Starter-generator with improved starter function

Abstract

The invention relates to a brushless starter-generator (1) comprising: - a main electrical machine (10) having a stator (11) comprising stator windings (12a, 12b, 12c) and a rotor (13) comprising a rotor winding (14) and intended to be mechanically coupled to the combustion engine (200); - an exciter (20) comprising a stator winding (24) and rotor windings (23a, 23b, 23c) connected to the rotor winding (14) of the main electrical machine (10), the rotors of the main electrical machine (10) and of the exciter (20) forming the rotor (75) of the starter-generator; and - a regulator (50) configured to supply the stator winding of the exciter with AC voltage with duty cycle modulation of alternation width when the starter-generator operates in starter mode, the regulator being configured to vary the duty cycle so as to vary an excitation delivered by the rotor (22) of the exciter to the rotor winding of the main electrical machine.

Description

DESCRIPTION Title of the invention: Starter-generator with improved starter function

[0001] The present invention relates to a starter-generator coupled to a combustion engine. The field of application of the invention is more particularly that of starter-generators for gas turbine aeronautical propulsion engines mounted on aircraft. The invention is however applicable to other types of combustion engines, for example internal combustion engines, industrial turbines, helicopter turbines or auxiliary power unit turbines or APUs.

[0002] Such a starter-generator comprises a rotating electrical machine intended to be mechanically coupled to a shaft of a combustion engine. The starter-generator is capable of operating in a generator mode, during a so-called generation phase, during which the combustion engine drives the shaft in rotation and the rotating electrical machine transforms the mechanical rotational energy of the shaft into electrical energy intended to electrically supply an electrical network, for example an on-board network of an aircraft. The starter-generator is also capable of operating in starter mode, during a so-called start-up phase, during which the rotating electrical machine supplies mechanical power to the shaft of the combustion engine to and rotate the shaft of the combustion engine so as to start the combustion engine.

[0003] The rotating electrical machine, often referred to as a S / G for its English acronym: "Starter / Generator", usually includes a main electrical machine, an exciter and possibly an auxiliary generator. These elements of the rotating machine are mounted on a common shaft mechanically coupled to a shaft of the combustion engine. Such a starter-generator is a brushless starter-generator.

[0004] The main electrical machine forms a main electrical generator (or alternator) operating in synchronous mode. The electrical machine The main generator has a rotor winding and stator windings which, when operating in synchronous generator mode, convert the mechanical energy supplied by a shaft mechanically coupled to the combustion engine into alternating electrical energy, for example three-phase, supplying an aircraft on-board network. For aeronautical applications, the aircraft's alternating on-board network, supplied by the voltage delivered by the starter-generator operating as a generator, can be at a fixed frequency, generally 400 Hz, or at a variable frequency set to the alternator drive speed. In the case of a fixed frequency network, the network can be supplied by an APU auxiliary power unit operating at constant speed. It is also possible to drive an alternator at constant speed by means of a mechanical regulator drawing its energy from an engine running at variable speed.It is also possible to implement a variable frequency alternator and to insert between this alternator and the on-board network a converter generating an alternating voltage at a fixed frequency.

[0005] The exciter comprises a wound rotor supplying the rotor of the main machine through a rotating rectifier and a stator comprising a stator winding supplied with direct current during the generation phase and with alternating current during the start-up phase. The stator winding of the exciter is generally designed for a direct voltage with a large number of turns suitable for generator mode. As a result, its inductance is large, which would require a high voltage to excite it in alternating current during the starting phase. To limit this voltage to an acceptable level, it is possible to provide two separate stator windings, one supplied with direct current for generator mode and the other with alternating current for starter mode.

[0006] It is possible to use the 400 Hz AC mains voltage to power the AC winding of the exciter but this frequency may be too low for the exciter's performance and furthermore does not allow the excitation level to be adjusted.

[0007] Due to the asymmetry between the numbers of turns usually required for the two windings, a high voltage can be induced on the continuous excitation winding during the starting phase where the excitation winding AC is supplied. This voltage can significantly exceed the thresholds usually accepted on on-board networks. One solution consists of using numerous contactors and cables to switch from a winding configuration used for starting to that used for electrical generation. These solutions are heavy and bulky. Another solution described in patent application FR 2 348 594 consists of arranging the two windings in quadrature, which limits the currents induced in the DC winding due to the power supply of the AC winding. Yet another solution described in patent application WO 2017 / 211838 A1 provides for short-circuiting the DC winding during the starting phase.

