WO2015040529A9

WO2015040529A9 - Hybrid operating synchronous electrical machine

Info

Publication number: WO2015040529A9
Application number: PCT/IB2014/064434
Authority: WO
Inventors: Gianluca STANIC; Silvano BONATO
Original assignee: Marelli Motori S.P.A.
Priority date: 2013-09-19
Filing date: 2014-09-11
Publication date: 2015-12-23
Also published as: WO2015040529A3; WO2015040529A2; ITVR20130217A1

Abstract

The present disclosure relates to a synchronous electrical machine (1) that is adapted to alternately work as an electric generator or as an electric motor. The synchronous electrical machine (1) comprises a rotor (2), that is provided with a main rotor winding (25) configured to cooperate with a main stator winding (15), and an excitation system (3) for exciting the main rotor winding (25) to a production of a main rotor magnetic field. The excitation system (3) comprises a first exciter (31), which in the specific case is of conventional type in direct current, and a second exciter (32), which in the specific case is in alternating current. The first exciter (31) is operational when the synchronous electrical machine (1) works as electric generator and it is non-operational when the synchronous electrical machine (1) works as a motor. The second exciter (32) is operational when the synchronous electrical machine (1) works as a motor, by guaranteeing excitation power at each speed; the second exciter (32) is non-operational when the synchronous electrical machine (1) works as electric generator.

Description

HYBRID OPERATING SYNCHRONOUS ELECTRICAL MACHINE

DESCRIPTION

The present disclosure relates in general to the field of the rotating electrical machines. In particular, the present disclosure relates to a synchronous electrical machine that can work both as electric generator and as an electric motor. In other words, it is a "hybrid" operating electrical machine which can be run by an alternator or a motor according to user needs.

In the state of art it is know that, in principle, a synchronous electrical machine can be used as generator and as motor, thanks the operation reversibility; however, a use as motor requires to solve the problem of starting from still position (motor with fixed speed mode) and/or using even a frequency converter (motor with adjustable speed mode).

This problem, in principle, would not be found by using asynchronous electrical machines, but the latter involve the absorption of reactive power from the electric network that is the missed control of the power factor (active-reactive power) which on the contrary is possible with the synchronous machines. For this reason - that is the control of the apparent power - the synchronous machines are preferred for producing electric energy. The present disclosure is then focused on the synchronous electrical machines and on the possibility of hybrid use of such electrical machines.

In order to overcome the difficulties connected to the phase of starting from the still position of the electrical machines used as motors, in some known embodiments the rotor of the electrical machine has been equipped with a damping cage working even as starting cage, that is as "squirrel cage": this allows supplying the torque requested for the starting from the still position. However, such damping cage is extremely expensive from the manufacturing point of view and generally it is little effective as the machine can be run at one single fixed speed - that is the synchronous speed of the electric network.

In other known embodiments, the starting of the synchronous machines as motor takes place by means of a cranking motor (for example a motor in direct current or an asynchronous motor): the cranking motor brings the rotor of the synchronous machine at the synchrony speed, then the cranking motor is disconnected and the synchronous machine is connected to the electric network.

Such configuration is found for example in some sea applications, wherein a diesel motor with constant regime actuates both a propeller and a synchronous electrical machine, which works as electric generator for optimizing the use of the diesel motor. In case the diesel motor breaks down or however is still, the propeller can be actuated by the synchronous electrical machine used as motor, prior starting with a cranking motor.

The need of the cranking motor involves a greater whole cost and a greater complexity both from the plant and the operation point of view, as - apart exceptions - the electrical machine is run at fixed speed and therefore it is not possible optimizing the performances of the moved machine (for example a propeller).

The present disclosure starts from the technical problem of providing a synchronous electrical machine adapted to alternately work as electric generator or as an electric motor and allowing to overcome the drawbacks mentioned above with reference to the known art and/or to obtain additional drawbacks, in particular by simplifying the starting of the electrical machine when it is used as motor and by increasing the versatility thereof, in case being able to run it at variable speed.

