Method of driving vibrating compressors

Abstract: In a method of driving vibrating compressors having such a construction that a-c voltage is fed to a vibrating compressor by converting d-c voltage into a-c voltage with two MOS-FETs, the two MOS-FETs as switching elements are driven to convert d-c voltage into a-c voltage by changing the ON time of each of the MOS-FETs corresponding to suction and compression strokes in one cycle of the vibrating compressor to improve the operating efficiency of the vibrating compressor. To this end, suction and compression time in one cycle of the vibrating compressor is changed by means of a timer IC.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to a method of driving vibrating compressors, and more particularly to a method of driving vibrating compressors having a vibrating drive coil in a magnetic field where the operating efficiency of the vibrating compressor is improved. DESCRIPTION OF THE PRIOR ART Vibrating compressors generally have such a construction as disclosed in U.S. Patent No. 4,632,645. A driving coil is vibratably supported by a mechanical vibrating system via resonating springs in a magnetic gap between an internal iron core and a ferrite magnet . Alternating current is fed to the driving coil to cause the driving coil to reciprocate in the magnetic gap and the reciprocating motion of the driving coil causes a piston mechanically connected to the driving coil to reciprocate. At this time, the natural vibration of the reciprocating piston is adapted to coincide with the vibration of the alternating current fed to the driving coil and thus the reciprocating motion of the piston compresses coolant, for example. Vibrating compressors have heretofore been driven by generating in a complicate circuit configuration an alternating voltage having a frequency agreeing with the mechanical vibration of the vibrating compressor to feed the alternating voltage of that frequency to the vibrating compressor. Now, the prior-art method of driving vibrating compressors will be described in the following, referring to Fig. 4. In the prior-art vibrating compressor, alternating voltage has been supplied to the vibrating compressor 5 via a transformer 4 by driving two MOS-FETs 2 and 3 by a PWM switching regulator controller IC 1 (TL494) that produces pulses of a duty ratio of 50% where one cycle of the frequency coinciding with the mechanical vibration of the vibrating compressor 5, that is, the reciprocating motion of the piston, is equally divided into the suction time and the compression time, as shown by gate signals of the MOS-FETs 2 and 3 in Fig. 5. Numeral 6 denotes a d-c power supply. A study conducted by the present applicant, however, revealed that the vibrating compressor, that is, the piston vibrates in such a manner as shown in Fig. 6 in the suction and compression strokes of one cycle of the vibrating compressor, and that generating pulses for driving the two MOS-FETs 2 and 3 by the PWM switching regulator controller 1C 1 (TL494) in such a manner that the frequency coinciding with the mechanical vibration of the vibrating compressor 5 is equally divided into the suction and compression strokes, as practiced in the prior-art method of driving vibrating compressors, would lead to lowered efficiency of the vibrating compressor 5. Based on this observation several methods for driving the vibrating compressor by pulse of different duration have been developed. European Patent Application No. EP 0 726 394 discloses method for generating pulses by using a timer IC. With regard to its publication date this document is prior art according to Article 54(3) EPC. U.S. Patent No. 4,706,470 discloses an electrical arrangement and a corresponding method for generating pulses having widely variable pulse duty factor and occurring alternately at two output ports of the circuit. This arrangement uses level conversion of a saw-tooth waveform input. Therefore, altering the upper threshold value allows to adjust the pulse duty factor of the generated pulses. However, this kind of operation is complicated and leads to rather poor timing precision of pulse duration. U.S. Patent No. 4,320,448 discloses another method for generating a pulse sequence having variable pulse duty factor. SUMMARY OF THE INVENTION It is an object of this invention to cause two MOS-FETs to be driven alternately so as to correspond to a suction time and a compression time of a vibrating compressor. It is another object of this invention to cause a timer IC to generate ON/OFF outputs corresponding to the suction duration and the compression duration. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a method of driving a vibrating compressor according to this invention. Fig. 2 is a diagram of assistance in explaining waveforms of each circuit. Fig. 3 is a diagram showing another embodiment of the method of driving a vibrating compressor according to this invention. Fig. 4 is a diagram of assistance in explaining a prior-art method of driving a vibrating compressor. Fig. 5 is a diagram of assistance in explaining the state of driving MOS-FETs in the prior-art method of driving a vibrating compressor. Fig. 6 is a diagram of assistance in explaining the operation of a piston of a vibrating compressor according to this invention. DETAILED DESCRIPTION OF THE EMBODIMENT Fig. 1 shows an embodiment of the method of driving a vibrating compressor of this invention. In the figure, numerals 2 through 5 refer to like parts indicated by like numerals in Fig. 4, numeral 7 refers to a timer IC, 8 to a transistor as a pulse reversing circuit, 9 to a capacitor, and 10 through 14 to resistors, respectively. The timer IC 7 (NE555, for example) operates as an astable multivibrator, the ON/OFF time of whose output is determined by the resistance values R10 and R11 of resistors 10 and 11, and the capacitance C9 of a capacitor 9, both connected to the pin numbers 6 and 7 thereof in such a manner as shown in the figure; and is normally expressed by ON time T1 = 0.693(R10 + R11)•C9, and OFF time T2 = 0.693•R11•C9. The MOS-FETs 2 and 3 are connected in the so-called push-pull fashion. The MOS-FET 3 is turned on by an H-level pulse signal ((1) of Fig. 2) generated on the pin number 3 of the timer IC 7, while the transistor 8 is turned on by that pulse signal, causing the MOS-FET 2 to be turned off. On the contrary, the MOS-FET 3 is turned off by a pulse signal of L level generated on the pin number 3 of the timer IC 7, while the transistor 8 is turned off by the same pulse signal, causing the MOS-FET 2 to be turned on. When the MOS-FET 2 is turned on, therefore, a current I 2 as shown by (3) of Fig. 2 flows, changing over the vibrating compressor 5 to the suction stroke. When the MOS-FET 3 is turned on, a current I 1 as shown by (2) of Fig. 2 flows, changing over the vibrating compressor 5 to the compression stroke. As the currents I 1 and I 2 flow, a current I 3 as shown by (4) of Fig. 2 flows in the vibrating compressor 5 via a transformer, thereby causing the suction and compression strokes to perform in one cycle. As described in Fig. 6, the results of a study shows that the vibrating compressor 5 can operate efficiently when the compression and suction time in one cycle of the vibrating compressor 5 are in a predetermined relationship between the suction time and the compression time, i.e., Suction time < compression time at a frequency corresponding to the mechanical vibration of the vibrating compressor 5. (This predetermined relationship between the compression time and the suction time is virtually determined at the design stage.) Consequently, the resistance values R10 and R11 of the resistors 10 and 11, and the capacitance C9 of the capacitor 9 are selected at appropriate values to cause the H-level time T1 of the pulse signal generated on the pin number 3 of the timer IC 7 to coincide with the compression time of the mechanical vibration of the vibrating compressor 5. Needless to say, one cycle of the pulse signal generated on the pin number 3 of the timer IC 7 at this time coincides with one cycle of the mechanical vibration of the vibrating compressor 5. Fig. 3 shows another embodiment of the method of driving a vibrating compressor according to this invention. In the figure, two MOS-FETs 2 and 3 are connected in series, and the vibrating compressor 5 is connected in parallel to the MOS-FET 3 via a capacitor 15. The operations of the timer IC 7, the transistor 8, the MOS-FETs 2 and 3, etc. are similar to those in Fig. 1; when the MOS-FET 2 is turned on, the vibrating compressor 5 performs almost the same operation as that in Fig. 1 via the capacitor 15. When the MOS-FET 3 is turned on, the vibrating compressor 5 is driven by a voltage charged in the capacitor 15, performing almost the same operation as that in Fig. 1. That is, the vibrating com-0 pressor 5 can be operated efficiently by causing the H-level time T1 of the pulse signal generated on the pin number 3 of the timer IC 7 to coincide with the compression time of the mechanical vibration of the vibrating compressor 5 to cause one cycle of the pulse signal to coincide with one cycle of the mechanical vibration of the vibrating compressor 5. As described above, this invention makes it possible to operate the vibrating compressor efficiently because in driving MOS-FETs as switching elements for converting direct current into alternating current, a timer IC is used to generate pulses to turn on the MOS-FETs to accomplish control in accordance with the movement of the piston, that is, to cause the pulse signals to coincide with the suction time and the compression time of the mechanical vibration of the vibrating compressor. In other words, in feeding alternating power to the vibrating compressor, a timer IC is caused to generate pulses to switch two MOS-FETs to change the time of suction and compression strokes that are performed in one cycle of the alternating power. Thus, the vibrating compressor can be operated efficiently since alternating voltages for suction and compression strokes can be applied to the vibrating compressor at a predetermined timing.