[0008] Furthermore, the exciter, in starting mode, has a low power factor, due to the high magnetizing current caused by the presence of an air gap between rotor and stator in the magnetic circuit. This limits the performance of the exciter and to overcome this defect, it is necessary to increase the dimensions of the starter-generator only to ensure the starting function. This increase in size is once again at the expense of mass and size.

[0009] The low power factor of the exciter also requires an oversized exciter power converter due to the high currents to be supplied to the AC winding for starting compared to the active power to be supplied. In addition, the generation of the voltage by switching requires the addition of an output filter for electromagnetic compatibility issues in order to limit the impact of voltage switching on the exciter winding and on the rotating rectifier.

[0010] US patent 6,998,726 proposes two solutions to improve the power factor: either by injecting a harmonic component 3 added to the sinusoidal voltage supplying the alternating winding or by supplying this winding by means of a square signal.

[0011] This allows, from the same converter input voltage, to reduce the effective value of the current that must be supplied by the exciter, for the same starting system performance. This solution may however require the use of an output filter on the converter.

[0012] The square wave signal significantly reduces switching losses in the converter forming the power supply signal for the AC starting winding, as the output voltage is no longer cut. However, the solutions proposed by US patent 6,998,726 do not allow the output current of the exciter to be regulated, unless a variable input voltage is available on the converter or an additional stage to adjust it.

[0013] The invention aims to overcome all or part of the problems mentioned above by proposing a starter-generator whose starting function is particularly well adapted.

[0014] To this end, the invention relates to a brushless starter-generator, capable of operating in starter mode so as to drive a combustion engine in rotation, and in synchronous electric generator mode so as to transform mechanical energy supplied by the combustion engine into electrical energy, the starter-generator comprising: - a main electrical machine having a stator comprising stator windings and a rotor comprising a rotor winding and intended to be mechanically coupled to the combustion engine, - a rotating rectifier, - an exciter comprising a stator, comprising a stator winding, and a rotor comprising rotor windings connected to the rotor winding of the main electrical machine via the rotating rectifier, the rotors of the main electrical machine and of the exciter forming the rotor of the starter-generator, the rotor being intended to be mechanically coupled to the combustion engine, - a regulator connected to the stator winding of the exciter, configured to supply the stator winding of the exciter with alternating voltage with a duty cycle modulation of the alternation width when the starter-generator operates in starter mode, and configured to vary the duty cycle so as to vary an excitation delivered by the rotor of the exciter to the rotor winding of the main electrical machine.

[0015] Advantageously, the alternating voltage alternations have a square wave shape of constant amplitude.

[0016] Advantageously, the frequency of the alternating voltage alternations is fixed.

[0017] Advantageously, the regulator is configured to generate a duty cycle comprising an increasing phase at the start of rotation of the rotor.

[0018] Advantageously, the starter-generator comprises an auxiliary generator configured to supply the stator winding with direct current when the starter-generator is operating in generator mode.

[0019] Advantageously, the stator winding comprises two end terminals and an intermediate terminal and wherein, the alternating current is applied between one of the end terminals and the intermediate terminal, and the direct current is applied between two end terminals.

[0020] Advantageously, the stator winding of the exciter is supplied only with alternating current, the exciter comprises a second stator winding and the regulator is configured to supply the second stator winding with direct current when the starter-generator is operating in generator mode.

[0021] The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, a description illustrated by the attached drawing in which:

[0022] Figure 1 schematically represents a starter-generator according to one embodiment of the invention;

[0023] Figure 2 represents in the form of a timing diagram an example of a signal applied to a stator winding of an exciter of the starter-generator in starting mode;

[0024] Figure 3 represents an example of a converter for supplying the stator winding of the exciter in starter mode;

[0025] Figure 4 represents in the form of a timing diagram an example of voltage and current applied to the terminals of the stator winding by the converter of Figure 3;

[0026] Figure 5 shows a variant of the stator of the starter-generator exciter;

[0027] Figure 6 shows another variant of the stator of the starter-generator exciter.

[0028] For the sake of clarity, the same elements will have the same references in the different figures.

[0029] Figure 1 schematically represents a brushless starter-generator 1 with wound rotor excitation according to the invention. The starter-generator 1 comprises a rotating electrical machine 100 intended to be mechanically coupled to a shaft 201 of a combustion engine 200. The starter-generator 1 is capable of operating in generator mode, during a so-called generation phase, during which the combustion engine 200 provides motive power to the rotating electrical machine 100. During this phase the rotating machine 100 transforms the mechanical rotational energy of the shaft 201 into electrical energy intended to supply an electrical network, for example an on-board network of an aircraft. The starter-generator 1 is also capable of operating in starter mode, during a start-up phase, during which it rotates the shaft 201 of the combustion engine 200 so as to start it.

