Frequency converting device and control method therefor

A frequency converting device is provided with a converter for converting ac input power into dc power. The device includes an inverter for inverting an output (dc power) of the converter into an ac output power, and adjustment device for adjusting an output waveform of the inverter in accordance with the voltage values of the ac input power. The adjustment device comprises a memory which stores adjustment data corresponding to the voltage value of the ac input power source connected to the converter, and a selector for selecting a certain adjustment data from the memory in accordance with the voltage value of the ac input power to which the device is connected. Thus, even when the voltage of the ac input power is changed, the effective output of the inverter can be kept substantially constant by adjusting on/off times of the switching elements constituting the inverter to vary a duty factor of the inverter output.

BACKGROUND OF THE INVENTION 
The present invention relates to a frequency converting device, and more 
particularly to a frequency converter applied to a speed control 
(revolution number control) of an ac motor, e.g. an induction motor for 
use in an air conditioner, refrigerating device or the like. 
For control of the revolution speed of an ac motor, e.g. an induction 
motor, there has been widely used a frequency converting device comprising 
in combination an inverter and a converter. In an air conditioning system, 
such as a home air conditioner, the refrigerating device or the like often 
utilizes the frequency converting device. For instance, the home air 
conditioner realizes an optimum air conditioning performance in responsive 
to the air conditioning load to accomplish improved comfort and energy 
savings. 
Meanwhile, a commercial ac power supply for feeding power to a frequency 
converting device has different voltage values. For example, they are 
typically 100 V and 200 V in Japan. In some other countries, typical 
voltage values are 115 V and 230 V. For this reason, different types of 
air conditioners, refrigerating devices or the like had to be provided in 
accordance with the voltage values of the commercially available power 
supplies. This has been an obstacle in improving the efficiency of 
manufacture, sale, etc. 
On the other hand, frequency converting devices with PWM (pulse width 
modulation) control system have heretofore been widely known. For 
instance, U.S. Pat. No. 4,410,937 to Michika Uesugi teaches a technique in 
which a signal for controlling a three phase inverter is stored in a ROM 
(read only memory) of a relatively small capacity. Further, in the paper 
entitled "Relationship between output waveform of PWM inverter and gate 
control signal" by Takahashi et al., (Journal of Japanese electric society 
Vol. 95, No. 2, Feb., 1975), there is exhibited a theoretical analysis in 
regard to improvement in the relationship between output waveform of a PWM 
inverter and the load terminal voltage or output voltage waveform, etc. 
However, these prior teachings do not take into consideration the 
adjustment of the inverter output in accordance with voltage values of the 
commercial power source. 
There is also known a method of converting a voltage value of a power 
source into a desired value by using a transformer. However, because this 
method requires the transformer, the device is large and heavy, and has a 
large energy loss. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to ensure that an effective 
value of an ac output power supplied from a frequency converting device to 
an ac motor is kept substantially constant, even when the converting 
device is connected to ac power sources with different voltages. 
A second object of the present invention is to make it possible to suppress 
an energy loss of a frequency converting device which can properly operate 
an ac motor regardless of the voltage values of the ac power sources 
connected to the device. 
A third object of the present invention is to provide the above-featured 
frequency converting features in a device which is small and light. 
To achieve these objects, the present invention provides a frequency 
converting device in which data for adjusting the on/off time of the 
switching elements constituting an inverter are stored in advance in a ROM 
corresponding to the ac power sources to adjust the on/off time of the 
switching elements in accordance with adjustment data corresponding to the 
voltage value of the ac power source. This technique ensures that an 
effective output of the inverter is kept substantially constant regardless 
of the voltage value of the ac power source connected to the device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an embodiment of a frequency converting device according to 
the present invention which can be connected to any of a plurality of ac 
power sources having different voltage values of, for example, 100 V, 115 
V, 200 V and 230 V. The ac input power from the ac power source 1 is fed 
to a converter 5 through a reactor 2 and a voltage detector 4. The 
converter 5 comprises bridge-connected four rectifier diodes D.sub.1 to 
D.sub.4, a power supply change-over switch 3, capacitors C.sub.1 and 
C.sub.2 for voltage doubler rectification, and a smoothing capacitor 
C.sub.3. The voltage detector 4 has an output terminal connected to a 
voltage detector 6. The switch 3 is operative in dependence upon a voltage 
value of the voltage detector 6. 
