U.S. patent application number 09/234714 was filed with the patent office on 2001-08-09 for power supply device for electromotive railcar.
Invention is credited to NOMURA, YOSHIHITO.
Application Number | 20010012207 09/234714 |
Document ID | / |
Family ID | 11983673 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012207 |
Kind Code |
A1 |
NOMURA, YOSHIHITO |
August 9, 2001 |
POWER SUPPLY DEVICE FOR ELECTROMOTIVE RAILCAR
Abstract
A power supply device for an electromotive railcar comprises a
first capacitor connected to receive a DC voltage for outputting a
first DC voltage, a DC/AC/DC converter, including an inverter
bridge including power transistors connected to the first capacitor
in parallel, an insulating transformer with high carrier frequency
having a primary winding connected to an output of the inverter
bridge and a rectifier circuit connected to a secondary winding of
the insulating transformer to receive a second DC voltage, and a
three-phase inverter including a bridge circuit of power
transistors for generating a three-phase AC voltage on the basis of
the second DC voltage.
Inventors: |
NOMURA, YOSHIHITO; (TOKYO,
JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
11983673 |
Appl. No.: |
09/234714 |
Filed: |
January 21, 1999 |
Current U.S.
Class: |
363/17 |
Current CPC
Class: |
H02M 7/08 20130101; H02M
3/33569 20130101; H02M 3/01 20210501; H02M 3/33573 20210501; H02M
1/0074 20210501 |
Class at
Publication: |
363/17 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 1998 |
JP |
P10-018873 |
Claims
What is claimed is:
1. A power supply device for an electromotive railcar adapted to
receive an external DC voltage from an external electric line,
comprising: a first capacitor connectable to receive said external
DC voltage and outputting a first DC voltage in response to the
external DC voltage; a DC/AC/DC converter including an inverter
bridge having a plurality of power transistors connected to said
first capacitor in parallel, an insulating transformer with a high
carrier frequency and having a primary winding connected to an
output of said inverter bridge, and a rectifier circuit connected
to a secondary winding of said insulating transformer to receive a
second DC voltage; and a three-phase inverter including a bridge
circuit having a plurality of power transistors for generating a
three-phase AC voltage on the basis of said second DC voltage.
2. The power supply device for an electromotive railcar as recited
in claim 1, wherein a carrier frequency band of said insulating
transformer of said DC/AC/DC converter is in the range of 1 to 6
KHz.
3. The power supply device for an electromotive railcar as recited
in claim 1, wherein said inverter bridge is a commutation system
having a partial resonance which switches each of said power
transistors at the time a zero-voltage or a zero-current is applied
to each of said power transistors.
4. The power supply device for an electromotive railcar as recited
in claim 1, further comprising a smoothing circuit including a DC
reactor and a second capacitor for smoothing said second DC voltage
from an output of said rectifier circuit of said DC/AC/DC
converter.
5. The power supply device for an electromotive railcar as recited
in claim 4, wherein a capacity of said second capacitor is large
enough so as to remove a ripple wave with a predetermined frequency
caused by load fluctuation.
6. The power supply device for an electromotive railcar as recited
in claim 1, further comprising: a discharge circuit including a
series circuit of a power transistor and a discharge resistor
connected to discharge said second DC voltage; and a voltage
surveillance circuit for detecting said second DC voltage and
turning on said power transistor at a time said voltage
surveillance circuit detects that said second DC voltage is over a
predetermined voltage in order to discharge regeneration energy
from load objects.
7. The power supply device for an electromotive railcar as recited
in claim 1, further comprising: an AC filter including a series
circuit of an AC reactor and an AC capacitor connected to an output
of each phase of said three-phase inverter; and one terminal of
said AC capacitor being connected to receive a neutral point of
said second DC voltage.
8. The power supply device for an electromotive railcar as recited
in claim 1, further comprising: an AC filter including a series
circuit of an AC reactor and an AC capacitor connected to an output
of each phase of said three-phase inverter; and one terminal of
said AC capacitor being connected to a neutral point of said
secondary winding of said insulating transformer.
