Motor drive apparatus having power failure detection unit for determining presence or absence of power failure

A motor drive apparatus includes a rectifier which converts AC power to DC power and DC power to AC power, an inverter which converts the DC power output by the rectifier to AC power and supplies the AC power to a motor, and which converts regenerative power from the motor to DC power and returns the DC power to the rectifier, a DC voltage detection unit which detects a DC output voltage of the rectifier, an AC voltage detection unit which detects an AC output voltage of the rectifier, a frequency calculation unit which calculates the frequency of the AC voltage; a storage unit which stores as a reference value the DC voltage at the start of the regenerative operation, and a power failure detection unit which determines the presence or absence of a power failure by using the DC voltage, the reference value, and the AC voltage frequency.

RELATED APPLICATIONS

The present application is based on, and claims priority from, Japanese Application Number 2011-262064, filed Nov. 30, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor drive apparatus which supplies drive power to a motor by first converting AC power supplied from an AC power supply to DC power and then converting the DC power to AC power for driving the motor, and more particularly, the invention relates to a motor drive apparatus having a power failure detection unit for determining the presence or absence of a power failure.

2. Description of the Related Art

In a motor drive apparatus for driving motors used in such machines as machine tools, forging presses, injection molding machines, and various kinds of robots, a motor speed, torque, or rotor position command is generated to control the operation of each of the motors provided one for each drive axis.FIG. 8is a diagram showing the configuration of a conventional motor drive apparatus for driving a plurality of motors. It is to be understood that, throughout the different drawings given herein, the same reference numerals designate component elements having the same functions. The motor drive apparatus100includes a 120-degree conduction mode rectifier11which converts AC power supplied from a commercial AC three-phase power supply3to DC power, and an inverter12which converts the DC power output by the 120-degree conduction mode rectifier11to AC power of a desired frequency and supplies the AC power as drive power to a motor2, or which converts AC power regenerated by the motor2to DC power, and the motor drive apparatus100controls the speed, torque, or rotor position of the motor2connected to the AC side of the inverter12.

For every inverter12there are motors2provided in order to drive and control the motors2by separately supplying drive power to each of the motors2provided one for each of the plurality of drive axes. On the other hand, usually only one 120-degree conduction mode rectifier11is provided for the plurality of inverters12in order to save the cost and installation space of the motor drive apparatus100.

When the motor is being decelerated under the control of the motor drive apparatus100, regenerative power is produced by the motor2. The regenerative power is fed through the inverter12back to the 120-degree conduction mode rectifier11. Since the 120-degree conduction mode rectifier11is a relatively low-cost rectifier having a power regeneration function for returning power recovered during motor deceleration to the power supply, and is capable not only of powering operation for converting AC power to DC power but also of regenerative operation for converting DC power back to AC power, the 120-degree conduction mode rectifier11can feed the regenerative energy recovered through the inverter12back into the AC line connecting to the commercial three-phase AC power supply3.

In the motor drive apparatus100, if a power failure occurs on the AC side of the 120-degree conduction mode rectifier11, the input supply voltage drops, and the motor2becomes unable to continue normal operation. To avoid such a situation, it is standard practice to provide a power failure detection unit52on the AC side of the 120-degree conduction mode rectifier11so that, when the occurrence of a power failure is detected by the power failure detection unit52, the operation of the motor drive apparatus100is switched so as to protect the motor drive apparatus100, the tool connected to each motor being driven by the motor drive apparatus100, the object being worked on by the tool, etc. The power failure detection unit52detects the presence or absence of a power failure based on the AC voltage detected by an AC voltage detection unit51. In normal operation when no power failure is detected by the power failure detection unit52, a host control apparatus53sets a switch55-1ON and a switch55-2OFF in a switch unit55. With this setting, the AC power from the commercial three-phase AC power supply3is converted by the 120-degree conduction mode rectifier11into DC power which is supplied to each inverter12. The host control apparatus53sends a motor drive command to each inverter12and controls the DC-to-AC conversion operation of the inverter12(more specifically, the switching operation of the switching device in the inverter12) so that desired AC power is output from the inverter12. Since the AC power output from the inverter12is used as the drive power to drive the motor2, the speed, torque, or rotor position of the motor2connected to the AC side of the inverter12can be controlled by controlling the AC power to be output from the inverter12. On the other hand, when a power failure is detected by the power failure detection unit52based on the AC voltage detected by the AC voltage detection unit51, the host control apparatus53sets the switch55-1OFF and the switch55-2ON in the switch unit55. As a result, DC power stored in a power storage device54is supplied to each inverter12. The host control apparatus53transmits to each inverter12a motor drive command for protecting the motor drive apparatus100, the tool connected to each motor being driven by the motor drive apparatus100, and the object being worked on by the tool.

