Current-source power converting apparatus with self-extinction devices

A current-source power converting apparatus having a three phase AC-DC converter composed of self-extinction devices for converting an AC power furnished from an three phase AC power source into a DC power and a DC-AC inverter connected with the AC-DC converter through a DC reactor for re-converting the DC power into a three phase AC power to supply the re-converted power for a load. When the failure of the AC power source is detect according to one embodiment, the AC power source is detached from the AC-DC converter and a battery is connected between arbitrary two phases at the input end of the AC-DC converter. After that, the DC power of the battery is supplied for the DC-AC inverter intermittently by switching the corresponding self-extinction devices of the AC-DC converter and controlled by varying the duty ratio of the switching operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a current-source power converting 
apparatus with self-extinction devices, and more particularly to the power 
converting apparatus which includes an AC-DC converter for converting an 
alternating current power furnished from an AC power source into a direct 
current power and a DC-AC inverter connected with the AC-DC converter 
through a direct current reactor for re-converting the direct current 
power into an alternating current power to supply the reconverted power 
for a load, and which is suitable for continuing operation of the DC-AC 
inverter even at the time of failure of the AC power source. 
2. Description of the Related Art 
There is a demand that, even if an AC power source fails, a power 
converting apparatus fed from the AC power source must continue to operate 
for a while. For example, a driving motor for an elevator is required to 
continue to operate until an elevator cage running at that time reaches 
the most neighboring floor safely. The power converting apparatus 
supplying such a driving motor with the electric power has to continue the 
feeding of the necessary power, the performance thereof being more or less 
derated. 
Usually, two types of the power converting apparatus are known; one type is 
a so called voltage-source type and the other a current-source type. The 
former has been used rather more frequently from the reason as follows. 
In the voltage-source power converting apparatus, an AC-DC converter 
included in such apparatus has not been required to be capable of 
controlling its output DC voltage. The AC-DC converter was sufficient only 
to output the DC power of the constant voltage, because the control of the 
voltage applied to a load can be easily realized by a DC-AC inverter 
connected to the AC-DC converter. Accordingly, at the time of failure of 
an AC power source, the AC-DC converter is replaced by a battery which can 
supply the DC-AC inverter with the DC power of the constant voltage, and 
the voltage of the AC power supplied for the load is controlled by the 
usual control method of the DC-AC inverter. 
However, when the voltage-source power converting apparatus conducts a 
regenerative operation, it becomes necessary to provide another converter 
exclusively used for the regenerative operation. On the other hand, in the 
current-source one, the power converting apparatus can achieve the 
regenerative operation by only the gate control of one converter without 
any further converter. Therefore, the current-source power converting 
apparatus is used, when the regenerative operation is required. 
Contrary to the case of the voltage-source type, however, a DC-AC inverter 
used in a current-source power converting apparatus is very difficult to 
control the voltage of its output AC power. Such control has been scarcely 
feasible in a practical use. Therefore, an AC-DC converter connected with 
the DC-AC inverter through a DC reactor has to fill the role of voltage 
control of the AC power supplied for a load as the final output of the 
current-source power converting apparatus. That is to say, the voltage of 
the DC power furnished for the DC-AC inverter has to be controlled by the 
AC-DC converter. Accordingly, the DC-AC inverter can not continue to 
operate by merely substituting a battery for the AC-DC converter at the 
time of failure of an AC power source. Such substitution of the battery 
for the converter made possible the continuous operation of the inverter 
at the time of failure of the AC power source in a case of the foregoing 
voltage-source power converting apparatus. 
Now, in a conventional example, a current-source power converting apparatus 
employing self-extinction devices utilizes the DC short circuiting mode of 
operation for a usual control of the DC voltage by means of the AC to DC 
conversion. In order to make such a power converting apparatus operate 
continuously at the time of failure of an AC power source, a battery is 
connected to the apparatus and the control of DC voltage must be performed 
by the DC to DC transformation. Since, however, the DC to DC 
transformation necessiates the provision of a freewheel diode, the 
arrangement of a main circuit of the usual converter is not suited for 
control of the DC voltage. Further, in case the freewheel diode is 
connected to the output end of the AC-DC converter, there arises a defect 
that the regeneration becomes impossible in the usual operation. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a current-source power 
converting apparatus which comprises an AC-DC converter connected to an AC 
power source and a DC-AC inverter supplied with the DC power from the 
AC-DC converter through a DC reactor, at least the AC-DC converter being 
composed of self-extinction devices, and which is capable of making the 
DC-AC inverter operate continuously by supplying the DC power from a 
battery at the time of failure of the AC power source. 
