VOLTAGE REGULATOR FOR AN AC GENERATOR

Abstract: A voltage regulator for an AC generator (1) with capacitor excitation is didsclosed, comprising a stator unit (2) having a stator body (4) on which at least an armature winding (5) is wound having at least a couple of terminals (6a, 6b) connected to a load (Z) and at least an auxiliary winding (5) and electrically series connected to at least an excitation capacitor (C>1<); a rotor unit (3) having a rotary body (10) on which at least an excitation winding (11) is wound electrically connected to at least a rectifying circuit (12). Said regulator comprises also at least a switch (13) electrically connected to said auxiliary winding (8) and to said at least one capacitor (C>1<), said switch being controlled by a control device (15) opening or closing said switch (13) to regulate circulation of current (ic) in said auxiliary winding (8) as a function of the variations of the voltage acoss said armature winding (5).

Description: VOLTAGE REGULATOR FOR AN AC GENERATOR The present invention relates to a voltage regulator for an alternated current (AC) with excitation of the kind with capacitor provided with a self- regulating device of the generated voltage. It is well known that an AC generator or alternator is a synchronous rotary electrical machine transforming the mechanical power of the shaft into electrical power. More particularly the electrical power is generated in the form of sinusoidal AC having a frequency rigidly connected to the operative velocity of the machine. In such a machine the rotor has the function of inductor, that is to generate the magnetic filled field with the primary and auxiliary induced windings of the stator, respectively. More particularly the rotor comprises a rotary body on which an excitation winding is wound, closed on one or more rectifying diodes to form the inductor circuit. As to the stator, it comprises a body on which a primary winding feeding the load and an auxiliary winding closed on a capacitor are wound and arranged orthogonal to one another. When the machine is being rotated, the residual magnetic field of the rotor body generates in the auxiliary circuit a current increasing the current in the inductor circuit so as to increase the induction flux. Increase of the induction flux increases the current in the auxiliary circuit that in turn causes an increase of current in the inductor circuit so as to increase further the induction flux. In this way a positive reaction is initiated, leading the machine to increase the induction flux until a maximum value is reached corresponding to the saturation value of the material of the rotor body. The main drawback of these machines consists in that the output voltage applied to the load is varying with the variation of the load. Indeed it is known that in the idling operation the induction flux of the machine is produced only by the magneto motive force generated by the inductor circuit and is directly proportional to the induced electro motive force, while in the loaded operation the induction flux depends also from the armature reaction of the primary winding. Such a reaction occurs in the inductor circuit where it is a function of the current intensity in the armature circuit and of the phase difference between current and its induced electro motive force and in the auxiliary circuit as well. More particularly when the primary winding is closed on a merely resistive load, the current circulating on the excitation circuits tend to decrease opposed by the action of the auxiliary circuit. The machine therefore is led to work in such a balance condition as to warrant the desired voltage to the load. When the primary winding is closed on a mainly inductive load, the current on the load, the conditions being equal, causes an armature reaction on the auxiliary circuit lower than the preceding case. Consequently the machine is led to work in a balance condition characterized by a voltage lower than the desired voltage. On the contrary when the primary winding is closed on a mainly capacity load, the machine behaves in a complementary mode to the preceding case, leading to work in a balance condition characterized by a higher voltage relative to the desired voltage. Another drawback consists in that the output voltage applied to the load varies considerably when the number of revolutions of the rotor is varying. A last but not least drawback consists in that the output voltage varies according to the variation of the operative temperature of the generator, which increases as it is well known, during generator operation in view of the losses due to the Joule effect. The object of the present invention is overcome the foregoing drawbacks. More particularly a first object of the invention is to provide a voltage regulator for an AC generator which is self - regulating the voltage applied to the load so as to keep the voltage on the load substantially constant when the load is varying. Another object is to provide a self- regulating device having a cost weighing in a limited quantity on the total cost of the generator. It is a further object to provide a device that can be applied also to already installed generators generally lacking of a device regulating the output voltage. Still another object is to provide a strong and reliable regulating device to be applied to a generator. The foregoing objects are attained by a voltage regulator for an AC generator with capacitor excitation that according to the main claim comprises: - a stator unit having a stator body on which at least an armature winding is wound, having at least a couple of terminals adapted to be connected to a load and at least an auxiliary winding arranged generally at 90 electrical degrees relative to said armature winding and electrically connected to at least an excitation