JP2001025290A - Control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect the rotational speed or the rotational direction of an AC motor in a free-run state without using an expensive and small-sized current detector having a limit in miniaturization. Obtain the control device. SOLUTION: An inverter control circuit 5a for simultaneously turning on a first or second switching element (Q1, Q3, Q5 or Q2, Q4, Q6) of a plurality of series connection circuits, and a series connection circuit Voltage polarity detection for detecting at least one phase of a voltage change between the connection point of the first and second switching elements and the negative DC bus or the positive DC bus of the inverter circuit 3 as a fixed potential point. A circuit 6a, a frequency measuring device 7a for detecting a rotation speed of the AC motor 4 in a free-run state by measuring a cycle of an output signal of the residual voltage polarity detecting circuit 6a, and the like,
Control device equipped with

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for driving an AC motor at a variable speed, and more particularly to a control device capable of detecting a rotation speed or a rotation direction of an AC motor in a free-run state.

[0002]

2. Description of the Related Art A system for driving an AC motor at a variable speed using an inverter has been widely used. In such a variable speed drive system, when the AC power supply is cut off due to a momentary power failure or the like, the voltage output from the inverter to the AC motor is cut off, and the AC motor enters a free-run state. When the load of the AC motor is a wind power device such as a fan, the inertia force is large, so the AC motor in the free-run state does not stop for a long time and rotates by tens of seconds to several minutes or more due to inertia. Sometimes. When the AC motor in the free-run state is restarted by the inverter after the power is restored, the rotation speed of the AC motor in the free-run state must be adjusted to avoid the inrush current and excessive torque and restart without shock. It is necessary to output the frequency from the inverter, and therefore, it is necessary to detect the rotational speed of the AC motor in the free-run state. A speed sensor is normally used to detect the rotational speed of the AC motor. However, when a speed sensor is added, there are problems such as an increase in the size of the device, an increase in cost, and troublesome cable routing.

FIG. 7 is a diagram showing the configuration of a conventional variable speed drive system for detecting the rotational speed of an AC motor in a free-run state, which is disclosed in Japanese Patent Application Laid-Open No. 8-331894. FIG. 8 is a diagram showing a circuit at the time of detecting the rotational speed of the AC motor in the free-run state in the variable speed drive system of FIG. In the figure,
1 is a three-phase AC power supply, 2 is a rectifying and smoothing circuit connected to the three-phase AC power supply 1, 3 is an inverter circuit connected to the rectifying and smoothing circuit 2, 4 is an induction motor connected to the inverter circuit 3, and 5b is an inverter. An inverter control circuit for controlling the circuit 3, 7b is a frequency measuring device, and 8 is a frequency setting device.
Reference numeral 10 denotes a current detector which is coupled to the output line of the inverter circuit 3 and detects an inverter output current, that is, a motor input current. The inverter circuit 3 has six IGBs
The first to sixth semiconductor switches Q1 to Q6 made of T are connected in a three-phase bridge connection, and the freewheel diodes D1 to D6 are connected to the respective semiconductor switches Q1 to Q6 in antiparallel.

During normal operation, the frequency setting device 8
The inverter control circuit 5b controls the inverter circuit 3 so that the induction motor 4 rotates in accordance with the desired frequency set in step 3.
Are turned on and off.

Here, when an instantaneous power failure or the like occurs in the three-phase AC power supply 1, all the semiconductor switches Q1 to Q6 of the inverter circuit 3 are turned off by the inverter control circuit 5b, and the induction motor 4 enters a free-run state. Next, when the three-phase AC power supply 1 is restored to a state where power can be supplied to the induction motor 4 again, the lower three semiconductor switches Q2, Q4, and Q6 in the inverter circuit 3 are simultaneously turned on, and FIG. As shown in FIG. 8, a closed circuit including the inverter circuit 3 and the induction motor 4 is formed.

At this time, a current flows through the closed circuit due to the residual voltage generated in the induction motor 4 in the free-run state. For example, when the U-phase current flows from the inverter circuit 3 toward the induction motor 4, it flows through the freewheel diode D2 as shown by the solid arrow in FIG. When the semiconductor switch Q2 flows, as shown by the dotted arrow,
Flow through This residual voltage is an AC voltage having a frequency proportional to the rotation speed caused by the influence of the residual magnetic flux of the induction motor 4, and the current flowing through the closed circuit in FIG. 8 also becomes a current having a frequency proportional to the rotation speed. The current flowing in this closed circuit, that is, the input current of the induction motor 4 is detected by the current detector 10 and the frequency of the detected current is obtained by the frequency measuring device 7b, thereby detecting the free-running rotation speed of the induction motor 4. Was.

