WO2008135532A1

WO2008135532A1 - Pole-changing asynchronous machine for variable supply frequency

Info

Publication number: WO2008135532A1
Application number: PCT/EP2008/055420
Authority: WO
Inventors: Stephan Risse; Michael Van Der Giet; Kay Hameyer
Original assignee: Airbus Operations GmbH
Current assignee: Airbus Operations GmbH
Priority date: 2007-05-03
Filing date: 2008-05-02
Publication date: 2008-11-13
Anticipated expiration: 2009-11-03
Also published as: DE102007020706A1

Abstract

The invention relates to an asynchronous motor which can be connected to variable- frequency alternating voltages and can be operated at these alternating voltages with at least two different numbers of poles, comprising a stator with a stator winding consisting of a plurality of groups of coils or a plurality of stator windings and a rotor, wherein tap lines and/or the ends of groups of coils are guided separately out from the motor to a change-over switch which makes it possible to limit the rotational speed of the asynchronous motor to a predetermined range by pole changing. The invention further relates to a method for operating such an asynchronous motor at variable-frequency alternating voltages.

Description

Pole-changing asynchronous machine for variable supply frequency

Reference to related application This application claims the benefit of the filing date of German Patent Application No. 10 2007 020 706.0 filed May 3, 2007 the disclosure of which application is hereby incorporated herein by reference.

Field of the invention The invention relates to an asynchronous motor which can be connected to and operated at variable-frequency alternating voltages.

Technological background of the invention

Such asynchronous motors are already sufficiently known. In frequency- variable power supply networks such as are frequently used, for example, in modern passenger aircraft, asynchronous machines are generally connected to frequency converters which supply a voltage having pre-definable frequencies. According to the triggering of the frequency converter, rotational speeds and torques of the asynchronous machine are thereby influenced over a wide range, thus providing the greatest possible flexibility in the respective application. In such a case, a usual electrical drive accordingly consists of a frequency converter having a corresponding triggering logic and power semiconductor components and the electrical machine itself.

For some applications requiring an electrical drive, however, high requirements are not imposed on the rotational speed of the machine so that this quantity need not be adjustable over a wide range. The exact rotational speed of the electrical machine is possibly unimportant for the operation as long as it merely lies within a permissible range of rotational speed. The drive of a pump serves as an example: if a sufficient pressure is produced in the pipe system by the electrical drive, the rotational speed of the drive is not of interest. If a converter-controlled drive is nevertheless used for such an intended application, some distinct disadvantages are obtained compared with a drive without a frequency converter. Firstly, additional weight and additional space are incurred by the frequency converter. If such machines are used in a modern passenger aircraft, these may be particularly serious disadvantages. Furthermore, as a result of the possible failure of the frequency converter or the power semiconductors contained therein, the failure probability of the entire arrangement comprising electrical machine and frequency converter may be increased. In addition, the power supply network may be increasingly endangered by interference effects due to operation of the frequency converter. Finally, the efficiency of the entire system may be reduced due to the lossy operation of the frequency converter and the heat input produced by the electrical power loss may be increased.

Summary of the invention

Accordingly, there may be a need to reduce or eliminate one or more of said shortcomings. In particular, there may be a need to provide a robust and reliable low- weight alternating-current drive for operation on a frequency-variable power supply which is suitable for largely rotational-speed-independent applications having a restricted range of rotational speed.

According to an exemplary embodiment of the present invention, an asynchronous motor is provided which can be operated at alternating voltages with at least two different numbers of poles at variable frequency. In addition to a rotor, this asynchronous machine comprises a stator with a stator winding comprising a plurality of groups of coils or a plurality of stator windings, wherein tap lines and/or the ends of groups of coils are guided separately out from the motor to a change-over switch or commutator which makes it possible to operate the asynchronous motor in different frequency ranges of the power (or current) supply by pole changing. When the frequency of an alternating voltage is variable (hereinafter also called "supply frequency"), a conventional asynchronous motor without a frequency converter would adopt a rotational speed proportional to the frequency. By pole changing and increasing or reducing the number of pole pairs of the stator windings which is thereby achieved, the rotational speed may be reduced or increased since the rotational speed of an asynchronous motor also depends inversely proportionally on the number of pole pairs.

If a permissible range of rotational speed for an asynchronous motor is predefined for an application which is substantially independent of rotational speed, this must be maintained by specific pole changing of the motor. In this case, the lower limit of the rotational speed range is obtained by the minimal system requirements (e.g. minimal pressure for a pump drive, minimal air mass flow rate for a fan). The upper limit of the rotational speed range is predefined by the mechanical loading capacity of the electrical machine. If a rotational speed of the asynchronous motor which goes outside the permissible range were to be obtained, for example, when a relatively low number of pole pairs is set and at the same time, the frequency of the supply network is relatively high, it may be necessary to specifically increase the number of poles to reduce the rotational speed of the asynchronous motor so that this again lies in the permissible range. Conversely, if the number of poles is too high and the frequency of the supply network is too low, the rotational speed of the asynchronous motor may fall below the required rotational speed range so that a specific reduction in the number of poles is required to increase the rotational speed.

