SE2250749A1 - Salient pole electrical machine - Google Patents

Abstract

An electrical machine (1) comprises a stator (20) having armature windings (24), a rotor (10) having salient poles (12) and being rotatably arranged with respect to the stator (20), and a rotor excitation system (30). The rotor excitation system (30) has a rotor winding arrangement (14), a rotor power supply (34) for exciting the salient poles (12), and a rotor driving unit (32) configured to control currents provided to the rotor winding arrangement (14). In a first operation mode, the rotor excitation system (30) is configured for providing a rotating magnetic field by supplying alternating currents to the rotor winding arrangement (14), while the armature windings (24) being shortcircuited or closed on external resistors. A method for operating an electrical machine is also disclosed.

Description

lO &MflENTPOLEEEECTRHMULMACHHW) TECHNK%LFElD The present technology relates in general to salient pole electrical machines, and in particular to devices and methods for starting Wound field synchronous machines.

BACKGROUND A synchronous machine can produce a net positive torque, When operated as a synchronous motor (SM), or a net negative torque, When operated as a synchronous generator (SG). Such torques are only possible When the armature rotating magnetic field and the excitation field are synchronous, thereby the name. For this reason, a SM needs an excitation DC Winding, usually one on the rotor, to first be sped up till the synchronism in order to be connected to the network and used for its main purpose.

There are many starting techniques of a SM Within the state of the art.

In a pony motor arrangement, an electric motor or combustion engine, permanently installed in the machine facility and smaller in rated power than the machine to be started, provides the mechanical torque needed for bringing the rotor up to the rated speed, i.e. into synchronism.

In a hydropoWer plant (HPP) a SG uses the driving torque of a coaxial hydraulic turbine in order to be accelerated up to the synchronism. In a pumped storage HPP (PSHPP), the synchronous machine commuting from SG to SM can use a separate hydraulic turbine for the start-up, Which spins the machine rotor in the opposite direction of that used during the generation. lO A Back-to-back synchronous start technique requires the availability of two synchronous machines in the PSHPP, one working as driving SG and the other being the SM to be started. Both machines are excited at the start and their armatures are connected in order to form an island. When the SG starts to spin, the SM follows it up to the rated speed.

In a solution of synchronous start by inverter (VFD) the SG in the back-to- back technique is surrogated by a variable frequency drive (VFD) which keeps the excited SM rotor permanently at the synchronism.

In an asynchronous start, the SM is started as an induction motor by making use of the rotor damper bars as a squirrel cage. In this case, the rotor winding is closed on an external resistor, typically showing a resistance between 3 and 5 times the excitation winding DC resistance, in order to prevent induced high voltage into the field winding and to limit the rippled torque contribution to the start of a single-phase like winding.

When applying asynchronous start with a reactor, an inductor is connected in series to the SM in order to prevent an excessive drop of the supplying voltage at the supplying bus-bar during the highly inrush of the reactive starting current.

In an asynchronous start with autotransformer/transformer, the goal of supplying the starting SM with a lower voltage than the rated one is that of reducing the starting current without penalizing the available apparent power needed for building up the torque.

A partial frequency start technique is an alternative back-to-back technique which does not requires the two SM in the parallel to be both excited. In fact, in the first part of the starting procedure the SM with short-circuited rotor winding runs after the fully excited SM, which has been previously sped-up up to 20-40% of the rated speed. In this way the SM can be brought close to the synchronism with the driving SG, with rated power ~ 15% of that of the lO driven motor, by using an asynchronous start. When the two machines are almost at the same speed, e.g. at a slip s < 5%, the field winding of the SM is opened and excited so to make finally the two machines synchronous.

A strategy of feeding an electrical rotating machine through the rotor winding in order to generate or to affect the electromagnetic torque can be found in the asynchronous machine with wound rotor which is called dual fed induction machine (DFIM). The rotor winding of such a machine is a multiphase one, i.e. at least a two phase winding, capable of generating a rotating magnetic field as well as the armature winding.

Since the synchronous machine needs a stationary rotor magnetic field for working at the rated speed, it has been provided with a DC heteropolar winding. The winding can be a distributed winding in the cylindrical type of synchronous machines, where the active sides of the conductors are led down in slots. Alternatively, the winding can be a concentrated winding in a salient pole synchronous machine. In both cases, the winding presents a start- and an end-terminal as a single-phase winding does. Such a winding is not capable of generating a magnetic rotating field, even though it gets supplied with an AC current.

The published US patent US 5,315,225 discloses, in a synchronous machine, which is already excited by a DC field current or by a set of permanent magnets, a frequency converter providing a variable frequency current to the stator windings, i.e. the armature. A stator current with lower frequency is applied during the start-up and is gradually increased to higher frequency up to the operating frequency. In this way the armature rotating field and the excitation rotor field are kept permanently at synchronism being able to provide a unipolar accelerating torque in the sense of rotation. Such arrangement may be complex, in particular for large machines and requires an external frequency converter. lO In the published US patent US 3,020,463, an asynchronous starting strategy is represented, where the armature provides the rotating magnetic field and the damper bars, amortisseur windings, are the main induced circuits. During this start the field winding is completely short-circuited. When the rotational speed approaches the synchronous speed the DC eXcitation of the rotor field winding is applied. This approach presents the problem that the rotating stator magnetic field is also the inducing magnetic field which must be large in order to induce large currents in the rotor winding. However, this magnetic field is not synchronous with the rotor and, if the rotor is a salient pole one, a periodically varying reluctance torque superposes to the electromagnetic one produced by electromagnetic induction. The resulting torque oscillations cause high mechanical stresses on the stator frame and on the rotor shaft.

In the published US patent US 8,508,179 B2, the asynchronous starting strategy sees the field winding as the main induced winding.

In the published US patent US 6,051,953, a brushless rotor concept is proposed where a damper winding is provided for the asynchronous start-up.

Asynchronous start of synchronous machines in prior art has a number of disadvantages. All the previous solutions, except the first one, experience the "inrush" current when the power of the stator is turned on. There is typically a current inrush during the start-up that typically is 4-8 times the rated current. This is due to the fact that the rotor is in standstill when the machine is magnetized so that the rotating inducing field shows the highest relative speed to the rotor at the starting. Such situation is also called short circuit condition. The procedure has a very low power factor. Furthermore, there is a thermal stress on damper bars and pole shoes. Moreover, the mechanical stress is huge, due to large torque ripple, typically 40 to 120%. In order to let the machine components to survive this tough transient, they need to be designed for withstanding the short lived but large inrush current. Many synchronous machines are designed in an electrical and mechanical view to withstand such short but intense strains, which means that many parts of a synchronous machine are heavily over-dimensioned with respect to the demands of a continuous synchronous operation. Otherwise, additional measures and component are required such as the reduction of the supply voltage by means of transformers/ autotransformers or the insertion of current limiting reactors during the start-up phase. They have of course the drawback of jeopardizing the machine efficiency and performances.

The published international patent application WO 2018/ 160122 A1 claims to solve the problem of the inrush current and that of the oscillating accelerating torque by synchronizing the appearance of the north and south polarities on the rotor with the speed of the armature rotating magnetic field. It uses the excited rotor and the armature rotating field at the same time. The rotating magnetic field and the rotor are always asynchronous except at the end of the start-up. In order to avoid the phenomenon of the inrush current, the supplying voltage on the armature must be artificially reduced by means of a transformer/autotransformer or by the insertion of a reactor in series to the armature phases. Moreover, the single-phase nature of the field winding injected by the AC-current is such to produce an excitation field which represents a standing wave with reference to the rotor. So, whereas the progressive component of this field interacts with the armature rotating field by producing a net accelerating torque, the regressive one generates an AC torque as large as the previous one, responsible for torque ripple. This problem is well known in the asynchronous start of synchronous machines when the external resistance of the field winding is too small, i.e. the field winding is close to short-circuit condition, or when the damper bars on all poles are not connected as a multiphase system. In other words, the application of single phase circuits or single-phase-like circuits in the asynchronous start always superposes a nasty large ripple on the useful average electromagnetic torque.