capacitor; - a rotor unit having a rotary body on which at least an excitation winding is wound, electrically connected to at least a rectifying circuit; said device being characterized by comprising at least a piloted switch electrically connected to said auxiliary winding and to said at least a capacitor, said switch having at least a control input electrically connected to a control device opening/closing said piloted switch as a function of the voltage variations across said armature winding. The control device may be indifferently made according to an analog or digital or even mixed technology. According to a preferred embodiment of the invention, the switch piloted by a control device is series connected to the auxiliary winding and the excitation capacitor. According to a further embodiment of the invention, the switch is series connected to a further generally capacitive impedance and both are paralleled relative to the excitation capacitor. According to another embodiment of the invention the switch is directly paralleled to the excitation capacitor. Advantageously a generator provided with the regulator of the invention tolerates higher variations of velocity of the rotor unit in comparison with the known generators, so as to warrant generally constant output voltages. Still advantageously the generator with the device of the invention allows to control the voltage applied to the load by acting on the current circulating on the auxiliary winding that is known to be of lower intensity in comparison with the other currents of the system, so as to allow control of great powers with little powers. Further advantages and features of the invention will be better understood by reading the description of three preferred embodiments of the invention given as illustrative and non limiting examples, with reference to the accompanying sheets of drawings in which: - Fig. 1 is a schematic illustration of a generator provided with a first embodiment of the device of the invention; - Fig. 2 is a block diagram of the control device of the generator of Fig. 1 ; - Fig. 3 shows a particular embodiment of the switch of the device shown in Fig. 1 ; - Fig. 4 is a graph relative to voltages and currents plotted during operation of the generator of Fig. 1 excited according to the diagrams of Fig. 7 or Fig. 8; - Fig. 5 is a block diagram of a version of the regulator of Fig. 1 ; - Fig. 6 is a block diagram of another version of the regulator of Fig. 1 ; - Fig. 7 is a basic scheme of the auxiliary winding with the switch of the invention series arranged with said winding; - Fig. 8 shows a version of the device of the invention wherein the switch is series connected to a capacitor and both are paralleled with the excitation capacitor; - Fig. 9 shows a further version of the device of the invention wherein the switch of the invention is arranged directly in parallel with the excitation capacitor belonging to the auxiliary circuit; and - Fig. 10 shows the development of voltage and current of the switch during the opening and closing periods of the switch in each half cycle according to the circuit of Fig. 9. The AC generator with capacitor excitation of the present invention is diagrammatically shown in Fig. 1 where it is generally indicated with reference numeral 1 and substantially comprises a stator unit 2 and a rotor unit 3. The stator unit 2 comprises a stator body 4 in the slots of which a single phase armature winding 5 is wound, having a couple of terminals 6a, 6b connected to the load Z and an auxiliary winding 8 arranged at 90 electrical degrees to the armature winding 5, and electrically connected to a capacitor C-j. The rotor unit 3 comprises a rotary body 10 on which an excitation winding 1 1 is wound and electrically connected to a rectifying circuit generally indicated with 12. According to the invention the self- regulating device of the generator 1 comprises a piloted switch 13 series arranged to the auxiliary winding 8 and capacitor Ci, having a control input 14 electrically connected to a control device generally indicated with 15, opening or closing switch 13 in order to prevent or allow circulation of current IC in the auxiliary winding 8 as a function of the voltage variations across the armature winding 5 and load Z. Opening and closing the piloted switch 13 causes the current IC circulating in the auxiliary winding 8 to be varied, so as to vary the electro motive force induced on the excitation winding 11 and therefore on the load Z. One can see that by varying properly the opening and closing duration ratio of the switch, a variable excitation current in the auxiliary winding is obtained, so as to induce in the excitation circuit such a current as to keep constant the voltage on the load. The control device 15 shown in detail in Fig. 2, controls opening of switch 13 when the voltage across the armature winding 5 is different from the set voltage. More particularly the device controls opening of switch 13 when voltage across the auxiliary winding 8 is higher than the difference between a reference voltage and the average value in one half cycle of the voltage across the armature 5. The control device setting the duration of opening and closing time of the switch in one half cycle comprises: - a first input 16 electrically connected to a reference voltage generator 17; - a second input 18 connected across the armature winding 5; - a third input 19 electrically connected across the auxiliary winding 8 through differential devices 51 a; - and an output 20 connected to the control input 14 of the piloted switch 13. As to the first 16 and second 18 input, they are electrically connected to the inputs of a summing block 21 supplying an output signal proportional to the difference between inputs 18 and 16 and constituting the first input of a comparator block 22. With