The solution to the technical problem is obtained by providing a synchronous electrical machine according to the independent claim 1. Particular embodiments of the subject of the present disclosure are defined in the corresponding depending claims.

One aspect underlying the present disclosure lies in the fact of providing a synchronous electrical machine having two separate exciters for exciting a main winding rotor and producing a main rotor magnetic field. The two exciters, even if they are part of the same machine, are independent therebetween and works alternatively: the first exciter is operational only when the synchronous electrical machine is used as electric generator, whereas the second exciter is operational only when the synchronous electrical machine is used as an electric motor.

This allows optimizing the structure and the operation of each exciter based upon the specific aim and application thereof. For example, the first exciter is of conventional type for the operation of a synchronous electrical machine as electric generator, whereas the second exciter is specific for an operation as an electric motor and it allows even an operation at variable speed by using a frequency converter. In particular, the first exciter is an exciter in direct current and the second exciter is an exciter in alternating current.

Therefore, it is possible providing a synchronous electrical machine which is intrinsically hybrid and it can operate to produce electric power or to produce mechanical power, by avoiding both a pushing motor and the implementation of a starting cage to make the machine to break away at the torque requested at the still position.

In particular, the second exciter is configured to be supplied in alternating current and to create a rotating magnetic field that acts on the rotor winding of the second exciter. In this way an excitation of the main rotor winding can be obtained even when the rotor is still: even when the machine is still one succeeds in producing the main rotor magnetic field which is requested for the operation. Consequently, there is a breakaway torque at the starting moment which is much higher than the one which would be present with a conventional synchronous motor: in fact, already when the rotor is still, there is a rotor magnetic field which tends to set the rotor into rotation.

The main stator winding, in the operation by motor, is supplied with an alternated current with an adjustable frequency thanks to a frequency converter, which is a component which can be provided separately from the machine: upon starting, the main stator winding is supplied with a current with zero frequency and thereafter with alternated current with an increasing frequency until reaching the wished speed.

The combined control of the excitation tension of the second exciter and of the main tension through the frequency converter allows optimizing the torque curves of the operation as motor.

A particular application of the synchronous electrical machine according to the present disclosure is a propelling apparatus, for example in a ship. The propelling apparatus comprises an endothermic engine (for example a diesel motor with constant regime) which actuates a propeller by means of a reduction gear. The endothermic engine actuates even the synchronous electrical machine operating as electric generator which then allows converting the mechanical power not used by the helix in electric power, thus optimizing the operation of the endothermic engine. If the endothermic engine breaks down or if one wishes to use the helix for positioning the ship (which requires to actuate the helix at less and variable speed), the endothermic engine is disconnected from the helix and the latter is actuated by the synchronous electrical machine used as motor. The starting as motor does require any cranking motor and it can be managed easily.

Additional advantages, features and use modes of the subject of the present disclosure will result evident from the following detailed description of embodiments thereof, shown by way of example and not with limitative purpose.

It is however evident that each embodiment of the subject of the present disclosure can have one or more of the above-mentioned advantages; in any case it is not requested that each embodiment has all the listed advantages simultaneously.

The figures of the enclosed drawings will be referred to, wherein:

- figure 1 represents a side view of a synchronous electrical machine according to the present disclosure;

- figure 2 represents a front view of the electrical machine of figure 1 ;

- figure 3 represents a side, partially cross-section view of the electrical machine of figure 1 , wherein some inner components of the electrical machine are partially visible;

- figure 4 represents a perspective view of the body of the electrical machine of figure 1 , therefrom some components have been removed;

- figure 5 represents a side view of the rotor of the electrical machine of figure 1 ;

- figure 6 represents a functional wiring scheme of the electrical machine of figure 1 ;

- figure 7 represents, in an enlarged view, a detail of the functional wiring scheme of figure 6;

- figure 8 represents a front view of a diode-carrying wheel of the electrical machine of figure 1 ;

- figure 9 represents schematically a propelling apparatus according to the present disclosure, comprising an electrical machine of figure 1.