Claims: A method of driving vibrating compressors having such a construction that a-c voltage is fed to a vibrating compressor by using two switching devices (2, 3) as means for converting d-c voltage into a-c voltage characterized in that a suction time and a compression time within one cycle of said vibrating compressor are varied by a timer IC (7); and a pulse generated by the timer IC (7) and a pulse reversed in a pulse reversing circuit are used to drive said switching devices (2, 3 for alternately turning ON and OFF. A method of driving vibrating compressors as set forth in Claim 1 wherein said switching devices (2, 3) are MOS-FETS. A method of driving vibrating compressors as set forth in Claim 1 wherein said switching devices (2, 3) are bipolar transistors. A method of driving vibrating compressors as set forth in Claim 1 wherein said timer IC (7) generates signals having an ON duration and an OFF duration in accordance with two kinds of time; a first time corresponding to the suction time of a vibrating compressor, and a second time corresponding to the compression time of said vibrating compressor using a resistor (11) and a capacitor (9) connected to terminals of said timer IC. A method of driving vibrating compressors as set forth in Claim 1 wherein a transistor (8) for performing ON/OFF operation in accordance with output signals from said timer IC is provided, and said two switching devices (2, 3) are connected in push-pull so that said two switching devices are turned on alternately in accordance with the ON/OFF operation of said transistor to drive said vibrating compressor. A method of driving vibrating compressors as set forth in Claim 1 wherein a transistor (8) for performing ON/OFF operation in accordance with output signals from said timer IC (7), two switching devices (2, 3) connected in series with respect to a power source, and said vibrating compressor having a capacitor (15) connected in series is connected in parallel with any one of said two switching devices.

Inventors: AKAZAWA NAOKI, NAMAI MASAO, AKAZAWA, NAOKI, NAMAI, MASAO, Akazawa, Naoki, c/o Sawafuji Elec. Co., Ltd., Namai, Masao, c/o Sawafuji Elec. Co., Ltd.
Applicants: SAWAFUJI ELECTRIC CO LTD, SAWAFUJI ELECTRIC CO., LTD.
Application year: 1996
Main classification: F04B35
IPC codes: F04B17, H02K33, F04B35