regard to the third input 19, it is electrically connected to the other input of the comparator block 22, which compares the voltages at its inputs and supplies an output signal 20 of logic type which is fed back to the output of the control device 15 and controls opening or closing of switch 13. More particularly the comparator 22 controls closure of a switch 13 when the voltage across the auxiliary winding 8 is lower than the voltage fed at the output of the summing block 21. With regard now to the piloted switch 13, it behaves like a bi-directional power switch controlled through the control terminal 14. More particularly as shown in detail in the example of Fig. 3, the bi-directional switch comprises a plurality of rectifying diodes 23 mutually connected to a power transistor 24, preferably consisting of an IGBT or a MOSFET or a bipolar transistor. More particularly the diodes 23 constitute a GRAETZ bridge having the input terminals 25, 26 electrically connected across the auxiliary winding 8 and the positive 27 and negative 28 output terminals connected to collector and emitter of the power transistor 24 respectively. As to the control terminal 14, it consists of the base of the power transistor 24 which allows or prevents passage of current between collector and emitter. It is important to note that the described configuration allows to use a single power transistor that it is known to be a unidirectional controlled switch to obtain a bi-directional controlled switch. It is also to be noted that opening of switch 13 causes reaction of the auxiliary winding 8 generating a reaction voltage that is known to be proportional to the variation of the current ic circulating in said winding and could cause damage of transistor 24 if the latter was not properly dimensioned. In order to remove such a limitation the invention provides for a double pole Z 2 , shown in Fig. 1 , connected in parallel to switch 13 keeping even at open switch a minimum value of current ic circulating in the auxiliary winding 8. More particularly the double pole Z 2 has across its terminals a generally capacitive impedance preferably consisting of a capacitor C . In this way the auxiliary winding 8 has always a capacitive load applied across its terminals, which is variable according to the opening or closing position of switch 13. The capacitive load varies from a maximum value Ci when the switch is closed, to a minimum value equal to (CιxC 2 )/(Cι+C 2 ) when the switch is open. This allows to have alternatively a maximum excitation and a minimum excitation of the excitation winding 4 at the maximum and minimum capacitive load applied to the auxiliary winding 8. It is important to note also that voltages taken from windings 8 and 5 and used by the control device 15 are all previously rectified through a plurality of rectifying circuits 32a and 32b of a kind known per se which therefore will be not described in detail hereinafter. The operation of the generator provided with the device of the invention will be now described in greater detail, making reference to the circuit of Fig. 2 with the aid of the graph shown in Fig. 4 illustrating the wave forms relative to the current ic circulating on the auxiliary winding 8 and the rectified voltages across the auxiliary winding V aux and switch V in t respectively as a function of the opening and closing state of switch 13. More particularly Fig. 4 shows the development: - with broken line of the current ic circulating in the auxiliary winding 8; - with dash dot line of voltage V aux shown rectified across the auxiliary winding 8; - with solid line the voltage Vj n t shown rectified across the piloted switch 13; and - with dotted line the value Vι_ of the voltage at the output of the summing block 21. As one can see, the control device 15 controls opening of switch 13 as soon as the voltage across the auxiliary winding V aux is greater than the reference voltage VL. At the same time of opening switch 13, the current Ac on the auxiliary winding 8 is annulled while voltage of switch Vin t that previously was clearly null, is increasing across the switch 13. It is to be noted that opening of switch 13 may generate a number of oscillations although signal Vj nt remains substantially with a sinusoidal development. The closing control of switch 13 is given by the control device 15 as soon as voltage V aU χ across the auxiliary winding 8 becomes lower than the reference voltage VL. Opening and closing of switch 13 causes a modulation of the current ic circulating in the auxiliary winding 8 and a consequent variation of excitation of generator 1 that is known to cause a variation of voltage across the load Z. A constructional version shown in detail in Fig. 5 according to the invention is different from the preceding embodiment because the switch 13 is brought to the closure position only when voltage at its terminals V in t is close to zero. This arrangement allows advantageously to use as a switch a power transistor 24 having characteristics of maximum admissible current lower than the previously described solution, so as to decrease power dissipation and total cost of the regulator. Indeed if closure of switch 13 occurs when a high voltage V, n is present at its terminals, transistor 24 is heated and if it is not properly dimensioned, can be irreparably damaged. To this purpose the generator 4 of the invention comprises a device 35 enabling closure of switch 13 and having the output 36 electrically connected to the control terminal 14 for opening and closing the switch. The enabling device consists of an AND logic gate 37 combining output 20 of the control device 15 with the signal coming from a detector block 38 of the zero crossings of voltage across switch Vj nt . This voltage V in t is the output voltage from the differential device 51 b at whose