9. A power supply device for an electromotive railcar adapted to
receive an external DC voltage from an external electric line,
comprising: a series circuit of a plurality of first capacitors
connected to receive said external DC voltage for outputting a
first DC voltage; a plurality of DC/AC/DC converters, each of said
DC/AC/DC converters including an inverter bridge having a plurality
of power transistors respectively connected to one of said
plurality of first capacitors in parallel, an insulating
transformer with a high carrier frequency and having a primary
winding connected to an output of said inverter bridge, and a
rectifier circuit connected to a secondary winding of said
insulating transformer to receive an second DC voltage; and a
three-phase inverter including a bridge circuit having a plurality
of power transistors for generating a three-phase AC voltage on the
basis of said second DC voltage.
10. The power supply device for an electromotive railcar as recited
in claim 9, wherein: each of said insulating transformers has a
common core; and the number of turns of said primary windings are
the same in said DC/AC/DC converters and the number of turns of
said secondary windings are the same in said DC/AC/DC converters
respectively.
11. The power supply device for an electromotive railcar as recited
in claim 1 or 9, further comprising: a discharge circuit including
a series circuit of a power transistor and a discharge resistor
connected to discharge said second DC voltage; and a voltage
surveillance circuit for detecting said second DC voltage and
turning on said power transistor at a time said voltage
surveillance circuit detects that said second DC voltage is over a
predetermined voltage in order to discharge regeneration energy
from load objects.
12. The power supply device for an electromotive railcar as recited
in claim 1 or 9, further comprising: an AC filter including a
series circuit of an AC reactor and an AC capacitor connected to an
output of each phase of said three-phase inverter; and one terminal
of said AC capacitor being connected to receive a neutral point of
said second DC voltage.
13. The power supply device for an electromotive railcar as recited
in claim 1 or 9, further comprising: an AC filter including a
series circuit of an AC reactor and an AC capacitor connected to an
output of each phase of said three-phase inverter; and one terminal
of said AC capacitor being connected to a neutral point of said
secondary winding of said insulating transformer.
14. A power supply device for an electromotive railcar adapted to
receive an external DC voltage from an external electric line,
comprising; a first capacitor having a first terminal to receive
said DC voltage and for providing a first DC voltage in response to
the external DC voltage and having a second terminal; a DC/AC/DC
converter having a pair of inputs connected in parallel to the
first and second terminals of the first capacitor and having a pair
of outputs for providing a second DC voltage in response to the
external DC voltage; and a three-phase inverter having a pair of
inputs connected in parallel to the pair of outputs of the DC/AC/DC
converter and having three outputs for generating a three-phase AC
voltage in response to said second DC voltage.
15. The power supply device for an electromotive railcar as recited
in claim 14, wherein the DC/AC/DC converter comprises: an inverter
bridge having a plurality of power transistors and having a pair of
inputs connected in parallel to the first capacitor and having a
pair of outputs; an insulating transformer having a primary winding
connected in parallel to the outputs of the inverter bridge and
having a secondary winding for providing the second DC voltage; and
a rectifier circuit connected to the secondary winding of the
insulating transformer to receive the second DC voltage.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a power supply device for an
electromotive railcar which insulates a three-phase alternating
current (AC) voltage from a direct current (DC) voltage from an
external electric line and, more particularly to a power supply
device having DC/AC/DC converters.
[0003] 2. Discussion of the Background
[0004] FIG. 5 is a circuit diagram of a conventional power supply
device for an electromotive railcar.
[0005] In FIG. 5, the power supply device obtains a DC voltage from
an electric power line 1. The DC voltage charges an electrolytic
capacitor 8 via a pantograph 2, a fuse 3, a contactor 4 (contact
breaker ), a DC reactor 5 and an initial charge resistor 7. When
the electrolytic capacitor 8 is charged to a predetermined voltage,
a conducting thyristor 6 connected in parallel with the initial
charge resistor 7 turns on. Then a three-phase inverter 9 is
operated.
