Device for the voltage adjustment in a slow-running alternator

Abstract: The invention realizes a device for the adjustment of the terminal voltage of a single-phase alternator (1) coupled to an endothermic motor, said device including a current sensor (10) of the alternator load transferring a signal (Is) to the entry of a comparator logic circuit (21), said comparator circuit emitting a first outgoing signal (25) suitable for controlling a shutter (23) connected to the accelerator of the endothermic motor. Said comparator circuit (21), in case of a substantial loss lack and of reduced revolutions per minute, emits a second electric signal (26) suitable to close a switch (22) connecting at least an auxiliary capacitor (17) to the excitation circuit (18) of the alternator (1), so that the excitation current causes a voltage to the alternator terminals substantially not lesser than 50% of the rated voltage.

Description: The invention concerns a device for the voltage adjustment in a self-energized and self-regulated single-phase alternator when said alternator is working at a reduced speed with respect to the rated one. It is known that especially single-phase alternators, coupled to endothermic motors and having a low capacity, for example 1 to 10 kW, are variously used, for example during markets or during open-air maintenance-repairs for which it is necessary to use electric tools and it is difficult to have a nearby electric power supply. The alternators which are used that way undergo very changeable loads. Generally the alternators work mostly loadless because loads are connected just occasionally to them. That is why it is important, in the operation of units using these alternators; to obtain both energetic saving, that is a decrease in the consumption of the alternator traction motor, and a decrease in the air and noise pollution. Therefore it is important that, when the alternators work loadless, their speed is drastically reduced, so that all the polluting emissions, the noise, as well as the fuel consumption, are decreased. It is known that there are solutions for obtaining the above -mentioned result in alternators in which the adjustment of the current of the alternator inductive circuit is carried out directly, so that the increase or the decrease of the traction motor speed cause proportional increases or decreases of the outgoing voltage. This type of control is disclosed, for example, in Patent Abstracts of Japan, vol.15, no. 185 (E-1066), 13 May 1991 & JP 03 045 199 A. In single-phase alternators the adjustment often is not made directly, but through a capacitor. Said single-phase alternators have a excitation winding placed at 90° with respect to the power winding. The excitation winding is closed on a capacitor. Such an arrangement is disclosed in Murthy et al: 'A novel self-excited self-regulated single phase induction generator, Part II: Experimental Investigation' IEEE Transactions on Energy Conversion, US, IEEE Inc. New York, vol. 8, no. 3, 1 September 1993, pages 383-387, XP000415560 ISSN: 0885-8969.} The function linking the speed of the alternator traction motor to the outgoing voltage in the machines having this type of control is not linear. In fact the result is that, in case of a reduction of the motor speed, a much higher per cent reduction of the voltage is produced, so much that it makes the restart of certain loads impossible. It is important that the voltage is not over-decreased. Sometimes this is necessary because the outgoing voltage of the alternator powers as well as the electronic circuits connected to the alternator the loads connected to it. Said electronic circuits need a threshold voltage which is not lower than a certain value. That is why it is important and necessary that the alternator voltage, even if it is reduced because of the decrease of the traction motor speed, has a value not lower than the half of the rated voltage or just above. For example in case of an alternator supplying a load at 220 Volt it is important that its loadless and reduced speed voltage is not lower than 100-120 Volt. In order to remedy the above-mentioned drawback single-phase alternators of the known type adjusted through a capacitor either cannot reduce the motor speed or supply a speed reduction to not very low values, just in order to avoid an over-reduction of the outgoing voltage. The aim of the invention is that of overcoming the above-mentioned drawbacks. One of the aims of the invention is that of realizing a device which can reduce the speed of the alternator traction motor between 50% and 75% of the rated speed when the load current is almost zero, without decreasing the value of the alternator terminal voltage to substantially below the half of the rated voltage. Another aim is that the adjustment of the invention does not intervene too immediately in case of load lack, but after a delay of some seconds, in order to allow the working of intermittent loads. Another aim of the adjustment device of the invention is that of ensuring the return to normal working conditions, that is to the rated voltage and to the rated speed in the presence of even a low load. All the above-mentioned aims and others which will be better underlined later on are achieved by a device for the adjustment of the terminal voltage of a single-phase alternator coupled to an endothermic motor, said device including a current sensor of the alternator load transferring a signal to the entry of a comparator logic circuit, said comparator circuit emitting a first outgoing signal suitable for controlling a shutter connected to the accelerator of the endothermic motor, the device being characterized in that said comparator circuit, in case of a substantial absence of load and of reduced revolutions per minute, emits a second electric signal suitable to close a switch connecting at least an auxiliary capacitor to the excitation circuit of the alternator, so that the excitation current causes a voltage at the alternator terminals substantially not less than 50% of the rated voltage. Further features and particularities of the invention will be better underlined in the description of an embodiment of the invention given illustratively but not restrictively, and shown in the enclosed drawings where: fig. 1 shows a principle diagram of the device of the invention; fig. 2 shows a block diagram of the device of the invention; fig. 3 shows a detail of the circuit realizing the invention. Referring to the mentioned drawings it can be observed that the alternator 1 is connected through its own outgoing terminals 12 to a load 13, so that a current Ic is determined in the load input circuit. Such a current Ic is detected by a load sensor 10, for example an amperometric transformer (TA). An ingoing signal goes through said TA to the device of the marked with 11, for adjusting the voltage. Such a device, when the sensor of the load current 10 feels the load lack, emits two outgoing signals, a signal 14 and a signal 16. The signal 14 will control the actuators adjusting the endothermic motor speed by decreasing it. The signal 16 will control an auxiliary capacitor 17 connected to the excitation circuit, so that the current circulating therein makes the terminal voltage 12 of the alternator not lower than the half of the rated voltage. The device of the invention is powered through connections 19 resulting from the connection with the outgoing terminal voltage 12 of the alternator 1. More particularly, in the block diagram of fig. 2, the,device 11 of the invention is powered through the terminals 19, which in their turn power both a comparator circuit 21, and a control circuit of the actuator connected with the accelerator 23. More particularly the signal Is coming from the load current sensor 10, for example a TA, comes to the entry of the comparator circuit 21, which is provided with hysteresis and with a possible delay, in order to absorb, as it has been said, possible instantaneous load variations or intermittent loads. From said comparator circuit 21 two signals 25 and 26 come out: the signal 25 operates the control 23 relative to the coil 15 of the accelerator, while the signal 26, in case of load lack, closes the static switch 22 and therefore puts the auxiliary capacitor 17 in parallel with the excitation circuit. The static switch 22 can be advantageously be a SCR component, or a TRIAC component or an IGBT component. Fig. 3 shows more particularly and schematically how the excitation circuit 18 is built and how the auxiliary capacitor 17 is connected to it In fact in this case the closing of the static switch 22 by the control 26 in case of load lack puts the auxiliary capacitor 17 in parallel with the excitation capacitor 31 forming, together with the excitation winding 32, the excitation circuit of the alternator 1. Another embodiment not shown in the enclosed drawings has the auxiliary capacitor connected through a switch in parallel with respect to a semi-winding of the excitation circuit. As it is clear, by increasing the whole capacity of the excitation circuit of the alternator, the current circulating in the excitation circuit increases too. Consequently the induced voltage at the terminals 12 of the alternator increases too. This is the result that was aimed at and as it can be observed such an effect is obtained rather simply and by practical means. It is important to underline that the comparator circuit 21 should be provided with a certain delay so that the load variation, due for example to an intermittent load or to a rather sudden load connection or disconnection, does not disturb the speed rate of the traction motor of the alternator and consequently also does not disturb the voltage at the alternator terminals.