According to a feature of the present invention, there is provided in the 
current-source power converting apparatus as described above a DC power 
supplying means which is capable of supplying the DC power of the 
intermittent voltage for a load during the failure of the AC power source, 
wherein the DC power supplied for the load is controlled by varing the 
degree of the intermittence. Further the DC power supplying means includes 
a battery for supplying the DC power which is connected on the side of the 
AC-DC converter with respect to the DC reactor arranged between the AC-DC 
converter and the DC-AC inverter. 
In one of the embodiments according to the present invention, the DC power 
supplying means supplies the DC power of the battery for the inverter 
intermittently by using the self-extinction devices of the AC-DC 
converter. 
According to another embodiment, the DC power supplying means includes a 
particular switching means for supplying the DC power of the battery for 
the inverter intermittently. 
According to still another embodiment, the DC power supplied for the DC-AC 
inverter from the battery is controlled intermittently by the DC-AC 
inverter itself. 
Other objects and features of the present invention will become apparent 
upon reading the specification and inspection of the drawings and will be 
particularly pointed out in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be explained in detail with 
reference to the accompanying drawings, wherein like parts in each of the 
several figures are identified by the same reference numerals and 
characters. 
Referring at first to FIG. 1, a reference numeral 1 denotes a three phase 
AC power source, which supplies a three phase AC power for an AC-DC 
converter 3 through a three phase contactors 5. At an input end of the 
converter 3, three capacitors are provided, which are formed in a known 
star-connection and denoted as a whole by a reference numeral 7. 
The AC-DC converter 3 is formed by a three phase bridge circuit, each arm 
of which is composed of a so called self-extinction device, such as a gate 
turn-off thyristor or a transistor. The self-extinction devices included 
in the respective arms are denoted by reference numerals 31, 32, 33, 34, 
35 and 36. Since such an arrangement of the converter is well known, the 
further description thereabout is omitted here. A battery is provided 
between arbitrary two among three phases U, V and W of the AC power in 
order that it supplies a DC power for the converter 3 when the AC power 
source 1 fails. In this embodiment, the battery 11 is connected between 
the phases V and W through a contactor 13. 
The DC power as an output of the converter 3, voltage and current of which 
are represented by reference characters V.sub.d and I.sub.d, respectively, 
is supplied to a DC-AC inverter 9 through a DC reactor 15 which smooths 
the DC current I.sub.d flowing therethrough. In this embodiment, the DC-AC 
inverter 9 is also formed by a three phase bridge circuit which has six 
arms each including a self-extinction device. The arrangement of the 
inverter of this kind is also known. The inverter 9 inverts the DC power 
supplied from the converter 3 into an AC power, which is furnished to a 
load, i.e., a three phase induction motor 17 in a case of this embodiment. 
Further, capacitors, denoted as a whole by a reference numeral 19, are 
connected at an output end of the inverter 9. 
There is further provided a control unit 21 consisting of a power failure 
detector 23 and a control circuit 25. The power failure detector 23 has a 
sensing part (not shown) equipped at an appropriate portion of the main 
circuit where the failure of the AC power can be watched. When the power 
failure occurs, the detector 23 produces output signals. One of the output 
signals is applied to the contactor 5, so that the contactor 5 is opened 
to detach the AC power source 1 from the converter 3. Another output 
signal is given to the contactor 13 to close it. Thereby the battery 11 is 
connected between the phases V and W. The last output signal is led to the 
control circuit 25 as a signal indicating occurence of the failure of the 
AC power. 
The control circuit 25 consists mainly of gate control means for the 
converter 3 and the inverter 9. During the normal condition of the AC 
power source 1, the control circuit 25 operates in almost the same manner 
as a known gate control means for the converter or inverter of this kind. 
The operation thereof at the time of the power failure will be explained 
in detail later. 
Next, the description is made of the operation of this embodiment, 
referring to FIG. 2 and FIGS. 3a and 3b. 
