GB2489523A

GB2489523A - Energy storage system using switched reluctance motor

Info

Publication number: GB2489523A
Application number: GB1105563.9A
Authority: GB
Inventors: George Prassinos
Original assignee: Oxto Ltd
Current assignee: Oxto Ltd
Priority date: 2011-04-01
Filing date: 2011-04-01
Publication date: 2012-10-03
Anticipated expiration: 2031-04-01
Also published as: GB2489523B; GB201105563D0

Abstract

An energy storage system comprises at least one switched reluctance motor 22 comprising a rotor 24 and a stator 32. The stator includes a plurality of electrical windings 36 around the rotor, and the rotor acts as an energy-storage flywheel. An electrical control circuit (44, fig. 4) controls electric currents in the electrical windings so as to transfer electrical power to or from the rotor. The rotor may be housed in an evacuated casing 33. Two switched reluctance motors may be provided, having rotors rotating at the same angular velocity, but in the opposite direction. The energy storage system may be installed on a vehicle to provide power when the vehicle is parked.

Description

Energy Storage System The present invention relates to an energy storage system, particularly although not exclusively to an energy storage system suitable for storing energy on a motor vehicle, for example to provide electrical power to the vehicle when the vehicle is parked.

It is known that energy can be stored in a flywheel, although this can ceuse problems if the flywheel is in a motor vehicle, because of the flywheel's gyroscopic effect. There is also a problem in transferring power to the flywheel, and in transferring power from the flywheel, without introducing significant energy losses.

According to the present invention an energy storage system comprising at least one switched reluctance motor, each switched reluctance motor comprising a rotor and a stator, the stator comprising a plurality of electrical windings around the rotor, wherein the rotor acts as an energy-storage flywheel; at least one casing enclosing the rotors; and an electrical control circuit to control electric currents in the electrical wlndings so as to transfer electrical power to or from the rotors.

The rotor defines projecting regions that act as magnetic poles, and at least those regions of the rotor are of ferromagnetic material. Since the rotor is required to act as a flywheel, it is desirable that outer regions of the rotor are of a high density material. The entire rotor may be made of a high-density ferromagnetic material such as steel; or for higher-speed operation the outer regions of the motor may be of high-density ferromagnetic material, while remaining regions of the rotor are of a high-strength material such as a carbon-fibre composite material.

It will be understood that the switched reluctance motor will act as a motor if power is transferred to it, and will act as a generator when power is transferred from it. The structure of the motor itself is not changed when it act as a generator, the only change is in the electrical control circuit.

In particular, such an energy storage system is preferably installed in a vehicle.

Preferably the at least one casing is evacuated, at least during operation, to minimise the frictional forces acting on the rotor. For good magnetic coupling, the periphery of the rotor is in close proximity to the windings of the stator; if the casing were not evacuated, there would be significant viscous drag between the rotor and the stator.

Preferably each rotor is supported by a magnetic bearing, as this minimises friction.

Preferably the energy storage system comprises at least two switched reluctance motors, arranged to suppress external torque when the rotors are rotating.

For example it may comprise two switched reluctance motors, and preferably these are arranged to rotate at the same angular velocity, but in opposite directions.

The axes of rotation may be parallel, or they may be Inclined relative to each other, preferably at less than 30°, for example at less than 15°. In one embodiment the two axes are tilted at 5.4° from the vertical in opposite directions, so the inclination is 10.8° relative to one another.

Alternatively the energy storage system may comprise a single switched reluctance motor. In this case it must be arranged within the vehicle so that gyroscopic torques do not have an impact on the directional stability of the vehicle. For example it may be arranged so that the gyroscopic torques are such as to cause the vehicle to tilt slightly from front to back or to roll slightly from side to side, so that those torques are accommodated by the vehicleTs suspension. A suitable position would be in the vicinity of the centre of gravity of the vehicle, or for example between the rear wheels.

As is known, a switched reluctance motcr is a brushless motor with an unequal number of salient rotor and stator poles. There are no windings or permanent magnets on the rotor. Preferably the number of poles, in each case, is an even number. The stator windings on opposite poles are energised simultaneously. In use, if an electric current is supplied to a pair of stator windings which are not aligned with poles on the rotor, the rotor experiences a torque. By controlling the electric currents supplied to the stator windings relative to the rotor angular position, the motor can be driven in either direction of rotation, or can be arranged to generate electrical power.

