Known alternators supply the electrodes of a welder with a current of about 165 A at the nominal welding voltage, i. e. about 26,6 V. These known alternators weigh at least 80 Kg, motor included, so that they are difficult to be transported.

It is therefore an object of the present invention to provide an alternator which is much lighter, and therefore transportable, than the known alternators. Said object is achieved with an alternator, the main features of which are disclosed in the first claim and other features are disclosed in'the subsequent claims.

Thanks to the particular compact structure of the rotor and the stator, the alternator according to the present invention weighs less than 30 Kg, motor included, with the same supplied currents and voltages. Furthermore, the particular structure of the rotor allows to firmly keep in position the magnets even when the rotor turns at a high speed and therefore the centrifugal force acting on them is high.

According to a particular innovative aspect of the invention, at least three windings of the stator are star-connected, while the other windings are delta- connected, so that the star-connected windings supply at least 70 V necessary for starting the welder and the delta-connected windings supply the current necessary for the working of the same welder at the nominal welding voltage.

According to another particular innovative aspect of the invention, three windings of the stator can be star-connected and distributed on more radial protrusions for supplying an electric energy at about 110 V to other devices besides the welder. Furthermore, the rotation speed of the motor which drives the rotor can be automatically controlled by a control unit according to the electric energy absorbed by the welder and/or by said devices.

Further advantages and features of the alternator according to the present invention will become evident to those skilled in the art from the following detailed and non-limiting description of an embodiment thereof with reference to the attached drawings, wherein:

figure 1 shows a front view of the alternator according to said embodiment of the invention ; - figure 2 shows a top cross-sectioned view along plane II-II of the alternator of figure 1; - figure 3 shows a first electric scheme of the windings of the stator of the alternator of figure 1; - figure 4 shows a second electric scheme of the windings of the stator of the alternator of figure 1; and - figure 5 shows the electric scheme of the alternator of figure 1.

Referring to figures 1 and 2, it is seen that the alternator according to the present embodiment of the invention comprises a rotor R having a substantially cylindrical shape in which a stator S provided with a plurality of radial protrusions, for example 27 protrusions P1... P27 having a substantially parallelepiped shape, is arranged in a coaxial manner. A plurality of magnets Mn, for example 18 permanent magnets M1... M18 having the polarities arranged along a circumference substantially concentric to the rotor R, is, arranged on the inner cylindrical surface of rotor R. The number of magnets Mn is two thirds of the number of protrusions Pm. A hub H in . which is keyed a shaft A connected to a motor (not shown in the figure), for example an internal-combustion engine having a power comprised between 5 and 8 kW and provided with an accelerator, is fixed in the middle of rotor R : The turns of one or more windings suitable for generating an alternated electromotive force according to the variation of the magnetic field of magnets Mn when the motor drives rotor R are wound around protrusions Pm.

With. reference to figure 3, it is seen that a, plurality a of groups of three windings Wx, for example 8 groups of three windings W1... W8, is arranged around a corresponding plurality a of groups of three protrusions Pm separated with a pitch a, for example around the protrusions P1, P9 and P17 and the subsequent seven groups of three protrusions P2... P8, P10... P16 and P18... P24. At least the-group of three windings W1 is connected to three terminals T1 by means of a star scheme, while the remaining seven groups of three windings W2... W8 are connected to seven groups of three terminals T2... T8 by means of a delta scheme (in the figure the windings from

W3 to W7 are shown with broken lines). Each winding W1... W8 comprises 40 to 60 turns with a wire having a diameter comprised between 0,7 and 1,1 mm, in particular 47 turns of a wire having a diameter of 0,9 mm.