In FIG. 2, it is assumed that the failure of the AC power occurs at a time 
point t.sub.0, and therefore, through all figures of FIGS. 2 (a) to (g), 
the waveforms illustrated in the duration before the time point t.sub.0 
are those during the normal operation and the waveforms in the duration 
after the time point t.sub.0 are those under the condition of the power 
failure. Further, in these figures, FIG. 2 (a) shows the waveform of 
voltage of the AC power source 1, in which the broken lines V.sub.U, 
V.sub.V and V.sub.W represent phase voltages of the phases U, V and W, 
respectively, and the solid line V.sub.VW the line voltage with respect to 
the phases V and W between which the battery 11 is connected. 
Gate signals of the converter 3 are as shown in FIG. 2 (b). The gate 
signals P.sub.31, P.sub.32, P.sub.33, P.sub.34, P.sub.35 and P.sub.36 are 
applied to gates of the self-extinction devices 31, 32, 33, 34, 35 and 36 
of the converter 3, respectively. As is understood from the application 
manner of these gate signals, a so called PWM (Pulse Width Modulation) 
control method is applied to the converter 3 of this embodiment in order 
to adjust its output DC voltage during the normal operation. 
The power failure detector 23 produces an output as shown in FIG. 2 (c) at 
the time point t.sub.0 when it detects the power failure, and the 
contactors 5 and 13 operate as shown in (d) and (e) of FIG. 2, 
respectively, upon occurence of the output of the detector 23. Voltage 
V.sub.d and current I.sub.d of the DC power as an output of the converter 
3 become as shown in (f) and (g) in the same figure. 
Now, when the power failure is detected by the detector 23 at the time 
point t.sub.0, the contactor 5 which was of ON state till then changes to 
OFF state and, on the contrary, the contactor 13 becomes ON state. Namely, 
the AC power source 1 is detached from the converter 3 and the battery 11 
is connected between the phases V and W of the converter 3. As a result, 
the line voltage V.sub.VW between the phases V and W becomes equal to 
voltage E of the battery 11 (cf. FIG. 2 (a)). 
After the time point t.sub.0, the self-extinction device 36 is kept at ON 
state by applying the continuing gate signal thereto (cf. P.sub.36 in FIG. 
2 (b)), and the self-extinction devices 32 and 33 repeat ON and OFF states 
alternately (cf. P.sub.32 and P.sub.33 in the same). Here assuming that 
the gate signal P32 becomes a high level at a time point t.sub.1 and the 
gate signal P.sub.33 at a time point t.sub.2, i.e., that the 
self-extinction device 32 becomes conductive at t.sub.1 and the 
self-extinction device 33 turns on at t.sub.2. In this case, the states of 
the circuit of the converter 3 for the durations of t.sub.0 
.ltoreq.t&lt;t.sub.1 and t.sub.1 .ltoreq.t&lt;t.sub.2 are as shown in FIGS. 3a 
and 3b, respectively. 
As is apparent from FIG. 3a, the self-extinction devices 33 and 36 are both 
in ON state, so that a DC circuit side falls into the short circuit state 
and the voltage V.sub.d becomes zero. At this time, the series connection 
of the self-extinction devices 33 and 36 functions as a freewheel diode, 
when being viewed from a side of the load. Therefore, this state of the 
circuit is called a freewheel-state. FIG. 3b shows the circuit state for 
the duration of t.sub.1 .ltoreq.t&lt;t.sub.2. In this duration, the 
self-extinction device 36 is continuously conductive. The self-extinction 
device 33 changes to OFF state and the self-extinction device 32 becomes 
ON state. Accordingly, the battery 11 is connected to the DC circuit, so 
that the voltage V.sub.d becomes equal to the voltage E of the battery 11. 
As is understood from FIG. 2, especially from P.sub.32 and P.sub.33 in the 
figure (b) thereof, the operation as described above is repeated over the 
duration of the power failure. As a result, the DC power with the voltage 
V.sub.d and the current I.sub.d as shown in (f) and (g) of FIG. 2 is 
obtained from the converter 3, and the DC power thus obtained is supplied 
for the inverter 9 through the DC reactor 15. Here, if the ON term of the 
self-extinction device 32, and accordingly the OFF term of the 
self-extinction device 33, is varied, the pulse width of the DC voltage 
V.sub.d changes so that the average value of the DC voltage V.sub.d 
changes. In this manner, the voltage of the DC power supplied for the 
inverter 9 can be controlled even at the time of failure of the AC power. 