[0006] The three-phase inverter 9 generates a three-phase AC
voltage 13 on the basis of the DC voltage from the electric power
line 1. Output waveforms of the three-phase inverter 9 are
well-known PWM (Pulse Width Modulation ) sinewaves including many
higher harmonics.
[0007] Therefore, the higher harmonics are removed by passing the
voltage signal through an AC filter comprising an AC reactor 10 for
smoothing and an AC capacitor 11, and then a commercial power
signal with 50 Hz or 60 Hz and 200 V is obtained. The electrolytic
capacitor 8 and the three-phase inverter 9 are coupled to a ground
14.
[0008] The commercial power signal is mainly used for operating air
conditioners and lighting on railcars. Moreover, the commercial
power signal is insulated through an insulating transformer 12 with
a commercial carrier frequency for the purpose of insulating the
three-phase AC voltage 13 from the DC voltage from the electric
power line 1.
[0009] After a commercial voltage (for example 270V ) is obtained,
the commercial voltage is insulated by the insulating transformer
12 whose carrier frequency is a commercial frequency of 50 Hz or 60
Hz. A control device disclosed in Japanese Patent Disclosure
(kokai) No. 7-31156 is applicable for the controller of the
three-phase inverter 9.
[0010] However, there are some problems in the conventional power
supply device of FIG. 5.
[0011] First, the insulating transformer 12 becomes heavy and
large, because the carrier frequency is a relatively low commercial
frequency. Moreover, the insulating transformer 12 causes noise of
the commercial frequency.
[0012] Further, the same voltage as that of the electric power line
1 is applied to the three-phase inverter 9, the AC reactors 10 and
the AC capacitors 11. Therefore, the conventional power supply
device must be suitably insensitive to voltage fluctuations and
becomes collectively large and costly.
[0013] Furthermore, load fluctuation from load objects, such as air
conditioners or lighting, causes an adverse influence on the
current of the DC voltage from the electric power line 1.
Therefore, the electrolytic capacitor 8 charged with the DC voltage
must be large enough to remove a ripple wave (50 Hz or 60 Hz)
caused by the load fluctuation.
SUMMARY OF THE INVENTION
[0014] Accordingly, one object of this invention is to provide a
miniaturized, light weight, low noise and low price power supply
device for an electromotive railcar. The present invention provides
a power supply device for an electromotive railcar, and comprises a
first capacitor connected to receive the DC voltage for outputting
a first DC voltage. The power supply device also comprises a
DC/AC/DC converter that includes an inverter bridge having power
transistors connected to the first capacitor in parallel, an
insulating transformer with high carrier frequency having an
primary winding connected to an output of the inverter bridge, and
a rectifier circuit connected to a secondary winding of the
insulating transformer to receive a second DC voltage. The power
supply device further comprises a three-phase inverter having a
bridge circuit of power transistors for generating a three-phase AC
voltage on the basis of the second DC voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0016] FIG. 1 is a circuit diagram showing a power supply device
for an electromotive railcar according to a first embodiment of the
present invention;
[0017] FIG. 2 is a circuit diagram of a partial resonance switching
circuit of a third embodiment of the present invention;
[0018] FIG. 3 is a circuit diagram of a discharge circuit of a
fourth embodiment of the present invention;
[0019] FIG. 4 is a circuit diagram showing a smoothing circuit of a
fifth embodiment of the present invention; and
[0020] FIG. 5 is a circuit diagram showing a conventional power
supply device for an electromotive railcar.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Referring now to the drawings, wherein like reference
numerals designate identical or corresponding parts throughout the
several views, the embodiments of the present invention are
described below.
