Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2002-04-02
2004-01-27
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S132000, C363S056020
Reexamination Certificate
active
06683799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a protecting apparatus for an inverter constituted by a plurality of switching devices connected between an anode and a cathode of a DC power source.
2. Description of the Related Art
Hitherto, a motor for driving a hermetic electric compressor constituting, for example, an air conditioner or a refrigerating cycle of an electric refrigerator or the like, a synchronous motor that has a permanent magnet in its rotor and is driven by direct current, or an induction motor driven by alternating current has been adopted. In recent years, a synchronous induction motor having a permanent magnet for its rotor and squirrel-cage secondary conductors have also been used. These motors are driven by inverters.
A drive circuit of such a conventional motor will now be described with reference to FIG.
10
. Shown in the drawing are an inverter main circuit power source Vcc
1
, an inverter
101
that generates three-phase pulse width modulation (PWM) outputs by a plurality of high-frequency output switching devices Tr
1
, Tr
2
, Tr
3
, Tr
4
, Tr
5
, and Tr
6
, a motor
102
installed in an electric compressor (not shown), a drive circuit
103
, a microcomputer
104
, an operational amplifier AMP, and current transformers
105
and
106
for detecting currents passing through the motor
102
. The microcomputer
104
generates drive signals to cause the motor
102
to continuously rotate, and drives the inverter
101
through the intermediary of the drive circuit
103
. The inverter
101
generates three-phase PWM substantially sinusoidal waveforms, using a series connection point, as an output, of each pair of the switching devices Tr
1
and Tr
4
, Tr
2
and Tr
5
, and Tr
3
and Tr
6
that are connected in parallel to the power source Vcc
1
, thereby continuously running the motor
102
at a predetermined number of revolutions.
The currents of two phases, namely, the current output between the switching devices Tr
1
and Tr
4
and the current output between the switching devices Tr
2
and Tr
5
are detected by the current transformers
105
and
106
. The detection outputs are amplified by the operational amplifiers AMP and AMP, then supplied to the microcomputer
104
. The microcomputer
104
presumes the position of the rotor of the motor
102
on the basis of the detection outputs of the current transformers
105
and
106
, controls the drive circuit
103
to drive the inverter
101
, and generates the three-phase PWM substantially sinusoidal waveforms to rotatively control the motor
102
.
If the detection outputs of the current transformers
105
and
106
are higher than a preset value, i.e., if overcurrent flows from the inverter
101
to the motor
102
, the microcomputer
104
controls the drive circuit
103
to stop the inverter
101
and also stop the motor
102
so as to protect the inverter
101
and the motor
102
from overcurrent.
Thus, in the prior art, the output currents of the inverter
101
are detected by the current transformers
105
and
106
to presume the position of the rotor of the motor
102
and to protect the motor
102
from overcurrent. The current transformers, however, are expensive; therefore, as shown in
FIG. 10
, current transformers for only two phases are usually inserted, and the microcomputer
104
has to compute a formula I
T
=−(I
R
+I
S
) to calculate the current value for the remaining one phase (e.g., T phase).
SUMMARY OF THE INVENTION
The present invention has been made with a view toward solving the problem with the prior art, and it is an object of the invention to provide a protecting apparatus capable of securely and inexpensively effecting protection of an inverter from overcurrent.
According to one aspect of the present invention, there is provided a protecting apparatus that is applied to an inverter constituted by a plurality of switching devices connected between an anode and a cathode of a DC power source, and includes a plurality of detecting devices that convert the currents passing through the plurality of switching devices into voltages and generate detection outputs, a plurality of comparing devices that receive the detection outputs of the detection devices and generate an abnormality output if the absolute values of the detection outputs exceed a predetermined value, and a protecting device that interrupts the drive of all the switching devices if any of the comparing devices generates the abnormality output.
In a preferred form of the present invention, the inverter is constructed of a plurality of pairs of the switching devices connected in parallel between the anode and the cathode of the DC power source, each of the pairs being formed of the switching devices connected in series. The inverter uses, as outputs, the connection points of the switching devices of the pairs mentioned above to convert DC to AC. The detecting devices convert the currents passing through the pairs of switching devices connected to the cathode into voltages, and generate detection outputs.
In another preferred form, the protecting device interrupts the drive of the switching devices if an abnormal output from the comparing devices occurs and lasts for a predetermined period of time.
In yet another preferred form, if the drive of the switching devices is interrupted, the protecting device maintains the interruption state until a predetermined release instruction is issued.
In still another preferred form, the detection outputs of the detecting devices are the information for detecting the condition of a motor driven by the inverter.
In a further preferred form, the motor is a synchronous motor having a permanent magnet in its rotor.
In another preferred form, the motor is a synchronous induction motor having a permanent magnet in its rotor.
In another preferred form, the motor is an induction motor.
REFERENCES:
patent: 6194856 (2001-02-01), Kobayashi et al.
Izawa Yuichi
Kato Hideaki
Nomoto Tetsuo
Ogawa Takashi
Darby & Darby
Patel Rajnikant B.
Sanyo Electric Co,. Ltd.
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