According to this embodiment, the main circuit of the current-source power 
converting apparatus is additionally provided by only a DC power supplying 
means as measures against the failure of the AC power, which is simply 
constructed by the battery 11 and the contactor 13. The main circuit 
usually has the contactor 5 as a main switch and therefore the signal from 
the power failure detector 23 can be applied to an actuator for the main 
switch. Accordingly, the main circuit is considerably simple in its 
structure, since the voltage control is performed by the gate control of 
the self-extinction devices of the converter 3. A circuit for such gate 
control as renders the self-extinction devices 32 and 33 conductive 
alternately and controls the duty ratio of the pulsing DC voltage V.sub.d 
can be easily achieved with a usually known electronic circuit techniquue. 
FIG. 4 shows a current-source power converting apparatus according to 
another embodiment of the present invention. In this figure, the details 
of a converter 3 and an inverter 9 are omitted, since they are entirely 
the same as those in FIG. 1. 
Referring to this figure, a DC power supplying means 4 as measures against 
the failure of the AC power is provided at the output end of the converter 
3, i.e. in the DC circuit of the power converting apparatus. The DC power 
supplying means 4 comprises a series connection of a battery 41 and a 
self-extinction device 42 and another self-extinction device 43 connected 
in parallel with the series connection. The self-extinction device 42 has 
a diode 44 connected in reverse parallel therewith. Further, this 
arrangement of circuit is connected across output terminals of the 
converter 3 through contactors 45 and 46. 
A power failure detector 23 in this embodiment produces four output 
signals, when it detects the power failure. The first output signal is 
sent to a contactor 5 to make it open and detach an AC power source 1 from 
the converter 3. The second output signal is given to the contactors 45 
and 46 to close them. The third one is a gate signal for the 
self-extinction devices 42 and 43, which renders these devices 42, 43 
conductive alternately, as described more in detail later. The last output 
signal of the detector 23 is led to a control circuit 25 as a signal 
indicating the occurence of the power failure. The first, second and third 
among the four output signals of the detector 23 are almost the same in 
their function as the signals produced by the power failure detector in 
FIG. 1. 
Referring now to FIG. 5 and FIGS. 6a and 6b, the description will be made 
of the operation of this embodiment, hereinafter. 
Similarly to a case of FIG. 2, it is assumed in FIG. 5 that the failure of 
the AC power occurs at a time point t.sub.0, and therefore, through all 
figures of FIGS. 5(a) to (i), the waveforms illustrated in the duration 
before the time point t.sub.0 are those during the normal operation and 
the waveforms in the duration after the time point t.sub.0 are those under 
the condition of the power failure. Further, FIGS. 5(a) to (e), (h) and 
(i) are the same as the corresponding figures of FIG. 2. As is apparent 
from these figures, the operation during the normal conditon is the same 
as that of the first embodiment shown in FIG. 1. Therefore, the 
explanation thereabout is omitted. 
When the power failure occurs at the time point t.sub.0, the detector 23 
produces the output signal (cf. FIG. 5(c)). In response to the output 
signal, the contactor 5 becomes OFF state and the contactors 45 and 46 
become ON state (cf. FIGS. 5(d) and (e)). Namely, the AC power source 1 is 
detached from the converter 3 and the DC power supplying means 4 is 
connected to the inverter 9 through a DC reactor 15. Further, as shown in 
FIG. 5(b), the gate signals P.sub.31 P.sub.32, P.sub.33, P.sub.34, 
P.sub.35 and P.sub.36 to the converter 3 are all suppressed. After the 
time point t.sub.0, the self-extinction devices 42 and 43 are given their 
gate signals P.sub.42 and P.sub.43 (cf. FIGS. 5(f) and (g)). Here assuming 
that the gate signal P.sub.42 becomes a high level at a time point t.sub.l 
and the gate signal P.sub.43 at a time point t.sub.2, that is to say, that 
the self-extinction device 42 becomes conductive at t.sub.1 and the 
self-extinction device 43 at t.sub.2. In this case, the state of the 
circuit of the converter 3 for the durations of t.sub.0 .ltoreq.t&lt;t.sub.1 
and t.sub.1 .ltoreq.t&lt;t.sub.2 are as shown in FIGS. 6a and 6b, 
respectively. 