By referring to the enclosed figures, a synchronous electrical machine according to the present disclosure is designated with the reference number 1. As already anticipated, the synchronous electrical machine 1 e adapted to alternately work as an electric generator or as an electric motor, according to the user needs, and therefore has a hybrid operation. In the operation mode as electric generator, the electrical machine 1 receives an inlet mechanical power and it converts it in outlet electric power, in particular in three-phase electric current; in the operating mode as an electric motor, the electrical machine 1 receives an inlet electric power and it converts it in outlet mechanical power.

The electrical machine 1 comprises a body or box-like casing 10 which supports the components of the electrical machine 1 and defines an inner chamber 11 wherein the inner components of the electrical machine 1 are arranged.

Substantially, the body 10 is or comprises a stator. A main stator winding 15 is arranged into the inner chamber 1 1 , which in particular is a three-phase winding. The electrical machine 1 comprises a rotor 2 having a shaft 21 which is mounted in the body 10 and extends through the inner chamber 1 1. The rotor 2 is mounted on suitable bearings, so as to be rotating with respect to the body 10 around a rotation longitudinal axis 200.

The rotor 2 further comprises a main winding rotor 25, that is configured to produce a main rotor magnetic field when it is run through by a direct electric current.

The main stator winding 15 and the main rotor winding 25 are configured to cooperate with each other by electromagnetic induction during the working of the synchronous electrical machine 1.

The constructive details of the main stator winding 15, of the main rotor winding 25 and of the respective portions of stator 10 and or rotor 2, as well as the modes of interacting therebetween and the operation thereof, are not described herein in greater detail and they can be considered of known art and substantially within the comprehension of the person skilled in the art of the field of the synchronous electric generators, in particular generators of three-phase alternating current.

The synchronous electrical machine 1 further comprises an excitation system 3 for exciting the main rotor winding 25, that is for supplying thereto a direct current producing the main rotor magnetic field.

The excitation system 3 comprises a first exciter 31 and a second exciter 32, which are distinct therebetween and they are mounted at a predetermined distance one with respect to the other one along the longitudinal axis 200, so as not to interfere with one another. In other words, the first exciter 31 and the second exciter 32 are independent therebetween.

One aspect underlying the present disclosure is that each one of the two exciters 31 , 32 is dedicated to a respective operation mode of the synchronous electrical machine 1.

In fact, the first exciter 31 is an exciter for the "electric generator" mode: it is operational when the synchronous electrical machine 1 works as electric generator and it is non-operational when the synchronous electrical machine 1 works as a motor. The second exciter 32 is an exciter for the "motor" mode: it is operational when the synchronous electrical machine 1 works as a motor and it is non- operational when the synchronous electrical machine 1 works as electric generator. The passage from an operating mode to the other one and the corresponding activation/deactivation of the two exciters 31 , 32 can be carried out manually by an operator or automatically by a controlling and handling unit with an automatic "switching".

Each exciter 31 , 32 comprises a respective exciter stator winding and a respective exciter rotor winding.

The first exciter 31 comprises a stator winding 41 and a rotor winding 51 ; the second exciter 32 comprises a stator winding 42 and a rotor winding 52.

The stator windings 41 , 42 are mounted in the inner chamber 1 1 of the body 10 and are fastened integrally to the stator. The rotor windings 51 , 52 are mounted on the shaft 21 of the rotor 2 and rotate together with the rotor 2. In other words, each exciter 31 , 32 has a portion which is fastened to the stator 10 and a portion which is fastened to the rotor 2.

In the specific case, the main rotor winding 25, the rotor winding 51 of the first exciter 31 and the rotor winding 52 of the second exciter 32 are mounted on the same shaft 21 of the rotor 2.

In each exciter 31 , 32, the stator winding 41 , 42 and the rotor winding 51 , 52 are configured to cooperate with each other by electromagnetic induction during the working of the synchronous electrical machine 1. In particular, the stator winding 41 , 42 is supplied with an electric current and it generates an excitation magnetic field which, as it will be clearer hereinafter, induces an electric current in the respective rotor winding 51 , 52.

