WO1996042133A1

WO1996042133A1 - Energy transfer system of an electric machine having magnetic bearings

Info

Publication number: WO1996042133A1
Application number: PCT/FI1996/000342
Authority: WO
Inventors: Olli Lindgren; Erkki Lantto
Original assignee: High Speed Tech Oy Ltd.
Priority date: 1995-06-13
Filing date: 1996-06-10
Publication date: 1996-12-27
Also published as: FI952908A; FI952908A0; AU6006496A

Abstract

An energy transfer system for an electric machine having magnetic bearings comprises an electric machine (6), a machine director (3) in energy transferring connection with the electric machine (6), magnetic bearings (7), a magnetic director (4) in energy transferring connection with the magnetic bearings (7), a control system (5), and an energy supplying means (1). The energy transfer system further includes an energy reserve (2) to which the energy supplying means (1), the machine director (3) and the magnetic director (4) are arranged in energy transmitting connection.

Description

Energy transfer system of an electric machine having magnetic bearings

The invention relates to an energy transfer system of an electric machine having magnetic bearings, which system comprises an electric machine, a machine director in energy transferring connection with the electric machine, magnetic bearings, a magnetic director in energy transferring connection with the magnetic bearings, a control system, an energy supplying means and an energy reserve.

Known energy transfer systems of electric machines having magnetic bearings are usually implemented by transmitting electric energy from an energy supplying means to a machine director, which is advanta¬ geously a frequency converter. From the machine director the electric energy is supplied to the electric machine. To magnetic bearings the energy is supplied through a separate magnetic director to which the energy can be transferred either from the energy supplying means of the machine director or a separate energy supplying means of the magnetic director. Known energy transfer systems have separate back- up systems for cases of a power failure both for the machine director and the magnetic director. In connection with a brake of the electric machine braking energy can be transferred from the electric machine either to an electric network or to a brake resistance.

Particularly in connection with high speed electric machines energy supply of the magnetic bearings has to be secured even during cases of power failure. The speed area of high speed electric machines is usually expressed as the round speed of the rotor of the electric machine or the peripheral speed of the rotor of the electric machine. The high speed area is typically 15 000 to 400 000 rpm, however, not more than 2 000 000 rpm, usually 20 000 to 200 000 rpm, advanta¬ geously 20 000 to 100 000 rpm. The peripheral speed of the rotor of the electric machine is in high speed electric machines typically 100 to 500 m/s, usually 200 to 500 m/s, advantageously 200 to 300 m/s.

Energy supply during cases of power failure in prior art energy transfer systems is usually implemented by controlling the electric machine to operate as a generator by using a frequency converter, whereby elec- trie energy is generated from the motion energy of the rotor of the elec¬ tric machine. The energy is mainly used for keeping the machine direc¬ tor in operation and for compensating the losses of the electric machine and the machine director. In connection with a power failure the energy supply of the magnetic bearings is arranged for example by using accumulators or a separate permanent-magnet generator in the rotor shaft of the electric machine.

European patent EP-430 009 describes an energy supplying equipment of an electric machine with magnetic bearings, in which a permanent- magnetic motor, such as a synchronous motor, is used as the electric machine. In the method, electric energy is taken from an electric net¬ work and it is transmitted after rectification to converter means, where three-phase pulse-width modulated voltage suitable for controlling the electric machine is formed from direct voltage. To the magnetic bear¬ ings energy is transmitted through a separate converter unit, which takes electric energy from the rectified mains voltage. Publication EP- 430 009 further describes a separate electric coupling circuit, which switches on in connection with a mains failure, wherein a relay switched to rectified voltage drops-out and the switch of the relay closes. In this situation, the electric coupling circuit takes its operating voltage from the voltage generated in the rotary movement of the rotor of the electric machine. The electric coupling circuit supplies electric energy to the converter means of the magnetic oearings. Further, the switch has a separate comparison means, with which the level of the voltage coming from the electric coupling circuit is compared to the voltage of an accumulator operating as a reserve source of energy. When the rotary movement of the electric machine retards decreases the voltage of the electric coupling circuit, wherein after the pre-set threshold is undershot electric energy is generated to the magnetic bearings by using an accumulator. In this manner, it is attempted to secure sufficient electric energy to the magnetic bearings until the rotor of the electric machine stops and thus damage to the rotor can be avoided.

