THE FREQUENCY AND THE AMPLITUDE OF THE INDUCED VOLTAGE ON

THE STATOR WINDINGS OF THE SYNCHRONOUS GENERATOR THEREOF"

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102020000003632 filed on 21/02/2020, the entire disclosure of which is incorporated herein by reference.

SECTOR OF THE ART

The present invention relates to a synchronous electric generator and the related method for controlling the frequency and amplitude of the voltage induced on the stator windings of the synchronous electric generator. In particular, reference will be made hereinafter to a

"brushless" synchronous electric generator.

PRIOR ART

They are known synchronous and asynchronous electric machines, used both as motors and as electric generators. Hereinafter, reference is made to synchronous electric machines of the "brushless" type used as electric generators.

Over the last years, synchronous electric generators, in particular of the "brushless" type, have been used in different application fields, such as aerospace or wind- fields, (in particular, in wind turbines). In greater detail, in a known-type synchronous electric generator, the frequency of the voltage induced on the stator windings (hereinafter also referred to as stator frequency) fst is strictly linked to the rotation velocity of the rotor through the equation (1):

(1) where N _r is the rotation velocity of the rotor expressed in revolutions per minute (RPM) and P is the number of poles of the synchronous electric generator (which are defined in the construction step). As it is clear from the equation

(1), once the number of poles P is set, the stator frequency f _st directly depends on the rotation velocity N _r of the rotor.

However, in case the rotation velocity N _r of the rotor undergoes non-negligible variations as a result, for example, of variations in a velocity regime of a main motor adapted to start the rotor of the "brushless" synchronous generator, also the stator frequency f _st will undergo corresponding non-negligible variations.

Known solutions make available "brushless" synchronous generators using adjustment systems, such as systems to stabilize the frequency of the power energy produced by mechanical systems which stabilize the velocity input to the

"brushless" synchronous generator or, alternatively, electronic converters of the static type adapted to convert a variable frequency voltage, provided as input to the "brushless" synchronous generator, into an output fixed- frequency alternating voltage. However, such known solutions make such electric machines, in particular such adjustment systems, bulky and heavy.

In addition, the manufacturing processes of the aforesaid electric machines are complex and expensive.

DESCRIPTION OF THE INVENTION

The object of the present invention is to make available a synchronous electric generator and a related control method which overcome the drawbacks of the prior art.

According to the present invention a synchronous electric generator and a related method for controlling the frequency and amplitude of the voltage induced on the stator windings of the synchronous electric generator are made, as defined in the enclosed claims.

The claims describe preferred embodiments of the present invention forming an integral part of the present disclosure .

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention a preferred embodiment thereof will be now described, provided by way of non-limiting example with reference to the accompanying drawings, wherein:

- figure 1 schematically shows the present synchronous

"brushless" generator according to a preferred embodiment; and figure 2 schematically shows the present method for controlling the frequency and amplitude of the voltage implemented by the present synchronous "brushless" generator.

PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a "brushless" synchronous electric generator 1 (hereinafter referred to as generator 1), which comprises a stator 2 and a rotor 3, which are magnetically coupled between each other; and a control unit 4, operatively coupled to the stator 2.

The stator 2 comprises: an excitation winding 5, which is operatively coupled to the control unit 4; and a first and second stator winding 7, 9 which are separate and independent from the excitation winding 5, as well as operatively coupled to the control unit 4. In particular, the first stator winding 7 represents a main stator winding of the stator 2 of the generator 1.

In the embodiment of figure 1, the excitation winding

5 and the first and second stator winding 7, 9 are three- phase systems (shown in a star-shaped configuration in figure

1), coupled to the control unit 4 by a respective first, second and third three-phase line 6, 8, 10. In greater detail, the first stator winding 7 comprises one first, one second and one third connection terminal A, B, C connected to the control unit 4 by the second three-phase line 8. Given the star-shaped configuration of the first stator winding 7, the latter further has a neutral terminal N, connected to the connection terminals A, B, C of the generator 1.