FIG. 9is a circuit diagram for explaining currents flowing in the 120-degree conduction mode rectifier during the powering operation by the conventional motor drive apparatus. In the illustrated example, each inverter12connected to the 120-degree conduction mode rectifier11via a smoothing capacitor C is omitted from illustration. When supplying drive power to the motor, the 120-degree conduction mode rectifier11turns off the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2on all of the R-, S-, and T-phases. This operation mode will hereinafter be referred to as “powering operation of the 120-degree conduction mode rectifier.” During the powering operation of the 120-degree conduction mode rectifier11, if there is current from the commercial three-phase AC power supply3, for example, on the R-phase in a given cycle, this AC current is output to the DC side through a diode DR1on the R-phase, and the current from the DC side returns to the commercial three-phase AC power supply3through a diode DT2on the T-phase. The same is applied for other phase currents that can occur at other times.

FIG. 10is a circuit diagram for explaining currents flowing in the 120-degree conduction mode rectifier during the regenerative operation by the conventional motor drive apparatus. In the illustrated example, each inverter12connected to the 120-degree conduction mode rectifier11via the smoothing capacitor C is omitted from illustration. The 120-degree conduction mode rectifier11controls the on/off operations of the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2on the R-, S-, and T-phases as needed so that the regenerative power recovered from the motor2via the inverter is converted to AC power and returned to the commercial three-phase AC power supply3. This operation mode will hereinafter be referred to as “regenerative operation of the 120-degree conduction mode rectifier.”During the regenerative operation of the 120-degree conduction mode rectifier11, the R-phase switch SWR1and the T-phase switch SWT2is turned on at a given time and the other switches SWR2, SWS1, SWS2, and SWT1are turned off. As a result, the current that occurs when the regenerative power from the motor2at the given time is fed back to the 120-degree conduction mode rectifier11via the inverter is passed via the R-phase switch SWR1back to the commercial three-phase AC power supply3, and the current from the commercial three-phase AC power supply3is supplied to the DC side via the T-phase switch SWT2. The same is applied for R-phase and T-phase currents that can occur at other times.

As a method for determining a power failure in the motor drive apparatus, Japanese Unexamined Patent Publication No. 2006-14546, for example, discloses a method that converts the three-phase AC input voltage into a voltage vector in a two-phase coordinate system, calculates the amplitude of the input voltage from the amplitude of the vector, and determines that a power failure has occurred when the condition in which the amplitude value is lower than a predetermined reference voltage value has continued for a predetermined reference period of time.FIG. 11is a circuit diagram showing a power failure detection unit for determining the presence or absence of a power failure based on the amplitude of the three-phase AC input voltage. The voltage from the commercial three-phase AC power supply3is detected by an AC voltage detection unit122, and the amplitude of the voltage is calculated by a voltage amplitude detection unit126. When the condition in which the value of the voltage amplitude calculated by the voltage amplitude detection unit126is lower than a predetermined reference voltage value has continued for a predetermined reference period of time, the power failure detection unit125determines that a power failure has occurred.

As another method for determining a power failure in the motor drive apparatus, Japanese Unexamined Patent Publication Nos. H06-169501 and H06-189411, for example, propose a method that calculates the frequency of the input voltage (power supply frequency) and determines that a power failure has occurred when the frequency calculated falls outside a predetermined range.FIG. 12is a circuit diagram showing a power failure detection unit for determining the presence or absence of a power failure based on the frequency of the three-phase AC input voltage, andFIG. 13is a diagram for explaining the basic concept of the power failure detection unit that determines the presence or absence of a power failure based on the frequency of the three-phase AC input voltage. The voltage from the commercial three-phase AC power supply3is detected by an AC voltage detection unit122, and the frequency is calculated by a frequency calculation unit123. The power failure detection unit125determines, based on the frequency calculated by the frequency calculation unit123, the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier11. For example, a predetermined range of frequencies centered about the commercial power supply frequency (50 Hz or 60 Hz) is predefined as a normal frequency range, as shown inFIG. 13. As long as there is no power failure on the AC side of the 120-degree conduction mode rectifier11, the frequency calculated by the frequency calculation unit123shows a substantially constant value, but when a power failure occurs, for example, at time A inFIG. 13, the frequency being calculated by the frequency calculation unit123begins to fluctuate. In the illustrated example, the frequency is shown as gradually increasing after the occurrence of the power failure but, depending on the situation of the power failure, the frequency may decrease, oscillate, or diverge. Then, when the frequency deviates outside the normal frequency range at time B, the power failure detection unit125determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11.