[0007]

In the conventional variable speed drive system as described above, the method for detecting the number of revolutions of the induction motor 4 in the free-run state requires the current detector 10, which increases the cost. And the size cannot be reduced. In addition, since the residual voltage of the induction motor 4 is attenuated by the time constant determined by the motor constant and the rotation speed, the residual voltage is reduced to some extent when the power supply stop period for the induction motor 4 is long or when the rotation speed is low. In the state, the semiconductor switches Q2, Q4, Q6
At the same time, only a very small current flows through the induction motor 4, and it is difficult for the current detector 10 to accurately detect the current. For example, in the case of a generally available three-phase induction motor with a rated voltage of 200 V and several kW, after several tens of seconds have elapsed since the power supply was stopped, the induction motor 4
Is in a free-run state near the rated rotation speed, the current flowing through the output line of the inverter circuit 3 is only a few tenths or less of the rated current. Due to noise included in the output,
In some cases, the frequency of the current was erroneously detected by the frequency measuring device 7b.

In Japanese Patent Application Laid-Open No. 9-163776, a voltage dividing circuit using a voltage dividing resistor is provided between the negative DC bus of the inverter circuit 3 and the output line to the induction motor 4 so that the residual voltage of the induction motor 4 is reduced. A method of detecting the rotation direction of the induction motor 4 in the free-run state by detecting the rotation direction is disclosed. However, since the phase voltage of the induction motor 4 is applied to the voltage dividing circuit during normal operation, there is a problem that the voltage dividing resistance constituting the voltage dividing circuit has a large loss. Although the loss of this resistor can be reduced as the resistance value increases, increasing the resistance value of the voltage-dividing resistor becomes extremely susceptible to noise and is not practical.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is free of cost and low loss and accurately in a free-run state without using a current detector that is expensive and has a limit in miniaturization. It is an object of the present invention to obtain a control device capable of detecting a rotation speed or a rotation direction of an AC motor.

[0010]

A control device according to the present invention comprises a plurality of series-connected circuits of first and second switching elements having diodes connected in anti-parallel to provide a variable frequency / variable DC power. An inverter circuit that converts the voltage into AC power, and on / off control of the first or second switching element based on a commanded frequency, and a rotational speed or a rotational direction of the AC motor to be controlled in a free-run state. And an inverter control circuit for simultaneously turning on one of the first and second switching elements in the plurality of series-connected circuits, and first and second constituent elements of the series-connected circuit.
A residual voltage polarity detection circuit for detecting at least one phase of a voltage change between the connection point of the switching element and the negative DC bus or the positive DC bus of the inverter circuit as a fixed potential point; A frequency measuring device for detecting a rotation speed of the AC motor in a free-run state by measuring a cycle of an output signal of a circuit or the like.

The residual voltage polarity detection circuit may be a negative DC bus or a positive DC bus of the inverter circuit as a fixed potential point and a connection point of the first and second switching elements constituting the series connection circuit. At least two phases are detected, and information on the rotational direction of the AC motor in the free-run state is detected based on the detected voltage changes.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a diagram showing a configuration of a variable speed drive system according to one embodiment of the present invention. In the figure, reference numerals 1 to 4, and 8 are the same as those of the above-described conventional device, and the description thereof is omitted. 5a is an inverter circuit 3
, 6a is a residual voltage polarity detection circuit, and 7a is a frequency measuring device.

FIG. 2 is a diagram showing a configuration of the residual voltage polarity detection circuit 6a in the variable speed drive system according to one embodiment of the present invention. In the figure, 60 is a series connection point of the semiconductor switches Q1 and Q2 of the inverter circuit 3, that is, an input terminal connected to the U-phase output to the induction motor 4, and 61 is an input connected to the negative DC bus of the inverter circuit 3. Terminal, 62 is an output terminal connected to the frequency measuring instrument 7a,
_CC is the control power supply voltage. D100 is a small signal diode, D101 is a small signal high voltage diode, and R100
R103 is a resistor, Q100 is a transistor, C100
Is a capacitor.