[0030] The rotating electrical machine 100 comprises three sub-machines whose rotors are mechanically coupled: a main electrical machine 10, an exciter 20, and optionally an auxiliary generator 30, as well as a rotating rectifier bridge 40. The rotating electrical machine 100 comprises a casing 150 housing the main electrical machine 10, the exciter 20 and the optional auxiliary generator 30. The stator of the rotating electrical machine 100 is fixed to the casing 150.

[0031] The rotors of the main electrical machine 10, of the exciter 20 and of the auxiliary generator 30 form the rotor 75 of the electrical machine 100. They are mounted on a common shaft 101 of which only the part outside the casing 150 is shown in FIG. 1. The common shaft 101 is intended to be mechanically coupled to the shaft 201 of the combustion engine 200. The combustion engine 200 is for example a piston engine or a gas turbine. commonly used for the propulsion of an aircraft. On board an aircraft, there is also an auxiliary generator known in English as: "Auxiliary Power Unit" or by its acronym APU which also requires a starter-generator. More generally, the invention can be implemented in a starter-generator coupled to any type of combustion engine. The coupling of the shafts 101 and 201 can be disconnected for example by means of a dog clutch device which can be actuated in the event of malfunction of either the combustion engine 200 or the starter-generator 1.

[0032] The casing 150 is intended to be mounted on a support of the equipment comprising the combustion engine 200, for example a support of an aircraft. In starter mode, the rotating electrical machine 100 drives the common shaft 101 in rotation. The common shaft 101 rotates relative to the casing 150.

[0033] The starter-generator 1 comprises elements for accelerating the rotor 13 of the main electrical machine 10 mechanically coupled to the combustion engine 200, so as to drive the combustion engine 200, in starter mode, to a rotational speed where it can maintain its speed thanks to the combustion of fuel. No auxiliary starting device outside the starter-generator 1 is necessary to start the combustion engine 200. When the starter-generator 1 operates in starter mode, the main electrical machine 10 forms a synchronous electric motor which provides the torque necessary to rotate the combustion engine 200. When the starter-generator operates in starter mode, the exciter 20 is supplied with alternating current so as to start the combustion engine.When the rotational speed of the shaft 201 of the combustion engine 200 reaches a first threshold, this power supply is cut off because the torque produced by the combustion engine 200 is sufficient for the combustion engine 200 to maintain the movement of the shaft 201 alone. The starter-generator 1 then no longer operates in starter mode. The rotational speed of the shaft 201 of the combustion engine 200 accelerates by itself, that is to say autonomously, until it reaches a second threshold, higher than the first threshold, from which the starter-generator 1 will be able to operate in generator mode. The generator and starter modes are not simultaneous.

[0034] The main electrical machine 10 comprises a stator 11 comprising stator windings 12a, 12b, 12c which can be connected in star, as shown, or in delta if the stator 11 is three-phase. Other numbers of phases are also possible. The rotor 13 comprises a rotor winding 14.

[0035] In generator mode, the combustion engine 200 having been started, the main electrical machine 10 constitutes a synchronous electrical generator which transforms the mechanical rotational energy of the common shaft 101 mechanically coupled to the shaft 201 of the combustion engine 200 into electrical energy of electrical voltage U delivered at the output of its stator 11 on a supply line 60 which conveys the electrical energy to a device intended to be electrically powered, for example an on-board network 202 of the aircraft. The voltage U is a three-phase electrical voltage in the example shown. The invention can be implemented regardless of the number of phases of the stator 11.

[0036] The exciter 20 comprises a stator 21 and a rotor 22 secured to the shaft 101. The rotor 22 comprises rotor windings 23a, 23b, 23c, which can be connected in star or in any other way. As previously, the invention can be implemented regardless of the number of phases of the rotor 22. The stator 21 comprises a stator winding 24 forming a wound inductor which can be traversed either by an alternating current in starter mode or by a direct current in generator mode. The alternating currents developed at the rotor 22 of the exciter 20 are rectified by the rotating rectifier 40, such as a rotating diode bridge 41, to supply the rotor winding 14 of the main electrical machine 10 with direct current.