There is thus still a need for reducing the pulsating torque variations caused by asynchronous start-up of synchronous electrical machines. lO SUMMARY A general object is to find methods and devices for reducing start load and strains of a synchronous electrical machine.

The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims.

In general Words, in a first aspect, an electrical machine comprises a stator having armature Windings, a rotor having salient poles and being rotatably arranged With respect to the stator, and a rotor excitation system. The rotor excitation system has a rotor Winding arrangement, a rotor power supply for exciting the salient poles, and a rotor driving unit configured to control currents provided to the rotor Winding arrangement. In a first operation mode, the rotor excitation system is configured for providing a rotating magnetic field by supplying alternating currents to the rotor Winding arrangement, While the armature Windings being short-circuited or closed on external resistors.

In a second aspect, a method for operating an electrical machine comprises, in a first operation mode, short-circuiting or closing on external resistors of armature Windings of a stator. In the first operation mode, a rotating magnetic field is provided from a rotor having salient poles and being rotatably arranged With respect to the stator. The provision of the rotating magnetic field from the rotor in turn comprises supplying of alternating currents to a rotor Winding arrangement of a rotor excitation system for exciting the salient poles.

One advantage With the proposed technology is that it makes it possible to asynchronously start a salient pole synchronous machine With at least two parallel strands per phase and four poles, up to a hypersynchronous speed, by AC feeding the motor through the rotor Winding.

Other advantages Will be appreciated When reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: FIG. 1 is a schematic drawing of an embodiment of a salient pole synchronous machine; FIGS. 2A and 2B are illustrations of instantaneous situations during an asynchronous start-up of a synchronous machine; FIGS. 3A and BB are illustrations of instantaneous situations of a rotor having rotor windings giving a rotating magnetic field; FIG. 4 is a flow diagram of steps of an embodiment of a method for operating an electrical machine; FIG. 5 is a flow diagram of steps of an embodiment of a switching step; FIG. 6 is a schematic drawing of an embodiment of a salient pole synchronous machine allowing a rotating rotor magnetic f1eld; FIG. 7A is a schematic drawing of salient poles of a rotor of a two-phase rotor winding in a steady-state operation mode; FIGS 7B-C are schematic drawings of salient poles of a rotor of a two- phase rotor winding in a start-up operation mode; FIG. 8 is a diagram illustrating alternating currents provided to two- phase rotor windings in a start-up operation mode; FIG. 9A is a schematic drawing of salient poles of a rotor of a three- phase rotor winding in a steady-state operation mode; FIGS 9B-C are schematic drawings of salient poles of a rotor of a three- phase rotor winding in a start-up operation mode; FIG. 10 is a diagram illustrating alternating currents provided to three- phase rotor windings in a start-up operation mode; FIG. 11 illustrates schematically electromotive forces to be expected in 3-phase armature strands with a rotor winding supplied with DC currents; FIG. 12A illustrates schematically electromotive forces to be expected in armature strands with a rotor winding supplied with two-phase AC currents; FIG. 12B illustrates schematically electromotive forces to be expected in armature strands With a rotor Winding supplied With three-phase AC currents; FIG. 13A illustrates armature strands rearranged for obtaining symmetrical electromotive force systems With a rotor Winding supplied With two-phase AC currents; FIG. 13B illustrates armature strands rearranged for obtaining symmetrical electromotive force systems With a rotor Winding supplied With three-phase AC currents; FIG. 14 is a diagram illustrating electromagnetic torque provided to a synchronous machine during the start-up operation mode; FIG. 15 is a schematic draWing of an embodiment of armature connections for a two-phase excitation system; FIG. 16 is a schematic draWing of an embodiment of armature connections With external resistors for a two-phase excitation system; FIG. 17 is a schematic draWing of an embodiment of armature connections for a three-phase excitation system; FIG. 18 is a schematic draWing of an embodiment of armature connections With external resistors for a three-phase excitation system; FIG. 19 is a schematic draWing of an embodiment of rotor Windings and a slip ring arrangement for a two-phase excitation system; and FIG. 20 is a schematic draWing of an embodiment of rotor Windings and a slip ring arrangement for a three-phase excitation system.

DETAILED DESCRIPTION Throughout the draWings, the same reference numbers are used for similar or corresponding elements.

For a better understanding of the proposed technology, it may be useful to begin With a brief overview of an ordinary synchronous machine operation.

Figure 1 illustrates schematically an embodiment of a salient pole synchronous electrical machine 1. A rotor 10 has a number of salient poles 12 that are excited by rotor Windings 14. The rotor Windings are supplied With a constant DC current If, by applying a constant DC voltage Vf over the rotor Windings 14. A stator 20 has armature Windings 24 that are, When the synchronous electrical machine 1 is used as a motor, supplied With alternating voltages Vs. In this embodiment, the armature Windings 24 comprise three phases are therefore supplied With a three-phase voltage. The armature Windings 24 give rise to a rotating magnetic field rotating With the angular speed of (oa. The magnetic field created by the rotor Windings 14 crosses the air gap between the rotor 10 and the stator 20 as illustrated by a schematic example magnetic field line 26. When a rotating speed (of. is the same as the speed (oa of the applied rotating magnetic field from the armature Windings, the magnetic fields are coupled to keep them in synchronism. The operation is thus intended to be performed at synchronism and any deviations cause problems, as been discussed above, e.g. during the start-up of the machine.

When the synchronous electrical machine 1 is used as a generator, voltages are induced in the armature Windings 24 in synchronism With the rotational speed of the rotor 10.

Figure 2A illustrates a part of a synchronous electrical machine 1 during an asynchronous start-up Where the field Winding excitation is kept. The armature Windings of the stator 20 created a rotating magnetic field illustrated schematically by an area denoted as N, indicating a north pole. The magnetic field rotates With a speed of (oa, determined by the applied alternating voltage frequency and the design of the armature Windings. A south pole 12 of the rotor is created by the constant rotor Windings and rotates therefore With the same speed as the rotor (of. During asynchronous start-up, these speeds are not the same and the south pole 12 therefore performs a rotation relative the armature Winding magnetic field being equal to the difference (oa- for., illustrated by the dotted arrow. In the momentary situation illustrated in Figure 2A, the attraction of the rotor south pole to the north pole of the rotating lO magnetic field creates a force F on the rotor, tending to increase the rotating speed thereof.

However, at other moments, e.g. the one illustrated in Figure 2B, the relative positions between the rotating magnetic field of the stator and the rotor poles 12 may instead give rise to a force F, tending to retard the rotor speed. Since these situations are alternating, in the beginning of a start-up with a relatively high frequency, the electrical and mechanical loads become large. This is the reason for all previous attempts to provide different start-up arrangements, as presented above.