The rotor 3 comprises: a rotor winding 15, including one first and one second rotor sub-windings 11, 12. The rotor sub-winding 11 is magnetically coupled to the first stator winding 7 and the second rotor sub-winding 12 is magnetically coupled to the excitation winding 5. The rotor 3 further comprises at least a magnetic element 13, independent from the rotor winding 15 and magnetically coupled to the second stator winding 9 in such a way to form an auxiliary electric generator 16.

In the embodiment of figure 1, the first and the second rotor sub-winding 11, 12 are three-phase systems (shown in a star-shaped configuration in figure 1), connected between each other by a fourth three-phase line 14. In addition, in the embodiment shown in figure 1, the at least one magnetic element 13 is a permanent magnetic generator (PMG) configured to rotate integrally with the rotor 3 when in use; in particular, when the rotor 3 (and, thus, the at least one magnetic element 13) is rotated, the at least one magnetic element 13 induces a supply voltage V _SUpply, as well as a corresponding power supply current i _supply (in particular, of the AC type) in the second stator winding 9, which are thus provided as an input to the control unit 4 such to supply the latter. In other words, the auxiliary electric generator

16 is configured to supply the control unit 4 taking advantage of the rotation of the rotor 3 and without using power supply systems outside the generator 1.

The connection terminals A, B, C are the terminals at which the control unit 4 is able to detect and, therefore, measure the voltage induced in the first stator winding 7 as a result of a rotation of the rotor 3 when it is excited by the excitation winding 5; in particular, the control unit 4 is able to detect the oscillation frequency and the voltage amplitude induced by the rotor 3 to the stator 2 when it is in use.

In the embodiment of figure 1, the control unit 4 comprises an inverter configured to convert direct currents

(DCs) or, in alternative, the power supply current i _supply coming from the second stator winding 9 as a result of the induction produced by the at least one magnetic element 13, into three-phase alternating currents having a desired frequency, the latter being controlled according to a frequency control method hereinafter described in greater detail. In addition, the control unit 4 comprises a controller (not shown), configured to adjust the amplitude of the voltages provided by the inverter of the control unit

4 according to the modes described with reference to a method for controlling the voltage amplitude hereinafter described in more detail.

In another embodiment not shown, the at least one magnetic element 13 is a multipolar machine and the control unit 4 comprises a converter (for example, a cycle converter) instead of the inverter.

The excitation winding 5 is configured, in use, to induce an induced current in the rotor 3 (in particular, on the second rotor sub-winding 12) by means of an electric excitation current iex/ generated by the control unit 4 and made flown in the aforesaid excitation winding 5; such induced current thus flows also in the first rotor sub- winding 11 and produces a rotating magnetic field which induces a voltage on the first stator winding 7 according to the modes disclosed hereinafter in greater detail.

In use, the rotor 3 is rotated by an external stimulus

(e.g •/ provided by a main motor coupled to the rotor 3) at a rotation velocity N _r '; in particular, the rotation velocity

N _r ' varies over time.

Furthermore, the control unit 4 provides an electric excitation current iex/ which is a three-phase alternating current and has an excitation frequency f _ex and an excitation current amplitude Δϊex/ at the excitation winding 5. The electric excitation current i _ex flowing into the excitation winding 5 creates a rotating magnetic field B _rot , integral with the stator 2, and that has an excitation frequency f _ex of the electric excitation current i _ex ; in other words, the rotating magnetic field Brot rotates at the excitation frequency f _ex velocity.

The rotating magnetic field B _rot thus generated couples with the second rotor sub-winding 12, inducing on the latter an induced electric excitation current i _ex,ind ; in detail, the induced electric excitation current i _ex,ind is alternating, tree-phase and has a frequency equal to the excitation frequency fex· In addition, the induced electric excitation current i _ex,ind flows also in the first rotor sub-winding 11 by virtue of the connection between the first and the second rotor sub-winding 11, 12 by means of the fourth three-phase line 14; in other words, the induced electric excitation current i _ex,ind , induced by the rotating magnetic field B _rot , flows in the entire rotor winding 15 of the rotor 3.