[0037] The auxiliary generator 30 is for example a synchronous generator comprising a rotor 31 secured to the shaft 101 and comprising permanent magnets 32. In addition, the auxiliary generator 30 comprises a stator 33 comprising stator windings 34a, 34b, 34c.

[0038] The stators of the possible auxiliary generator 30, of the exciter 20 and of the main machine 10 form the stator of the rotating electrical machine. It is fixed relative to the casing 150. The aforementioned rotors and stators each comprise a structure on which the windings of the corresponding rotor or stator are fixed.

[0039] The starter-generator 1 further comprises a regulator 50 configured to supply the stator winding 24 with alternating current when the starter-generator 1 operates in starter mode. In generator mode, the stator winding 24 is for example supplied by the auxiliary generator 30 through a rectifier 52 which may be accompanied by a regulator, not shown. The change from starter mode to generator mode is for example carried out by means of a contactor 25 when the same winding 24 is supplied either with alternating current for the starter mode, or with direct current for the generator mode. It is possible to do without the contactor 25 when the stator 21 comprises two separate windings, one for the starter mode and the other for the generator mode, as will be seen later with the aid of FIG. 5.For the present invention, we are mainly interested in the starter mode and the alternating current injected into the stator winding 24.

[0040] The electrical energy used to power the stator winding 24 can come from different sources: the on-board network 202, the auxiliary generator 30, a battery 60 or a ground socket 70. On board an aircraft, when it is on the ground, when starting the first engine, in particular that of the auxiliary power unit, the only source of electrical energy available on board may be that of the battery 60. Some airports are equipped with ground power units allowing the aircraft to be connected through its ground socket 70, which makes it possible to avoid the operation of the auxiliary power unit when the aircraft is stationary. Once the auxiliary power unit has been started, the on-board network 202 can be energized, which makes it possible to power other starter-generators associated in particular with the propulsion engines. Power supply through the auxiliary generator 30 cannot be provided as long as the shaft 101 is stationary.The auxiliary generator 30 can however take over from another power source in starter mode. In Figure 1, several energy sources are shown to power the stator winding 24. In practice, the invention can be implemented regardless of the source(s) used to power the stator winding 24.

[0041] In the example shown in Figure 1, the regulator 50 comprises several converters. In practice, the number of converters and the function of each are adapted to the number and specificities of the different energy sources used to power the stator winding 24. A first rectifier 51 transforms the energy from the on-board network 202 or from the park socket 70 into direct voltage. In starter mode, the starter-generator 1 does not produce electrical energy and the stator 11 is then not connected to the on-board network 202. A contactor 15 makes it possible to disconnect the stator 11 from the on-board network 202 in starter mode and to connect it in generator mode. An inverter 53 receives the direct voltages from the rectifier 51 or possibly from the battery 60 to power the stator winding 24 with alternating voltage.The regulator 50 may further comprise a control module 54 managing the various converters, in particular the rectifier 51 and the inverter 53. The regulator 50 may also comprise other contactors, not shown in FIG. 1 and making it possible in particular to switch from one source to another: 60, 70, 202. These contactors are controlled by the control module 54.

[0042] Figure 2 represents in the form of a timing diagram an example of a signal applied to the stator winding 24 in start-up mode. More precisely, the regulator 50 is configured to apply to the stator winding 24 an alternating voltage whose amplitude width is modulated. The modulation comprises an increasing phase of the amplitude width at the start of rotation of the rotor 75.

[0043] By applying a constant sinusoidal alternating voltage to the stator winding 24 during the starting phase, when the rotor 75 is stopped, a significant current draw may appear on the stator winding 24, which may be detrimental to it. By modulating the duty cycle of the alternation width of a constant amplitude alternating voltage, it is possible to limit the occurrence of this current draw. For example, the width of the alternation is defined as the time separating the rise and fall in voltage at mid-voltage value from the maximum amplitude. The duty cycle represents the ratio between the width of two successive alternations, one positive and the other negative, and the duration of a period of the alternating signal.

[0044] The frequency of the alternating voltage applied to the stator winding 24 can vary during the start-up phase. However, to simplify the control of the inverter 53, a fixed frequency is preferable.

[0045] Using the inverter 53, it is possible to give the alternations any form of voltage evolution as a function of time. For example, it is possible to give the alternations a sinusoidal shape, for example by generating the sinusoidal shape using pulse width modulation (PWM) cutting. This makes it possible to vary the voltage smoothly across the stator winding 24. To simplify the shape of the alternations, and limit the number of switchings of electronic switches of the inverter 53, it is possible to give the alternations a shape substantially in square waves of constant amplitude. More precisely, the switches of the inverter 53 only switch twice per alternation, once to establish the voltage and a second time to interrupt it. The square wave shape is directly given by the switching of the electronic switches of the inverter 53.It is then not necessary to add a filter at the output of inverter 53.