During asynchronous start-up, the rotor windings may also be short-circuited or closed on resistances, since the relative shift of the armature rotating field with respect to the rotor induces relevant electromotive forces in the field winding, especially at the beginning of the starting process. The closure of the field winding in short circuit or through an external resistance has two beneficial effects in preventing an insulation breakdown in the rotor winding due to severe overvoltage. On one side it allows a field winding reaction which rejects the cause of the induction, i.e. the armature flux. On the other side, it forces the potentials of the field winding terminals to be stay close. However, the most effective armature flux rejection, the one achieved through the pure short circuit of the rotor winding, causes the largest ripple in the starting torque without contributing to the net starting torque. This is the reason why the alternative closure on an external resistance is preferred, since it represents a trade-off between the beneficial effect of the armature flux rejection, the reduced amplitude of the torque ripple and a positive net contribution to the starting torque.

According to the present technology, it is suggested to use another approach during start-up and/ or slowing-down. By short-circuiting the armature windings and instead producing a rotating magnetic field by means of the excited salient poles of the rotor, a number of advantages are achieved. 11 In one embodiment, an electrical machine comprises a stator having armature windings, and a rotor having salient poles and being rotatably arranged with respect to the stator. The electrical machine is preferably a wound field synchronous machine. A rotor excitation system has a rotor winding arrangement and a rotor power supply for exciting the salient poles. A rotor driving unit is configured to control currents provided to the rotor winding arrangement. In a first operation mode, the rotor excitation system is configured for providing a rotating magnetic field by supplying alternating currents to the rotor winding arrangement, while the armature windings are short-circuited or closed on external resistors.

In a preferred embodiment, the electrical machine further comprises a stator excitation system comprising the armature windings, and a stator power supply arrangement configured to provide armature currents to the armature windings. In a second operation mode, the rotor excitation system is configured for providing a static, with respect to the rotor, magnetic field. This is achieved by the rotor driving unit supplying direct currents to the rotor winding arrangement. The stator power supply arrangement is configured for providing armature currents for causing the armature windings to provide a rotating magnetic field. The electrical machine further comprises means for switching between the first operation mode and the second operation mode.

The first operation mode is typically a start-up phase and the second operation mode is typically a steady-state synchronous operation phase.

Figure 3A illustrates schematically a part of an embodiment of an electrical machine 1 in a particular moment during a start-up. The rotor windings 14 are supplied with an alternating current, which in this particular moment created a magnetic field 25 with a south pole S in front of one of the salient poles 12 and a north pole N in front of a next neighbouring one of the salient poles 12. A magnetic circuit, as illustrated by the line 26 is created via the stator 20. The rotor 10 is rotating with a speed (of and the alternating currents to the rotor windings 14 are such that the so created magnetic field 25 rotates 12 in an opposite direction with a speed (oe. The magnetic field 25 rotation (oe is slightly higher that the rotor speed (of, which results in that the magnetic field 25 rotated with a low speed (oe - (of with respect to the stator 20. This slowly rotating magnetic field 25 induced currents in the short-circuited armature having the same low frequency. These induced currents generate an armature rotating magnetic field which is in synchronism with the rotor magnetic rotating field. This results in turn in an accelerating torque is applied to the rotor 10, thereby increasing the rotational speed of the rotor 10.

Figure BB illustrates another particular moment of the electrical machine 1 of Figure 3A. The rotor 10 has rotated somewhat and the magnetic field 25 has rotated in the opposite direction. This is achieved by using two pairs of adjacent salient poles 12 together forming a south pole S and a north pole N, respectively, controlled by the alternating current through the rotor windings 14. The low speed (oe - (of moving of the magnetic field with respect to the stator still gives rise to an accelerating torque.

In this mode, the machine armature is not energized with any external power supply. Instead, the machine armature is kept short-circuited, typically without using any additional resistance. However, in particular cases, the machine armature may be closed on external resistors. This can be used when, in spite of the very low frequency of the armature currents, the stray reactances of the stator phases are large enough to prevent the building of active currents. This would in turn decrease the accelerating torque and therefore, the overall resistance of the armature windings are artificially increased by adding external resistors.

This solution does not make use of any single-phase circuits during the asynchronous start made. Therefore, it does not suffer from the problem of large ripple on the accelerating torque. Moreover, while the armature rotating magnetic field in traditional asynchronous start senses the magnetic saliencies of the rotor by producing a nasty ripple in the accelerating torque, the rotating field generated from the rotor according to the present technology lO 13 does not sense any saliency on the armature. These facts in turn solve the problem of the stress produced on the synchronous machine frame and shaft during an asynchronous start.

The above presented start-up mode typically makes use of a frequency converter. However, unlike prior art frequency converters, it is connected to the rotor windings rather than to the armature one. It provides two functions at the same time - the function as eXcitation system and the function of starting the drive.

The approach of using the rotor windings for creating the rotating magnetic field has further advantages. The induced currents during start-up are produced in the armature phases rather than in the damper bars or in the short-circuited field winding as in other approaches. The favorable position of the armature windings for dissipating the heat, normally in the outer part of the machine, and their robustness mean a reduction of the machine thermal stress during the start-up and thereby a reduced failure risk.

Furthermore, the applied voltage to the multiphase field winding arrangement can be controlled by a frequency converter, so that no inrush current can arise during the start period.

The rotating magnetic field is produced by the phase currents in the rotor winding. In case the rotor is a salient pole one, no reluctance torque can be produced by the rotor currents, since the mutual inductances between the rotor phases do not change depending on the rotor angular position. However, the mutual inductances between the stator phases still change according to the angular rotor position when the rotor presents salient poles. Nevertheless, the currents induced in the stator phases are essentially the active ones, neglecting the effect of the armature stray inductance, since the magnetization current is now provided by the rotor currents. Due to this fact, the reluctance torque produced by the variation of the armature mutual inductances is drastically reduced by the lack of the magnetization current. 14 The rotor phase current may be kept constant Whilst increasing its frequency. This results in that one may achieve the same constant frequency of the armature induced currents in spite of the changing rotor speed. This kind of drive ensures a constant excitation flux in the motor and a constant reaction current in the rotor Winding. Both evidence of a constant amplitude of the induced armature currents. As a consequence of this, the accelerating average torque is also constant.

Figure 4 is a floW diagram of steps of an embodiment of a method for operating an electrical machine. The electrical machine is preferably a Wound field synchronous machine. A first operation mode comprises the steps S10 and S20. In step S10, armature Windings of a stator are short-circuited or closed on external resistors. In step S20, a rotating magnetic field is provided from a rotor having salient poles and being rotatably arranged With respect to the stator. The step S20 of providing the rotating magnetic field from the rotor in turn comprises the step S22, in Which alternating currents are supplied to a rotor Winding arrangement of a rotor excitation system for exciting the salient poles.

In step S30, a sWitching between the first operation mode and a second operation mode is performed.

The second operation mode comprises the steps S50 and S40. In step S40, a static magnetic field is provided from the rotor. The magnetic field is static With respect to the rotor. The step S40 of providing the static magnetic field in turn comprises the step S42, in Which direct currents are supplied to the rotor Winding arrangement. In step S50, a rotating magnetic field is provided from the stator. This step S50 in turn comprises the step S52, in Which armature currents are provided to the armature Windings.

In preferred embodiments, the arrangements of the rotor Windings are adapted for giving an as simple transfer in step S30 between the above described first operation mode, i.e. typically the start-up phase, and the second operation mode, i.e. typically the steady-state synchronous operation.

Figure 5 illustrates an embodiment of the step S30 of a switching between the first and second operation modes. In a typical process, the rotor is originally in a stationary condition with the field windings de-eXcited. The armature winding is short-circuited or connected via external resistances. The first operation mode is then entered, and the rotor is accelerated by use of AC eXcitation of the field windings. An accelerating torque is generated, increasing the rotating speed of the rotor.