With the method that detects the occurrence of a power failure based on the amplitude of the three-phase AC input voltage, such as disclosed in the above-cited Japanese Unexamined Patent Publication No. 2006-14546, when a 120-degree conduction mode rectifier is used as the converter for converting the AC power supplied from the commercial three-phase AC power supply to DC power, if a power failure occurs during the regenerative operation of the 120-degree conduction mode rectifier, it is not possible to detect the power failure.FIGS. 14 and 15are diagrams for explaining the problem associated with the power failure detection performed based on the voltage amplitude when the 120-degree conduction mode rectifier is used:FIG. 14shows the power failure detection during the powering operation of the 120-degree conduction mode rectifier, andFIG. 15shows the power failure detection during the regenerative operation of the 120-degree conduction mode rectifier. In the illustrated example, each inverter12connected to the 120-degree conduction mode rectifier11via the smoothing capacitor C is omitted from illustration.

As shown inFIG. 14, when a power failure occurs on the AC side of the 120-degree conduction mode rectifier11during the powering operation of the 120-degree conduction mode rectifier11by the motor drive apparatus, since the switches SWR1, SWR2, SWS1, SWS2, SWT1, and SWT2in the 120-degree conduction mode rectifier11are all OFF, the voltage that the voltage detection unit122detects on the AC side of the 120-degree conduction mode rectifier11drops to almost zero, upon detection of which it can be determined that a power failure has occurred on the AC side.

However, as shown inFIG. 15, when a power failure occurs on the AC side of the 120-degree conduction mode rectifier11during the regenerative operation of the 120-degree conduction mode rectifier11by the motor drive apparatus, since the DC output voltage appears at the voltage detection unit122through the ON switches, the voltage amplitude does not drop, and therefore, it is not possible to detect the power failure. In the illustrated example, since the switches SWR1and SWT2are ON, the DC output voltage appears on the AC side of the 120-degree conduction mode rectifier11through the switches SWR1and SWT2. As a result, even when a power failure has actually occurred, it is not possible to detect the occurrence of the power failure, because the amplitude of the voltage on the AC side of the 120-degree conduction mode rectifier11does not drop.

On the other hand, in the case of the method that determines the presence or absence of a power failure based on the frequency of the three-phase AC input voltage, such as disclosed in the above-cited Japanese Unexamined Patent Publication Nos. H06-169501 and H06-189411, unlike the method that determines the presence or absence of a power failure based on the amplitude of the three-phase AC input voltage, the occurrence of a power failure at the AC power supply side can be detected even during the regenerative operation of the 120-degree conduction mode rectifier. However, if the normal frequency range is narrowed in order to advance the power failure detection timing, the possibility of an erroneous power failure detection increases; conversely, if the normal frequency range is widened in order to prevent an erroneous power failure detection, the time taken to detect the occurrence of a power failure increases, resulting in an inability to protect the motor drive apparatus, the tool connected to each motor being driven by the motor drive apparatus, the object being worked on by the tool, etc.FIGS. 16 and 17are diagrams for explaining the problem associated with the power failure detection performed based on the voltage frequency when the 120-degree conduction mode rectifier is used:FIG. 16shows the power failure detection when the normal frequency range is narrowed, andFIG. 17shows the power failure detection when the normal frequency range is widened.