Each rotor winding 51 , 52 of the exciters 31 , 32 is connected to the main rotor winding 25, with interposition of at least a rectifier 35. In particular the rectifier 35, belonging to the excitation system 3, is a diode-bridge rectifier (in the specific case, a diode bridge of Graetz) and it is mounted on a diode-carrying wheel 28 that is jointly rotatable with the rotor 2. It is then a rotating rectifier.

The current induced in a rotor winding 51 , 52 is then rectified and supplied to the main rotor winding 25.

In the specific case, in order to obtain an as much as possible direct rectified current, each rotor winding 51 , 52 of the exciters 31 , 32 has three phases (511 , 512, 513; 521 , 522, 523), connected like a star. The rectifier 35 comprises a diode bridge for each phase. The terminals of each diode bridge are connected to the terminals 251 , 252 of the main rotor winding 25.

In the rectifying circuit there are also a condenser 38 to dampen the oscillations of the rectified voltage and a protector 39 against the overvoltages; these aspects are known on themselves to the person skilled in the art. The terminals of the rectifying circuit are connected to the terminals 251 , 252 of the main rotor winding 25.

The rotor windings 51 , 52 of the two exciters 31 , 32 operate on the same main rotor winding 25. Since during the operation of the electrical machine 1 only one of the two exciters 31 , 32 is operational, it is convenient that the rotor windings 51 , 52 of the two exciters 31 , 32 are separated and independent, in order to avoid unwished currents. Therefore, each exciter 31 , 32 is connected to the main rotor winding 25 with interposition of its own dedicated rectifier: the rotor winding 51 of the first exciter 31 is connected to a first rectifier 351 , the rotor winding 52 of the second exciter 32 is connected to a second rectifier 352. Both rectifiers 351 , 352 are connected to the terminals 251 , 252 of the main rotor winding 25.

Thanks to that, the rotor winding 51 of the first exciter 31 does not communicate with the rotor winding 52 of the second exciter 32, and viceversa. As shown in figure 7, the two rectifiers 351 , 352 are arranged so that there cannot be a current passage between the two rotor windings 51 , 52 of the exciters 31 , 32, as the current passing through one of the two rectifiers 351 , 352 is stopped by the rectifier 352, 351 towards the other rotor winding 52, 51.

Each one of the two rectifiers 351 , 352 comprises three diode-bridges, one for each phase.

The two rectifiers 351 , 352 are mounted on the same diode-carrying wheel 28, that is on the same supporting framework. In order to ease the maintenance operations, the three diode bridges 351 a, 351 b, 351c of the first rectifier 351 are all mounted on half of the diode-carrying wheel 28, whereas the three diode bridges 352a, 352b, 352c of the second rectifier 352 are all mounted on the other half of the diode- carrying wheel 28 (see figure 8).

To increase safety of the electrical machine 1 , each diode bridge is redundant, that is each branch of each diode bridge comprises a pair of diodes 350a, 350b which are arranged in series: if one of the two diodes 350a, 350b of the pair damages and short-circuits, however the other diode 350b, 350a of the pair remains, which continues to carry out its function of rectifying and locking unidirectionally the current.

The two exciters 31 , 32 are different therebetween, being destined to different operation modes.

The first exciter 31 is an exciter in direct current, that is its stator winding 41 is configured to be supplied in direct current and to produce a stationary magnetic field acting on the respective rotor winding 51. During the operation of the electrical machine 1 as electric generator, the rotor 2 is made to rotate by an outer driving force and then the rotor winding 51 of the first exciter 31 rotates with respect to such stationary magnetic field produced by the stator winding 41 : in the rotor winding 51 then there is the electro-magnetic induction that produces the excitation current of the main rotor winding 25.

The first exciter 31 can be an exciter in direct current of conventional type for the synchronous alternators.