Accumulator safety of magnetic bearings, as presented above, is easy to implement. However, disadvantages in accumulator safety include that the life term of accumulators is limited and their condition has to be controlled regularly. Further, the set of accumulators has to be over- sized, wherein the set of accumulators develops large in size. Further¬ more, control of the loading and charging stages of the accumulator requires supplementary electronics.

A permanent-magnet generator connected to the rotor of the electric machine decreases the bending-critical rotation speed of the rotor, which particularly in high speed machines may become a restricting factor to the rotation speed. In order to avoid this, the thickness of the rotor has to be increased, but in this case the size and the weight of the electric machine increase. The voltage given by the permanent-mag¬ netic generator is directly comparable to the rotation speed, wherein a separate voltage adjusting system has to be added to the energy sup¬ ply system.

The object of the present invention is to eliminate the above mentioned disadvantages and to improve the level of technology in the field. The invention is based on the idea that the energy transfer system further includes an energy reserve to which an energy supplying means, a machine director and a magnetic director are arranged in an energy transferring connection. Thus, energy from the electric machine can be transferred to the energy reserve particularly when the rotation speed of the electric machine retards, and in connection with a mains failure, wherein the electric machine is controlled to operate as a generator. Further, by using a magnetic director magnetic bearings can be adjusted in a manner that lateral forces of the rotor shaft can be com¬ pensated. Thus, the magnetic director can also transfer energy from the magnetic bearings to the energy reserve. In connection with a power failure, the energy coming from the electric machine is transferred through the energy reserve to the magnetic director, which converts the energy into a form suitable for magnetic bearings. In this manner, the magnetic bearings maintain their operating condition advantageously as long as the rotor of the electric machine is rotating. Energy may be supplied from the energy reserve also back to the mains for instance when braking the electric machine.

The invention is characterized in that the energy reserve is arranged in a continuous energy transferring connection with the energy supplying means, the machine director and the magnetic director, wherein during the use of the electric machine, the capacity of the energy reserve is arranged to be maintained by the energy which exists in and/or is sup¬ plied to the energy transfer system.

The energy transfer system according to the invention provides consid¬ erable advantages over solutions of prior art. In the energy transfer system of the invention the machine director supplies energy to the electric machine when the supply frequency is greater than the rotation speed of the rotor of the electric machine. In a corresponding manner, the machine director supplies energy to the energy reserve when the supply frequency is smaller than the rotation speed of the rotor of the electric machine. Thus, the electric machine operates as a generator generating electric energy which can be transmitted to the energy reserve and to the magnetic bearings preferably through the magnetic director. Further, the energy transfer system of the invention provides the advantage that the magnetic director can be used for controlling the direction of the powers directed to the rotative rotor, as the magnetic director operates as an intensifier when energy is transferred to the energy reserve. Damaging of the electric machine can be avoided, as the energy transfer system of the invention advantageously secures the energy supply of the magnetic bearings even in case of a mains failure as long as the rotor of the electric machine is rotating.

The energy reserve is advantageously to be formed substantially capacitive or substantially inductive, because in this case the voltage of the electric energy supplied to the energy reserve can vary within a broad range, which facilitates the sizing of the means belonging to the energy transfer system.