The small-signal high-voltage diode D101 is a high-voltage diode for cutting off a current flowing from the inverter circuit 3 to the residual voltage polarity detection circuit 6a when the potential of the input terminal 60 becomes high. Residual voltage polarity detection circuit 6
a is one of the connection point 60 of the first and second switching elements (Q1, Q2) forming the series connection circuit and the negative DC bus or the positive DC bus of the inverter circuit 3 as a fixed potential point. Is connected through a high-voltage diode, the main circuit voltage of the inverter applied during normal operation can be cut off by the high-voltage diode. The rotational speed or rotational direction of the AC motor in the free-run state can be detected.

Next, the operation will be described. During normal operation, the inverter control circuit 5a operates the semiconductor switches Q1 to Q4 of the inverter circuit 3 so that the induction motor 4 rotates according to the desired frequency set by the frequency setting device 8.
The on / off control of Q6 is the same as in the prior art.
Here, the inverter control circuit 5a monitors the output voltage of the rectifying / smoothing circuit 2, and when the output voltage of the rectifying / smoothing circuit 2 becomes equal to or less than the set value due to an instantaneous power failure of the three-phase AC power supply 1, etc. It operates to turn off all the semiconductor switches Q1 to Q6, and the induction motor 4 enters a free-run state.

When the three-phase AC power supply 1 is restored, the rectifying and smoothing circuit 2
When the output voltage of the inverter circuit 3 becomes equal to or higher than the set value again, the inverter control circuit 5a simultaneously turns on the three lower semiconductor switches Q2, Q4, and Q6 in the inverter circuit 3 and the inverter circuit 3 and the induction motor 4 A closed circuit similar to that of FIG. 8 is formed.

FIG. 3 is a diagram showing a closed circuit formed by the inverter circuit 3 and the induction motor 4 by simultaneously turning on the lower three semiconductor switches Q2, Q4, and Q6 in the inverter circuit 3. 4) is a circuit diagram showing a circuit at the time of detecting the rotation speed of the induction motor in the free-run state in the variable speed drive system, and FIG. 4 (b) is a U-phase current Iu and the U-phase output of the inverter circuit 3 viewed from the negative DC bus. FIG. 9 is a diagram showing a voltage, that is, a collector potential of the semiconductor switch Q2 or a cathode potential V _UN of the freewheel diode D2. The lower three semiconductor switches Q2,2 in the inverter circuit 3
In the closed circuit formed by the inverter circuit 3 and the induction motor 4 by simultaneously turning on Q4 and Q6, a current flows due to the residual voltage generated in the induction motor 4 in the free-run state as in the conventional example. For example, the U-phase current is
3 flows out of the motor toward the induction motor 4 in FIG.
(A) flows through the freewheel diode D2 as shown by the solid arrow, and conversely, flows from the induction motor 4 toward the inverter circuit 3 through the semiconductor switch Q2 as shown by the dotted arrow.

Here, in FIG. 3A, the case where the current flows through the freewheeling diode D2 as shown by the solid arrow is a positive polarity, and the case where the current flows through the semiconductor switch Q2 as shown by the dotted arrow. Is a negative polarity. FIG.
As shown in (b), when the current has a positive polarity, V _UN has a negative potential by the forward voltage V _F (D2) of the freewheeling diode D2, and when the current has a negative polarity, V _UN is a semiconductor switch. The potential becomes positive by the ON voltage V _CE (Q2) of Q2.

The ON voltage V _CE (Q2) at the semiconductor switch Q2 and the forward voltage V
_As a general characteristic of _F (D2), its magnitude changes depending on the main circuit current, and is about 1 to 2 V during normal operation, but V _CE (Q2) and V _F (D
2) is about 0.6 V or more. Therefore, small residual voltage of the induction motor 4, even the current flowing through a closed circuit shown in FIG. 3 (a) micro, by utilizing the difference in electric potential of the V _UN for the polarity of the current, residual voltage Polarity can be detected.

FIG. 4 is a diagram for explaining the operation of the residual voltage polarity detection circuit 6a in the variable speed drive system according to one embodiment of the present invention, wherein (a) shows that the U-phase current has a positive polarity (Iu>
0), (b) shows a case where the U-phase current is negative (Iu <0). Since the input terminal 60 is connected to the U-phase output of the inverter circuit 3 and the input terminal 61 is connected to the negative DC bus of the inverter circuit 3, the input terminals 60, 6
In other _words , the voltage of V _UN is applied between the two. Note that the potential of the negative DC bus is set to the GND potential (0
This will be described below as V).