[0022] FIG. 1 is a circuit diagram showing a power supply device
for an electromotive railcar according to a first embodiment of the
present invention. The power supply device of the first embodiment
of the present invention omits the electrolytic capacitor 8 and the
insulating transformer 12 of the conventional power supply device
of FIG. 5 and adds some elements as described below.
[0023] The power supply device for an electromotive railcar
receives a first DC voltage from an electric power line 1.
[0024] A series circuit of a pair of first capacitors 21, 22 is
connected to receive the first DC voltage from the electric power
line 1 via a pantograph 2, a fuse 3, a contactor 4, a DC reactor 5,
and either a conducting thyristor 6 or an initial charge resistor
7.
[0025] An H-type inverter bridge 58 comprises a plurality of power
transistors 23-26 and a plurality of diodes 60-63. An H-type
inverter bridge 59 comprises a plurality of power transistors 27-30
such as IGBT (Insulated Gate Bipolar Transistor) and a plurality of
diodes 64-67. The H-type inverter bridge 58 is connected in
parallel to a first capacitor 21. The H-type inverter bridge 59 is
connected in parallel to a first capacitor 22. The first capacitor
22 and the H-type inverter bridge 58 are coupled to a ground
14.
[0026] Thus, the H-type inverter bridges 58, 59 are respectively
connected to the first capacitors 21, 22 in parallel and are
connected to the first DC voltage in serial.
[0027] A pair of insulating transformers 31, 32 with a high carrier
frequency and having primary windings are respectively connected to
outputs of the H-type inverter bridges 58, 59.
[0028] Rectifier circuits 68, 69 each are connected to a respective
secondary winding of the insulating transformers 31, 32 and
comprise a corresponding bridge circuit of diodes 33-36 and 37-40,
respectively, in the form of a center tap. The outputs of rectifier
circuits 68, 69 are connected in parallel to each other.
[0029] A pair of DC/AC/DC converters comprises a respective one of
the H-type inverter bridges 58, 59, a respective one of the
insulating transformers 31, 32, and a respective one of the
rectifier circuits 68, 69.
[0030] A smoothing circuit 70 comprises a DC reactor 41 and a
second capacitor 42 and is connected to the outputs of the
rectifier circuits 68, 69 to form a second DC voltage. The
three-phase AC voltage 13 is obtained from the three-phase inverter
9 on the basis of the second DC voltage.
[0031] Further, each of the AC filters comprising a series circuit
of the AC reactor 10 and the AC capacitor 11 is connected to the
output of each phase of the three-phase inverter 9. Terminals of
the AC capacitors 11 are connected to a neutral point (0V) of the
secondary windings of the insulating transformers 31, 32, that is,
the neutral point (0V) of the second DC voltage.
[0032] A control circuit for the power transistors 23-30 of the
H-type inverter bridges 58, 59 of the DC/AC/DC converters is
described as follows.
[0033] A reference voltage of the DC/AC/DC converters' output is
determined by a voltage setter 43. An adder 44 is coupled to the
voltage setter 43 and the output of the smoothing circuit 70 and
calculates a difference between the reference of a DC voltage and
the second DC voltage. An amplifier 45 amplifies the difference
with a proportional integral operation. A PWM (Pulse Width
Modulation ) generator 46 compares the amplified difference with
the output of a triangular wave generator 47 and modulates the
pulse width. A plurality of gate drive amplifiers 48, 49 for
driving the power transistors 23-26 and 27-30, respectively,
amplifies the modulated digital wave signal and insulates the power
transistors 23-26 and 27-30 from the signal.
[0034] The DC/AC/DC converters control the high voltage of the
first DC voltage so as to obtain a constant DC voltage suited for
the three-phase AC voltage 13 generated by the three-phase inverter
9. Although the first DC voltage is changeable, the DC/AC/DC
converters keep the output steady.
[0035] A description of the operation of the power supply device of
FIG. 1 follows.