The induced electric excitation current i _ex,ind , passing into the first rotor sub-winding 11, generates an induction rotating magnetic field B _ind , which affects the first stator winding 7 and induces an alternating voltage V _s , having a frequency f _s and a voltage amplitude ΔV _s , on the first stator winding 7. In particular, the induction rotating magnetic field B _ind is rotating and has a rotation velocity which depends both on the rotation velocity N _r ' of the rotor 3 and on the rotation velocity of the rotating magnetic field B _rot . In greater detail, the frequency f _s and the voltage amplitude ΔV _s of the alternating voltage V _s induced on the first stator winding 7 by the induction rotating magnetic field B _ind can be measured at the connection terminals A, B, C of the generator 1 by the control unit 4. Furthermore, considering the above discussed, the frequency f _s is defined according to the equation (2):

2.

That is, the frequency f _s of the alternating voltage V _s induced on the first stator winding 7 depends in a directly proportional way both on the rotation velocity N _r ' of the rotor 3 and on the excitation frequency f _ex of the electric excitation current i _ex ; therefore, the induction rotating magnetic field B _ind affects the first stator winding 7 with a velocity which is equal to the sum of the rotation velocity

N _r ' of the rotor 3 and the velocity of the rotating magnetic field B _rot produced in the excitation magnetic field. In other words, at the first stator winding 7, a voltage induction occurs which depends both on the rotation velocity N _r ' and on the excitation frequency fex·

It must be noted that, as it can be inferred from the equation (2), if the rotor 3 is still, i.e. the rotation velocity N _r ' is null, the frequency f _s of the alternating voltage V _s induced on the first stator winding 7 is equal to the excitation frequency f _ex of the electric excitation current i _ex provided from the control unit 4.

By contrast, without excitation by the excitation winding 5 (e.g •, without the electric excitation current i _ex and, therefore, the excitation frequency f _ex ), the first rotor sub-winding 11 does not produce the induction rotating magnetic field B _ind , as there is neither the induction of the rotating magnetic field B _rot nor, thus, the induction of the induced excitation electric current i _ex,ind ; therefore, there is no induction of the alternating current V _s on the first stator winding 7.

Furthermore, it must be noted that, by supplying the excitation winding 5 with an electric excitation current i _ex having a null excitation frequency f _ex (i.e •, the control unit 4 provides a direct electric excitation current i _ex ), a rotor electric current i _r is induced on the second rotor sub-winding 12; in particular, the rotor electric current i _r is an alternating current having a frequency f _r that is directly proportional to the rotation velocity N _r '. In other words, the frequency f _r of the rotor electric current i _r is equal to the frequency of the rotation velocity N _r '.

Consequently, by virtue of the connection between the first and the second sub-winding 11, 12 by means of the fourth three-phase line 14, the rotor electric current i _r flows in the first rotor sub-winding 11, thus producing a rotating magnetic field B with a rotation velocity equal to the rotation velocity N _r ' of the rotor 3; in particular, the magnetic field B induces a corresponding alternating current

V _s , having a corresponding frequency f _s and a corresponding voltage amplitude ΔV _s on the first rotor winding 7. Therefore, as it is clear from the equation (2), the frequency f _s of the alternating voltage V _s is directly proportional only to the rotation velocity N _r ' of the rotor

3.

It must also be noted that, as previously said, the mechanical rotation of the rotor 3 at the rotation velocity

N _r ' involves a corresponding rotation also of the at least one magnetic element 13, which, therefore, induces the supply voltage V _supply , as well as the corresponding power supply electric current i _supply in the second stator winding 9; thereby, the rotation of the rotor 3 allows to supply the control unit 4 by means of the auxiliary electric generator

16. It must also be noted that, as known, the frequency of the supply voltage V _supply is strictly linked to the rotation velocity N _r '; in other words, the frequency of the supply voltage V _supply is indicative of the rotation velocity N _r '.