In the duration of t.sub.0.ltoreq.t&lt;t.sub.1, (cf. FIG. 6b), the 
self-extinction device 43 is in ON state, therefore a DC circuit side 
falls into the short circuit state and the voltage V.sub.d becomes zero. 
At this time, the self-extinction device 43 functions as a freewheel diode 
when being viewed from a side of the load. This state is the same as that 
shown in FIG. 3a, and therefore, this is also called a freewheel-state. 
FIG. 6b illustrates the circuit state for the duration of 
t.sub.1.ltoreq.t&lt;t.sub.2, in which the self-extinction device 42 is in ON 
state and the battery 41 is connected to the inverter 9. In this duration, 
the DC voltage V.sub.d becomes equal to the voltage E of the battery 41. 
As is seen from FIGS. 5(f) and (g) showing the gate signals P.sub.42 and 
P.sub.43 applied to the self-extinction devices 42 and 43, the operation 
as mentioned above is repeated over the duration of the power failure. As 
a result, the DC power of the voltage V.sub.d and the current I.sub.d as 
shown in FIGS. 5(h) and (i) are obtained from the DC power supplying means 
4, and the DC power thus obtained is supplied for the inverter 9 through 
the DC reactor 15. Similarly to the case in FIG. 2, the average value of 
the output voltage of the DC power supplying means 4 can be adjusted by 
controlling the duty ratio of the pulsing DC voltage V.sub.d. 
According to the second embodiment, the voltage of the DC power supplied 
for the inverter 9 can be controlled, not only when the AC power source 1 
fails, but also at the time of the trouble of the converter 3. 
FIG. 7 is a schematic diagram showing a current-source power converting 
apparatus in accordance with still another embodiment of the present 
invention, in which, similarly to FIG. 4, the details of a converter 3 is 
omitted. However, an inverter 9 is illustrated in detail although it is 
the same as that in FIG. 1, because the following description of this 
embodiment is concerned with the control manner of the inverter 9. 
Therefore, the detailed illustration of the converter 9 in this figure is 
only for the purpose of convenience of understanding of this embodiment. 
In the embodiment of FIG. 7, there is provided a DC power supplying means 6 
of the simpler structure, compared with the DC power supplying means 4 in 
FIG. 4. Namely, the DC power supplying means 6 in this embodiment 
comprises a battery 61 and contactors 62, 63 connecting the battery 61 
with a DC circuit of the power converting apparatus. The contactors 62, 63 
are closed in response to a signal from a power failure detector 23, so 
that the DC power is supplied to an inverter 9 from the battery 61 through 
a DC reactor 15 at the time of failure of an AC power source 1. 
Referring to FIG. 8, the explanation is done of the operation of this 
embodiment. As is understood from FIG. 8(b), the converter 3 is operated 
in the same manner as those in FIGS. 1 and 4, i.e. in the PWM mode, during 
the normal condition of the AC power source 1. However, the converter 3 in 
this embodiment is so controlled that its output DC power is maintained 
constant. The DC voltage V.sub.d and the DC current I.sub.d of the output 
DC power of the converter 3 are as shown in FIGS. 8(f) and (g), and the 
constant DC power is supplied for the inverter 9 through the DC reactor 
15. 
On the other hand, the inverter 9 is given gate signals P.sub.91, P.sub.92, 
P.sub.93, P.sub.94, P.sub.95 and P.sub.96 as shown in FIG. 8(h) and 
operated in the PWM mode. Consequently, the inverter 9 produces the output 
currents I.sub.U, I.sub.V, I.sub.W as shown by rectangular waveforms of 
FIG. 8(i) and the currents flowing through a load 17 become as I.sub.UL, 
I.sub.VL and I.sub.WL shown by broken sinusoids of the same figure. 
Different from the inverters in two embodiments already described, the 
inverter 9 in this embodiment is able to control its output AC power by 
means of the PWM control operation. 