The supply in direct current of the stator winding 41 of the first exciter 31 is provided by a voltage regulator 63 ("Automatic Voltage Regulator") belonging to a controlling and managing unit 6 of the electrical machine 1. The voltage regulator 63 receives the power need by a generator with permanent magnets 68 which is mounted on the rotor 2. On the line of the generator with permanent magnets 68, or alternatively on the line going to the stator winding 41 of the first exciter 31 , a group of switches 61 is provided which are closed when the electrical machine 1 works as generator and they are opened when the electrical machine 1 works as motor.

The second exciter 32 is an exciter in alternating current, that is the stator winding thereof 42 is configured to be supplied in alternating current and to produce a rotating magnetic field acting on the respective rotor winding 52. In particular, the stator winding 42 is a three-phase winding.

When the electrical machine 1 has to be worked as motor, the rotor 2 at the beginning is still and then even the rotor winding 52 of the second exciter 32 at the beginning is still with respect to the stator winding 42. In this case, the electromagnetic induction needed to produce the excitation current of the main rotor winding 25 is obtained thanks to the rotation of the magnetic field produced by the stator winding 42, such magnetic field being rotating with respect to the stator 10 and with respect to the rotor winding 52 of the second exciter 32, instead of thanks to the rotation of the rotor with respect to the stator as it happens for the first exciter 31. Therefore, even when the rotor 2 is still, in the rotor winding 52 there is the electro-magnetic induction that produces the excitation current of the main rotor winding 25.

The supply in three-phase alternating current of the stator winding 42 of the second exciter 32 is provided for example by an outer source 69. On the line between the outer source 69 and the stator winding 52 of the second exciter 32, a group of switches 62 is provided which are closed when the electrical machine 1 works as motor and are opened when the electrical machine 1 works as generator.

The electrical machine 1 , in the operation as motor, is actuated by a frequency converter or inverter 70 (which can be provided separately by the electrical machine 1) which can be managed by the controlling and managing unit 6 of the system. When the electrical machine 1 works as a motor, the frequency converter 70 supplies the main stator winding 15 with an alternating current (in particular, a three- phase alternating current) with an adjustable frequency.

In case, even the outer source 69 for the second exciter 32 can be derived by the frequency converter 70.

The controlling and managing unit 6 can be configured to control the opening/closing of the switches 61 , 62 according to the operation mode, apart from the fact of actuating and managing the frequency converter 70. The passage from a mode to another one and in particular the activation and the deactivation of the respective exciters 31 , 32 must take place with the still rotor 2 to avoid damages. The controlling and managing unit 6 can further include its own supplying unit 66, a current detector/measurer 64 and a voltage detector/measurer 65 in the circuit of the main stator winding 15 and other control and safety devices which are known to the person skilled in the art.

A cooling fan 27, for example, can be mounted on the rotor 2.

The electrical machine 1 further comprises an absolute encoder 16 for controlling the throw from the still position, a cooling system 17 with water circulation, a lubrication system 18. These elements and other accessory elements which usually are present in the synchronous electric machines can be considered substantially within the comprehension of the person skilled in the art and then they will be not described in detail.

The base operation of the synchronous electrical machine 1 in the operating modes thereof will be described hereinafter.

When the synchronous electrical machine 1 has to be operated as electric generator, the switches 61 are closed (thus connecting the stator winding 41 of the first exciter 31 to the direct current supply) and the switches 62 are opened (thus disconnecting the stator winding 42 of the second exciter 32). The frequency converter 70 is disconnected from the main stator winding 15.

The rotor 2 is then connected to the external motive power and it is placed in rotation at the synchrony speed. II generator with permanent magnets 68 produces a current supplying the voltage regulator 63, which in turn supplies the stator winding 41 of the first exciter 31 with a direct electric current.