As previously said, the variability of the rotation velocity N _r ' may determine a corresponding variability of the frequency f _s of the alternating voltage V _s , in particular with respect to a predetermined reference frequency value f _ref (e.g •, equal to 400 Hz, in particular in the aeronautical field). In addition, the voltage amplitude ΔV _s of the alternating voltage V _s may vary with respect to a predetermined reference voltage amplitude value ΔV _ref (e.g •, in particular in the aeronautical field, equal to an average quadratic value equal to 115 V or 200 V, corresponding to a maximum value equal to 162 V or, respectively, 282 V in a sinusoidal regime) due to the induction to the air gap, i.e. due to a corresponding excitation current amplitude Δi _ex of the excitation current i _ex provided to the excitation winding

5 by the control unit 4 according to the previously described modes. In other words, the alternating voltage V _s measured at the connection terminals A, B, C may vary due to the variation of the current delivered by the generator 1 to a load, the latter being connected to connection terminals A,

B, C, N as a result of an inner impedance thereof.

Referring to figure 2, methods for controlling the frequency and, respectively, the voltage amplitude induced by the rotor 3 in the stator 2 will now be described; in particular, such control methods allow the generator 1 to manage the possible variations of frequency f _s and of the voltage amplitude ΔV _s of the alternating voltage V _s to the determined connection terminals A, B, C as a result of the variations in the rotation velocity N _r ' and, respectively, of the amplitude of excitation current Δi _ex of the electric excitation current iex· In the frequency control mode, the control unit 4 detects at the connection terminals A, B, C the frequency f _s of the alternating voltage V _s on the first stator winding 7, the latter being induced according to the modes previously described; if, at a first comparison block 17 of the control unit 4 (in particular, of the inverter of the control unit

4), the latter determines that there is a deviation (i.e. a frequency error ε _f ), between the frequency f _s detected and the reference frequency value f _ref , the control unit 4 generates an error signal indicative of the aforesaid frequency error ε _f .

Thereafter, such error signal is transmitted to a first processing block 18, which is further configured to receive a signal indicative of the rotation velocity N _r ', that can be detected from the frequency of the supply voltage V _supply on the second stator winding 9.

On the basis of the frequency error ε _f and of the rotation velocity N _r ', the first processing block 18 determines a new value of excitation frequency fex/ hereinafter indicated as a further excitation frequency f _ex ', which characterizes a further corresponding electric excitation current i / ex / the latter generated by an excitation block 20, configured to receive the further excitation frequency f _ex ' and determine the further electric excitation current i _ex ' based on the aforesaid further excitation frequency f _ex '. In greater detail, similarly to the electric excitation current iexf the further electric excitation current i _ex ' is a three-phase alternating current and it is the current that the control unit 4 is adapted to provide to the excitation windings 5 whose frequency allows to compensate the variations of the frequency f _s of the alternating voltage V _s detected at the connection terminals

A, B, C as a result of the variations of the rotation velocity

N _r ' of the generator 1 (schematized in figure 2 by means of the connection between the excitation block 20 and a block

1, the latter indicative of the generator 1, and, therefore, identified with the same reference number).

The control unit 4 thus operates similarly to what previously described, i.e. provides the further electric excitation current i _ex ' to the excitation winding 5 in such a way to cause the previously discussed electromagnetic inductions referring to the induction by means of the electric excitation current iex·

In detail, the aforesaid operations determine, finally, the induction of a further alternating voltage V _s ", having a further frequency f _s " and a further voltage amplitude ΔV _s ", on the first stator winding 7.

Similarly to what above discussed, the further frequency f _s ' and the further voltage amplitude ΔV _s ' of the further alternating voltage V _s ' induced on the first stator winding 7 can be measured at the connection terminals A, B,

C of the generator 1 by the control unit 4; in particular, referring to figure 2, a first feedback block 21, being part of the control unit 4, detects the aforesaid further frequency f _s ', which, similarly to what discussed with reference to the frequency f _s (in particular, with reference to the equation (2)), also depends directly both on the rotation velocity N _r ' of the rotor 3 and on the further excitation frequency f _ex ' of the further electric excitation current i _ex '.