Preferably the energy storage system is supplied with electrical energy while the vehicle is moving. For example the vehicle may include electrical generators linked to one or more of the wheels, and actuated when the brakes are applied, or independently of the brakes, so that some energy is transferred to the energy storage system. while the vehicle is under way, the energy storage system may be used to provide the power for electrical components in the vehicle, while receiving electrical power from the generators that are actuated when the brakes are applied, so that the electrical components are powered by energy derived from the kinetio energy of the vehicle when slowing down, or from its potential energy when going downhill. This can therefore reduce the fuel consumption of the vehicle. Alternatively or additionally, when the vehicle is parked, the energy storage system may be used as a generator to supply electrical energy to power electrical components in the vehicle, avoiding the need to run an engine for this purpose. As a further alternative the electrical energy may be supplied to the energy storage system from a generator driven by the engine of the vehicle.

The dimensions and rate of rotation of the rotor are selected in accordance with the energy requirements of the vehicle, so that the energy storage system can supply the requisite electrical power for a predetermined period, for example 12 hours. If the electrical power is not required, then the energy storage system would store the energy for a period that may be several days.

Thus, as an alternative aspect of the present invention, there is provided a vehicle comprising a generator of electricity, and an energy storage system comprising at least one switched reluctance motor, each switched reluctance motor comprising a rotor and a stator, the stator comprising a plurality of electrical windings around the rotor, wherein the rotor acts as an energy-storage flywheel; and an electrical control circuit to control electric currents in the electrical windings so as to transfer electrical power to or from the rotors; wherein the generator of electricity provides electrical power to the electrical control circuit.

Typically the generator or generators of electricity transfer power to the rotors while the vehicle is under way, either when the vehicle is braking, or when coasting down a hill. While the vehicle is under way and when the vehicle is parked power can then be transferred from the rotors to power electrical components of the vehicle.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic view of a vehicle incorporating an energy storage system of the invention; Figure 2 shows a cross-sectional view of a switched reluctance motor of the energy *storage system of figure 1; Figure 3 shows a longitudinal sectional view of the switched reluctance motor of figure 2; and Figure 4 shows a circuit diagram of an electrical control circuit of the energy storage system of figure 1.

Referring to figure 1 there is shown a schematic view of a truck 10 which in use may be used to tow a trailer (not shown) . The truck 10 includes a chassis 12 supported by three pairs of wheels 14, the middle pair of wheels 14 being shown raised off the ground. The chassis 12 carries a cab 16. Immediately behind the cab 16 is an energy storage system 20 of the invention; and between the rear pair of wheels 14 is an electricity generator 18. Preferably a similar electricity generator 18 would also be provided in conjunction with one or more of the pairs of wheels of the trailer. Whenever the driver applies the brakes, the electricity generator 18 is coupled to the rear pair of wheels 14, and so generates electricity; this may be described as regenerative braking, and a variety of suitable generating systems are known. For example a generator shaft may be connected to the wheels by a clutch, the clutch being actuated to rotate the shaft when regenerative braking is reguired; or the generator may be always linked to the wheels, and regenerative braking achieved by changing electrical connections. Regenerative braking may also be utiuised independently of the brakes. This electricity is provided to the energy storage system 20, in which it is stored as described below. When the truck 10 is parked with its engine off, then electrical power can be provided by the energy storage system 20 to operate auxiliary electrical components both inside and outside the cab 16.

The energy storage system 20 consists of two switched reluctance motors 22 (see figure 2) whose rotors are arranged to rotate at the same angular velocity but in opposite directions, to counteract any gyroscopic effects, and in this example the axes of rotation of the rotors are both vertical. In an alternative, there may be more than one such pair of switched reluctance motors 22.

Each pair of switched reluctance motors 22 may be enclosed within a common casing (not shown) Referring now to figure 2, each switched reluctance motor 22 comprises a rotor 24 of a soft ferromagnetic material such as laminated silicon steel, of generally cylindrical shape but with four equally-spaced protruding regions 26 that act as magnetic poles in use. The rotor 24 is supported on stub axles 28 (see figure 3), supported by magnetic bearings 30 (shown in figure 3) at each end, so it can rotate freely. Surrounding the rotor 24 is a stator 32 which includes a thick-walled aluminium casing 33 defining six equally-spaced rectangular recesses 34 along its inner surface. Each recess 34 is occupied by an electrical winding 36 (represented schematically), so that there are six such electrical windings 36; and diametrically opposite pairs of electrical windings 36 are always energised together, with the electrical currents flowing such as to create North and South poles; they may therefore be connected in series. The directions of the electrical currents in one such pair of diametrically-opposite windings 36 are shown schematically. Depending on the power requirements, the casing 33 may also define flow channels 35 for a coolant (only three such channels 35 are shown). The coolant may for example be a liquid, being recirculated through a radiator (not shown) to exchange heat with the ambient air; or the coolant may be air; or it may be a recirculated refrigerant fluid that evaporates during passage through the coolant channels 35.