The acceleration is preferably continued until a hypersynchronous speed is reached, typically 10-20% higher than a nominal steady-state rotational speed. The switching of step S30 then begins. In step S32, the rotor is de- excited. The rotor then starts to slow down, and the armature currents fade away. When the armature currents are small enough, preferably zero, the armature is made open-circuited in step S34. At this point, the rotor is also DC-excited, as illustrated by step S40, which will be kept into the completion of the second operation mode, as illustrated by the dotted line. Hypersynchronous EMFs are induced in the open phases of the armature. The speed of the rotor decreases now faster due to the iron losses produced by the rotor eXcitation. When the rotor reaches the synchronous speed and the armature EMFs are in phase with the EMFs of the grid, the second operation mode is entered. The armature is in step S50 galvanically connected to the grid and the synchronization process is completed.

Figure 6 illustrates schematically an embodiment of an electrical machine 1. Most parts are similar to the embodiment of Figure 1. A stator 20 has armature windings 24. A rotor 10 has salient poles 12 and is rotatably arranged with respect to the stator 20. A rotor eXcitation system 30 has a rotor winding arrangement 14 and a rotor power supply 34 for exciting the salient poles 12. The rotor eXcitation system 30 further comprises a rotor driving unit 32 conf1gured to control currents provided to the rotor winding arrangement 14. 16 In a first operation mode, the rotor excitation system 30 is configured for providing a rotating magnetic field by supplying alternating currents to the rotor winding arrangement 14. At the same time, the armature windings 24 are short-circuited or closed on external resistors.

A stator excitation system 40 comprises besides the armature windings 24, also a stator power supply arrangement 42. The stator power supply arrangement is configured to provide armature currents to the armature windings 24. In a second operation mode, the rotor excitation system 30 is configured for providing a static magnetic field, with respect to the rotor 10. This static magnetic field is achieved by the rotor driving unit 32, supplying direct currents to the rotor winding arrangement 14. The stator power supply arrangement 42 is configured for providing the armature currents to cause the armature windings 24 to provide a rotating magnetic field. There are also means for switching between the first operation mode and the second operation mode, as will be discussed further below.

In order to be able to control the currents supplied to the rotor winding arrangement for controlling the speed of the rotating magnetic field from the rotor in the first operation mode, feedback information is of benefit. A rotor current meter 54 is an obvious option, whereby a targeted current to the rotor can be compared to an actual current to the rotor. Here, both amplitude and frequency can be considered. Furthermore, in one embodiment, a rotor shaft speed meter 50 is employed. The rotor shaft speed meter 50 is connected to the rotor driving unit 32 and may give the rotor driving unit 32 information about the actual rotor speed. This information can then be used for selecting appropriate frequencies of the applied rotor winding currents.

Alternatively, or in combination, an armature current-frequency meter 52 may be provided, measuring e.g. the frequency of zero passing in the armature windings. The armature current-frequency meter 52 is connected to the rotor driving unit 32 for providing information about such frequency measures. 17 As Was discussed above, the first operation mode is a mode intended for changing the speed of the rotor. Therefore, in one embodiment, the rotor driving unit 32 is conf1gured to provide the rotor 10 With a torque. This is performed by supplying alternating currents to the rotor Winding arrangement 14 having a different frequency than a present rotor frequency.

The typical situation in the first mode is an acceleration of the rotor 10, however, also controlled retardation of the rotor 10 may alternatively be performed.

Upon accelerating the rotor 10, the rotor driving unit 32 is conf1gured to provide the rotor 10 With a torque in a same direction as a present rotor rotation direction. This is performed by supplying alternating currents to the rotor Winding arrangement 14 having a higher frequency than a present rotor frequency.

In a preferred embodiment, the rotor driving unit 32, in the first operation mode, is conf1gured to accelerate the rotor 10 by supplying alternating currents to the rotor Winding arrangement 14 of a frequency giving a constant frequency of an induced electromotive force in the armature Windings 24.

When changing operation mode, it is preferred if any physical changes only are performed at the stator side. Changes at the rotor side may of course be feasible e.g. by using Wireless control approaches. HoWever, it is preferred to have all changes at the stator side. This put some restriction on how to design the rotor pole eXcitation.

Figure 7A illustrates schematically a two-phase embodiment of a rotor 10. The illustration is made With the salient poles 12 of the rotor 10 visualized in a linear fashion, in order to facilitate the understanding. The rotor driving unit 32 is conf1gured for the second operation mode, i.e. steady-state synchronous operation. Four sets 15 of strands are energizing the rotor 10, causing every second salient pole to be a north pole N and every other second pole to be a 18 south pole S. The distance between adjacent poles, i.e. the pole pitch, is denoted as t. This is caused by applying a constant voltage from a constant voltage supply 31 to each set 15 of strands. The physical interface between the movable rotor 10 and the stationary rotor driving unit 32 is illustrated by the dotted line 21.

Figure 7B illustrates schematically the same embodiment as in Figure 7A, but in the first operation mode. Note that no physical changes are made at the rotor 10 side. Instead of providing a constant current to the rotor windings, alternating currents are provided to the set 15 of strands with alternating current supplies 33. Neighbouring rotor windings are provided with alternating currents in quadrature. In other words, the current iQ is phase shifted 90 degrees compared to iß, and i-Q and i-ß, have the opposite phases compared to iQ and iß, respectively. This connection scheme gives rise to a moving (rotating) magnetic field with a pole distance twice as large 2t as for the second operation mode. The notation of the poles refers to a situation where the instantaneous current has a same magnitude in all rotor windings.

Figure 7C illustrates schematically the same embodiment as in Figure 7B, but with the pole notations for a situation where the current iß is at its minimum, which means that iQ and i- q are instantaneous zero and i-ß at its maximum.

The pole pitch is always 2t.

The present technology thereby presents special rotor windings for a salient pole synchronous machines in order to produce a rotating magnetic field from the rotor side showing a reduced set of pole-pairs, during a start-up phase. In the particular embodiment presented here above, the asynchronous start of a synchronous machine utilizes at least four poles and two parallel strands per phase by AC supplying its special multiphase excitation winding arrangement.

The present technology proposes preferably a rotor winding with x phases, where x being an integer number, which is able to produce with respect to the rotor either an alternated sequence of static N and S magnetic rotor poles over lO 19 p-pole pairs or a rotating sequence of alternated N and S magnetic rotor poles over p / X-pole pairs.

Figures 7A-C show the application of these ideas to the case of a rotor of a salient pole synchronous machine having p=4 pole-pairs. A two-phase winding provides either 8 static poles or a rotating sequence of 4 poles. Figure 7A shows how it is possible to obtain the static 8-poles needed by the synchronous machine for its eXcitation during the normal behavior in steady- state. The two DC currents needed for producing the static magnetic sequence of poles, are shown in the same picture in direction. Figures 7B-C show how it is possible to obtain the rotating magnetic sequence of 4-poles from the same rotor winding. The two AC currents in quadrature ia and iß, needed for producing the rotating magnetic sequence of 4 poles, are shown in the same picture.

In other words, the embodiment of Figures 7A-C illustrates a rotor having n sets of strands of rotor windings, where n=2, and wherein each set of strands constitute a phase in the first operation mode. Each set of strands of rotor windings comprises m strands of rotor windings, where m=2, wherein each strand of rotor windings excites every 4:th salient pole of the rotor. In the first operation mode, the two strands of rotor windings in each set of strands of rotor windings are provided with alternating currents of opposite instant direction. Preferably, in the second operation mode, the two strands of rotor windings in each set of strands of rotor windings are provided with direct currents of the same direction.