Namely, when the voltage value of the ac power source 1 connected to the 
device is 100 V or 115 V, the contact of the switch 3 is connected to a 
terminal 3a, thus forming a voltage doubler rectifier circuit comprising 
rectifier diodes D.sub.1 and D.sub.2 and capacitors C.sub.1 and C.sub.2. 
On the other hand, when the voltage value of the ac power source 1 is 200 
V or 230 V, the contact of the switch 3 is connected to a terminal 3b, 
thus forming a bridge rectifier circuit comprising rectifier diodes 
D.sub.1 to D.sub.4. 
An output (dc power) of the converter 5 is supplied to an inverter 7. The 
inverter 7 comprises npn transistors TR1 to TR6 serving as switching 
elements, and diodes D.sub.5 to D.sub.10 inversely connected in parallel 
with the transistors TR1 to TR6, respectively. A gate array 8 is operative 
to deliver gate signals to bases of the transistors TR1 to TR6, 
respectively. Thus, the switching operation of the transistors is effected 
to produce a pulse signal of e.g. 4 KHz. The pulse width of the pulse 
signal corresponds to each on-time of the transistors TR1 to TR6. 
The voltage detector 6 is also operative to deliver a select signal to a 
selector 9. The selector 9 is operative to select one of four ROMs (Read 
Only Memories) labelled as 100VROM, 115VROM, 200VROM and 230VROM 
constituting a memory 10 in accordance with a signal indicating one of ac 
input power supply voltages of 100 V, 115 V, 200 V and 230 V. Data related 
to adjustment of each on-time of the transistors TR1 to TR6, i.e., data 
related to adjustment of a pulse width of an output pulse of the inverter 
7, are stored in the four ROMs, respectively. 
Inside the room in which air is conditioned, there are provided an indoor 
controller 20, a room temperature sensor 21, a temperature setter 22 and 
an air-conditioner change-over switch 23 which are connected to the indoor 
controller 20. Outside the room, there are provided an outdoor controller 
24 and a four-way valve drive circuit 25. Fed from the indoor controller 
20 to the outdoor controller 24 are a frequency command signal Sa related 
to rotational speed of an ac motor and an air-conditioning signal Sb for 
changing over the four-way valve. 
The operation of the embodiment shown in FIG. 1 will be described with 
reference to FIGS. 2 and 3. FIG. 2 shows waveforms when the ac input power 
of the ac power source 1 has a voltage value of 100 V and a frequency of 
60 Hz wherein FIG. 2a shows a voltage waveform of the input power. In this 
instance, because the contact of the power supply change-over switch 3 is 
connected to the terminal 3a, the converter 5 becomes operative to serve 
as a voltage doubler rectifier circuit to output a dc power of 230 V as 
shown in FIG. 2b. The selector 9 becomes operative to select the 100VROM 
of the memory 10. 
The 100VROM stores data in regard to pulse width of an output from the 
inverter 7 in the case where the voltage value of the ac input power from 
the ac power source 1 is 100 V. The frequency command signal Sa 
corresponding to the waveform shown in FIG. 2 is fed to the 100VROM 
through the selector 9. Thus, the gate array 8 becomes operative to adjust 
a pulse width as shown in FIG. 2c in accordance with the data from the 
100VROM, thereby to control on/off operation of the transistors TR1 to TR6 
so that an ac waveform as shown in FIG. 2d is obtained as an effective 
value. At this time, the inverter 7 produces an output pulse signal having 
an amplitude of 230 V. 