Now, when the failure of the AC power source 1 is detected at the time 
point t.sub.0 (cf. FIG. 8(a)), the detector 23 produces an output signal 
(cf. FIG. 8(c)). In reply to this output signal, the contactor 5 is opened 
and the contactors 62, 63 are closed so that the AC power source 1 is 
released and the battery 61 is connected to the DC circuit. Accordingly, 
after that, the DC voltage V.sub.d is kept at the voltage E of the battery 
61 (cf. FIG. 8(f)). Further, after the time point t.sub.0 the gate signals 
P.sub.31, P.sub.32, P.sub.33, P.sub.34, P.sub.35 and P.sub.36 of the 
converter 3 are all suppressed, similarly to the case of FIG. 5. However, 
the gate signals P.sub.91, P.sub.92, P.sub.93, P.sub.94, P.sub.95 and 
P.sub.96 continue to be applied to the corresponding self-extinction 
devices 91, 92, 93, 94, 95 and 96 of the inverter 9 (cf. FIG. 8(h)). 
Therefore, the output AC power of the inverter 9 is continuously 
controlled (cf. FIG. 8(i)). 
Referring next to FIG. 9, the AC output power control by the inverter 9 
after the time point t.sub.0, i.e. during the failure of the AC power, is 
explained hereinafter. 
In this figure, distribution signals R.sub.U, R.sub.Z, R.sub.V, R.sub.X, 
R.sub.W and R.sub.Y (cf. FIG. 9(a)) are signals each of which has a pulse 
width T corresponding to an operational period 60.degree. of the inverter 
9 and is shifted by 60.degree. in phase from one another. These 
distribution signals are obtained by dividing one cycle of the voltage of 
the AC power source 1 into six equal sections I to VI. A signal Q (cf. 
FIG. 9(c)) is a triangular wave signal which has a peak or a maximum value 
I.sub.RMAX at the time point of the leading edge of pulses of a reference 
pattern P for the PWM control (cf. FIG. 9(b)). The periods of individual 
triangular waves of the signal Q are determined by the width of the pulses 
of the reference pattern P and hence not uniform. A short-circuit pulse 
train S is made by comparing the triangular wave signal Q with an 
instruction I.sub.R.sup.* of the AC output current (cf. FIGS. 9(c) and 
(d)). Next, the reference pattern P and the short-circuit pulse train S 
are inverted into signals P and S, respectively. By taking the logical 
product between the signals P and S and between the signals P and S, 
signals P.sub.F and P.sub.R are obtained (cf. FIGS. 9(e), and (f)). 
Further, signals P'.sub.F, P'.sub.R and S' are obtained by the logical 
product of the signals P.sub.F and R.sub.Y, of the signals P.sub.R and 
R.sub.Z, and of the signals S and R.sub.X (cf. FIGS. 9(g), (h) and (i)), 
and the logical summation of the thus obtained signals P'.sub.F, P'.sub.R, 
S' and the signal R.sub.U is conducted to get the gate signal P.sub.91 to 
the self-extinction device 91 (cf. FIG. 9(j)). In the similar way, the 
gate signals P.sub.92, P.sub.93, P.sub.94, P.sub.95 and P.sub.96 of the 
self-extinction devices 92, 93, 94, 95 and 96 of the inverter 9 can be 
made. The output current of the inverter 9 controlled by the gate signals 
P.sub.91 to P.sub.96 becomes the pulse-width-modulated rectangular current 
as shown by I.sub.U, I.sub.V, I.sub.W in FIG. 9(k). The currents flowing 
through the load are as shown by broken sinusoids I.sub.UL, I.sub.VL, 
I.sub.WL in the same figure. 
In order to facilitate a good understanding, the output control in the 
inverter 9 is explained more in detail, referring to FIG. 10 which, taking 
the section I shown in FIG. 9 as an example, illustrates the operation in 
the expanded form and to FIGS. lla to llc which indicate the circuit 
states in the duration from a time point t.sub.0 to a time point t.sub.3 
shown in FIG. 10. 