The invention is now described in detail with reference to the appended figures, in which

Fig. 1 shows a reduced block diagram of an energy transfer sys¬ tem according to a first embodiment of the invention,

Fig. 2 shows a reduced circuit diagram of an energy transfer sys¬ tem according to a first embodiment of the invention, Fig. 3 shows a reduced block diagram of an energy transfer sys¬ tem according to a second embodiment of the invention,

Fig. 4 shows a reduced circuit diagram of an energy transfer sys¬ tem according to a second embodiment of the invention implemented with an inductive energy reserve,

Fig. 5 shows a reduced block diagram of an energy transfer sys¬ tem according to a third embodiment of the invention,

Fig. 6a shows an application of an energy transfer means used in energy transfer systems according to the second and third embodiments of the invention,

Fig. 6b shows another structure of an energy transfer means used in connection with an energy transfer system according to the second and third embodiments of the invention,

Fig. 7 shows a reduced block diagram of the structure of an energy transfer system according to a fourth embodiment of the invention,

Fig. 8 shows a reduced block diagram of the structure of an energy transfer system according to a fifth embodiment of the invention,

Fig. 9 shows a reduced block diagram of the structure of an energy transfer system according to a sixth embodiment of the invention,

Fig. 10 shows a reduced block diagram of the structure of an energy transfer system according to a seventh embodiment of the invention,

Fig. 11 shows a reduced block diagram of the structure of an energy transfer system according to an eight embodiment of the invention, and Fig. 12 shows a reduced block diagram of the structure of an energy transfer system according to a ninth embodiment of the invention.

An energy transfer system according to the first embodiment of the invention illustrated in Figures 1 and 2 will now be closer described. In normal operation situations electric energy is supplied to the energy transfer system from an electric network 8 through an energy supplying means 1 , which is also called as a network director 1. The network director 1 is advantageously a rectifier bridge, with which full-wave rectified direct voltage is generated from alternating^" voltage, which is known to an artisan in the field. Electric energy is transmitted from the network director 1 to an energy reserve 2 and further both to a machine director 3 and a magnetic director 4, for example by using a wiring influencing between the parts 1 to 4. The machine director 3 is an ordinary transistor director controlled by a control system 5. The control is advantageously implemented by supplying control voltage to the base of a transistor T1 , wherein the collector-emitter circuit of the transistor T1 conducts electric current. Bases of other transistors do not have control voltage, wherein hardly any electric current runs through their collector-emitter circuits. In the following step, control voltage is switched to the base of a transistor T2 and the control voltage in the base of the transistor T1 is removed. Thus, the transistor T2 is changed to a conducting state and the transistor T1 is changed to a non- conducting state. In a corresponding manner, control voltage is switched to the bases of transistors T3, T4, T5 and T6 in turn. By changing the switching frequency of the control voltage it is possible to change the rotation speed of the rotor of the electric machine 6. When the rotation speed of the rotor of the electric machine 6 is greater than the switching frequency of the control voltage, the rotation speed of the rotor of the electric machine retards and energy is transferred from the electric machine 6 to the energy reserve 2. In a corresponding manner, when the rotation speed of the rotor of the electric machine 6 is smaller than the switching frequency of the control voltage, electric energy is transferred from the energy reserve 2 to the electric machine 6. By changing the switching frequency of the control voltage it is possible to change the transfer direction of the electric energy. The electric machine 6 is advantageously an asynchronous machine, such as a squirrel-cage motor

The control system 5 comprises preferably a processor unit, such as a microcontroller, for example a microcontroller of the series 8051. Implementation of the control system 5 with a microcontroller is known as such to an artisan in the field

The energy transfer system according to the first embodiment of the invention illustrated in Figures 1 and 2 has a control switch of the type 1/2-H bridge as the magnetic director The magnetic director 4 supplies electric energy to the magnetic bearings 7 The magnetic director 4 is also controlled by the control system 5 by switching control voltage to the bases of transistors T7 and T8 by using a method known as such

In case of a power failure in the electric network 8 the energy transfer system according to the first embodiment of the invention operates in the manner that the control system 5 controls the switching frequency of the control voltage of the machine director 3, wherein when the switch- ing frequency is smaller than the rotation speed of the rotor of the elec¬ tric machine 6, electric energy is transferred to the magnetic director 4 to be further transferred to the magnetic bearings 7 Also the electric energy of the control system 5 is derived from the rotation energy of the rotor of the electric machine 6

In the energy transfer system according to the first embodiment of the invention the network director 1 and the machine director 3 may also be current controlled, wherein the energy reserve 2 is preferably an induc¬ tance such as a coil The implementation of the network director 1 and the machine director 3 in a current controlled manner is known to an artisan in the field.