[0036] In FIG. 1, the power supply device obtains the first DC
voltage from the electric power line 1 via the pantogragh 2. The
first DC voltage charges the first capacitors 21, 22 via the fuse
3, the contactor 4, the DC reactor 5 and the initial charge
resistor 7. When the first capacitors 21, 22 are charged to a
predetermined voltage, the conducting thyristor 6 connected in
parallel with the initial charge resistor 7 turns on.
[0037] The output signals of the first capacitors 21, 22 are
respectively converted into AC voltages by the H-type inverter
bridges 58, 59 controlled by the respective gate drive amplifiers
48, 49. The AC voltages are insulated by the insulating
transformers 31, 32 and then converted into a DC voltage by the
rectifier circuits 68, 69.
[0038] The smoothing circuit 70 smoothes the DC voltage to obtain
the second DC voltage. The three-phase inverter 9 generates the
three-phase AC voltage 13 on the basis of the second DC
voltage.
[0039] The AC filters composed of AC reactors 10 and AC capacitors
11 remove the higher harmonics of the three-phase AC voltage 13 to
obtain a commercial voltage with a fundamental wave such as 50 Hz
or 60 Hz frequency.
[0040] Thus, a stable DC voltage for the second DC voltage is
obtained without being influenced by the voltage of the electric
power line 1 (the first DC voltage).
[0041] The power supply device of the first embodiment has the
following effects.
[0042] First, since the insulating transformers 31, 32 are designed
with a high carrier frequency and excited with a several KHz
carrier frequency generated by the power transistors of the H-type
inverter bridges 58, 59, it makes both size and weight of the
insulating transformers 31, 32 smaller by 1/4 to 1/8 of
corresponding elements of the conventional power supply device of
FIG. 5, and further attenuates noise.
[0043] Second, the voltage susceptibility of the secondary side of
the insulating transformers 31, 32 can be different from that of
the primary side. Thus, the components of the primary side handle
high voltages, such as a high voltage of 1500V for the first DC
voltage. In general, for a three-phase AC voltage 13 of less than
440V, the DC/AC/DC converters output voltage is about 600V. So the
components of the secondary side can be designed with relatively
low voltage susceptibility and use both small-sized and low cost
equipment.
[0044] Third, if the sharing of loads between the H-type inverter
bridges 58, 59 changes, the H-type inverter bridges 58, 59 are not
able to share the first DC voltage by halves. Further, if almost
all the first DC voltage is applied to one of the H-type inverter
bridges 58, 59, the power transistors 23-26 or 27-30 may fail.
However, since each of the outputs of the rectifier circuits 68, 69
is connected in parallel, the unbalance load sharing between the
H-type inverter bridges 58, 59 is canceled.
[0045] Specifically, if the load in the H-type inverter bridges 58
increases, the voltage of the first capacitor 21 connected to the
H-type inverter bridge 58 decreases. Consequently, the output
voltage of the secondary winding also decreases. On the other hand,
the voltage of the other first capacitor 22 increases and the
output voltage of the secondary winding connected to the other
H-type inverter bridge 59 increases.
[0046] As a result, the load concentrates on the H-type inverter
bridge 59 with a higher voltage. This operation is taken quickly
and finally the load sharing between the H-type inverter bridges
58, 59 is equal.
[0047] Furthermore, each of the DC voltages applied to the H-type
inverter bridges 58, 59 becomes equal. Consequently, the power
transistors 23-30 can be used as low voltage-proof elements.
[0048] Fourth, since the terminals of the AC capacitors 11 are
connected to a neutral point (0V ) of the secondary windings of the
insulating transformers 31, 32 (e.g., the neutral point (0V ) of
the second DC voltage), the inductive interference caused by the
three-phase inverter 9 is attenuated. Specifically, since the peak
current applying to the AC capacitors 11 is half of the amplitude
of the current applying to the AC capacitors 11, the inductive
interference caused by the switching noise of the three-phase
inverter 9 is attenuated.