A series circuit of a resistor R100 and a small signal diode D100 is connected from the control power supply voltage V _CC to GND, and the small signal diode D100 is connected to the control power supply voltage V _CC.
Connected forward to _CC . The transistor Q100 is connected to the anode terminal of the small signal diode D100.
Are connected. Accordingly, the base potential of the transistor Q100 is positive by the forward voltage V _F (D100) of the small-signal diode D100, and is usually about 0.6V.

Here, when the current has a positive polarity as shown in FIG. 4 (a), the current is
_Therefore , V _UN has a negative potential by the forward voltage V _F (D2) of the freewheeling diode D2. Further, since the reflux diode D2 is a diode main circuit current flows, the forward voltage V _F (D2) is generally about 1 to 2 V, about 0.6V even in a region the current is small. Also,
Forward voltage V _F of the small-signal high-voltage diode D101 for the control circuit (D101) is almost the same about 0.6V as the forward voltage V _F of the small signal diode D100 (D100). From these relationships, when the current is positive polarity, approximately the same about the voltage V _X of the base terminal of the transistor Q100 as seen from the anode terminal of the small-signal high-voltage diode D101 is forward voltage V _F of the freewheeling diode D2 (D2) 0.6 ~
It becomes 2 V, and a base current flows through the transistor Q100 via the resistor R101. Thereby, the transistor Q1
00 operates so that the collector current flows through the path shown by the dotted line in FIG. 4A, and if the value of the resistor R103 is selected to be sufficiently larger than the value of R101, the voltage of the output terminal 62 becomes almost GND. Potential.

When the current has a negative polarity as shown in FIG. 4B, the current flows through the semiconductor switch Q2 as described above, so that V _UN is positive by the ON voltage V _CE (Q2) of the semiconductor switch Q2. Potential. The ON voltage V _CE (Q2) of the semiconductor switch Q2 is usually about 1-2 V,
It is about 0.6 V even in a region where the current is small. Further, the forward voltage V _F (D10
1) is the forward voltage V _F (D
Because it is almost the same as about 0.6V and 100), the voltage V _X of the base terminal of the transistor Q100 as seen from the anode terminal of the small-signal high-voltage diode D101 becomes a negative potential, the transistor Q100 base current is cut off.
As a result, the output terminal 62 is pulled up by the resistor R103, and its voltage becomes the control power supply voltage V _CC . The capacitor C100 is used for removing noise, and may be omitted.

That is, the output terminal 62 of the current polarity detector 6
The output of almost becomes GND potential when the current, as shown in FIG. 4 (a) of the positive polarity, the V _CC potential when the current, as shown in FIG. 4 (b) of negative polarity.

Next, the frequency measuring device 7a receives a signal from the output terminal 62 of the residual voltage polarity detecting circuit 6a and detects the free-running rotational speed of the induction motor 4 by measuring the period of the signal. .

FIG. 5 is a diagram showing the relationship between the direction of rotation of the induction motor 4 and the operation of the closed circuit when detecting free-running rotation information.
(A) is a diagram showing the operation of the closed circuit, (b) is the induction motor 4
FIG. 7C is a diagram showing the phase relationship between the residual voltage of each phase and the current of each phase when the motor is free-running in the CCW direction;
FIG. 4 is a diagram showing a phase relationship between each phase residual voltage and each phase current when the induction motor 4 is free-running in the CW direction.

The free running of the induction motor 4 in the free-run state
-Residual electricity generated by the induction motor 4 depending on the run rotation direction
The pressure phase relationship is reversed. Therefore, the inverter circuit 3
Lower three semiconductor switches Q2, Q4, Q6
At the same time, the inverter circuit 3 and the induction motor 4
When a closed circuit as shown in FIG.
Fig. 5 also shows the phase relationship of each phase current flowing through the closed circuit by voltage.
Different as shown. For example, if the induction motor 4 is in the CCW direction
Is free running, I_U, I_V, I _WPhase order
If current flows at, rotate in the opposite CW direction
If you have_U, I_W, I_VA current flows in the order of the phases.

As a result, U, viewed from the negative DC bus,
_Assuming that the output voltages of the V and W phases are V _UN , V _VN , and V _WN , as described above, the current has a negative potential when the current has a positive polarity, and has a positive potential when the current has a negative polarity. , V
The phase relationship between _UN , V _VN and V _WN is shown in FIGS.
As shown in FIG. Therefore, the rotation direction can be detected by providing the residual voltage polarity detection circuit 6a in at least two phases and measuring the phase difference between the output signals.