Figure 8 is a diagram illustrating the two currents iQ and iß, phase shifted 90 degrees. At the time indicated by the line 100, both currents are instantaneously the same and this corresponds to the situation of Figure 7B. At the time indicated by the line 102, iQ is zero and this corresponds to the situation of Figure 7C.

Figure 9A illustrates schematically a three-phase embodiment of a rotor 10. The illustration is made with the salient poles 12 of the rotor 10 visualized in a linear fashion, in order to facilitate the understanding. The rotor driving unit 32 is conf1gured for the second operation mode, i.e. steady-state synchronous operation. Three sets 15 of strands are energizing the rotor 10, causing every second salient pole to be a north pole N and every other second pole to be a south pole S. The distance between adjacent poles is denoted as t. This is caused by applying a constant voltage from a constant voltage supply 31 to each set 15 of strands. The physical interface between the movable rotor 10 and the stationary rotor driving unit 32 is illustrated by the dotted line 21.

Figure 9B illustrates schematically the same embodiment as in Figure 9A, but in the first operation mode. Note that no physical changes are made at the rotor 10 side. Instead of providing a constant current to the rotor windings, alternating currents are provided to the set 15 of strands by alternating current supplies 33. Neighbouring rotor windings are provided by alternating currents with 120° phase difference. In other words, the current iQ is phase shifted 120 degrees compared to iß, and iß is phase shifted 120 degrees compared to iv. This connection scheme gives rise to a moving (rotating) magnetic field with a pole distance three times as large 3t as for the second operation mode. The notation of the poles refers to a situation where the instantaneous current has a maximum magnitude for iQ.

Figure 9C illustrates schematically the same embodiment as in Figure 9B, but with the pole notations for a situation where the momentary current iQ is zero, which means that iß and iv have the same instantaneous magnitude, but opposite signs. The pole distance is always St.

In other words, Figures 9A-C show an embodiment of a rotor of a salient pole synchronous machine having p=3 pole-pairs. Three-phase windings providing either 6 static poles or a rotating sequence of 2 poles. It is thus possible to obtain the static 6-poles needed by the synchronous machine for its excitation during the normal behavior in steady-state. The three DC currents needed for 21 producing the static magnetic sequence of poles are shown in Figure 9A with its direction. It is also possible to obtain the rotating magnetic sequence of 2- poles from the same rotor winding. The three symmetric AC currents iQ, iß and iv, needed for producing the rotating magnetic sequence of 2 poles, are shown in Figures 9B and 9C.

In other words, the embodiment of Figures 9A-C illustrates a rotor having n sets of strands of rotor windings, where n=3. Each set of strands of rotor windings comprises one strand of rotor windings. The strand of rotor windings eXcites every 3:rd salient pole of the rotor. In the first operation mode, the strands of rotor windings are provided with alternating currents phase-shifted by 120 degrees with respect to a neighbouring salient pole.

Preferably, in the second operation mode, each strand of rotor windings is provided with direct currents of the same direction.

Figure 10 is a diagram illustrating the three currents iQ, iß and iv, phase shifted 120 degrees. At the time indicated by the line 104, iQ is at its maximum and this corresponds to the situation of Figure 9B. At the time indicated by the line 106, iQ is zero and this corresponds to the situation of Figure 9C.

At the common point of the winding strands in Figures 7A-C, 9A-C, it is possible to connect a terminal in order to collect the possible unbalance of the currents. In the 2-phase solution of Figures 7A-C, which has four phase terminals and a common point terminal, it can be observed that the phase currents add up to zero for both operation modes. In the 3-phase solution of Figures 9A-C, which has three phase terminals and a common point terminal, the latter collects the sum of the three equal DC currents in the second operation mode.

The above presented embodiments may be further generalized. In one embodiment, the rotor has N salient poles, where N is twice a product of two lO 22 integers n, k, Where n>1 and kzl, N=2nk, and Wherein the rotating magnetic field has N/n poles.

In a further embodiment, the rotor Winding arrangement comprises n sets of strands of rotor Windings. Each set of strands constitute a phase in the first operation mode. The rotor driving unit, in the first operation mode, is then configured to supply each set of strands of rotor Windings separately With an alternating current.

In a further embodiment, each set of strands of rotor Windings comprises m strands, Where mzl, and eXcites every n:th salient pole of the rotor.

In a further embodiment, the rotor driving unit, in the first operation mode, is configured to supply each set of strands of rotor Windings separately With alternating currents phase shifted 360/ n/ m degrees With respect to the set of strands of rotor Windings of adjacent salient poles.

Both configurations in Figures 7B-C and 9B-C are able to generate a rotating magnetic field Which shows a pole pitch twice or three times as large as that of the armature Winding, respectively. This fact has consequences When the synchronous machine is used With the rotor providing a rotating magnetic field, as for example during an asynchronous start of the machine by feeding the rotor Winding With alternating currents. A dissymmetry is created in the armature induced electromotive forces (EMFs) due to the reduced set of pole- pairs.

In the second operation mode, i.e. the normal synchronous machine operation, a three-phase armature Winding Will experience rotating salient poles as illustrated in Figure 11. Three parallel strands, illustrated in full, broken and dotted lines, respectively, are assumed here, all experiencing three phases, phase shifted by 120 degrees. The first strand is associated With the voltages VA, VB and VC, the second strand is associated With the voltages VU, Vv and VW and the third strand is associated With the voltages VX, Vy and V; lO 23 This is in accordance With a typical prior-art synchronous machine. The rotor Winding is here supplied With DC current.

However, the change in pole pitch during the first operation mode, results in another behaviour. The fundamentals of the electromotive forces (EMFs) induced in the 2k- or Bk- parallel strands (With k being an integer) of the armature phases do not form a symmetric system of voltages anymore. This is illustrated in Figures 12A and 12B, respectively, for a 2-phase and 3-phase rotor Winding setup, respectively. In Figure 12A, the armature has 2k-strands in parallel and the rotor is supplied With 2-phase currents. Preferably, in the first operation mode, every second strand of the armature Windings are short- circuited With each other or closed on external resistors. In Figure 12B, the armature has Bli-strands in parallel and the rotor is supplied With 3-phase currents. Preferably, in the first operation mode, all the armature Windings are short-circuited With each other or closed on external resistors.

In order to obtain a symmetric system of armature EMFs on the armature side, even though a rotating magnetic field With a reduced set of pole pairs is produced by the rotor, the 3X2Xk strands relative to two-phase fed rotor solution must be rearranged as a double 3-phase Winding (Figure 13A) according to the strand sequences (A, C, V) and (U, W, B).

For the three-phase fed rotor instead the 3X3Xk strands must be rearranged as a nine phase Winding (Figure 13B) or triple three phase Winding according to the strand sequences (A, U, X), (B, V, Y) and (C, W, Z).

The first operation mode is typically a start of the salient pole synchronous machine. In a two-phase system, e.g. as illustrates in Figures 7B-C, by supplying the rotor Winding by two AC currents ia and iß having the same amplitude, being in quadrature and showing a frequency f, a rotating magnetic field With mechanical relative frequency to the rotor: ff=2f/P (1) lO 24 is generated. If n is assumed to be the mechanical rotor speed in rpm, the mechanical frequency of the rotating magnetic field With reference to the stator is: fs=2f/P- 11/60- (2) Since the rotor presents p/2 pole pairs, the electrical frequency fsß of the induced EMFs in the stator is: fs,e=Pfs/2=f-f1v/2, (3) Where fN represents the electrical frequency related to the rated rotor speed.