[0049] As described above in the first embodiment, the power supply
device obtains a steady DC voltage as the second DC voltage with no
influence of the first DC voltage from the electric power line 1
and achieves miniaturization, light weight, low level noise and
attenuation of inductive interference.
[0050] Further, the number of the first capacitors 21, 22 and the
DC/AC/DC converters can be designed in response to the first DC
voltage (for example, 600V or 1500V). For example, in the case of
600V, the number of the first capacitors 21, 22 and the DC/AC/DC
converters may be two as shown in FIG. 1. In the case of 1500V,
three first capacitors and three DC/AC/DC converters may be used.
Therefore, this embodiment optimizes the elements of cost, size and
weight and achieves miniaturization, light weight and low cost.
[0051] Furthermore, the insulating transformers 31, 32 in the first
embodiment are designed the carrier frequency for the range of 1 to
6 KHz. Therefore, the first embodiment optimizes the elements of
cost, size and weight and achieves miniaturization, light weight
and low noise.
[0052] To achieve low noise, the generated frequency should be less
than the audio range (15 KHz). Accordingly, the carrier frequency
should be less than 7.5 KHz, half of 15 KHz, as calculated in
accordance with conventional theory.
[0053] On the other hand, to achieve miniaturization and light
weight, a carrier frequency less than 6 KHz is useful. More than a
6 KHz carrier frequency may not achieve sufficient tradeoffs in
view of the switching loss of the power transistors 23-30.
Consequently, a 1 to 6 KHz carrier frequency is useful from the
point of view of miniaturization, lightweight and low noise.
[0054] Further, the capacity of the second capacitor 42 is large
enough, the same as that of the capacitor 8 (FIG. 5), so as to
remove a ripple wave with a predetermined frequency which is a
commercial frequency (50 Hz or 60 Hz). Consequently, the load
fluctuation caused by the load objects connected to the three-phase
inverter 9 does not influence the current of the first DC voltage
from the electric power line 1. As a result, the capacity of the
first capacitors 21, 22 take no account of a ripple wave with a
commercial frequency caused by the load fluctuation and take
account of a 180 Hz or 360 Hz ripple wave in the electric power
line 1. As a practical matter, the capacity of the first capacitors
21, 22 is 1/3 or 1/6 of the second capacitor 42.
[0055] The ripple wave passed through the first capacitors 21, 22
is 180 Hz or 360 Hz under the influence of the electric power line
1 and the ripple wave passed through the second capacitor 42 is 50
Hz or 60 Hz under the influence of three-phase inverter 9. If the
second capacitor 42 can filter the 50 Hz ripple wave, the first
capacitor 21, 22 accounts for only the 180 Hz or 360 Hz ripple
wave.
[0056] Consequently, since the ripple wave with commercial
frequency caused by the load fluctuation is filtered by the second
capacitor 42, a small capacitor can be used as the first capacitors
21, 22.
[0057] According to a second embodiment of the present invention,
each of the insulating transformers 31, 32 has a common core; the
number of turns of the primary windings are the same in the
DC/AC/DC converters; and the number of turns of the secondary
windings are the same in the DC/AC/DC converters respectively. In
this embodiment, the sharing of the load of the DC/AC/DC converters
is substantially equal. Further, the DC/AC/DC converters are
simultaneously driven with the pulse width modulated digital wave
signal.
[0058] FIG. 2 is a circuit diagram showing a partial resonance
switching circuit of a third embodiment of the present
invention.
[0059] As shown in FIG. 2, this embodiment deletes the first
capacitor 22, the H-type inverter bridge 59 with the transistors
27-30, the insulating transformer 32 and the rectifier circuit with
the diodes 37-40 in FIG. 1. The circuit of this embodiment has one
DC/AC/DC converter comprising the first capacitor 21, the H-type
inverter bridge 58 having the transistors 23-26, the insulating
transformer 31 and the rectifier circuit (not shown in FIG. 2) with
the diodes 33-36. Further, partial resonance switching circuits 50,
51 are connected in parallel to the H-type inverter bridge 58. The
commutation system of the H-type inverter bridge 58 is a partial
resonance type which switches the power transistors 23-26 at the
time zero-voltage or zero-current is applied to the power
transistors 23-26.