Figure 3 shows an energy transfer system according to the second embodiment of the invention which differs from the energy transfer system of the first embodiment of the invention chiefly due to the fact that the energy supply circuit of the magnetic director 4 further includes an energy transfer means 10, which is capable of supplying electric energy from the energy reserve 2 to the magnetic director 4 and if needed also from the magnetic director 4 to the energy reserve 2.

Figure 4 shows a reduced circuit diagram of an energy transfer system according to the second embodiment of the invention, in which the energy reserve 2 is implemented inductively, for example with a coil L. In a normal operating situation the energy transfer system is supplied electric energy from the electric network 8 through the network direc¬ tor 1. The network director 1 is advantageously a rectifier bridge imple- mented by transistors, with which full-wave rectified DC is formed from alternating current, as is known to an artisan in the fiel^'d. Electric energy is transmitted from the network director 1 to the energy reserve 2 and further both to the machine director 3 and to the energy transfer means 10. The machine director 3 is an ordinary transistor director controlled by the control system 5. The control can be implemented advantageously in a corresponding manner as described in connection with the specification of Figure 2.

Furthermore, from the energy reserve 2 energy is supplied through the energy transfer means 10 to the magnetic director 4. The energy transfer means in this second embodiment of the invention is advanta¬ geously an up/down-converting voltage converter, whose output voltage to the magnetic director 4 is maintained substantially constant in spite of variations in input voltage. The energy transfer means 10 can form constant output voltage, which is for example 100 V, when the input voltage can vary from a few volts up to 500 V. The control system 5 fur¬ ther controls the energy transfer means 10 preferably by using control voltage switched to the bases of transistors T9 and T10, which is known to an artisan in the field.

Alongside with the network director 1 an extra network director 11 can be used and in connection with it a supplementary energy reserve 12, wherein in case one network director 1 , 11 is damaged, electric energy is obtained from the energy network 8 through the other network direc- tor 1 , 11. The third embodiment of the invention of this type is described in Figure 5. This energy transfer system provides the further advantage that damage to the network director 1 and/or the energy reserve 2 and/or the machine director 3 and/or the electric machine 4 does not cause discharging of the electric energy in the supplementary energy reserve 12, wherein energy supply of the magnetic director 4 is not interrupted. Electric energy to the supplementary network director 11 can also be taken from another electric network 13, wherein the energy supply of the whole energy transfer system and particularly of the mag¬ netic director 4 is well secured against situations of failure.

Between the energy reserve 2 and the supplementary energy reserve 12 a separate energy transfer means 10 is connected, the alter- native couplings of which are shown in Figures 6a and 6b. The energy transfer means 10 is advantageously composed of diodes 9, 9a and 9b.

Figure 7 shows an energy transfer system according to the fourth embodiment of the invention. In the rotor shaft of the electric machine 6 an auxiliary generator 14 is connected from which electric energy is derived when the rotor is rotating. The electric energy is directed from the auxiliary generator to an auxiliary generator 15 from where the energy can be further transferred to the energy reserve 2.

Figure 8 shows an energy transfer system according to the fifth embodi¬ ment of the invention, in which the electric machine 6 is not used. Instead of this the auxiliary generator 14 is used, from which electric energy can be transferred through the auxiliary generator director 15 to the energy reserve 2. One known application of this type is an expan- sion turbine 3, in which a turbine is formed in one end of the shaft and a compressor is formed in the other end, wherein the rotation movement of the shaft is derived from the turbine. Thus, it is possible to transfer electric energy from the auxiliary generator 14, which is connected in the same shaft, through the auxiliary generator director 15 to the energy reserve 2.