The method of restarting the induction motor on the basis of the rotation speed and the rotation direction of the induction motor in the free-run state detected here is well known, and a description thereof will be omitted.

Embodiment 2 FIG. FIG. 6 is a diagram showing a configuration of the residual voltage polarity detection circuit 6b in the variable speed drive system according to one embodiment of the present invention. In the figure, 60
Is an input terminal connected to the series connection point of the semiconductor switches Q1 and Q2 of the inverter circuit 3, that is, an input terminal connected to the U-phase output to the induction motor 4, 61 is an input terminal connected to the negative DC bus of the inverter circuit 3, and 62 is Output terminals connected to the frequency measuring device 7a, D100 and D101 are diodes, R10
0 and R102 are resistors, and IC100 is a comparator. The diode D101 is a high withstand voltage diode for cutting off a current flowing from the inverter circuit 3 to the residual voltage polarity detection circuit 6b when the potential of the input terminal 60 becomes high. Note that components denoted by the same reference numerals as those in FIG. 2 perform the same operation.

The potential of each part of the residual voltage polarity detection circuit 6b is
The potential of each part of the residual voltage polarity detection circuit 6a shown in FIG. 4 is almost the same, and the potential of the negative input terminal of the comparator IC100 is equal to the forward voltage V _F (D10
0), usually about 0.6V. The potential of the positive input terminal becomes lower than the potential of the negative input terminal of the comparator IC100 when the current flows through the freewheel diode D2 of the inverter circuit 3 in a positive polarity. The voltage at the output terminal 62 of the voltage polarity detection circuit 6b becomes the GND potential.

Conversely, when the current has a negative polarity, the current flows through the semiconductor switch Q2 as described above, so that the potential of the input terminal 60 viewed from the input terminal 61 is only the ON voltage V _CE (Q2) of the semiconductor switch Q2. The potential of the output terminal 62 becomes positive, the potential of the positive input terminal of the comparator IC 100 becomes higher than the potential of the negative input terminal, and the voltage of the output terminal 62 becomes the control power supply voltage V _CC .

The signal from the output terminal 62 of the residual voltage polarity detection circuit 6b is input to the frequency measuring device 7a, and the period of the signal is measured to detect the free-running rotation speed of the induction motor 4 as described above. This is the same as the first embodiment.

In the residual voltage polarity detection circuit 6b, a resistor is provided in place of the diode D100, and the potential of the negative input terminal of the comparator IC 100 is created by dividing the control power supply voltage V _CC by this resistor and R100. You may.

In the above description, the three lower semiconductor switches Q2, Q4, Q
6 is turned on simultaneously to form a closed circuit by the inverter circuit 3 and the induction motor 4, but the upper three semiconductor switches Q1, Q3, Q5 in the inverter circuit 3 are turned on simultaneously to turn on the inverter circuit 3 And the induction motor 4 may form a closed circuit.

Further, the residual voltage polarity detection circuit is not limited to the above-described residual voltage polarity detection circuit 6a and residual voltage polarity detection circuit 6b, and at least one-phase AC motor of a switching element which is simultaneously turned on. Any circuit may be used as long as it detects a voltage change between the output potential and the fixed potential point and outputs a signal corresponding to the detected voltage change.

In the above description, the induction motor is described as the free-running AC motor.
The present invention is not limited to this, and similar effects can be obtained with other AC motors as long as the method can be applied to the AC motor.

In the above description, a configuration in which diodes are connected in antiparallel to the first and second switching elements has been described. However, in the case where a parasitic diode exists inside the switching elements, such as an FET, etc. The same effect can be obtained by using the diode.

In the above description, the inverter control circuit and the frequency measuring device have been described as separate components. However, part or all of the inverter control circuit and the frequency measuring device may be realized by software of a microcomputer.

[0040]

Since the present invention is configured as described above, it has the following effects.

In the control device according to the present invention, a plurality of series-connected circuits of the first and second switching elements having diodes connected in antiparallel are connected, and the DC power is converted into AC power having a variable frequency and a variable voltage. And an on / off control of a first or second switching element of a plurality of serially connected circuits constituting the inverter circuit based on a commanded frequency, and a control circuit for controlling a free-run state of the AC motor to be controlled. An inverter control circuit that simultaneously turns on one of the first and second switching elements of the plurality of series-connected circuits when detecting a rotation speed or a rotation direction; and a first circuit that forms the series-connected circuit. And a connection point of the second switching element and a negative DC bus or a positive DC bus of the inverter circuit as a fixed potential point. And a frequency measurement for detecting the rotational speed of the AC motor in a free-run state by, for example, measuring the period of an output signal of the residual voltage polarity detection circuit. , It is possible to accurately and inexpensively detect the rotational speed of the AC motor in the free-run state without using an expensive and small-sized current detector.