Still referring to figure 2, the first feedback block

21, once determined the frequency f _s ', provides the latter to the comparison block 17.

The operations previously described with reference to the further frequency f _s ' are repeated once more if the first comparison block 17 determines that there is again a deviation (e.g •, a further frequency error ε _f ') between the further frequency f _s ' and the reference frequency f _ref ·

Alternatively, if the first comparison block 17 detects that the further frequency f _s ' and the reference frequency f _ref are substantially equal (e.g •, the frequency error ε _f ' is substantially null), the control unit 4 does not modify the further excitation frequency f _ex ' of the further electric excitation current i _ex ' provided to the excitation windings 5, i.e. the variations of the rotation velocity N _r ' are compensated by the further excitation frequency f _ex ' of the further electric excitation current i _ex ' provided.

Therefore, in a frequency control mode, the control unit 4 is configured to adjust the frequency of the current provided to the excitation winding 5 as a result of the rotation speed N _r ' of the rotor 3 and of the reference frequency value f _ref . In other words, in a frequency control mode, the control unit 4 is configured to compensate variations of the rotation velocity N _r ' of the rotor 3 by a feedback control which allows to virtually modify the rotation velocity N _r ' by means of the current frequency provided by the control unit 4 to the excitation winding 5.

It must be noted that the aforesaid frequency control of the voltage induced on the first stator winding 7 is performed in a constant way, i.e. the control unit 4 monitors at any time instant the aforesaid frequency value; therefore, the present generator 1 allows to monitor and adjust the frequency of the induced voltage on the first stator winding

7 substantially in real time.

In the control mode of the voltage amplitude, the control unit 4 detects at the connection terminals A, B, C the voltage amplitude ΔV _s of the alternating current V _s on the first stator winding 7; if, at a second comparison block

22 of the control unit 4, the latter determines that there is a deviation (i.e. an error in the voltage amplitude ε _ν ) between the detected voltage amplitude ΔV _s and the value of the reference magnitude amplitude Av _ref , the control unit 4 generates an error signal indicating the aforesaid error of voltage amplitude ε _ν ·

Thereafter, such error signal is transmitted to a second processing block 23.

Therefore, on the basis of the voltage amplitude error εv, the second processing block 23 determines a further excitation current Δi _ex ", which characterizes a further electric excitation current i _ex ", the latter generated by the excitation block 20 on the basis of the aforesaid further excitation current Δi _ex "

The control unit 4 thus operates similarly to what previously described, i.e. provides the further electric excitation current i _ex ' to the excitation winding 5 in such a way to cause the electromagnetic inductions previously discussed referring to the induction by means of the electric excitation current i _ex and of the further electric excitation current i _ex 'of the frequency control mode.

In detail, the aforesaid operations determine, finally, the induction of a further alternating voltage V _s ", having a further frequency f _s " and a further voltage amplitude ΔV _s ", on the first stator winding 7. In particular, the further voltage frequency ΔV _s " can be measured at the connection terminals A, B, C by a second feedback block 24, which is further configured to provide the further voltage amplitude ΔV _s " detected at the second comparison block 22.

The operations described with reference to the further voltage amplitude ΔV _s " are repeated once more if the second comparison block 22 determines that there is again a deviation (i.e •9 a further voltage error ε _ν ') between the further voltage amplitude ΔV _s " and the reference voltage amplitude AV _ref . Alternatively, if the second comparison block 22 detects that the further voltage amplitude ΔV _s " and the reference voltage amplitude AV _ref are substantially equal

(i.e •, the voltage error ε _f ' is substantially null), the control unit 4 does not modify the further amplitude of excitation current Δi _ex " of the further electric excitation current i _ex ' provided to the excitation windings 5, i.e. the further voltage amplitude ΔV _s " is stabilized at the reference voltage amplitude AV _ref .