The stator winding 41 produces a magnetic field which, thanks to the motion of the rotor 2, generates an alternating current induced in the rotor winding 51 of the first exciter 31. Such induced alternating current is rectified and transformed in direct current by the first rectifier 351 ; the obtained direct current supplies the main rotor winding 25 and produces a main rotor magnetic field that rotates with the rotor 2 and induces an alternating electric current in the main stator winding 15. The so- obtained alternating electric current is collected at the terminals 150 to be used. In particular, the main stator winding 15 is a three-phase winding and then the obtained current is a three-phase alternating current.

When the synchronous electrical machine 1 has to be operated as an electric motor, the switches 61 are opened (thus disconnecting the stator winding 41 of the first exciter 31 from the direct current supply) and the switches 62 are closed (thus connecting the stator winding 42 of the second exciter 32 to the outer source 69 of three-phase alternating current). These operations are performed with the still machine.

The stator winding 42 of the second exciter 32, supplied with the three-phase alternating current of the outer source 69, generates a rotating magnetic field that acts on the (still) rotor winding 52 of the second exciter 32 and produces therein an induced alternating current. Such induced alternating current is rectified and transformed in direct current by the second rectifier 352; the obtained direct current supplies the main rotor winding 25 and produces a main rotor magnetic field. It is known that the main rotor magnetic field is present already with the still 2 rotor. The main stator winding 15 is supplied, by means of the frequency converter 70, with a three-phase alternating current with an increasing frequency, at the beginning equal to zero. A rotating magnetic field (at an increasing speed with the frequency) is obtained, thereto the main rotor magnetic field tends to align. Therefore there is the breakaway torque which is necessary to put the rotor 2 in rotation. The frequency of the supplying current of the main stator winding 15 is increased until reaching the wished rotation speed. The rotor 2 delivers mechanical power to the shaft 21.

It is to be noted that the second exciter 32 continues to produce the excitation current of the main rotor winding 25 even when the rotor 2 is in motion: whereas in the starting phase from the still position the induction is due only to the rotation of the rotating magnetic field generated by the stator winding 42, when the rotor 2 is in motion the induction is due to the combined effect of the rotation of the rotating magnetic field and of the rotation of the rotor winding 52 with the rotor 2. An excitation power of the main rotor winding 25 is then guaranteed at each speed of the rotor 2.

The combined control of the voltage of the alternating current from the outer source 69 and of the voltage of the current supplied to the main stator winding 15 allows optimizing the torque curves of the operation as motor.

It is to be noted that, in the operation mode as motor, the outer source 69 could be an existing electric network. Therefore, it would be not necessary to provide an inverter for supplying the second exciter 32 during the operation as motor.

Alternatively, the second exciter 32 is controlled in voltage by a main inverter.

An application example is present in a propelling apparatus 8, in particular for sea use on a ship; the propelling apparatus 8 is shown schematically in figure 9. Such propelling apparatus 8 comprises an endothermic engine 81 (for example a diesel motor), a synchronous electrical machine 1 , a drive system 83, a propeller 85 and the shaft thereof 86.

The helix 85 is mounted on the shaft 86 which is connected to the drive system 83. Even the rotor 2 of the synchronous electrical machine 1 is connected to the drive system 83.

The endothermic engine 81 can be connected or disconnected to/from the drive system 83. In particular, the endothermic engine 81 is a propelling engine which has a substantially constant operating speed.

The drive system 83 comprises a reduction gear to make the helix 85 to rotate at a fixed speed. The drive system 83 even acts as reduction gear to make the rotor 2 to rotate at the speed requested for the wished frequency of the generated electric current, when the electrical machine 1 is used as electric generator. Obviously, the drive system 83 can be configured so as to allow that the speed of the helix 85 and the speed of the rotor 2 are different therebetween.

The propelling apparatus 8 can run in two operating modes.

In a first mode, for example corresponding to a ship navigation normal condition, the endothermic engine 81 is connected to the drive system 83: it supplies power to the helix 85 and the synchronous electrical machine 1 , which works as electric generator. Therefore, there is production of propelling power to the helix 85 and production of electric power to the terminals 150 of the synchronous electrical machine 1.

