sensors are iised, a magnetic piece or strip may be attached to or embedded in the rotor 10 and the sensor attached to any element which is not the rotor 10.

Referring now to Fig. 2, a detailed diagram showing the magnetic flux paths and the specific coil windings and laminations 36 without showing the stator housing 34 nor the cap 40 of Fig. 1.

Regarding Phase A (Á), the electric current propagates along the wire 120 to a laminated post (or tooth) 122. The wire 120 is wrapped around the laminated post 122 such that the magnetic field is right to left, as indicated by an arrow 124. The wire 120 exits the post 122 and is fed to a post 126 around which the wire 120 is wound in the opposite direction, so as to create a magnetic field from left to right as indicated by an arrow 128.

The magnetic flux 124 travels from the post 122 through a laminated stator back iron 130 as indicated by a line 132 to the post 126 across the gaps (g2 + gl) to the south pole of a permanent magnet 134 in the flywheel 10. Flux exits the north pole of the magnet 134 along a magnet backing region 136 as indicated by a line 138 to the south pole of a permanent magnet 140. The flux exits the north pole of the magnet 140 and travels across the gaps (g2 + g) and then completes the flux path to the post 122.

The magnet backing region 136 is made from a magnetically conductive material, e. g., iron. The entire flywheel 10 may be made of this material if desired. However, the regions 142 between the magnets should be made of a non-magnetic conductive material to prevent the north and south poles of a given magnet from shorting out. Also, the spacing between the magnets, along the regions 142 should be at least greater than the depth dimension (left-toright) of the magnet (into the flywheel) to minimize losses.

Regarding a second flux path for Phase A (Á), the wire 120 leaves the post 126 and is fed to a post 150. The wire is wrapped around the post 150 to cause a magnetic field in the direction right to left as indicated by an arrow 152. From the post 150, the wire 120 is fed to another post 154 where it is wound around the post 154 in the opposite direction to cause a magnetic field from left to right, as indicated by an arrow 156. The wire then returns as the return wire 160 which makes up the other side of Phase A.

In a similar fashion to the first flux path for
Phase A described hereinbefore, the coil around the post 150 produces a magnetic flux field which exits the post 150 and travels along the stator back iron 130 to the post 154 as indicated by a line 162, to the post 154, across the gaps gl and g2 as indicated by a line 156 to the south pole of a magnet 164.

The flux exits the north pole of the magnet 164 and travels along the backing portion 136 of the permanent magnets, as indicated by a line 166, to a south pole of a permanent magnet 168. Flux exits the north pole of the permanent magnet 168 and travels across the gaps (gl + g2) back to the post 150 to complete the flux loop.

It should be understood that the stator back iron 130 and the posts (or teeth) of the stator 28 discussed hereinbefore are typically laminated metal to provide good magnetic field conductivity.

A similar arrangement exists for Phase B (°B) with the posts 170,172 and the posts 174,176. Also, a similar arrangement exists for Phase C (°C) with the posts 178,180 and the posts 182,184.

To cause the flywheel 10 to rotate about the shaft 16 in a direction indicated by the arrow 19 starting in the position indicated on Fig. 2, Phase
A is de-energized and Phase B is energized which causes the flux path 124,132,128,138 to move from the poles 122,126 to the poles 170,172 which causes the flywheel 10 to rotate slightly in the direction of the arrow 19. Then, Phase B is de-energized and
Phase C is energized which causes the flux path 124,132,128,138 to move to the poles 178,180. Then,
Phase C is de-energized and Phase A is again energized to complete the cycle. A similar arrangement exists for the lower half of Fig. 2.

This Phase driving operation is well known in the art for driving a permanent magnet rotor.

Instead of the coils being wrapped individually and independently around each post as indicated in
Fig. 2, an overlapping form of winding arrangement may be used where the wire is wrapped around two posts having a plurality of posts in between. For example, for Phase A, the wire 120 may be wrapped from the post 122 around the post 178 with the post 170 being in between and the wire also wrapped in the opposite direction from the post 126 around the post 180 and back to 126 again, having the post 172 in the middle. In that case, Phase B would be wrapped from the post 170 around the post 126 with the post 178 in the middle and oppositely wound around the post 172 and the post 150, having the post 180 in the middle. A similar arrangement would exist for Phase C.

Many other winding configurations and number of phases exist in the art any of which will work equally well for the present invention, such as that shown in Fig. 2.14 of the book Campbell P., "Performance Calculation of a Permanent-Magnet Axial
Field DC Machine Electric Power Application,"Vol.

2, No. 4, ppl39-144 (1979). It should be understood that the illustrated version shown in Fig. 2 herein is merl-y shown for clarity of demonstrating the invention. Also, other shapes for the flywheel may be used if desired.

Also, instead of having the pedals 52,60 (Fig.

1) drive the stator position directly through the linkage 46 and cables 56,64, the brake and accelerator position signals on the lines 74,78 may drive an electronic actuator (not shown) which positions the stator 28 along the shaft 16 to provide the appropriate gap based on the mode of the system.

Also, instead of the spring 43 (Fig. 1) being located between the stator 28 and rotor housing 12, any means may be used to apply a force on the stator 28 such that when not in spin-up or spin-down modes, the stator 28 is pushed to the left to a maximum gap g, spacing. For example, a coil spring may be placed around the pivot point 48 of the linkage 46, or separate springs may be connected to the pedals 52,60 or the associated lever arms 54,62, respectively, or the spring 43 may be located around the shaft 16 but to the left of the thrust plate 44.

Other locations for such means may be used if desired.