In the same Way, considering the eXcitation Winding of Figures 9B-C, by using a three phase system of currents io: , iß and iy having the same amplitude, being displaced of 120° from each other and showing a frequency f , the electrical frequency fw of the induced EMFs is: fs,e=f-f1v/3, (4) Where f N represents the electrical frequency related to the rotor speed.

Preferably, the machine is started asynchronously With a constant accelerating electromagnetic torque. This is illustrated in Figure 14. The figure illustrates electromagnetic torque provided by the synchronous machine With the armature Windings short-circuited or closed on external resistances When the rotor provides a rotating magnetic field at different rotor speed, expressed by the rotor slip s, so to keep the frequency of the induced currents in the armature phases constant and equal to 0f1v/ x.

The driving technique called "volt over hertz constant" is applied here. It consists in injecting two- (x=2) or three-phase (X=3) AC currents With constant amplitude into the field windings of Figures 7B or 9B, respectively, and increasing progressively their frequency proportionally to the rotor increasing speed. This is performed so to keep the frequency of the armature induced EMFs constant: fw = apnR/öOx = const, (5) where 0 is the constant slip (3-10%) between the rotating magnetic field produced by the rotor and the rotor speed at the rotor rated speed nR.

Since the slip s decreases from 1 to O during the acceleration of the rotor (s=1 rotor at stand still and s=O rotor at the synchronism), o fN is assumed to be the constant frequency of the currents induced in the armature phases. The frequency of the AC excitation currents injected into the rotor is then to be updated according to: f=(1+0-S)f1v/X- (6) The principle schemata of Figures 15-18 show the driving armature arrangements for implementing the present invention in the case of a two phase and three phase excitation system respectively. Figure 15 illustrates armature connections for a 2-phase excitation system. Figure 16 illustrates armature connections with external resistors 80 for a 2-phase excitation system. Figure 17 illustrates armature connections for the 3-phase excitation system. Figure 18 illustrates armature connections with external resistors 80 for the 3-phase excitation system. In the arrangement of Figure 15, the two double three-phase armature systems ACV and UWB are short-circuited separately. Since the targeted frequency in the armature phases is equal to eq. (5), by choosing: Û-:Rarmature/ (LarmatureQÜfN/X) lO 26 the maximal asynchronous torque is achieved during the start procedure, With no need of additional external armature resistances. In case the armature reactance should be very large in comparison to the armature resistance additional external resistances can help to increase the value of 0 artificially.

The actual value of the frequency fis for the induced armature currents is preferably detected by counting the number of zero-crossing for just one phase current in the time unit. The current in the phase can be sensed e. g. by a Hall sensor or by a shunt resistor. The detected frequency value is compared With the desired frequency value for the armature currents fshouzd given by eq. (5)- At the same time the rotational speed of the shaft is preferably sensed by a tachometer or via another equivalent device and is converted in electrical frequency. The rotor slip is determined as: s=1 - np/öOfzv, (8) Where f N represents the rated frequency of the machine. Through equation (6), the frequency of the excitation currents ia and iß can be determined so to instruct the driver unit how to set the frequency of the injected rotor currents. Another sensor may sense the amplitude of the rotor currents, so to keep their amplitudes constant by a negative feedback control loop.

In the arrangement of Figure 17, the triple three-phase armature systems AUX, BVY and CWZ are short-circuited altogether since they represent a nine phase system. The targeted frequency in the armature phases is still given by eq. (5) Where 0 is determined by eq. (7). Therefore, in this case too there is typically no need for external additional armature resistances, but they can be used When the resistance to reactance ratio should be too low.

The actual value of the frequency for the armature currents fis is preferably detected by counting the number of zero-crossing for just one phase current in the time unit. The detected frequency value is then compared With the desired frequency value for the armature currents fshouzd given by eq. (5). At lO 27 the same time the rotational speed of the shaft is preferably sensed by a tachometer or equivalent device and converted in electrical frequency so to determine the rotor slip s as it has been seen in eq. (8). Finally, through equation eq. (6) the frequency of the excitation currents io., iß and iv can be determined so to instruct the driver unit how to set the frequency of the injected rotor currents. Another sensor may sense the amplitude of the rotor currents so to keep their amplitudes constant by a negative feedback control loop.

In a method point of view, in one embodiment, in the method for operating an electrical machine, the rotor has N salient poles, where N is twice a product of two integers n, k, where n>1 and kzl, N=2nk, and wherein the rotating magnetic field has N / n poles.

Preferably, the rotor winding arrangement comprises n sets of strands of rotor windings, wherein each set of strands constitute a phase in the first operation mode. The step of providing, in the first operation mode, a rotating magnetic field from the rotor comprises supplying each set of strands of rotor windings separately with an alternating current.

Preferably, each set of strands of rotor windings comprises m strands, where szl, and eXcites every n:th salient pole of the rotor.

In order to perform preferred start-up procedures, the method for operating an electrical machine comprises preferably measuring of a rotor shaft speed. The step of supplying alternating currents to the rotor windings is then performed in response to the measured rotor shaft speed.

In order to perform preferred start-up procedures, the method for operating an electrical machine comprising preferably measuring of an armature current frequency. The step of supplying alternating currents to the rotor windings is then performed in response to the measured armature current frequency. lO 28 For changing a speed of the rotor, preferably, the step of supplying a1ternating currents to the rotor windings comprises supplying of a1ternating currents to the rotor winding arrangement having a different frequency than a present rotor frequency, thereby providing the rotor with a torque.

In the case the speed is to be increased, the torque has to be provided in a same direction as the rotor rotation. To this end, the step of supplying a1ternating currents to the rotor windings preferably comprises supplying a1ternating currents to the rotor winding arrangement having a higher frequency than a present rotor frequency. Thereby, the rotor is provided with a torque in a same direction as a present rotor rotation direction.

Furthermore, in a preferred embodiment, the step of supplying a1ternating currents to the rotor windings comprises supplying a1ternating currents to the rotor winding arrangement of a frequency giving a constant frequency of an induced e1ectromotive force in the armature windings, thereby acce1erating the rotor.

In one embodiment, the step of supplying a1ternating currents to the rotor windings comprises supplying each set of strands of rotor windings separately with a1ternating currents phase shifted 360/ n/ m degrees with respect to the set of strands of rotor windings of adjacent sa1ient po1es.

In a further embodiment, where n=2, each set of strands of rotor windings comprises two strands of rotor windings, i.e. m=2. Each strand of rotor windings eXcites every 4:th sa1ient pole of the rotor. The step of supplying a1ternating currents to the rotor windings then comprises providing the two strands of rotor windings in each set of strands of rotor windings with a1ternating currents of opposite instant direction.

In a further embodiment, the step of supplying direct currents to the rotor winding arrangement comprises providing the two strands of rotor windings 29 in each set of strands of rotor windings with direct currents of the same direction.

In one embodiment, in the first operation mode, every second strand of the armature windings are short-circuited with each other or closed on external resistors.

In one embodiment, where n=3, each set of strands of rotor windings comprises one strand of rotor windings. The strand of rotor windings then eXcites every 3:rd salient pole of the rotor. The step of supplying alternating currents to the rotor windings then comprises providing the strands of rotor windings with alternating currents phase-shifted by 120 degrees with respect to a neighbouring salient pole.