In a second mode, for example corresponding to a breakdown condition of the endothermic engine or to a procedure for positioning the ship, the endothermic engine 81 is disconnected from the drive system 83 and the synchronous electrical machine 1 works as a motor. The synchronous electrical machine 1 makes the helix 85 to rotate by means of the drive system 83, to a production of propelling power to the helix 85 and a consumption of electric power absorbed by the synchronous electrical machine 1.

It is to be noted that in this second operation mode the speed of the synchronous electrical machine 1 can be adjusted and therefore the speed of the helix 85 (and then the push thereof) can be varied according to the needs, as requested in a positioning operation.

For example, in this application, the synchronous electrical machine 1 has a nominal power of 1250 kW, provided at 100% in the operation as electric generator and at 70% in the operation as motor.

The subject of the present disclosure has been sofar described by referring to preferred embodiments thereof. It is to be meant that other embodiments belonging to the same inventive core may exist, all belonging to the protection scope of the here below illustrated claims.

Claims

1. A synchronous electrical machine (1 ) adapted to alternately work as an electric generator or as an electric motor, the synchronous electrical machine (1 ) comprising:

- a stator (10) that is provided with a main stator winding (15);

- a rotor (2) that is provided with a main rotor winding (25), the main stator winding (15) and the main rotor winding (25) being configured to cooperate with each other by electromagnetic induction during the working of the synchronous electrical machine (1 ); and

- an excitation system (3) for exciting the main rotor winding (25), to a production of a main rotor magnetic field,

wherein the excitation system (3) comprises:

- a first exciter (31 ) in direct current;

- a second exciter (32) in alternating current; and

- a first rectifier (351 ) and a second rectifier (352),

each of said first exciter (31 ) and second exciter (32) comprising a respective exciter stator winding (41 , 42) and a respective exciter rotor winding (51 , 52) that are configured to cooperate with each other by electromagnetic induction during the working of the synchronous electrical machine (1 ),

the rotor winding (51 ) of the first exciter (31 ) being connected to the main rotor winding (25) with interposition of said first rectifier (351 ), and the rotor winding (52) of the second exciter (32) being connected to the main rotor winding (25) with interposition of said second rectifier (352), the rotor winding (51 ) of the first exciter (31 ) and the rotor winding (52) of the second exciter (32) being not communicating with each other,

the synchronous electrical machine (1 ) being configured to operate the first exciter (31 ) or the second exciter (32) depending on the working mode of the synchronous electrical machine (1 ),

the first exciter (31 ) being operational when the synchronous electrical machine (1 ) works as electric generator and being non-operational when the synchronous electrical machine (1 ) works as an electric motor,

the second exciter (32) being operational when the synchronous electrical machine (1 ) works as an electric motor and being non-operational when the synchronous electrical machine (1 ) works as electric generator.

2. The synchronous electrical machine (1 ) according to claim 1 , wherein the synchronous electrical machine (1 ) is configured to supply the stator winding (41 ) of the first exciter (31 ) with a direct electric current, the stator winding (41 ) of the first exciter (31 ) being configured to produce a stationary magnetic field that acts on the rotor winding (51 ) of the first exciter (31 ).

The synchronous electrical machine (1 ) according to claim 1 or 2, wherein the synchronous electrical machine (1 ) is configured to supply the stator winding (42) of the second exciter (32) with an alternating electric current, the stator winding (42) of the second exciter (32) being configured to produce a rotating magnetic field that acts on the rotor winding (52) of the second exciter (32).

The synchronous electrical machine (1 ) according to any one of claims 1 to 3, comprising a diode-carrying wheel (28) that is jointly rotatable with the rotor (2), said first and second rectifiers (351 , 352) being diode-bridge rectifiers that are mounted on the diode-carrying wheel (28).

The synchronous electrical machine (1 ) according to any one of claims 1 to 4, wherein the rotor winding (51 ) of the first exciter (31 ) and the rotor winding (52) of the second exciter (32) have three phases each, said at least a rectifier (35, 351 , 352) comprising a respective diode bridge for each phase.