Further, even though the invention has been shown as being used with an automobile, the invention may be used in any energy storage/ retrieval system, e. g., elevators, air conditioners, any land, air, water, or space vehicles, and utility/energy providers, among others. For example, in an elevator application, energy from the weight of the car on descent could be used to spin up the flywheel instead of using the rotating car wheel, which can then be extracted upon ascent.

It should be understood that the means for invoking which mode (i. e., spin-up, spin-down, or freewheeling) the system should run in, i. e., brake and/or accelerator pedals, the means for electrically interfacing with the stator 28 (Fig.

1), i. e., the flywheel control circuit 76, and the means from which the energy is extracted (in spin-up mode) and to which the energy is provided (in spindown mode), i. e., the drive wheel motor/generator, are not critical to the invention. Thus, it suffices for the present invention that there be a stator that moves axially along a shaft which a flywheel also spins around, the stator being slidably mounted to the shaft, thereby allowing the air gap (and the electro-magnetic interaction) between the stator and the flywheel to be adjusted.

Also, instead of using a plain shaft, an optional uni-directional or bi-directional spiral 100 (Fig. 1) may be used to automatically decrease the gap g, when in spin-up or spin-down modes and to automatically increase the gap when in freewheeling mode, similar to that discussed in copending US
Patent Application, Serial No. (UTC Docket No.

3733), entitled,"Self-Adjusting Airgap
Motor/Generator For Flywheel System,"filed contemporaneously herewith.

Claims

Claims We claim: 1. A flywheel system, comprising: a shaft having a longitudinal axis; a flywheel capable of rotating about said shaft; at least one bearing which facilitates said rotating of said flywheel about said shaft; a plurality of permanent magnets embedded within said flywheel; and a stator disposed on said shaft which couples electro-magnetic energy to and from said flywheel and which moves axially along said longitudinal axis, thereby creating an adjustable gap between said stator and said flywheel.

2. The flywheel system of claim 1, wherein said flywheel is housed within a chamber.

3. The flywheel system of claim 2, wherein said chamber is an evacuated chamber.

4. The flywheel system of claim 2, wherein no electrical wires pass into said evacuated chamber.

5. The flywheel system of claim 1, wherein said adjustable gap is set to exert minimal electro magnetic drag on said flywheel when said flywheel is freewheeling.

6. The flywheel system of claim 1, wherein said stator comprises a plurality of coils wound around laminations to create a plurality of electrical phases.

7. The flywheel system of claim 1, further comprising stator positioning means for setting the axial position of said stator, thereby setting said gap.

8. The flywheel system of claim 7, wherein said stator positioning means comprises pedals for setting the desired position of said stator and the desired mode of said flywheel system.

9. The flywheel system of claim 8, wherein said stator positioning means comprises stator driving means, connected to said stator and said pedals, for exerting an axial force on said stator to adjust said gap in response to the position of said pedals.

10. The flywheel system of claim 9, wherein said stator driving means comprises a mechanical linkage connected to said stator and said pedals.

11. The flywheel system of claim 7, wherein said stator positioning means comprises a bi-directional spiral along a portion of said shaft and said stator which automatically pulls said stator toward said flywheel, thereby reducing said gap, during spin-up and spin-down of said flywheel.

12. The flywheel system of claim 8, further comprising position sensing means connected to said pedal means for detecting the position of said pedal means and for providing pedal position signals indicative thereof.

13. The flywheel system of claim 8, further comprising signal processing means responsive to said pedal position signals for driving said coils of said stator to spin-up and spin-down said flywheel based on said pedal position signals.

14. The flywheel system of claim 1, wherein said stator and said permanent magnets in said flywheel comprise a brushless DC pancake motor.

AMENDED CLAIMS [received by the International Bureau on 21 September 1995 (21.09.95); original claim 1 amended; original claim 7 cancelled; original claims 8-13 replaced by amended claims 7-12; original claim 14 renumbered as claim 13; remaining claims unchanged (3 pages)] 1. A flywheel system, comprising: a shaft having a longitudinal axis; a flywheel capable of rotating about said shaft ; at least one bearing which facilitates said rotating of said flywheel about said shaft; a plurality of permanent magnets embedded within said flywheel; a stator disposed on said shaft which couples electro-magnetic energy to and from said flywheel and which moves axially along said longitudinal axis, thereby creating an adjustable gap between said stator and said flywheel;

and stator positioning means, for setting the axial position of said stator, thereby setting said gap, such that said gap is reduced during spin-up and spin-down of said flywheel.

5. The flywheel system of claim 1, wherein said adjustable gap is set to exert minimal electromagnetic drag on said flywheel when said flywheel is freewheeling.

7. The flywheel system of claim 1, wherein said stator positioning means comprises pedals for setting the desired position of said stator and the desired mode of said flywheel system.

8. The flywheel system of claim 7, wherein said stator positioning means comprises stator driving means, connected to said stator and said pedals, for exerting an axial force on said stator to adjust said gap in response to the position of said pedals.

9. The flywheel system of claim 8, wherein said stator driving means comprises a mechanical linkage connected to said stator and said pedals.

10. The flywheel system of claim 1, wherein said stator positioning means comprises a bi-directional spiral along a portion of said shaft and said stator which automatically pulls said stator toward said flywheel, thereby reducing said gap, during spin-up and spin-down of said flywheel.

11. The flywheel system of claim 7, further comprising position sensing means connected to said pedals for detecting the position of said pedals and for providing pedal position signals indicative thereof.

12. The flywheel system of claim 7, further comprising signal processing means responsive to said pedal position signals for driving said coils of said stator to spin-up and spin-down said flywheel based on said pedal position signals.

13. The flywheel system of claim 1, wherein said stator and said permanent magnets in said flywheel comprise a brushless DC pancake motor.