FIG. 3 shows a waveform which occurs when the ac input power of the ac 
power source 1 has a voltage value of 230 V and a frequency of 60 Hz 
wherein FIG. 3a shows a voltage waveform of the input power. In this 
instance, because the contact of the switch 3 is connected to the terminal 
3b, the converter 5 becomes operative to function as a bridge rectifier 
circuit to produce a dc power of 320 V as shown in FIG. 3b. The selector 9 
becomes operative to select the 230VROM of the memory 10. 
The 230VROM stores data related to adjustment of a pulse width of an output 
from the inverter 7 in the case where the voltage value of the ac input 
power from the ac power source 1 is 230 V. Further, the frequency command 
signal Sa corresponding to the waveform shown in FIG. 3d is delivered to 
the 230VROM through the selector 9. The gate array 8 becomes operative to 
adjust a pulse width as shown in FIG. 3c in accordance with data from the 
230VROM to control on/off operation of the transistors TR1 to TR6 so that 
an ac waveform as shown in FIG. 3d is obtained as an effective value. At 
this time, the inverter 7 produces an output pulse signal having an 
amplitude of 320 V. 
A comparative study on the waveforms shown in FIGS. 2 and 3 is now made. In 
the case shown in FIG. 2, the converter 5 is operative to produce a dc 
output power of 230 V, while in the case shown in FIG. 3, it is operative 
to produce a dc output power of 320 V. Accordingly, as apparent from FIGS. 
2c and 3c, the inverter 7 produces output pulse signals having amplitudes 
of 230 V and 320 V which are different from each other, respectively. For 
this reason, in order to obtain an ac power having the same voltage value 
as shown in FIGS. 2d and 3d, it is necessary to adjust the pulse widths 
T.sub.100 and T.sub.230 of the output pulse signal from the inverter 7. 
Assuming now that the output pulse signal of the inverter 7 is 4 KHz, 
comparative study on the pulse width when the power supply voltage is 100 
V or 230 V is made in a modeling form. When the power supply voltage is 
100 V as shown in FIG. 2, the pulse width T.sub.100 is expressed as 
EQU T.sub.100 =(1/4000) sin .theta.. 
In contrast, when the power supply voltage is 230 V as shown in FIG. 3, the 
pulse width t.sub.230 is expressed as 
EQU T.sub.230 =(1/4000) sin .theta..times.(230/320). 
Namely, by adjusting on-times of the transistors TR1 to TR6 so that the 
ratio of the pulse widths T.sub.100 and T.sub.230 becomes equal to 1/1.39, 
it is possible to keep an effective output substantially constant even if 
the ac input power source is switched to a power source having the 
different voltage, thus making it possible to properly operate the ac 
motor. 
In the above-mentioned embodiment, it is described that the device is 
connectable to one of four ac power sources having different voltages of 
100 V, 115 V, 200 V and 230 V. However, the voltage value of the ac power 
source being connectable to the device is not limited to such values. The 
switching of the voltage value by the power changeover switch may be 
effected so that e.g. 200 V is converted into 100 V by removing two diodes 
for voltage doubler rectification or cutting off the circuit. The memory 
means for storing adjustment data is not limited to a ROM. The memory 
means and the selector, etc. may be integrally configured by using a 
microprocessor, RAM (random access memory), etc. Further, it is not 
essential to the present invention to effect switching of the voltage 
doubler rectification and the bridge rectification by using the power 
supply change-over switch. Namely, the present invention can provide the 
same advantages by using means which controls the on/off operation of the 
switching element based on the adjustment data stored in the memory. 
As stated above, the frequency converting device according to the present 
invention can provide a constant power supply to a load circuit e.g. a 
compressor, a fan motor, and a four-way valve, etc. even in the case of 
different power supply voltages (e.g. when the voltage value of the ac 
commercial power source connected to the device is different from the 
rating voltages of the device, or when an effective value is lowered 
depending on the area where the frequency converter is installed etc.), 
thus enabling the single device to be adapted for two or more different 
voltages of commercial power sources. Further, as compared to a system in 
which a power is converted with a transformer, etc., the system according 
to the present invention has less energy loss, and can be small in size 
and light in weight.