[0046] Figure 2 represents more precisely the shape of the alternating voltage applied to the stator winding at the beginning of the start-up phase. The time is represented on the abscissa and the voltage on the ordinate. The voltage is expressed as a percentage of the voltage present at the output of the rectifier 51 supplying the inverter 53. The square wave shape of the alternations represented in Figure 2 is a theoretical shape. In practice, the actual voltage undergoes variations around this square wave shape. The electronic switches of the inverter 53 are controlled by means of a square wave signal and parasitic phenomena are often found on the alternating voltage applied to the stator winding of the exciter, phenomena such as transients at the moment of switching of the electronic switches.

[0047] In Figure 2, the increasing phase of the duty cycle is represented over about twenty alternations to illustrate the invention at the beginning of the phase of start-up. In practice, the rising phase can be spread over a greater number of alternations. For example, the alternating voltage applied to stator winding 24 can have a fixed frequency of 1 kHz. The duration of the starting phase can be of the order of a second. It may then be necessary to vary the duty cycle over a thousand alternations.

[0048] The duration of the increasing phase can be predetermined in the regulator 50. It is also possible to predetermine the evolution of the duty cycle during the increasing phase. The predetermination is mainly defined as a function of the electrical time constant of the exciter. It is for example possible to define a ramp with a constant slope defining the duty cycle as a function of time. At the end of the increasing phase, the duty cycle can be varied as a function of the torque to be supplied to start the motor 200. More precisely, during the increase in speed of the rotor 75, it is possible to first deliver a constant torque to the motor 200 and then deliver a constant power.

[0049] The invention providing a variation in the duty cycle can be applied to the entire start-up phase, in particular at the beginning of the start-up phase with an increasing phase of the duty cycle even though the rotor 75 is still stationary and has barely begun to rotate. In addition or alternatively, a decreasing phase of the duty cycle can be applied in order to reduce the direct excitation current applied to the rotor winding 14 of the main electrical machine 10, for example, when switching from constant torque control to constant power control. Then, during constant power control, the speed of the rotor 75 increases and it is still desirable to reduce the direct excitation current by further reducing the duty cycle.

[0050] During constant torque driving, it is generally useful to maintain a constant direct excitation current and therefore to keep the duty cycle value constant.

[0051] Figure 3 shows an example of an inverter 53 comprising two branches 53a and 53b. Branch 53a comprises two electronic switches 53a+ and 53a- each associated with a freewheeling diode. The electronic switches 53a+ and 53a- are connected in series between two input terminals + and - receiving a direct voltage. Similarly, branch 53b comprises two electronic switches 53b+ and 53b- each associated with a freewheel diode and connected in series between the two input terminals + and The direct voltage coming from one of the rectifiers 51 or 52 is applied between the two input terminals + and The stator winding 24 is connected between the common points of two switches of each of branches 53a and 53b.

[0052] Figure 4 represents in the form of a timing diagram over two periods T, i.e. four alternations, the evolution over time of the voltage at the terminals of the stator winding 24 and of the current flowing in the stator winding 24. During a first step A, the switches 53a+ and 53b+ are closed, the switches 53a- and 53b- are open and the voltage applied to the stator winding 24 is zero. Following the first step, during a second step B, the switches 53a+ and 53b- are closed, the switches 53a- and 53b+ are open and the voltage applied to the stator winding 24 is positive. At the end of step B, a third step C follows during which switches 53a+ and 53b+ are open, switches 53a- and 53b- are closed and the voltage applied to stator winding 24 is again zero.After step C, step D follows during which switches 53a+ and 53b- are open, switches 53a- and 53b+ are closed and the voltage applied to stator winding 24 is negative. The four steps follow one another during a period T. Step B forms a positive half-wave and step D, a negative half-wave. Each step begins with a switching of switches and ends with a switching of another switch. No switching is carried out between the beginning and the end of each step.