Referring now to figure 3, the stub axles 28 project from the two ends of the rotor 24, and locate within magnetic bearings 30 that are fixed to end plates 37 and 38. The end plates 37 and 38 are fixed to the two ends of the casing 33 by screws 40, and the end plates 37 and 38 are sealed to the ends of the casing 33. One end plate defines an outlet port 42 whereby the space around the rotor 24 can be evacuated. This outlet port 42 is shown as being in the bottom end plate 38, but it might alternatively be in the top end plate 37. The motor 22 is arranged with the axis of rotation defined by the stub axles 28 oriented vertically. By way of example the rotor 24 may be of diameter 0.3 m, and of length 0.25 m.

Hence, in use, after the space around the rotor 24 has been evacuated, the rotor 24 can be rotated and act as a flywheel. There is very little friction opposing its rotation, because of the evacuation of the space in which it rotates, and because it is supported only by the magnetic bearings 30, which are non-contact supports. If electrical current is supplied to the pairs of windings 36 in succession, as the rotor 24 rotates, this applies a torque to the rotor 24 causing it to rotate faster. It will be understood that the phasing of the currents supplied to the windings 36 must be synchronised with the rotation of the rotor 24, so that each pole 26 is attracted to a winding 36 as it approaches it. The input electrical power is hence converted to kinetic energy of the rotating rotor 24. Because there is little friction opposing its rotation, this energy is stored in the rotating rotor 24; depending on the size of the rotor 24, the energy may be stored for a period of days after input electrical power has ceased, if no electrical energy is taken from it.

Referring now to figure 4 the electrical circuit 44 for supplying and withdrawing electrical power from the switched reluctance motor 22 is shown. Each electricity generator 18 (two such generators are shown in the circuit 44), when operating, provides a DC voltage which is applied in parallel via respective switches 45 between an earth line 50 and a power line 51. The earth line 50 and the power line 51 are connected by a capacitor C, and an inductor L is provided in the power line 51. The truck also has a battery B, and this is also connected between the earth line 50 and the power line 51 via a switch 46 and a diode 47. Diodes 48 are also provided in the power line 51 on each side of the junction with the second electricity generator 18. This arrangement ensures that electrical power can be provided between the earth line and the power line, whenever required, either from one or both of the electricity generators 18, or from the truck battery B. Each pair of diametrically-opposed windings 36, which are conneoted in series with each other (and are represented by a single coil in figure 4), is connected to the earth line 50 and the power line 51 via electronic switches 52 at each end. These electronic switches 52 are preferably insulated gate bipolar transistors (IGBT5), although alternative electronic switching elements may be

used instead, such as field effect transistors, or

thyristors. in addition, the end of each pair of windings 36 remote from the earth line 50 is connected through a flyback diode 54 to the earth line 50. Similarly, the end of each pair of windings 36 remote from the power line 51 is connected through a fly back diode 55 to the power line 51.

The electrical output from the switched reluctance motor 22, when it acts as a generator, therefore also appears between the power line 51 and the earth line 50.

These are connected by a smoothing output capacitor 61, and are also provided with an output filter consisting of a series inductor 62 and a second capacitor 63. The electrical output is connected to a load, represented by a resistor 65, via a fuse 66 and a switch 64.

The electrical circuit 44 also includes a microprocessor 70 connected to a sensor 72 (represented diagrammatically) for monitoring the angular orientation of the rotor 24, also connected to each of the electronic switches 52 (only two such connections are shown) . The microprocessor 70 can therefore control operation of the electronic switches 52 and hence the currents in the windings 36 in accordance with the rotation of the rotor 24. Vhenever electrical power is available from one or other of the electricity generators 18, the microprocessor 70 controls the electronic switches 52 to provide mechanioal energy to the switched reiuctanoe motor 22, so that some energy that would otherwise be dissipated in braking is instead stored in the energy storage system 20.

The electrical components of the stator 32, that is to say the windings 36, are within the portion of the circuit 44 identified by a broken line; during operation the microprocessor 70 controls the currents to the three pairs of windings 36 to achieve the required energy transfers.