Referring at first to FIG. 10, the short-circuit pulse train S obtained, in 
this section I, by comparing the triangular wave signal Q with the current 
instruction I.sub.R.sup.* corresponds to the gate signal P.sub.92 given to 
the self-extinction device 92 (cf. FIGS. 10(b) and (e)). On the other 
hand, in the section I, the self-extinction device 95 is continuously 
given the gate signal P.sub.95, as shown in FIG. 10(h). Therefore, the DC 
circuit of the power converting apparatus is short-circuited, as shown in 
FIG. llb which shows the circuit state of the inverter 9 in the duration 
from the time point t.sub.1 to the time point t2. Accordingly, the 
currents IU, IV, IW in each phase U, V, W of the load become zero in this 
duration (cf. FIGS. 10(i), (j) and (k)). Before this duration, i.e. in the 
durarion from the time point t.sub.0 to the time point t.sub.l, the 
self-extinction devices 93 and 95 are made conductive (FIGS. 10(g) and 
(h)), so that the current flows through the load/in the phases V and W 
(cf. FIGS. 10(j) and (k) and FIG. ll(a). In the duration from the time 
point t.sub.2 to the time point t3, the self-extinction devices 91 and 95 
become ON state (cf. FIGS. 10(c) and (g)), so that the current flows 
through the load in the phases U and V (cf. FIGS. 10(i) and (j), and FIG. 
ll(c). 
As ;is apparent from FIG. 10, the inverter 9 is able to control the current 
flowing throught the load by successively repeating the three circuit 
states as shown in FIGS. lla, llb and llc. 
Now, assuming here that the conductive and non-conductive durations of the 
current in the respective phases are denoted by d.sub.0 to d.sub.3 and 
d.sub.S1 to d.sub.S3 as shown in FIGS. 10(i) to (k). Then, if the current 
instruction I.sub.R.sup.* is varied from zero to the maximum value 
I.sub.RMAX, the short circuit durations d.sub.S1, d.sub.S2, d.sub.S3 in 
the current I.sub.U of the phase U changes from d.sub.1, d.sub.2 and 
d.sub.3 to zero, respectively, holding the following relation: 
##EQU1## 
The same relation is applicable to the current I.sub.W of the phase W. 
With respect to the current IV of the phase V, the following relation 
exists between d.sub.1 to d.sub.3 and d.sub.Sl to d.sub.S3 : 
##EQU2## 
In these circumstances, the effective value I.sub.RMS of the current 
I.sub.U, I.sub.V, I.sub.W becomes as follows: 
##EQU3## 
When I.sub.R.sup.* =0, the DC circuit of the power converting apparatus is 
in the short circuit state and therefore I.sub.RMS becomes equal to zero. 
Contrary, when I.sub.R.sup.* =I.sub.RMAX, I.sub.RMS becomes as follows: 
##EQU4## 
Namely, as is apparent from the equation (3) above, the effective value 
IRMS of the current I.sub.U, I.sub.V, I.sub.W changes in proportion to a 
square root of the current instruction I.sub.R.sup.*, since I.sub.d, 
I.sub.RMAX and d.sub.0 to d.sub.3 are all constant. 
As described above, the continuous operation of the power converting 
apparatus during the failure of the AC power is possible by only 
connecting the battery 61 in case the inverter 9 itself is able to control 
the output power. 
Further, in this embodiment, there is an additional feature as follows. 
Namely, in this embodiment, the short circuit state of the DC circuit 
includes the battery 61 within the circuit (cf. FIG. 7 and FIG. 11 b). 
Therefore, energy is stored in the DC reactor 15 by the current I.sub.d 
flowing therethrough during the short circuit state. When the short 
circuit state is broken, that is to say, the circuit state changes from 
the state shown in FIG. 11b to that shown in FIG. 11c, for example, the 
energy stored in the DC reactor 15 is released toward the load 17. At this 
time, voltage (L dI.sub.d dt) is produced across the DC reactor 15, 
wherein L represents an inductance value of the DC reactor 15. Therefore, 
the condensor 19 connected to the output end of the inverter 9 can be 
charged by the sum of the voltage E of the battery 61 and the voltage (L 
dI.sub.d / dt) produced by the DC reactor 15. Accordingly, the load 17 can 
be applied by the voltage higher than the voltage E of the battery 61. 
Although we have herein shown and described only limited number of forms of 
a current-source power converting apparatus embodying the present 
invention, it is to be understood that various changes and modifications 
may be made therein within the scope of the appended claims without 
departing from the spirit and scope of the present invention.