When the normal frequency range is narrowed in order to advance the power failure detection timing, as shown inFIG. 16, if a power failure occurs, for example, at time D inFIG. 16, the frequency being calculated by the frequency calculation unit123begins to fluctuate (increase in the example ofFIG. 16), and when the frequency deviates outside the normal frequency range at time E, the power failure detection unit125determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. The narrower the normal frequency range, the shorter the time taken to detect abnormality in frequency, i.e., the time taken from the moment a power failure occurs (time D) to the moment the power failure is detected (time E). The fluctuation of the power frequency that occurs when a power failure occurs during the regenerative operation of the 120-degree conduction mode rectifier depends on the load of the motor drive apparatus and the impedance, and there are cases where the fluctuation is mild; therefore, from the standpoint of detecting a power failure early, it is desirable that the detection sensitivity to the frequency fluctuation be higher. However, the actual frequency (50 Hz or 60 Hz) of the commercial three-phase AC power supply3more or less fluctuates even during normal operation and, even when there is no power failure, the frequency may deviate outside the normal frequency range, as indicated at C inFIG. 16, depending on the way the frequency fluctuates. For example, when the AC power supply is a small-sized power supply implemented in a distributed power supply system, the frequency fluctuation is large, and the possibility of an erroneous power failure detection increases.

On the other hand, when the normal frequency range is widened in order to prevent an erroneous power failure detection, as shown inFIG. 17, if a power failure occurs, for example, at time F inFIG. 17, the frequency being calculated by the frequency calculation unit123begins to fluctuate (increase in the example ofFIG. 17), but the length of time that elapses until reaching time G at which the power failure detection unit125determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11increases. This increases the possibility that the motor drive apparatus, the tool connected to each motor being driven by the motor drive apparatus, the object being worked on by the tool, etc., may not be protected before damage is caused.

SUMMARY OF THE INVENTION

In view of the above problems, it is an object of the present invention to provide a motor drive apparatus that can protect the apparatus by quickly detecting a power failure at the AC power supply side even during the regenerative operation of the 120-degree conduction mode rectifier, while making provisions to eliminate the possibility of an erroneous power failure detection.

To achieve the above object, a motor drive apparatus includes, a 120-degree conduction mode rectifier which, during powering operation, converts AC power supplied from an AC side to DC power for output and, during regenerative operation, converts DC power supplied from a DC side to AC power for output, an inverter which, during powering operation, converts the DC power output by the 120-degree conduction mode rectifier to AC power and supplies the AC power as drive power to a motor, and which, during regenerative operation, converts regenerative AC power recovered from the motor to DC power and returns the DC power to the 120-degree conduction mode rectifier, a DC voltage detection unit which detects a DC voltage on the DC output side of the 120-degree conduction mode rectifier, an AC voltage detection unit which detects an AC voltage on the AC output side of the 120-degree conduction mode rectifier, a frequency calculation unit which calculates the frequency of the AC voltage detected by the AC voltage detection unit, a storage unit which stores as a reference value the DC voltage detected by the DC voltage detection unit at the start of the regenerative operation of the 120-degree conduction mode rectifier, and a power failure detection unit which determines the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier during the regenerative operation of the 120-degree conduction mode rectifier, by using the DC voltage detected by the DC voltage detection unit, the reference value stored in the storage unit, and the AC voltage frequency calculated by the frequency calculation unit.

The power failure detection unit includes, a voltage evaluation unit which determines whether the DC voltage detected by the DC voltage detection unit during the regenerative operation of the 120-degree conduction mode rectifier is larger than the reference value, and a frequency abnormality detection unit which determines whether the AC voltage frequency calculated by the frequency calculation unit is outside a normal frequency range when it is determined by the voltage evaluation unit that the DC voltage detected by the DC voltage detection unit during the regenerative operation of the 120-degree conduction mode rectifier is larger than the reference value, and wherein when the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit has been outside the normal frequency range continuously for a predetermined period of time, the power failure detection unit determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier.

The motor drive apparatus further includes an amplitude calculation unit which calculates the amplitude of the AC voltage detected by the AC voltage detection unit, and the power failure detection unit includes: a voltage evaluation unit which determines whether the DC voltage detected by the DC voltage detection unit during the regenerative operation of the 120-degree conduction mode rectifier is larger than the reference value, a frequency abnormality detection unit which determines whether the AC voltage frequency calculated by the frequency calculation unit is outside the normal frequency range when it is determined by the voltage evaluation unit that the DC voltage detected by the DC voltage detection unit during the regenerative operation of the 120-degree conduction mode rectifier is larger than the reference value, a regeneration stopping unit which causes the DC-to-AC power conversion operation of the 120-degree conduction mode rectifier to stop when the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit has been outside the normal frequency range continuously for a predetermined period of time, and an amplitude evaluation unit which detects whether or not the AC voltage amplitude calculated by the amplitude calculation unit is equal to or smaller than a predetermined value, and wherein after the conversion operation of the 120-degree conduction mode rectifier has been caused to stop by the regeneration stopping unit, if the amplitude evaluation unit has detected that the AC voltage amplitude calculated by the amplitude calculation unit is equal to or smaller than the predetermined value, the power failure detection unit determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier.