Claims: Device for the adjustment of the terminal voltage of a single-phase alternator (1) coupled to an endothermic motor, said device including a current sensor (10) of the alternator load transferring a signal (Is) to the entry of a comparator logic circuit (21), said comparator circuit emitting a first outgoing signal (25) suitable for controlling a shutter (23) connected to the accelerator of the endothermic motor, characterized in that said comparator circuit (21), in case of a substantial absence of load and of reduced revolutions per minute, emits a second electric signal (26) suitable to dose a switch (22) connecting at least an auxiliary capacitor (17) to the excitation circuit (18) of the alternator (1), so that the excitation current causes a voltage at the alternator terminals substantially not less than 50% of the rated voltage. Device according to claim 1) characterized in that said comparator circuit (21) has such an hysteresis that, after an instantaneous load variation, the outgoing signals (25, 26) come with a certain delay from said circuit for the adjustment of the motor speed and for the adjustment of the alternator terminal voltage. Device according to claim 1) or 2) characterized in that said switch connecting the capacitor (17) to the excitation circuit (18) is of the electronically controlled static type. Device according to claim 3) characterized in that the static switch is realized through SCR. Device according to claim 3) characterized in that the static switch is realized through TRIAC (IGBT). Device according to any of the preceding claims characterized in that the capacitor (17) connected through the switch (22) to the excitation circuit (18) is placed in parallel with respect to said circuit Device according to any of the preceding claims characterized in that the capacitor (17) is connected through said switch (22) in parallel with respect to a semi-winding of the excitation circuit (18).

Inventors: CARRARO DIEGO, CARRARO, DIEGO
Applicants: CARRARO DIEGO, CARRARO, DIEGO
Application year: 1999
Main classification: H02P9
IPC codes: H02P9