[0053] The duty cycle of a period T is the ratio between the sum of the durations of steps B and D, where a voltage is applied to the stator winding 24, and the total duration of the period, i.e. the sum of the durations of the four steps A, B, C, D. Figure 2 represents an example of variation in duty cycle over about twenty periods during the starting phase of the combustion engine 200. At the start of the starting phase, when the rotor 75 is stopped, at the start of the growth phase, the duty cycle is low, for example of the order of 10%. Then, the duty cycle increases. At the end of the timing diagram in Figure 2, the duty cycle reaches 50%. It is understood that the duty cycle can still increase later before reaching a phase where the starter-generator is controlled at constant torque.

[0054] Figure 5 represents a variant of the stator 21 of the exciter 20 of the starter-generator 1. We have seen previously that the stator 21 of the exciter 20 can comprise two separate windings, one DC supplied with direct current during the generation phase and the other AC supplied with alternating current during the starting phase of the motor 200. The winding marked AC in Figure 5 corresponds to the winding 24 shown in Figure 1. Advantageously, the two AC and DC windings are arranged in quadrature, which limits the currents induced during the starting phase of the motor 200 in the DC winding due to the power supply of the AC winding.

[0055] Alternatively, as shown in Figure 1, it is possible to use only a single winding supplied first with alternating current during the start-up phase and then with direct current during the generation phase, in particular when the starting torque of the motor 200 coupled to the starter-generator 1 is low, for example as for an APU auxiliary generator of an aircraft. Indeed, the shape of the alternating excitation signal applied to the winding 24, in particular a square-wave signal, allows an increase in the number of turns of the winding 24, compared to the number of turns required by traditional excitation by sinusoidal voltage. This makes it possible to reduce the asymmetry between the numbers of turns of the AC and DC windings of the stator 21 of the exciter 20 and therefore to limit the problems of high voltage induced in the DC winding during excitation on the AC winding.

[0056] Figure 6 shows a variant of the stator 21 in which the winding 24 has three terminals, including two end terminals 24a and 24b and an intermediate terminal 24c. During the starting phase, only part of the winding 24 is supplied with alternating current AC by implementing the increasing phase as, for example, illustrated using Figure 2. More precisely, the winding 24 is supplied between its terminals 24a and 24c. During the generation phase, the entire winding 24 is supplied with direct current DC between its terminals 24a and 24b.

Claims

CLAIMS 1. Brushless starter-generator (1), capable of operating in starter mode so as to drive a combustion engine (200) in rotation, and in synchronous electric generator mode so as to transform mechanical energy supplied by the combustion engine (200) into electrical energy, the starter-generator (1) comprising: - a main electrical machine (10) having a stator (11) comprising stator windings (12a, 12b, 12c) and a rotor (13) comprising a rotor winding (14) and intended to be mechanically coupled to the combustion engine (200), - a rotating rectifier (40), - an exciter (20) comprising a stator (21), comprising a stator winding (24; AC), and a rotor (22) comprising rotor windings (23a, 23b, 23c) connected to the rotor winding (14) of the main electrical machine (10) via the rotating rectifier (40), the rotors of the main electrical machine (10) and of the exciter (20) forming the rotor (75) of the starter-generator, the rotor (75) being intended to be mechanically coupled to the combustion engine (200), - a regulator (50) connected to the stator winding (24; AC) of the exciter (20), configured to supply the stator winding (24; AC) of the exciter (20) with alternating voltage with a duty cycle modulation of the alternation width when the starter-generator operates in starter mode, and configured to vary the duty cycle so as to vary an excitation delivered by the rotor (22) of the exciter (20) to the rotor winding (14) of the main electrical machine (10).

2. Starter-generator according to claim 1, in which the alternating voltage alternations have a square wave shape of constant amplitude.

3. Starter-generator according to one of the preceding claims, in which the frequency of the alternating voltage alternations is fixed.

4. Starter-generator according to one of the preceding claims, in which the regulator (50) is configured to generate a duty cycle comprising an increasing phase at the start of rotation of the rotor (75).

5. Starter-generator according to one of the preceding claims, comprising an auxiliary generator (30) configured to supply the stator winding (24) with direct current when the starter-generator (1) operates in generator mode.

6. A starter-generator according to claim 5, wherein the stator winding (24) comprises two end terminals (24a, 24b) and an intermediate terminal (24c) and wherein alternating current (AC) is applied between one of the end terminals (24a) and the intermediate terminal (24c), and in which direct current is applied between two end terminals (24a, 24b).

7. Starter-generator according to one of claims 1 to 4, in which the stator winding (AC) of the exciter (20) is supplied only with alternating current, in which the exciter (20) comprises a second stator winding (DC) and in which the regulator (73) is configured to supply the second stator winding with direct current when the starter-generator operates in generator mode.

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