The above normal frequency range may be defined as a range of frequencies centered about the frequency calculated by the frequency calculation unit at the start of the regenerative operation of the 120-degree conduction mode rectifier.

DETAILED DESCRIPTION

A motor drive apparatus having a power failure detection unit for determining the presence or absence of a power failure will be described below with reference to the drawings. It should, however, be understood that the present invention is not limited to the accompanying drawings, nor is it limited to the particular embodiments described herein.

While each of the embodiments described herein deals with a motor drive apparatus for driving and controlling a plurality of motors, the number of motors to be driven and controlled does not limit the present invention.

FIG. 1is a circuit diagram showing a motor drive apparatus according to a first embodiment of the present invention. The motor drive apparatus1for driving motors2, according to the first embodiment of the present invention, includes a 120-degree conduction mode rectifier11, inverters12, a DC voltage detection unit21, an AC voltage detection unit22, a frequency calculation unit23, a storage unit24, and a power failure detection unit25.

The 120-degree conduction mode rectifier11is a power converter capable of converting power from AC to DC and DC to AC; more specifically, during powering operation, the converter converts AC power supplied from a commercial three-phase AC power supply3to DC power for output, and during regenerative operation, the converter converts DC power, supplied from the inverter12by converting regenerative power recovered from the motor2, to AC power for output.

Each inverter12is connected to the 120-degree conduction mode rectifier11via a DC link; during powering operation, the inverter12converts the DC power output by the 120-degree conduction mode rectifier11to AC power and supplies the AC power as drive power to the motor2, and during the regenerative operation, the inverter12converts regenerative AC power recovered from the motor2to DC power and returns the DC power to the 120-degree conduction mode rectifier11.

The DC voltage detection unit21detects the DC voltage on the DC output side of the 120-degree conduction mode rectifier11. On the other hand, the AC voltage detection unit22detects the AC voltage on the AC output side of the 120-degree conduction mode rectifier11.

The frequency calculation unit23calculates the frequency of the AC voltage detected by the AC voltage detection unit22.

The storage unit24stores as a reference value the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11.

When the regenerative operation of the 120-degree conduction mode rectifier11is started a short time after the occurrence of regenerative power from the motor2, the DC voltage on the DC output side of the 120-degree conduction mode rectifier11should thereafter drop below the DC voltage immediately after the start of the regenerative operation of the 120-degree conduction mode rectifier11, but if the DC output voltage after a short time from the start of the regenerative operation of the 120-degree conduction mode rectifier11is larger than the DC voltage immediately after the start of the regenerative operation of the 120-degree conduction mode rectifier11, this means that the energy is not flowing from the DC side to the AC side of the 120-degree conduction mode rectifier11, i.e., it is highly likely that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. In view of this, in the present invention, the power failure detection unit25determines the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier11during the regenerative operation of the 120-degree conduction mode rectifier11, by using the DC voltage detected by the DC voltage detection unit21, the reference value stored in the storage unit24, and the AC voltage frequency calculated by the frequency calculation unit23. The power failure detection unit25is implemented by an operational processor such as a DSP or an FPGA, and its operation is defined by a software program.

The configuration and operation of the power failure detection unit25according to the first embodiment of the present invention will be described in further detail below. The power failure detection unit25according to the first embodiment of the present invention determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11when the following two conditions are simultaneously satisfied: the first condition is that the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value which is the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11, and the second condition is that a frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for a predetermined period of time. With this provision, it is possible to advance the power failure detection timing by narrowing the normal frequency range, because the occurrence of a power failure is determined only when both of the first and second conditions are satisfied. For example, during the powering operation of the 120-degree conduction mode rectifier11, if the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, there is no possibility of erroneously detecting a power failure, since the first condition is not satisfied.