input voltages across the switch 13 are coming. The zero crossing detector 38 of voltage Vj nt across the switch 13 consists of a comparator 39 having at one input 40 the rectified voltage V| nt across switch 13 and at the other input a reference voltage substantially equal to zero. A further constructional version shown in detail in Fig.6 according to the invention is different from the preceding one because the switch 13 is closed at least once at each half wave of the voltage across the auxiliary winding 8, even if the control device 15 does not send the signal of closing the switch 13. To this purpose the generator of the invention is provided with an override device 40 of the closure of the piloted switch 13 having its output electrically connected to the control terminal 14 of switch 13. The override device 40 consists of an OR gate 41 combining the output signal 36 of the enabling device 35 with the signal coming from a zero crossing detector 47 of the voltage V aux across the auxiliary winding 8. The zero crossing detector 47 of voltage V aux across the auxiliary winding 8 also consists of a comparator 48 having at one input 49 the rectified voltage V aU χ across the auxiliary winding 8 and at the other input 50 a reference voltage substantially equal to zero. From the foregoing it is clear that the generator of the invention attains the above mentioned objects. According to a constructional version diagrammatically shown in Fig. 8, the piloted switch 13 instead of being series connected as shown in figs. 1 and 7 to the auxiliary winding 8 and the excitation capacitor C-i, is arranged in parallel to the excitation capacitor Cι. More particularly the parallel leg of the switch 13 has another capacitor C 3 series connected to switch 13. Also in this circuit condition opening of the switch occurs by the control device when voltage across the auxiliary winding V aux is equal to or higher than a limit voltage VL given by the control device. Closure of the switch occurs always when the voltage at its terminals is substantially null. The piloting action of the switch according to this version occurs substantially in the same way of the switch series arranged according to the circuit of Figs. 1 and 7. According to another possible version of the invention diagrammatically shown in Fig. 9, switch 13 is arranged in parallel to the exciting capacitor C-i, both being in turn series connected to the auxiliary winding 8. Under these circuit conditions, while the switch is always being closed by the control device when voltage at its terminals is substantially null, opening of the switch on the contrary occurs when the control device of the switch detects that the current crossing the switch is higher than a predetermined limit set by the control device. The development of tension and current of the switch in the embodiment of fig. 9 are shown in the diagrams of Fig. 10 showing the development of these electrical quantities during the opening and closing stage of the switch. Although the invention was described with reference to the accompanying sheets of drawings, it may undergo in the constructional stage modifications that are all falling within the inventive principle set forth in the appended claims and therefore covered by the present patent.

Claims: CLAIMS 1 ) A voltage regulator for an AC generator (1 ) with capacitor excitation comprising: - a stator unit (2) having a stator body (4) on which at least an armature winding (5) is wound, having at least a couple of terminals (6a, 6b) connected to a load (Z) and at least an auxiliary winding (8) generally arranged at 90 electrical degrees relative to said armature winding (5) and electrically series connected to at least an excitation capacitor (C-i); - a rotor unit (3) having a rotary body (10) on which at least an excitation winding (11) is wound electrically connected to at least a rectifying circuit; characterized by comprising at least a switch (13) electrically connected to said auxiliary winding (8) and to said at least one capacitor (Ci), said switch being controlled by a control device (15) opening or closing said switch (13) to regulate the circulation of current (ic) in said auxiliary winding (8) as a function of the voltage variations across said armature winding (5). 2) The regulator according to claim 1 ) characterized in that said switch is series connected to said auxiliary winding (Fig. 4, Fig. 7). 3) The regulator according to claim 2) characterized in that a double pole (Z 2 ) is arranged in parallel to said switch and has a substantially capacitive impedance at its terminals. 4) The regulator according to claim 3) characterized in that said double pole (Z 2 ) consists of at least a capacitor (C 2 ). 5) The regulator according to claim 1 ) characterized in that said switch is series connected to at least a further generally capacitive impedance, both being arranged in parallel to said capacitor C (Fig. 8). 6) The regulator according to claim 5) characterized in that said generally capacitive impedance consists of at least a capacitor (C 3 ). 7) The regulator according to claim 1 ) characterized in that said switch is directly connected in parallel with said excitation capacitor (Fig. 9). 8) The regulator according to any of claims 1 ) to 7) characterized in that said switch is closed by the control device when the voltage across its terminals is substantially null. 9) The regulator according to any of claims 2) to 5) characterized in that said switch is opened by the control device when voltage across the auxiliary winding (V aux ) is not lower than a limit voltage (VL) set by the control device. 10) The regulator according to claim 9) characterized in that said limit voltage (VL) is the difference between a reference voltage (V rif ) and the average value in one half cycle of the voltage across the armature (5). 