For specific pole changing, it may be necessary to divide the stator windings according to a specific scheme and connect them together again by specific wiring. For this purpose, the ends of the individual winding segments are guided out from the motor so that they are accessible for wiring to one another. The specific wiring can, for example, be a delta or a star circuit which is accomplished either by means of a manual, an automatic or a change-over switch (or commutator switch) controlled by an electronic device which can switch to and fro between different types of circuit. In order to avoid frequent switching processes in the event of slight frequency fluctuations, a switching hysteresis is provided in this case.

If the alternating voltage regularly adopts a frequency from a very large frequency range, possibly more than two different types of circuit and therefore more pole changing possibilities are therefore appropriate. The asynchronous machine should be designed according to the predefined frequency range, wherein in particular the frequency at the change-over operating point should be noted. It should further be noted that the number and size of the pole numbers also influences the physical size of the asynchronous machine.

Advantageous further embodiments of the asynchronous motor according to the invention are specified in the dependent claims. Likewise, a corresponding method for operating an asynchronous motor as well as the use of an asynchronous motor according to the invention in an aircraft having a frequency- variable power supply network are proposed.

The invention is explained in detail hereinafter with reference to the figures. In the figures the same objects are characterised by the same reference numerals.

Short description of the drawings

Figure 1 : shows an exemplary diagram giving the motor rotational speed as a function of the supply frequency and operating limits;

Figure 2a: shows a delta circuit;

Figure 2b: shows a double star circuit;

Figure 2c: shows an exemplary winding scheme and

Figure 3 : shows a schematic view of a method according to the invention for operating an asynchronous motor. Detailed description of exemplary embodiments

The illustration in the drawings is schematically. In different drawings, similar or identical elements are provided with the same reference numerals.

As an example, Figure 1 shows a diagram in which an operating range 2 of a motor required for a predetermined application is identified as a non-shaded white area. As an example, the application comprises a coolant pump used to supply a coolant circuit in a modern passenger aircraft.

The supply frequency in Hz is plotted as the parameter on the abscissa 4 and the motor rotational speed in revolutions per minute (min^" ) is plotted on the ordinate 6. For the exemplary application the motor rotational speed at all the supply frequencies attained must lie in the operating range 2 which is delimited by a minimum rotational speed 8 of, for example, 8000 min^"1 and a maximum rotational speed of, for example, 16000 min^"1. The supply frequency in this range meanwhile lies in a range of about 360 Hz to 800 Hz and is dependent on the respective rotational speed of the generators driven by the aircraft engines. As an example, this frequency range of 360 Hz to 800 Hz is delimited by a minimum frequency 12 and a maximum frequency 14.

Figure 1 also shows a first operating curve 16 for an asynchronous motor which is operated on a power supply having the exemplary supply frequency range described hereinbefore. In this case, the initial number of pole pairs is p = 2. At the minimum frequency 12 a motor rotational speed of about 11000 min^" is established, said speed increasing linearly up to the maximum value 10 of 16000 min^" permissible in this application up to a supply frequency of 533 Hz. If a further increase in the supply frequency occurs, the winding of the asynchronous motor will be wired in a different manner by a change-over switch so that the number of pole pairs of doubled and is now p = 4. The rotational speed of the motor is thereby reduced by half and is 8000 min^"1 at the exemplary change-over supply frequency of 533 Hz. The second operating curve shows the resulting behaviour of the motor rotational speed as a function of the supply frequency when the number of pole pairs is p = 4. With the number of pole pairs doubled, the maximum frequency 14 of 800 Hz can be reached without the permissible rotational speed limit 10 being exceeded.

The number of pole pairs of an asynchronous motor can then be actively influenced if the stator winding of the asynchronous motor is divided into a plurality of coils which are connected to one another externally in different ways. For example, all known types of delta and star circuits may be used. For this purpose, the ends of the individual stator windings are usually guided out separately from the motor and, for example, into a plugboard located externally on the motor, where they are then wired together to achieve a certain type of circuit. In addition, tap lines may also be used for tapping the windings, these also being guided out from the motor. The connection of the individual ends or tap lines may be made by fixed wiring but in the asynchronous motor according to the invention, a change-over switch not represented in detail may be particularly advantageous. By means of a change-over switch the wiring of the stator windings may be changed rapidly between different types of circuit with different numbers of pole pairs. In an industrial application this changeover takes place automatically whereby the change-over frequency is detected by a rotational speed sensor or by evaluating the mains frequency. Stable change-over processes may be achieved in conjunction with a switching hysteresis provided in the changeover logic.

