In recent years, a technique is being developed rapidly for reducing in size and for improving in efficiency of an inverter and a motor which are employed in an air conditioner. But, improvement in performance of a converter is a necessary matter which acts as a power source of the inverter and the motor for improving performance of an entire system. Therefore, technical development of a converter having high performance is required to be performed in a hurry. In addition, trouble to a transmission system or other devices has become a social problem which trouble is due to harmonics in power sources which harmonics flow out from a power converter. In Europe, standard values in harmonics in power sources (IEC100-3-2) have already been established by IEC (International Electrotechnical Commission) in the year of 1996. Further, in Japan, self-imposed control on harmonics currents (harmonics guide-lines for home electric devices and popular devices) has started from the year of 1996 under generalization of the resources and energy office.
A converter popularly employed from the past has an arrangement that a smoothing capacitor is connected between output terminals of a full-wave rectifier circuit through a reactor, as is illustrated in FIG. 1(a). When the converter having the arrangement is employed, enlarging effect in the conduction angle of an input current due to the reactor is expected, as is illustrated in FIG. 1(b). But, a great improvement effect in the power factor cannot be realized (the maximum power factor is about 80%) because delay in the phase of a fundamental wave is great. Further, the IEC standard (class A) cannot be satisfied in harmonics, as is illustrated in FIG. 1(c).
By taking the above problems into consideration, a converter which is called charge pump system, as is illustrated in FIG. 2(a), for improving power factor performance is employed as a converter which is especially employed in an air conditioner connected to a 200 V power system of a single phase. And, improvement performance in power factor exceeding 85% is realized. But, in this converter, a capacitor serially connected to a power source system and an inductance component of the power source system constitute a series resonance circuit. Therefore, an input current thereof greatly includes tenth through thirtieth harmonics, as is illustrated in FIG. 2(b), so that the converter becomes a power source which is not adapted to the harmonics guide-lines for home electric devices and popular devices, as is illustrated in FIG. 2(c). The IEC standard does not take system impedance into consideration, but it is not an adaptable level. Further, a DC voltage of a converter is nearly equal to a peak value of power source voltage (about 280 V for 200 V power source system) at its maximum (unloaded). The DC voltage is decreased following an increase in load due to the voltage drop caused by inserting the reactor. When the DC voltage is decreased, a current should be increased in inverse proportion to the DC voltage so that an increase in cost and increase in size of an inverter and motor are needed. Further, a driving frequency extent of a motor is practically determined based upon an application voltage so that lowering in DC voltage causes reduction in the driving frequency extent of a motor.
On the other hand, a converter of voltage doubler system is widely employed, as is illustrated in FIG. 3(a), as a converter of a 100 V power system of a single phase. It is known that an output voltage becomes a twice voltage of a power source voltage and that harmonics generation amount is suppressed to be a relatively low level when this system is employed. When this system is employed in a 200 V power system, the DC voltage is excessively raised when load is light and the DC voltage reaches twice voltage (560 V) of a peak value of the power voltage when load is opened (load is broken). Therefore, resisting voltages of parts should be increased which are connected downstream with respect to the converter. Especially, when a power converter or the like including switching elements such as an inverter is connected, a resisting voltage of the switching elements should be determined to be about a twice voltage. Consequently, the device is increased in size and is increased in cost greatly. Further, a voltage doubler current is greatly advanced in phase with respect to a power voltage phase basically, as is illustrated in FIG. 3(b). The converter of voltage doubler system employs a reactor of about 20% for maintaining a high power factor which converter is employed in an air conditioner which is input a 100 V power system. When a reactor having similar current smoothing power is realized in a 200 V power system, an inductance of the reactor becomes about four times inductance. Therefore, the reactor departs from a practical use level due to increase in size and increase in cost of a reactor.
In recent years, a power factor improving converter employing a boosting chopper is proposed, as is illustrated in FIG. 4(a), for solving problems of the above converters. This converter controls an ON-duty of the boosting chopper so as to control a DC voltage using control circuitry which is input an input voltage, input current, DC voltage, and DC voltage command value. A input power factor can be controlled to be about 1, as is illustrated in FIG. 4(b). Further, a DC current can freely be changed, and a voltage can be boosted to infinity in principal, so that this converter is an ideal converter.