The synchronous electrical machine (1 ) according to any one of claims 1 to 5, configured to be connected to a frequency converter (70) for supplying the main stator winding (15) with an alternating electric current with an adjustable frequency, when the synchronous electrical machine (1 ) works as an electric motor.

The synchronous electrical machine (1 ) according to any one of claims 1 to 6, wherein the main rotor winding (25), the rotor winding (51 ) of the first exciter (31 ) and the rotor winding (52) of the second exciter (32) are mounted on a same shaft (21 ) of the rotor (2).

A propelling apparatus (8), comprising an endothermic engine (81 ), a synchronous electrical machine (1 ) according to any one of claims 1 to 7, a drive system (83), a propeller (85), a shaft (86) for the propeller (85),

wherein the propeller (85) is mounted on the shaft (86), which is connected to the drive system (83),

and wherein the rotor (2) of the synchronous electrical machine (1 ) is connected to the drive system (83),

the propelling apparatus (8) being configured to run in a first mode and in a second mode such that,

in the first mode, the synchronous electrical machine (1 ) works as electric generator and the endothermic engine (81 ) is connected to the drive system (83), the endothermic engine (81 ) being rotating the propeller (85) and the rotor (2) of the synchronous electrical machine (1 ), to a production of propelling power and a production of electric power,

and that, in the second mode, the synchronous electrical machine (1 ) works as an electric motor and the endothermic engine (81 ) is disconnected from the drive system (83), the synchronous electrical machine (1 ) being rotating the propeller (85) to a production of propelling power and a consumption of electric power. A method for operating a synchronous electrical machine (1 ) adapted to alternately work as an electric generator or as an electric motor, comprising the steps of:

- providing a a synchronous electrical machine (1 ) comprising:

• a stator (10) provided with a main stator winding (15);

• a rotor (2) provided with a main rotor winding (25) that is configured to cooperate with the main stator winding (15) by electromagnetic induction;

• a first exciter (31 ) comprising a stator winding (41 ) of the first exciter (31 ) and a rotor winding (51 ) of the first exciter (31 ) configured to cooperate with the stator winding (41 ) of the first exciter (31 ) by electromagnetic induction, the rotor winding (51 ) of the first exciter (31 ) being connected to the main rotor winding (25) with interposition of a first rectifier (351 );

• a second exciter (32) comprising a stator winding (42) of the second exciter

(32) and a rotor winding (52) of the second exciter (32) configured to cooperate with the stator winding (42) of the second exciter (32) by electromagnetic induction, the rotor winding (52) of the second exciter (32) being connected to the main rotor winding (25) with interposition of a second rectifier (352);

- operating the synchronous electrical machine (1 ) as electric generator by carrying out the following sub-steps of:

• disconnecting the stator winding (42) of the second exciter (32);

• setting the rotor (2) into rotation by external motive power;

• supplying the stator winding (41 ) of the first exciter (31 ) with a direct electric current, to obtain an induced current in the rotor winding (51 ) of the first exciter (31 ) and a a rectified current in the main rotor winding (25) that produces a main rotor magnetic field that rotates with the rotor (2);

• drawing, from the main stator winding (15) an induced electric current that is produced by the main rotor magnetic field that rotates with the rotor (2); operating the synchronous electrical machine (1 ) as an electric motor by carrying out the following sub-steps of:

• disconnecting the stator winding (41 ) of the first exciter (31 ); supplying the stator winding (42) of the second exciter (32) with an alternating electric current, to obtain a rotating magnetic field that acts on the rotor winding (52) of the second exciter (32) and produces an induced current in the rotor winding (52) of the second exciter (32), such induced current generating a rectified current in the main rotor winding (25) that produces a main rotor magnetic field;

supplying the main stator winding (15) with an alternating electric current with an increasing frequency, to obtain a magnetic field that rotates at an increasing speed and cooperates with the main rotor magnetic field to set the rotor into rotation (2).