The output of the electrical circuit 44 is supplied to the load 65 whenever the switch 64 is closed. The windings 36 are also connected across the output of the electrical circuit 44, so that the system is self-energised. Hence the auxiliary electrical components of the truck 10 can be supplied by energy from the energy storage system 20, as they can act as the load 65, when the truck 10 is being driven. The energy storage system can take energy from the electrical generator 18 while at the same time electrical energy is supplied to the load 65.

As an additional or alternative mode of operation, the energy storage system 20 may be arranged to ensure that the rotors 24 are storing significant energy before the truck 10 is parked. Subsequently, while the truck 10 is parked, the microprocessor 70 may be arranged to control the electronic switches 52 so that the switched reluctance motor 22 acts as a generator, providing energy to operate the load 65. Hence auxiliary electrical components of the truck 10 can be operated for many hours without the need to run an engine. If the truck 10 is parked, but no energy is required, then all the switches 45, 46 and 64 would be turned off, so that the rotor 24 merely acts as a freewheeling flywheel.

In normal operation, while the truck 10 is under way, there will be sufficient power available from a single generator 18. If extra power is required, then the second electrical generator 18 may be switched into the circuit.

In the situation in which the truck 10 is parked and ii the energy storage system 20 is being used to power auxiliary electrical components, if a fault develops which causes the fuse 66 to blow, then the driver of the truck 10 would have to correct the fault, and replace the fuse 66. Excitation of the windings 36 can then be reinitiated for example by means of the truck battery B. Normal operation can then continue.

It will be appreciated that the energy storage system 20 described above is shown by way of example only. The switched reluctance motor 22 has a rotor 24 with four poles 26, and a stator 32 with six windings 36 (i.e. three pairs) . This arrangement is comparatively easy to control. An alternative design has a rotor with six poles, and a stator with eight windings, and is somewhat more efficient. Another alternative design has a rotor with eight poles, and a stator with ten windings, this providing the advantage of generating less noise.

It will also be appreciated that the rotor dimensions mentioned above are by way of example only, and that the rotor dimensions would be selected in accordance with the required energy storage, and the strength of the material forming the rotor.

It will also be appreciated that the circuit 44 may be modified in various ways. For example although the microprocessor 70 may control the output voltage, in a modification the circuit 44 may include a voltage regulator (not shown) . Such a voltage regulator may for example be connected in parallel with the second capacitor 63.

Claims

Claims 1. An energy storage system comprising at least one switched reluctance motor, each switched reluctance motor comprising a rotor and a stator, the stator comprising a pluraiity of electrical windings around the rotor, wherein the rotor acts as an energy-storage flywheel; at least one casing enclosing the rotors; and an electrical control circuit to control electric currents in the electrical windings so as to transfer electrical power to or from the rotors.
2. An energy storage system as claimed in claims 1 which is installed in a vehicle.
3. An energy storage system as claimed in claim 1 or claim 2 wherein the at least one casing is evacuated, at least during operation, to minimise the frictional forces acting on the rotor.
4. An energy storage system as claimed in any one of the preceding claims wherein each rotor is supported by a magnetic bearing.
5. An energy storage system as claimed in any one of the preceding claims comprising at least two switched reluctance motors, arranged to suppress gyroscopic torque effects when the rotors are rotating.
6. An energy storage system as claimed in claim 5 comprising two switched reluctance motors.
7. An energy storage system as claimed in claim 6 wherein the rotors of the two switched reluctance motors are arranged to rotate at the same angular velocity, but in opposite directions.
8. An energy storage system as claimed in claim 7 wherein the axes of rotation of the rotors are parallel.
9. An energy storage system as claimed in claim 7 wherein the axes of rotation of the rotors are inclined relative to each other, at less than 30°.
10. An energy storage system as claimed in any one of claims 1 to 4 comprising a single switched reluctance motor.
11. An energy storage system as claimed in any one of the preceding claims, installed in a vehicle, wherein the vehicle comprises an electrical generator arranged to provide power to the energy storage system.
12. An energy storage system as claimed in claim 11 wherein the electrical generator is actuated when the brakes of the vehicle are applied.
13. A vehicle comprising a generator of electricity, and an energy storage system comprising at least one switched reluctance motor, each switched reluctance motor comprising a rotor and a stator, the stator comprising a plurality of electrical windings around the rotor, wherein the rotor acts as an energy-storage flywheel; and an electrical control circuit to control electric currents in the electrical windings so as to transfer electrical power to or from the rotors; wherein the generator of electricity provides electrical power to the electrical control circuit.
14. A vehicle as claimed in claim 13 wherein the electrical generator is actuated when the brakes of the vehicle are applied.
15. An energy storage system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.