The specific configuration of an asynchronous motor according to an exemplary embodiment of the invention for providing a necessary power in a frequency-variable supply network is effected by designing using conventional methods. Various boundary conditions such as a maximum stator external diameter or a maximum length of the motor are frequently predefined, specifically with the severely restricted space in passenger aircraft. The general design process for an alternating current motor substantially consists of the steps:

• Determining the principal dimensions of the motor,

• Designing the stator winding, • Designing the rotor winding,

• Designing the magnetic circuit

• Verifying the design.

In many intended applications, especially with the planned integration of the asynchronous motor in the restricted spatial conditions of a passenger aircraft, its maximum external dimensions are already predefined. When designing the stator windings, parameters such as, for example, number of turns, number of grooves and tooth width will be determined depending on a rated voltage, a rated frequency and the power to be supplied. In particular the groove geometry is determined in the analytical design of the magnetic circuit. However, detailed calculation schemes are dispensed with at this point and reference is made to basic specialist literature.

The application of the conventional design scheme must be modified to specifically lead to a pole-changing alternating-current motor which can be used on a frequency- variable supply network. In the exemplary case of an electrical machine which can be switched over between a number of pole pairs p = 2 to a number of pole pairs p = 4, see Figure 1, the design of the stator windings must be made separately for each number of pole pairs to match the parameters determined therefrom. An important parameter for the operating behaviour of the pole-changing asynchronous motor is the ratio of the induction for both numbers of pole pairs

^B _P,

B_ which depends on the selected type of circuit of the turns. For the case where an asynchronous motor according to an exemplary embodiment of the invention, for example at 400 Hz and having the number of pole pairs pi = 2, is to supply the same rotational speed, the same mechanical power and accordingly the same torque as at 800 Hz and the number of pole pairs p₂ = 4, the ratio of the inductions must be equal to 1 as far as possible. Other induction ratios are possible but influence the mechanically delivered power of the machine for the selected number of pole pairs.

As an example, Figure 2a shows a delta circuit 20 with which the groups of coils of the stator winding are connected so that a number of pole pairs p = 2 is present in the stator. Separate winding parts of the stator are hereinafter also called "coils". For example, the groups of coils are identified by numbers from Sl to S 12 representing connections which are repeated in the following Figures 2b to 2c. These may be the ends of individual turns but also of tap lines.

In the delta circuit 20, two coils 22 in each case are connected in series to form a total of three coil groups 24 whose external connection points are connected to one another and to the individual phases of the supply network.

The double star circuit 26 shown in Figure 2b generates a number of pole pairs p = 4 in the stator and is built up from coil groups 28 connected in parallel, whose first connection points as shown are connected to a star point 30 to form a star and whose second connection points are connected to the individual phases.

An exemplary winding scheme pertaining to an experimental asynchronous motor according to another exemplary embodiment of the invention is shown in Figure 2c. The individual connection points Sl to S 12 of the individual coils 32 are identified to clarify the representation in Figures 2a and 2b. As a result of the requirement for an asynchronous motor that is as compact as possible, the number of grooves is selected to be as small as possible so that the number of grooves 30 of the stator is 24, taking into account three connecting strands and a maximum number of pole pairs p = 4 or a maximum number of poles of 8.

Finally, Figure 3 schematically shows a method for operating an asynchronous motor according to an exemplary embodiment of the invention. During operation 36 of the asynchronous motor according to an exemplary embodiment of the invention at a variable frequency, the pole number is reduced 40 when the actual rotational speed falls below the minimal rotational speed 38, so that the rotational speed of the asynchronous motor decreases. The change-over to reduce the rotational speed is preferably made to a delta circuit 42. After change-over, the operation 36 of the asynchronous motor proceeds continuously. If the maximum rotational speed is exceeded 44, the number of poles of the asynchronous motor is increased 46 to increase the rotational speed. The change-over to increase the rotational speed is preferably made to a double- star circuit 48. The operation 36 of the asynchronous motor is then continued with a new pole number.

The change-over may be effected automatically by means of mechanical or electronic aids and in a frequency-dependent manner but a manual change-over may also be advantageous in particular situations. During automatic change-over, control by a processing unit or an electronic device is feasible, said device being tuned to delimit the rotational speed of the asynchronous motor according to the invention to the permitted range by automatic connection of the stator windings.

The asynchronous motor described in the present application for operation at variable-frequency alternating voltages having at least two different numbers of poles may be particularly advantageous for supplying applications substantially independent of rotational speed with mechanical power and not departing from a restricted range of rotational speed regardless of the frequency of the power supply. Due to the feature of the frequency-dependent pole changing according to the invention, a frequency converter may be dispensed with, thus saving weight, reducing space and increasing the efficiency and reliability of the entire drive.