Figure 9 shows an energy transfer system of the sixth embodiment of the invention, in which an auxiliary machine director 16 is connected alongside with the machine director 3, wherein in case the machine director 3 or the auxiliary machine director 16 is damaged it is still pos¬ sible to generate electric energy to the magnetic director 4 from the rotation movement of the rotor of the electric machine 6 during a power failure of the electric network 8. Figure 10 shows an energy transfer system of the seventh embodiment of the invention, in which electric energy of the electric machine 6 is derived directly from the electric network 8. In this energy transfer sys- tern the auxiliary machine director 16 supplies electric energy to the energy reserve 2 particularly during a power failure when the rotor of the electric machine 6 is rotating.

Figure 11 shows an energy transfer system of the eight embodiment of the invention, in which alongside with the energy reserve 2 is coupled a load resistor R_L through a DC-interrupter 17. This coupling provides for example the advantage that, in case of a power failure or in case either the machine director 3 or the electric machine 6 is out of order, it is possible to control the discharging of the attenuation energy which is possibly derived from a magnetic bearing 7 through the magnetic direc¬ tor 4. The DC-interrupter 17 operates advantageously in a manner that when the voltage of the energy reserve 2 exceeds a pre-set threshold value the DC-interrupter 17 switches the load resistor R_L alongside with the energy reserve 2, wherein excess energy which is accumulated in the energy reserve 2 is discharged through the load resistor R . This is necessary particularly in such embodiments in which a diode bridge or the like is used as the energy supplying means 1 , wherein extra load energy cannot be transferred back to the electric network 8. The DC- interrupter 17 and the load resistor R_L are used to prevent over- voltages from forming between the poles of the energy reserve 2, which could damage to the machine director 3 and/or the energy supplying means 1.

Figure 12 shows an energy transfer system of the ninth embodiment of the invention, in which several electric machines 6, 6', 6", magnetic directors 4, 4', 4", machine directors 3, 3', 3", and magnetic bearings 7, 7', 7" are used. Further, the control system 5 comprises a control cir¬ cuit 5a of the network director and a motor control circuit 5b, 5b', 5b" in order to control each electric machine 6, 6' 6" and the machine directors 3, 3', 3", magnetic directors 4, 4', 4" and magnetic bearings 7, 7', 7" connected thereto. Fig. 12 shows a system of three electric ma¬ chines 6, 6', 6", but the system can be larger or it can comprise only two electric machines 6, 6', 6". The electric machine systems are coupled preferably to a single, common energy reserve 2, wherein in case of a power failure it is enough that even one electric machine 6, 6', 6" can produce energy to the energy reserve 2 for the magnetic bear¬ ings 7, T 7". Thus, a possible fault in one electric machine system does not damage the respective electric machine in case of a power failure.

According to an advantageous embodiment of the invention described in Fig. 12, the control system 5a, 5b, 5b', 5b" can be implemented advantageously by using one or several processor units. The imple- mentation of the control system is known as such to an artisan in the filed, so that it is not necessary to describe it in detail in this context.

The invention is not restricted entirely to the embodiments presented above but it can be modified within the accompanying claims.

Claims

Claims:

1. Energy transfer system of an electric machine (6) having magnetic bearings, which system comprises an electric machine (6), a machine director (3) in energy transferring connection with the electric machine (6), magnetic bearings (7), a magnetic director (4) in energy transferring connection with the magnetic bearings (7), a control sys¬ tem (5), an energy supplying means (1 ) and an. energy reserve (2), characterized in that the energy reserve (2) is arranged in continuous energy supplying connection with the energy supplying means (1 ), the machine director (3) and the magnetic director (4), wherein during the operation of the electric machine (6) the capacity of the energy reserve (2) is arranged to be maintained by the energy which exists in and/or is supplied to the energy transfer system.

2. Energy transfer system as set forth in claim 1 , character¬ ized in that a separate energy transferring means (10) is connected between the energy reserve (2) and the magnetic director (4).

3. Energy transfer system as set forth in claim 1 , character¬ ized in that the system further comprises an auxiliary network direc¬ tor (1 1 ) and a supplementary energy reserve (12) connected thereto which are both connected to the energy reserve (2) through the energy transferring means (10).