Further, a converter having an arrangement that a reactor is connected between an output terminal of each phase of a three phase AC power source and each input terminal of three phase rectifier circuit and that a smoothing capacitor is connected between output terminals of the three phase rectifier circuit, is known as a converter which is connected to the three phase AC power source, as is illustrated in FIG. 19(a). When the converter having the above arrangement is employed, input power factor improvement effect and reducing effect, to some degree, reduce harmonics currents due to the reactors, as is illustrated in FIG. 19(b). But, it is difficult to satisfy the IEC standard class A, as is illustrated in FIG. 19(c) when DC power is supplied to a device of equal to or more than several kW.
For solving the problem, a PWM (pulse width modulation) converter may be employed which employs six switching elements, as is illustrated in FIG. 20(a). When this PWM converter is employed, input currents are controlled by high frequency switching so that the input currents can be controlled not to include harmonics components and are controlled to determine input power factor to be 1. Specifically, an equivalent circuit of each phase of this PWM converter becomes a circuit which is illustrated in FIG. 20(c). Therefore, no harmonic components are included in the input current iu when the input voltage vu of the converter is determined to have a sine waveform. That is, a voltage vector diagram becomes a diagram which is illustrated in FIG. 20(d). Therefore, reduction of harmonic components in the input currents is realized by generating PWM patterns of converter input voltages so as to determine the converter input voltages to have sine waveforms, the PWM patterns being generated by a method which is recited in "current controlling method which takes parameter change of a three phase PWM converter into consideration (sansou PWM konbata no parame-ta hendou wo kouryoshita denryuu seigyo hou)", Takaharu Takeshita, Makoto Iwasaki, Nobuyuki Matsui, Dengakuron D, Vol. 107, No. 11, Sho-62.
Further, multiplex system or multiplex stage system using transformers is employed in many cases in a large capacity device having a large capacity to some degree such as a converter for transmitting a DC power, a rectification device for a furnace or the like. For example, a three phase 12-pulses rectifier circuit system is employed which is recited in Japanese Patent Laid-Open Gazette Tokukaihei 2-142357. The arrangement of this system is illustrated in FIG. 21. This system employs a transformer. The primary windings are connected in star connection, while the secondary windings are connected in a star connection and in a delta connection. Therefore, output voltage phases are shifted by .pi./6 from one another. The double system is realized by connecting a couple of three phase diode rectification circuits in parallel to one another to the secondary windings which are isolated from one another. In this system, the power voltage waveform of u-phase is a waveform which is illustrated in FIG. 22(a). The input currents of the three phase diode rectifier circuit are currents having a conduction width of 2 .pi./3, as are illustrated in FIG. 22(b), the three phase diode rectification circuit is connected to the star connection of the secondary windings of the transformer. The input currents of the three phase diode rectification circuit are currents which are in delay of phase by .pi./6 with respect to the input currents which are illustrated in FIG. 22(b), as are illustrated in FIG. 22(c), the three phase diode rectification circuit is connected to the delta connection of the secondary windings of the transformer. Therefore, the currents flowing in the primary windings of the transformer becomes currents {refer to FIG. 22(e)} which are obtained by adding the input currents illustrated in FIG. 22(b) and currents {refer to FIG. 22(d)} which are obtained by performing delta-star conversion to the input current illustrated in FIG. 22(c).
But, when the power factor improvement converter using a boosting chopper is employed, controlling of a switch becomes extremely complicated and a high-potency filter is necessary for working out a countermeasure for noises which flow out towards the power source system. Therefore, cost is increased greatly. Also, efficiency is lowered because losses are great which are due to high frequency components of a current which flows through the reactor.
Further, when the PWM converter having the arrangement illustrated in FIG. 20(a) is employed, efficiency is lowered following the high frequency switching, noises are increased, as the input current waveform and input voltage waveform are illustrated in FIG. 20(b), and the controlling becomes complicated, and the cost is increased.
Furthermore, when the three phase 12-pulses rectifier circuit system having the arrangement illustrated in FIG. 21 is employed, the transformer and a plurality of three phase diode rectification circuits are necessary. Therefore, the system is increased in size in its entirety, and the system is greatly increased in cost.