The invention is explained in detail with reference to exemplary embodiments which, however, are not to be understood as a restriction of the invention. In particular, a restriction to delta and double star circuits is not essential but all types of circuit can be taken into account in the technical implementation of the asynchronous motor according to the invention and the method according to the invention.

It should be noted that the term "comprising" does not exclude other elements or steps and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined.

It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

C l a i m s

1. An asynchronous motor adapted for being connected to variable-frequency alternating voltages and for being operated at the alternating voltages with at least two different numbers of poles, the motor comprising a stator with a stator winding comprising a plurality of groups of coils or a plurality of stator windings and a rotor, wherein tap lines and/or the ends of the groups of coils are guided separately out from the motor to a change-over switch which is operable to limit the rotational speed of the asynchronous motor at different frequencies of the power supply to a predetermined range by pole changing.

2. The asynchronous motor of claim 1, wherein the change-over switch for changing over the stator windings is designed for optionally achieving precisely two, three, four, five or more different numbers of pole pairs.

3. The asynchronous motor of at least one of the preceding claims, wherein the change-over switch for changing over between a delta circuit (20) and a double-star circuit (25) is designed with a pole pair ratio 1 :2.

4. The asynchronous motor of claim 3, wherein the change-over switch in the delta circuit (2) switches two coil groups (22) in series for each connecting strand and connects the coil groups pairwise to the connecting strands and in the double-star circuit (26) switches (28) two coil groups (22) in parallel, connects one end of the parallel-switched coil groups to a star point (30) and connects the respective other end of the parallel-switched coil groups to a connecting strand.

5. The asynchronous motor of at least one of the preceding claims, wherein the change-over switch increases the number of poles by changing over when a given maximum rotational speed (10) is attained or exceeded due to increasing frequency of the power supply.

6. The asynchronous motor of at least one of the preceding claims, wherein the change-over switch reduces the number of poles by changing over when a given minimum rotational speed (8) is attained or fallen below due to decreasing frequency of the power supply.

7. The asynchronous motor of at least one of claims 5 and 6, which has an electronic device which automatically controls the reduction or increase in the number of poles to limit the rotational speed to a rotational speed range which extends from the minimum rotational speed (8) as far as the maximum rotational speed (10).

8. The asynchronous motor of at least one of the preceding claims, wherein the electronic device produces a switching hysteresis which avoids frequent change-over by the set change-over frequency.

9. The asynchronous motor of at least one of the preceding claims, wherein the supplied torque of the asynchronous motor is substantially the same for all numbers of poles.

10. A method for operating an asynchronous motor at a variable-frequency alternating voltage, wherein the asynchronous motor comprises a stator with a stator winding comprising a plurality of groups of coils or a plurality of stator windings and a rotor; tap lines and/or the ends of groups of coils are guided separately out from the motor to a change-over switch; the method comprising the steps of: changing over the asynchronous motor between at least two different numbers of poles at various frequencies of the power supply to limit the rotational speed to a predetermined range.

11. The method of claim 10, wherein the change-over switch changes over the stator windings as desired to achieve two, three, four, five or more different numbers of pole pairs.

12. The method of at least one of the preceding claims 10-11, wherein the change-over switch switches optionally over between a delta circuit (20) and a double-star circuit (26) with the pole pair ratio 1 :2.

13. The method of at least one of the preceding claims 10-12, wherein the change-over switch in the delta circuit (2) switches two coil groups (22) in series for each connecting strand and connects the coil groups pairwise to the connecting strands and in the double-star circuit (26) switches (28) two coil groups (22) in parallel, connects one end of the parallel-switched coil groups to a star point (30) and connects the other end of the parallel switched coil groups to a connecting strand.

14. The method of at least one of the preceding claims 10-13, wherein the change-over switch increases the number of poles by changing over when a given maximum rotational speed (10) is attained or exceeded due to increasing frequency of the power supply

15. The method of at least one of the preceding claims 10-14, wherein the change-over switch reduces the number of poles by changing over when a given minimum rotational speed (8) is attained or fallen below due to decreasing frequency of the power supply.

16. The method of at least one of the preceding claims 14 and 15, wherein an electronic device automatically controls the reduction or increase in the number of poles to limit the rotational speed to a rotational speed range which extends from the minimum rotational speed (8) as far as the maximum rotational speed (10).

17. The asynchronous motor of at least one of the preceding claims, wherein a switching hysteresis is produced by the electronic device which avoids frequent change-over by the set change-over frequency.

18. Use of an asynchronous motor of claims 1 to 9 on a variable-frequency power supply network.