In a further embodiment, the step of supplying direct currents to the rotor winding arrangement comprises providing the two strands of rotor windings in each set of strands of rotor windings with direct currents of the same direction.

In a further embodiment, in the first operation mode, all the armature windings are short-circuited with each other or closed on external resistors.

As can be seen in Figures 7A-C, a number of connections pass between the rotor and stator. Figure 19 illustrates schematically four rotor winding strands numbered from 1 to 4 for a set of 4(k+1) poles. Each strand includes the series of (k+1) homopolar pole windings taken as it follows: strand 1 : 1+5+9+...+ (4k+1) strand 2 : 2+6+10+...+ (4k+2) strand 3 : 3+7+11+...+ (4k+3) strand 4 : 4+8+12+...+ (4k+4).

A slip ring arrangement 90 comprises four rings (1-4), one for each strand, and an additional center ring (C) connected to the common point of the rotor winding strands.

During the AC behavior, i.e. the first operation mode, the current ia entering the strand 1, illustrated by full line arrows, exits from the strand 3 whereas the current iß entering the strand 2 exits from the strand 4. In this way there is ideally no current coming out from the center point "C".

During the DC operation, i.e. the second operation mode, the DC current ia entering the strand 1 and 3 equals the DC current iß exiting the strand 2 and 4, illustrated by the broken line arrows, so that also in this mode, ideally no current emerges from the center point "C". The small slip-ring C is anyway foreseen for providing the degaussing of the magnetic circuit with small unbalances between the currents if needed.

The rotor winding of Figure 20 shows corresponding three strands numbered from 1 to 3 for a set of 3(k+1) poles. Each strand includes the series of (k+1) heteropolar pole windings taken as it follows: strand 1 : 1+4+7+...+ (3k+1) strand 2 : 2+5+8+...+ (3k+2). strand 3 : 3+6+9+...+ (3k+3) During the AC phase, i.e. the first operation mode, the currents ia, iß and iv entering the strands 1, 2 and 3, full line arrows, respectively, add up to zero so that there is no current coming out from the center point C and the large slip-ring does not bear any current.

During the DC phase, i.e. the second operation mode, three equal DC eXcitation currents, illustrated by broken line arrows, enters the three strands and a three times larger current emerges from the center point C. In this case the larger slip-ring allows the coming back to the supplying bridge of the three excitation currents injected into the rotor. lO 31 The technology presented here has several advantages. The eXcitation system, which is an already present and indispensable unit in present synchronous machine, performs the additional function of AC driving the rotor. There is no need of additional devices, as is common in many other start-up approaches, such as pony, VFD, reactor, autotransformer, dedicated turbine, etc. A more flexible eXcitation system that is capable of AC supplying the rotor winding can offer additional benef1cial performances such as the unbalanced magnetic pull compensation, active damping of the rotor oscillation, negative sequence harmonics energy recovery.

At the same accelerating torque, and therefore at the same starting time, compared to a traditional asynchronous start by means of damper bars, the present technology offers a more attractive power factor all over the starting process. Less reactive power is needed. The present ideas present a higher energy efficiency, much larger than 50%. In comparison, in a traditional asynchronous start, 50% is a theoretical limit which is never attained in practice. Preferred embodiment of the present technology also ensure a unipolar accelerating torque presenting a much lower torque ripple than in prior art. Furthermore, these arrangements also dissipate the thermal energy related to the asynchronous start in the machine armature instead of in the rotor. That prevents thermal stress and failure by making a more rational use of the machine cooling performance.

The obvious advantages of the present technology make it very attractive for many applications. Pumped Storage Hydro Power Plants (PSHPP) is one candidate. In PSHHP using a reversible turbine WT and showing low short circuit power at the bus bar, the present technology can provide the start and synchronization of the synchronous motor avoiding eXcessive voltage drop at the connection point. Furthermore, it eliminates the thermal stress of the electrical machine and prevents the eXcitation of vibrational modes in the shaft and in the structure. lO 32 In a Turbo Gas Generator (TGG) , the Brighton's cycle used in a gas turbine uses about 2/3 of the power delivered by the turbine for driving the Compressor. At the start, when the turbine is off, the mechanical work for the Compressor must be provided by a combustion or electric engine, a pony, or by the synchronous generator driven asynchronously or by a VFD. The group compressor does not need to be driven up to the rated speed but up to the f1ring speed which is around 40-50% of the rated speed. Without any need of additional units, the work for the compressor could be delivered by motoring the synchronous generator (SG) by the present technology. The fact that the f1ring speed is smaller than half the rated speed it reduces the rated power for the innovative excitation system.

Large Uninterruptible Power Source (UPS) units making use of a SG need a prime mover, usually a Combustion Engine (CE) in order to be started. The start is usually provided by a small sized DC motor geared on the flywheel of the CE. By making use of the present technology, both the starter motor and the gearing could be avoided. In this application a few percent of the rated speed must be attained for ensuring the CE start. This fact reduces drastically the size of the needed power inverter excitation system.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modif1cations, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims (34)

1. An electrical machine (1), comprising: - a stator (20) having armature windings (24); - a rotor (10) having salient poles (12) and being rotatably arranged with respect to said stator (20) ; - a rotor excitation system (30) having a rotor winding arrangement (14) and a rotor power supply (34) for exciting said salient poles (12), and a rotor driving unit (32) configured to control currents provided to said rotor winding arrangement (14); wherein, in a first operation mode, said rotor excitation system (30) being configured for providing a rotating magnetic field (25) by supplying alternating currents to said rotor winding arrangement (14), while said armature windings (24) being short-circuited or closed on external resistors (so).

2. The electrical machine according to claim 1, characterized by further comprising: a stator excitation system (40) comprising said armature windings (24), and a stator power supply arrangement (42) configured to provide armature currents to said armature windings (24); wherein, in a second operation mode, said rotor excitation system (30) being configured for providing a static, with respect to said rotor (10), magnetic field by said rotor driving unit (32) supplying direct currents to said rotor winding arrangement (14), and said stator power supply arrangement (42) being configured for providing said armature currents to cause said armature windings (24) to provide a rotating magnetic field; wherein said electrical machine (1) further comprises means for switching between said first operation mode and said second operation mode.

3. The electrical machine according to claim 1 or 2, characterized in that said rotor (10) has N salient poles, where N is twice a product of two integersn, k, Where n>1 and kzl, N=2nk, and Wherein said rotating magnetic field (25) has N/n poles.

4. The electrical machine according to claim 3, characterized in that said rotor Winding arrangement (14) comprises n sets (15) of strands of rotor Windings, Wherein each set (15) of strands constitute a phase in said first operation mode, Wherein said rotor driving unit (32), in said first operation mode, is configured to supply each set (15) of strands of rotor Windings separately With an alternating current.

5. The electrical machine according to claim 4, characterized in that each set (15) of strands of rotor Windings comprises m strands, Where mzl, and eXcites every n:th salient pole of said rotor.

6. The electrical machine according to any of the claims 1 to 5, characterized by further comprising a rotor shaft speed meter (50), connected to said rotor driving unit (32).

7. The electrical machine according to any of the claims 1 to 6, characterized by further comprising an armature current-frequency meter (52), connected to said rotor driving unit (32).

8. The electrical machine according to claim 6 or 7, characterized in that said rotor driving unit (32), in said first operation mode, is configured to provide said rotor (10) With a torque by supplying alternating currents to said rotor Winding arrangement (14) having a different frequency than a present rotor frequency.