Therefore, in a voltage control mode, the control unit

4 is configured to adjust the amplitude of the electric current provided to the excitation winding 5 as a result of the variations in the amplitude of the alternating current induced in the stator winding 7. In other words, in a voltage amplitude control mode, similarly to the frequency control mode, the control unit 4 is configured to compensate variations of the voltage amplitude induced on the first stator winding 7 with respect to a reference amplitude value by a feedback control by means of the amplitude of the electric current provided to the excitation winding 5.

It must be noted that, even in the voltage amplitude control mode, the aforesaid control of the amplitude of the voltage induced of the first stator winding 7 is performed in a constant way, i.e. the control unit 4 monitors at any time the aforesaid value of the amplitude of the induced voltage; therefore, the present generator 1 allows to monitor and adjust the amplitude of the voltage induced on the first stator winding 7 substantially in real time.

The aforesaid frequency and voltage amplitude control modes may be performed as separate from each other or in combination between each other, such to separately, or respectively control, simultaneously the frequency and amplitude of the voltage induced on the first stator winding

7 of the stator 2.

The synchronous electric generator and the herein described related control method have several advantages.

First of all, the present generator 1 allows to stabilize both the frequency and the amplitude of the voltage induced on the first stator winding 7 to predetermined respective frequency and amplitude values. In fact, the control unit 4 is able to compensate variations of the frequency f _s of the alternating current V _s induced on the first stator winding 7 with respect to a reference frequency value f _ret as a result of the variation in the rotation velocity N _r ' of the rotor 3; furthermore, the control unit 4 is configured to compensate variations of the voltage amplitude ΔV _s of the alternating voltage V _s with respect to the reference value of voltage amplitude AV _ref as a result of the variations of the excitation current amplitude Δi _ex ·

Furthermore, the present synchronous electric generator is less bulky and lighter than the known-type electric machines, as it does not use either external adjustment systems or supply systems, as it makes use of the excitation winding 5 for controlling the frequency and amplitude of the voltage induced on the first stator winding 7 and, respectively, of the auxiliary electric generator 16; in addition, the present synchronous electric generator is of easy construction.

It is finally clear that changes and variations can be made to the electric machine and method herein described and illustrated without departing from the protection scope of the present invention, as defined in the appended claims.

Furthermore, the windings of the stator 2 and rotor 3 may be multiphase, i.e. they may be systems with a number of phases greater than three.

LIST OF THE REFERENCE NUMBERS OF THE FIGURE

1 "brushless" synchronous electric generator

2 stator

3 rotor 4 control unit

5 excitation winding

6 first three-phase line

7 first stator winding

8 second three-phase line

9 second stator winding

10 third three-phase line

11 first rotor sub-winding

12 second rotor sub-winding

13 magnetic element

14 fourth three-phase line

15 rotor winding

16 auxiliary electric generator

A first connection terminal

B second connection terminal

C third connection terminal

17 first comparison block

18 first processing block

20 excitation block

21 first feedback block

22 second comparison block

23 second processing block

24 second feedback block fst stator frequency

N _r rotation velocity P poles

N _r · rotation velocity V _supply supply voltage i _supply power supply current iex electric excitation current fax excitation frequency

Δi _ex excitation current amplitude iex,ind induced excitation current amplitude

Brot rotating magnetic field

Bind induction rotating magnetic field

V _s alternating current ΔV _s voltage amplitude fs frequency

B magnetic field ir rotor electric current fr frequency f _ref reference frequency

AV _ref reference voltage amplitude ε _f frequency error f _ex ' further excitation frequency i _ex ' further electric excitation current

Vs' further alternating voltage fs' further frequency ΔV _s ' further voltage amplitude εf' further frequency error ε _ν voltage amplitude error

Δi _ex " further excitation current amplitude

-i _ex " further electric excitation current Vs'' further alternating voltage fs'' further frequency ΔV _s " further voltage amplitude ε _ν ' further voltage amplitude error