Accordingly, the power failure detection unit25according to the first embodiment of the present invention includes, a voltage evaluation unit31which determines whether the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value which is the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11, and the frequency abnormality detection unit32which determines whether the AC voltage frequency calculated by the frequency calculation unit23is outside the normal frequency range when it is determined by the voltage evaluation unit31that the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value. When the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, the power failure detection unit25determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. As described above, the motor drive apparatus1according to the first embodiment of the present invention determines the presence or absence of a power failure during the regenerative operation of the 120-degree conduction mode rectifier11, by using the reference value, i.e., the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11, in this case, after the regenerative operation of the 120-degree conduction mode rectifier11is terminated and switched to the powering operation, when the regenerative operation of the 120-degree conduction mode rectifier11is started again, the motor drive apparatus1updates the reference value stored in the storage unit24by the DC voltage detected by the DC voltage detection unit21at the start of the new regenerative operation, and determines the presence or absence of a power failure during the new regenerative operation by using the updated reference value.

FIG. 2is a diagram for explaining the operation of the power failure detection unit according to the first embodiment of the present invention. As an example, consider the situation where the operation mode of the 120-degree conduction mode rectifier11is switched from powering operation to regenerative operation at time I under the control of the motor drive apparatus1, and thereafter, at time J, a power failure occurs on the AC side of the 120-degree conduction mode rectifier11.

As shown inFIG. 2, when the 120-degree conduction mode rectifier11is in powering operation under normal conditions with no power failure on the AC side of the 120-degree conduction mode rectifier11, the DC voltage on the DC side of the 120-degree conduction mode rectifier11is substantially constant. In this situation, when regenerative power occurs from the motor2, the regenerative power energy flows through the inverter12into the DC side of the 120-degree conduction mode rectifier11; as a result, the DC voltage being detected by the DC voltage detection unit21begins to increase. When the DC voltage on the DC side of the 120-degree conduction mode rectifier11reaches a predetermined value, the operation mode of the 120-degree conduction mode rectifier11is switched from powering operation to regenerative operation (time I inFIG. 2) under the control of the motor drive apparatus1, and the regenerative power energy begins to flow into the AC side of the 120-degree conduction mode rectifier11; as a result, the DC voltage being detected by the DC voltage detection unit21begins to decrease. When a power failure occurs at time J inFIG. 2, the DC voltage being detected by the DC voltage detection unit21suddenly begins to increase, and the frequency being calculated by the frequency calculation unit123begins to fluctuate. The illustrated example shows as an example of the frequency fluctuation the case where the frequency being calculated by the frequency calculation unit123gradually increases. The storage unit24stores the reference value which is the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11(at time I inFIG. 2), and the voltage evaluation unit31in the power failure detection unit25determines whether or not the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24. When it is determined at time K inFIG. 2that the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24, the frequency abnormality detection unit32in the power failure detection unit25then determines whether or not the AC voltage frequency calculated by the frequency calculation unit23is outside the normal frequency range. At time L inFIG. 2, the AC voltage frequency calculated by the frequency calculation unit23deviates outside the normal frequency range; then, when the frequency abnormality detection unit32has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, the power failure detection unit25determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. More specifically, when it is determined that the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24(at time K inFIG. 2) and when the frequency abnormality detection unit32has detected abnormality in frequency, the power failure detection unit25according to the first embodiment of the present invention determines that a power failure has occurred; as a result, if the normal frequency range is narrowed in order to advance the power failure detection timing, there is no possibility of erroneously determining that a power failure has occurred when actually no power failure has occurred. Further, if the AC voltage frequency calculated by the frequency calculation unit23has exceeded the upper limit of the normal frequency range for any reason at time H inFIG. 2during the powering operation of the 120-degree conduction mode rectifier11, since the 120-degree conduction mode rectifier11is not in regenerative operation, there is no possibility of the power failure detection unit25erroneously detecting the situation as the occurrence of a power failure.

Next, a second embodiment of the present invention will be described.FIG. 3is a circuit diagram showing the motor drive apparatus according to the second embodiment of the present invention. The motor drive apparatus1according to the second embodiment of the present invention includes a 120-degree conduction mode rectifier11, inverters12, a DC voltage detection unit21, an AC voltage detection unit22, a frequency calculation unit23, a storage unit24, a power failure detection unit25, and an amplitude calculation unit26. The 120-degree conduction mode rectifier11, the inverters12, the DC voltage detection unit21, the AC voltage detection unit22, the frequency calculation unit23, and the storage unit24in the second embodiment of the present invention are the same as the corresponding circuit component elements in the first embodiment, and therefore, the same circuit component elements will be designated by the same reference numerals and will not be described in further detail.