Referring to figure 4, it is seen that three auxiliary windings W9 can be distributed with a pitch 3 around three consecutive protrusions Pb, P (b+l), P (b+2) and the subsequent ones, always with a pitch 3, for example around the protrusions PI, P2 and P3 and the subsequent seven groups of three consecutive protrusions P4... P22, P5... P23 and ! P6... P24. These three auxiliary windings W9 are connected to three terminals T9 by means of a star scheme. Each of the three auxiliary windings W9 comprises 7 to 11 turns for each of the eight protrusions Pb, P (b+1), P (b+2) with a wire having a diameter comprised between 0,7 and 1,1 mm, in particular 9 turns of a wire having a diameter of 0,9 mm. Furthermore, three auxiliary windings W10 can be arranged around the last three consecutive windings P25, P26 and P27. These three auxiliary windings W10 are connected to three terminals T10 by means of a delta scheme and comprise 40 to 60 turns with a wire having a diameter comprised between 0,7 and 1,1 mm, in particular 47 turns of a wire having a diameter of 0,9 mm. In other embodiments free from windings W10, stator S of the alternator according to the present invention may comprise only a number 3a of radial protrusions Pm, for example 24 radial protrusions, while rotor R may comprise a number 2a of magnets Mn, for example 16 magnets.

Figure 5 finally shows that the ten groups of three terminals T1... T10 of the ten groups of three windings W1... W10 are connected to the three inputs of ten three- phase rectifiers B, for example three-phase rectifiers with diode bridges. At least one current sensor CS of., the known kind arranged in a control unit CU is preferably. connected in series with one of the lines LI connecting a terminal Tl of the winding W1 to the relevant rectifier B, so that this sensor CS can detect the current passage through this line. A power unit PU of the known kind which supplies the control unit CU with electric energy is connected in parallel with the other two lines Ll connecting terminal Tl with rectifier B. The current sensor CS can be connected in series also with one of the lines L9 connecting a terminal T9 of winding W9 with the relevant rectifier B, so as to detect also the current passage, if any, through this line.

The control unit CU further comprises a control circuit CC of the known kind directly or indirectly connected with the current sensor CS, so that when the latter detects the presence of a current, the control unit CU supplies with a current a solenoid TS. The latter is provided with a mobile member mechanically connected with the accelerator of the motor of rotor R,. so as to increase its rotation speed when the control unit CU supplies it with a current.

The positive and negative outputs of rectifiers B of the windings Wl to W8 are connected in parallel with a pair of output terminals OT which can supply with an electric current, for example the electrodes of a welder, according to the rotation speed of rotor R. In particular, the outputs of rectifiers B of windings W6 and W8 are directly connected in parallel to the outputs of rectifiers B of windings W5 and W8, respectively. For varying the current between the output terminals OT during the rotation at full capacity of rotor R, a progressive selector PS is connected in series with the positive lines of rectifiers B of some windings, in particular of windings W3, W4, W5, W6, W7, W8 and W10, so as to sum the current contribution of these rectifiers according to the position of the same selector. The outputs of rectifier B of winding W9 are instead connected with an electric socket ES, preferably provided with an earthed connection and with a security switch SS of the known kind, which is connected in series with the line connecting the positive output of rectifier B with the same socket.

During the use, the motor of the alternator according to the present embodiment of the invention rotates rotor R at a relatively low speed, for example about 2000 round per minute. When an electric current passes through the electric socket and/or the output terminals OT, the current sensor CS sends a signal to the control circuit CC which, by means of solenoid TS, acts on the motor accelerator, thereby increasing its speed to about 4000 round per minute. In the present embodiment of the invention, the alternated electromotive force generated at the ends of windings Wx has a frequency f comprised between 500 and 700 Hz, for instance 600 Hz, for the known formula f = (number of magnets) * 4000/60 = 9 * 4000/60 = 600. Thus, the voltage at the electric socket ES is about 110 V with a continuous current of about 16 A, while the open-circuit voltage at the terminals OT, principally

s determined by windings Wl, is about 80 V. When a load, for example the electrodes of a welder, is connected with terminals OT, the voltage falls to about 24 V, while the current, according to the position of the progressive selector PS, changes between about 60 and 165 A with this welding voltage.

Modifications and/or additions, if any, may be made by those skilled in the art to the embodiment of the invention hereinabove described and illustrated while remaining within the scope of the same invention.