9. The electrical machine according to claim 8, characterized in that said rotor driving unit (32), in said first operation mode, is configured to provide said rotor (10) With a torque in a same direction as a present rotor rotation direction by supplying alternating currents to said rotor Winding arrangement (14) having a higher frequency than a present rotor frequency.

10. The electrical machine according to claim 9, characterized in that said rotor driving unit (32), in said first operation mode, is configured to accelerate said rotor (10) by supplying alternating currents to said rotor winding arrangement (14) of a frequency giving a constant frequency of an induced electromotive force in said armature windings (24).

11. The electrical machine according to claim 5 or any of the claims 6 to 10 when being dependent on claim 5, characterized in that said rotor driving unit (32), in said first operation mode, is configured to supply each set (15) of strand of rotor windings separately with alternating currents phase shifted 360/ n/ m degrees with respect to the set (15) of strands of rotor windings of adjacent salient poles (12).

12. The electrical machine according to claim 11, characterized in that n=2, wherein each set (15) of strand of rotor windings comprises two strands of rotor windings, m=2, wherein each strand of rotor windings eXcites every 4:th salient pole (12) of said rotor (10), and wherein, in said first operation mode, said two strands of rotor windings in each set (15) of strands of rotor windings are provided with alternating currents of opposite instant direction.

13. The electrical machine according to claim 12 when being dependent on claim 1, characterized in that in said second operation mode, said two strands of rotor windings in each set (15) of strands of rotor windings are provided with direct currents of the same direction.

14. The electrical machine according to claim 12, characterized in that, in said first operation mode, every second strand of said armature windings (24) are short-circuited with each other or closed on external resistors (80).

15. The electrical machine according to claim 11, characterized in that n=3, wherein each set (15) of strand of rotor windings comprises one strand, m=1, of rotor windings, wherein said strand of rotor windings eXcites every3:rd salient pole of said rotor, and Wherein, in said first operation mode, said strands of rotor Windings are provided With alternating currents phase-shifted by 360/ n/ m=120 degrees With respect to a neighbouring salient pole (12).

16. The electrical machine according to claim 15 When being dependent on claim 1, characterized in that in said second operation mode, each strand of rotor Windings is provided With direct currents of the same direction.

17. The electrical machine according to claim 15, characterized in that, in said first operation mode, all said armature Windings (24) are short- circuited With each other or closed on external resistors (80).

18. A method for operating an electrical machine (1), comprising the steps of: - in a first operation mode, short-circuiting (S10) or closing on external resistors (80) of armature Windings of a stator (20) ; - providing (S20), in said first operation mode, a rotating magnetic field (25) from a rotor (10) having salient poles (12) and being rotatably arranged With respect to said stator (20) ; said step of providing said rotating magnetic field (25) from said rotor (10) in turn comprises the step of: - supplying (S22) alternating currents to a rotor Winding arrangement (14) of a rotor excitation system(30) for exciting said salient poles (12).

19. The method according to claim 18, characterized by comprising the further steps of: - in a second operation mode, providing (S50) a rotating magnetic field from said stator (20), in turn comprising the step of: - providing (S52) armature currents to said armature Windings (24); - providing (S40), in said second operation mode, a static, With respect to said rotor (10), magnetic field from said rotor (10);said step of providing (S40) said static magnetic field in turn comprises the step of: - supplying (S42) direct currents to said rotor winding arrangement (14); and - switching (S30) between said first operation mode and said second operation mode.

20. The method according to claim 18 or 19, characterized in that said rotor (10) has N salient poles (12), where N is twice a product of two integers n, k, where n>1 and kzl, N=2nk, and wherein said rotating magnetic field (25) has N/n poles.

21. The method according to claim 20, characterized in that said rotor winding arrangement (14) comprises n sets (15) of strands of rotor windings, wherein each set (15) of strands constitute a phase in said first operation mode, wherein said step of providing (S20), in said first operation mode, a rotating magnetic field (25) from said rotor (10) comprises supplying each set (15) of strands of rotor windings separately with an alternating current.

22. The method according to claim 21, characterized in that each set (15) of strands of rotor windings comprises m strands, where mzl, and eXcites every n:th salient pole (12) of said rotor (10).

23. The method according to any of the claims 20 to 22, characterized by comprising the further step of: - measuring a rotor shaft speed, wherein said step of supplying alternating currents to said rotor windings is performed in response to said measured rotor shaft speed.

24. The method according to any of the claims 20 to 23, characterized by comprising the further step of: - measuring an armature current frequency,Wherein said step of supp1ying (S22) a1ternating currents to said rotor Winding arrangement (14) is performed in response to said measured armature current frequency.

25. The method according to c1aim 23 or 24, characterized in that said step of supp1ying (S22) a1ternating currents to said rotor Winding arrangement (14) comprises supp1ying of a1ternating currents to said rotor Winding arrangement (14) having a different frequency than a present rotor frequency, thereby providing said rotor (10) With a torque.

26. The method according to c1aim 25, characterized in that said step of supp1ying (S22) a1ternating currents to said rotor Winding arrangement (14) comprises supp1ying a1ternating currents to said rotor Winding arrangement (14) having a higher frequency than a present rotor frequency, thereby providing said rotor (10) With a torque in a same direction as a present rotor rotation direction.

27. The method according to c1aim 26, characterized in that said step of supp1ying (S22) a1ternating currents to said rotor Winding arrangement (14) comprises supp1ying a1ternating currents to said rotor Winding arrangement (14) of a frequency giving a constant frequency of an induced e1ectromotive force in said armature Windings (24), thereby acce1erating said rotor (10).

28. The method according to c1aim 23 or any of the c1aims 24 to 27 When being dependent on c1aim 23, characterized in that said step of supp1ying (S22) a1ternating currents to said rotor Winding arrangement (14) comprises supp1ying each set of strands of rotor Windings separate1y With a1ternating currents phase shifted 360/ n/ m degrees With respect to the set of strands of rotor Windings of adjacent sa1ient poles (12).

29. The method according to c1aim 28, characterized in that n=2, Wherein each set of strands of rotor Windings comprises two strands of rotor Windings, m=2, Wherein each strand of rotor Windings excites every 4:th sa1ient pole (12) lOof said rotor (10), wherein said step of supplying (S22) alternating currents to said rotor winding arrangement (14) comprises providing said two strands of rotor windings in each set of strands of rotor windings with alternating currents of opposite instant direction.

30. The method according to claim 29, characterized in that said step of supplying (S42) direct currents to said rotor winding arrangement (14) comprises providing said two strands of rotor windings in each set of strands of rotor windings with direct currents of the same direction.

31. The method according to claim 29, characterized in that, in said first operation mode, every second strand of said armature windings are short- circuited with each other or closed on external resistors (80).

32. The method according to claim 28, characterized in that n=3, wherein each set of strands of rotor windings comprises one strand, m=1, of rotor windings, wherein said strand of rotor windings excites every 3:rd salient pole (12) of said rotor (10), and wherein said step of supplying (S22) alternating currents to said rotor winding arrangement (14) comprises providing said strands of rotor windings with alternating currents phase-shifted by 360/ n/ m=120 degrees with respect to a neighbouring salient pole (12).

33. The method according to claim 32, characterized in that said step of supplying (S42) direct currents to said rotor winding arrangement (14) comprises providing said two strands of rotor windings in each set of strands of rotor windings with direct currents of the same direction.

34. The method according to claim 32, characterized in that, in said first operation mode, all said armature windings (24) are short-circuited with each other or closed on external resistors (80).