[0060] Therefore, the switching loss of the power transistors 23-26
is minimized or deleted and only the ON loss of the power
transistors 23-26 is accounted for.
[0061] In general, a switching loss is generated at a transient
stage when a power transistor switches ON to OFF or OFF to ON, and
it is calculated by the product of voltage and current. An ON loss
is generated at a steady state after the transient stage while a
power transistor is ON. It is also calculated by the product of
voltage and current. In a DC/AC/DC converter with a high frequency
insulating transformer, the switching loss of transistors increases
in addition to the ON loss of the transistors.
[0062] In the third embodiment of the present invention, since the
commutation system of the H-type inverter bridge 58 is a partial
resonance type which switches the power transistors 23-26 at the
time zero-voltage or zero-current is applied to the power
transistor, the switching loss of the power transistors is
minimized or deleted. The loss accompanied with high frequency
switching can be reduced.
[0063] FIG. 3 is the circuit diagram showing the discharge circuit
of a fourth embodiment of the present invention.
[0064] As shown in FIG. 3, this embodiment adds a discharge
circuit, connected to the second capacitor 42 in parallel,
comprising a series circuit of a power transistor 52 and a
discharge resistor 53, and a voltage surveillance circuit 54,
connected to a second capacitor 42 in parallel, for detecting the
second DC voltage.
[0065] If the voltage surveillance circuit 54 detects a voltage
over a predetermined voltage, it turns on the power transistor 52
via a transistor drive amplifier 55 in order to discharge
regeneration energy from load objects. The discharge circuit of
this embodiment protects the transistors of the three-phase
inverter 9 from high voltage of the second DC voltage.
[0066] In brief, the power supply unit in FIG. 1 may not discharge
regenerated energy from load objects, such as air conditioners and
lighting. Consequently once the second DC voltage rises over the
rated voltage of a transistor, the transistor may fail.
[0067] In this embodiment, since the voltage surveillance circuit
54 detects the second DC voltage, if the second DC voltage rises
over the predetermined voltage, first the voltage surveillance
circuit 54 outputs a detecting signal to the transistor drive
amplifier 55, then the transistor drive amplifier 55 turns on the
power transistor 52, and then the regenerated current is passed
through the discharge resistor 53. Finally, the regenerated energy
is discharged and the second DC voltage drops.
[0068] The power transistor 52 turns off and stops discharge when
the second DC voltage drops below the second predetermined voltage.
The second predetermined voltage for turning off the power
transistor 52 is lower than the predetermined voltage for turning
on the power transistor 52. The power transistor 52 keeps an
average voltage constant and discharges the regenerated energy by
repeatedly switching ON and OFF.
[0069] Therefore this embodiment controls an increase in the second
DC voltage caused by load objects, and protects the power
transistors of the three-phase inverter 9 from application of a
high voltage.
[0070] FIG. 4 is the circuit diagram showing a smoothing circuit
according to a fifth embodiment of the present invention.
[0071] As shown in FIG. 4, the smoothing circuit of this embodiment
comprises a series circuit of a pair of capacitors 56, 57 instead
of the second capacitor 42. Further, one terminal of each of the AC
capacitors 11 is connected to a neutral point between the
capacitors 56, 57.
[0072] Since the terminals of the AC capacitors 11 are connected to
a neutral point (0V) between the capacitors 56, 57 (e.g., the
neutral point (0V) of the second DC voltage), the peak current of
the AC capacitors 11 is half of that amplitude and the capacity of
AC capacitors 11 can be smaller. Further, the inductive
interference caused by the three-phase inverter 9 is
attenuated.
[0073] Consequently, the power supply device of the present
invention can be miniaturized, light weight, low noise and low
price.
* * * * *