11 ) The regulator according to claim 6) or 7) characterized in that said switch is opened by the control device when the current of said switch is higher than a limit set by the control device of the switch. 12) The regulator according to claim 2) characterized in that said control device (15) comprises: - a first input (16) electrically connected to a generator of a reference voltage (17); - a second input (18) electrically connected across said armature winding (5); - a third input (19) electrically connected across said auxiliary winding (8); - at least an output (20) electrically connected to said control input (14) of said piloted switch (13); said output (20) controlling opening of said switch (13) when voltage (V aux ) across said auxiliary winding (8) is greater than the difference between the reference voltage (V r j f ) and the average value in one half cycle of the voltage across said armature winding (5). 13) The regulator according to claim 12) characterized in that said control device (15) comprises a summing block (21 ) having an input electrically connected to said first input (16) and another input electrically connected to said second input (18), said summing block (21 ) supplying an output signal (21 1 ) proportional to the difference between said inputs and constituting an input of a comparator block (22) having another input electrically connected to said third input (19) and the output (20) electrically connected to said output, said comparator block (22) controlling the closure of said switch (13) when the voltage (V aux ) across said auxiliary winding (8) is lower than the voltage (VL) supplied as output from said summing block (21 ). 14) The regulator according to claim 12) or 13) characterized in that between said output (20) of said control device (15) and said control input (14) of said piloted switch (13), an enabling device (35) for the closure of said switch (13) is interposed, comprising: - a first input electrically connected to a detector block (38) of the zero crossings of voltage (Vj nt ) across said switch (13); - a second input electrically connected to output (20) of said control device (15); - an output electrically connected to said control input (14) of said piloted switch (13); said enabling circuit (35) enabling closure of said piloted switch (13) only when voltage (V| nt ) across said piloted switch (13) is substantially zero (Fig. 5). 15) The regulator according to claim 14) characterized in that said zero crossing detector (38) of the voltage (V in t) across said piloted switch (13) consists of a comparator (39) having at one input (391 ) the voltage across said piloted switch (13) and at the other input a reference voltage substantially equal to zero. 16) The regulator according to any of claims 12) to 15) characterized in that between said output of said control device (15) and said control input (14) of said piloted switch (13), an override device (40) of the closure of switch (13) is interposed comprising: - a first input electrically connected to a zero crossing detecting circuit (47) of the voltage (V aux ) across the auxiliary winding (8); - a second input electrically connected to the output of said control device (15) of the closure of said switch (13); - an output electrically connected to said control input (14) of said piloted switch (13); said override circuit (40) enabling closure of said piloted switch (13) when voltage (V aux ) across said auxiliary winding (8) is substantially zero (Fig. 6). 17) The regulator according to claim 16) characterized in that said override device (40) consists of an OR logic gate. 18) The regulator according to claim 16) or 17) characterized in that said zero crossing detecting circuit (47) of voltage (V aux ) across said auxiliary winding (8) consists of a comparator (48) having at one input (49) the voltage across said auxiliary winding (8) and at the other input a reference voltage substantially equal to zero. 19) The regulator according to claim 1) characterized in that said at least one piloted switch (13) comprises a bi-directional power switch. 20) The regulator according to claim 19) characterized in that said bi- directional power switch consists of a bridge of diode rectifier (23) having the input terminals (25, 26) electrically connected across said auxiliary winding (8) and the output terminals (27, 28) electrically connected to collector and emitter of at least a power transistor (24) having its base constituting said control input (14) of said piloted switch (13). 21 ) The regulator according to claim 20) characterized in that said at least one power transistor (24) has the collector electrically connected to the positive terminal (27) of said diode rectifying bridge (23) and the emitter electrically connected to the negative terminal (28) of said diode rectifying bridge (23). 22) The regulator according to any of claims 12) to 19) characterized in that a rectifying circuit (32b) is interposed between said second input (18) of said control device and said armature winding (5). 23) The regulator according to any of claims 12) to 19) characterized in that a second rectifying circuit (32a) is interposed between said second input (18) and said auxiliary winding (8). 24) The regulator according to any of the preceding claims characterized in that said piloted switch (13) is closed at least once for each half wave of voltage (V aux ) across the auxiliary winding (8). 25) The regulator according to any of the preceding claims characterized in that said rectifying circuits (12, 32a, 32b) comprise at least a rectifying diode.

Inventors: CARRARO DIEGO, MALESANI LUIGI, CARRARO, DIEGO, MALESANI, LUIGI
Applicants: CARRARO DIEGO, MALESANI LUIGI, CARRARO, DIEGO, MALESANI, LUIGI
Application year: 2001
Main classification: H02K19
IPC codes: H02K19, H02P9