Further, the residual voltage polarity detection circuit may comprise a negative DC bus or a positive DC bus of the inverter circuit as a fixed potential point and a connection point of the first and second switching elements constituting the series connection circuit. At least two phases are detected, and information on the rotational direction of the AC motor in the free-run state is detected based on the detected voltage change. It is possible to accurately and inexpensively and accurately detect the rotational direction of the AC motor in the free-run state without using a current detector having a current.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a variable speed drive system according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a residual voltage polarity detection circuit 6a in the variable speed drive system according to one embodiment of the present invention.

FIG. 3 is a diagram showing a closed circuit formed by the inverter circuit 3 and the induction motor 4 by simultaneously turning on three semiconductor switches Q2, Q4, and Q6 on the lower side of the inverter circuit 3;

FIG. 4 is a diagram for explaining the operation of the residual voltage polarity detection circuit 6a in the variable speed drive system according to one embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the rotation direction of the induction motor 4 and the operation of the closed circuit when free-running rotation information is detected.

FIG. 6 is a diagram showing a configuration of a residual voltage polarity detection circuit 6b in the variable speed drive system according to one embodiment of the present invention.

FIG. 7 is a diagram showing the configuration of a conventional variable speed drive system for detecting the rotational speed of an AC motor in a free-run state, which is disclosed in Japanese Patent Application Laid-Open No. 8-331894.

8 is a diagram showing a circuit at the time of detecting the rotation speed of the AC motor in a free-run state in the variable speed drive system of FIG. 7;

[Explanation of symbols]

1 three-phase AC power supply, 2 rectifier smoothing circuit, 3 inverter circuit, 4 induction motor, 5a, 5b inverter control circuit, 6a, 6b residual voltage polarity detection circuit,
7a, 7b frequency measuring device, 8 frequency setting device, 1
0 current detector, 60 input terminal connected to the U-phase output to the induction motor 4, 61 input terminal connected to the negative DC bus of the inverter circuit 3, 62 output terminal connected to the frequency measuring device 7a, Q1 ~Q6 semiconductor switches, D1 to D6 wheeling diodes, D100 small signal diodes, D101 small signal high-voltage diode, R100～R103 resistance, Q100 transistor, C100 capacitor, IC 100 _{comparator, I U, I V, I} W U, V, W Phase current, V _UN ,
V _VN, V _WN negative viewed from the DC bus U, V, the output voltage of the W-phase, V _F (D2) forward voltage of the freewheeling diode D2, V _F (D100) small signal diodes D10
Forward voltage of 0, V _F (D101) Forward voltage of small-signal high-voltage diode D101, V _CC control power supply voltage, V _X Transistor Q10 viewed from anode terminal
0 base terminal voltage.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Taro Ando 2-3-2 Marunouchi, Chiyoda-ku, Tokyo F-term in Mitsubishi Electric Corporation (reference) 5H007 AA12 BB06 CA01 CB02 CB05 CC01 CC23 DA01 DC05 5H576 BB06 BB10 CC05 DD02 DD04 DD05 EE22 HA02 HB02 LL16 LL24 LL37 LL60

Claims (2)

[Claims] 1. An inverter circuit connected to a plurality of series-connected circuits of first and second switching elements having diodes connected in anti-parallel, for converting DC power into AC power having a variable frequency and a variable voltage, The plurality of series-connected circuits when on / off control of the first or second switching element is performed based on the detected frequency and when the rotation speed or the rotation direction of the controlled AC motor in a free-run state is detected. And an inverter control circuit for simultaneously turning on one of the first and second switching elements, and the inverter as a fixed potential point and a connection point between the first and second switching elements constituting the series connection circuit. Residual voltage polarity detection circuit for detecting at least one phase voltage change between a negative DC bus or a positive DC bus of a circuit And a frequency measuring device for detecting a rotation speed of the AC motor in a free-run state by measuring a cycle of an output signal of the residual voltage polarity detection circuit. 2. A negative DC bus or a positive DC bus of the inverter circuit as a fixed potential point and a connection point of the first and second switching elements constituting the series connection circuit. And detecting the information on the rotation direction of the AC motor in a free-run state based on the detected voltage change. The control device as described.