4. Energy transfer system as set forth in claims 2 or 3, char¬ acterized in that the energy transferring means (10) includes a diode coupling (9, 9a, 9b).

5. Energy transfer system as set forth in any of the claims 1 to 4, characterized in that a separate auxiliary generator (14) and an auxiliary generator director (15) are further connected to the energy reserve (2).

6. Energy transfer system as set forth in any of the claims 1 to 4, characterized in that the energy transfer system further includes an auxiliary machine director (16) which is connected to an energy transferring connection between the energy reserve (2) the electric machine (6).

7. Energy transfer system as set forth in any of the claims 1 to 6, characterized in that the energy reserve (2) is a capacitance, such as a capacitor.

8. Energy transfer system as set forth in any of the claims 1 to 6, characterized in that the energy reserve (2) is an inductance, such as a coil.

9. Energy transfer system as set forth in any of the claims 1 to 6, characterized in that the energy reserve (2) is an accumulator.

10. Energy transfer system as set forth in any of the claims 1 to 7, 9, characterized in that the network director (1) and/or the auxiliary network director (11 ) is a diode bridge.

11. Energy transfer system as set forth in any of the claims 1 to 10, characterized in that the energy transfer system further com¬ prises a DC-interrupter (17) coupled alongside with the energy re¬ serve (2) and a load resistor (R_L) coupled to the DC-interrupter (17) for transferring energy from the energy reserve (2) to the load resistor (R_L) advantageously when the voltage in the energy reserve (2) exceeds the pre-set threshold value.

12. Energy transfer system as set forth in any of the claims 1 to 11 , characterized in that the auxiliary generator (14) or the like is connected to the rotor shaft of the electric machine (6) for generating electric energy from rotation movement of the rotor shaft of the electric machine (6).

13. Energy transfer system as set forth in any of the claims 1 to 11 , characterized in that the electric energy for a magnetic bear¬ ing (7) is arranged to be taken from the electric machine (6) or the auxiliary generator (14).

14. Energy transfer system as set forth in any of the claims 1 to 13, characterized in that the electric machine (6) or the auxiliary generator (14) is a high speed machine having a normal operating range of a typical 15 000 to 400 000 rpm, however, not more than 2 000 000 rpm, usually 20 000 to 200 000 rpm, advantageously 20 000 to 100 000 rpm.

15. Energy transfer system as set forth in any of the claims 1 to 12, characterized in that the electric machine (6) or the auxiliary generator (14) is a high speed electric machine having a typical periph¬ eral rotor speed of 100 to 500 m/s, usually 200 to 500 m/s.

16. Energy transfer system as set forth in any of the claims 1 to 15, characterized in that the electric machine (6) or the auxiliary generator (14) is an asynchronous machine, such as a short-circuit motor

17. Energy transfer system as set forth in any of the claims 1 to 15, characterized in that the electric machine (6) or the auxiliary generator (14) is a reluctance motor.

18. Energy transfer system as set forth in any of the claims 1 to 15, characterized in that the electric machine (6) or the auxiliary generator (14) is a brushless DC-electric machine.

19. Energy transfer system as set forth in any of the claims 1 to 15, characterized in that the electric machine (6) or the auxiliary generator (14) is a permanent-magnet electric machine.

20. Energy transfer system as set forth in any of the claims 1 to 15, characterized in that the electric machine (6) or the auxiliary generator (14) is a synchronous electric machine.

21. Energy transfer system as set forth in claim 1 , character- ized in that the energy reserve (2) is placed between the energy supplying means (1) and the machine director (3) in the direction of energy transfer and that the magnetic director (4) is coupled to the wiring between the energy supplying means (1 ) and the machine direc¬ tor (3).

22. Energy transfer system as set forth in any of the preceding claims, characterized in that the system comprises two or more electric machines (6, 6', 6") and for each electric machine a machine director (3, 3', 3"), a magnetic director (4, 4', 4") and a magnetic bearing unit (7, 7', 7").