During the regenerative operation of the 120-degree conduction mode rectifier11, the power failure detection unit25determines the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier11by using the DC voltage detected by the DC voltage detection unit21, the reference value stored in the storage unit24, and the AC voltage frequency calculated by the frequency calculation unit23. The power failure detection unit25is implemented by an operational processor such as a DSP or an FPGA, and its operation is defined by a software program.

The configuration and operation of the power failure detection unit25according to the second embodiment of the present invention will be described in further detail below. The power failure detection unit25according to the second embodiment of the present invention causes the DC-to-AC power conversion operation (i.e., regenerative operation) of the 120-degree conduction mode rectifier11to stop when the following two conditions are simultaneously satisfied: the first condition is that the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value which is the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11, and the second condition is that the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for a predetermined period of time. When the regenerative operation of the 120-degree conduction mode rectifier11is stopped, the semiconductor switches in the 120-degree conduction mode rectifier11are all turned off; as a result, the AC voltage that the voltage detection unit22detects on the AC side of the 120-degree conduction mode rectifier11drops below the rated voltage of the commercial AC three-phase power supply3. Accordingly, when the amplitude of the AC voltage detected by the AC voltage detection unit22drops to or below a predetermined value, it is determined that a power failure has occurred. The “predetermined value” may be set as desired by the user in accordance with the operating condition of the motor2, etc., and may be set, for example, to a value equal to the rated voltage of the commercial AC three-phase power supply3minus several to several tens of percent of the rated voltage. Alternatively, the “predetermined value” may be set appropriately in accordance with the detection limit of the AC voltage detection unit22.

By making provision to determine that a power failure has occurred when the amplitude of the AC voltage detected by the AC voltage detection unit22drops to or below the predetermined value, as described above, it becomes possible to advance the power failure detection timing by narrowing the normal frequency range, because the occurrence of a power failure is determined only when both of the first and second conditions are satisfied. For example, during the powering operation of the 120-degree conduction mode rectifier11, if the frequency abnormality detection unit has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, there is no possibility of erroneously detecting the situation as the occurrence of a power failure, since the first condition is not satisfied.

Accordingly, the power failure detection unit25according to the second embodiment of the present invention includes, a voltage evaluation unit31which determines whether the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value, the frequency abnormality detection unit32which determines whether the AC voltage frequency calculated by the frequency calculation unit23is outside the normal frequency range when it is determined by the voltage evaluation unit31that the DC voltage detected by the DC voltage detection unit21during the regenerative operation of the 120-degree conduction mode rectifier11is larger than the reference value, a regeneration stopping unit33which causes the DC-to-AC power conversion operation of the 120-degree conduction mode rectifier11to stop when the frequency abnormality detection unit32has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, the amplitude calculation unit26which calculates the amplitude of the AC voltage detected by the AC voltage detection unit22, and an amplitude evaluation unit34which detects whether or not the AC voltage amplitude calculated by the amplitude calculation unit26is equal to or smaller than the predetermined value. Then, after the conversion operation of the 120-degree conduction mode rectifier11has been caused to stop by the regeneration stopping unit33, if the amplitude evaluation unit34has detected that the AC voltage amplitude calculated by the amplitude calculation unit26is equal to or smaller than the predetermined value, the power failure detection unit25determines that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. In the motor drive apparatus1according to the second embodiment of the present invention, after the regenerative operation of the 120-degree conduction mode rectifier11is terminated and switched to the powering operation, when the regenerative operation of the 120-degree conduction mode rectifier11is started again, the motor drive apparatus1updates the reference value stored in the storage unit24by the DC voltage detected by the DC voltage detection unit21at the start of the new regenerative operation, and determines the presence or absence of a power failure during the new regenerative operation by using the updated reference value.

FIG. 4is a diagram for explaining the operation of the power failure detection unit according to the second embodiment of the present invention. As an example, if the operation mode of the 120-degree conduction mode rectifier11is switched from powering operation to regenerative operation at time N under the control of the motor drive apparatus1, and thereafter, at time O, a power failure occurs on the AC side of the 120-degree conduction mode rectifier11.

As shown inFIG. 4, when the 120-degree conduction mode rectifier11is in powering operation under normal conditions with no power failure on the AC side of the 120-degree conduction mode rectifier11, the DC voltage on the DC side of the 120-degree conduction mode rectifier11is substantially constant. In this situation, when regenerative power occurs from the motor2, the regenerative power energy flows through the inverter12into the DC side of the 120-degree conduction mode rectifier11; as a result, the DC voltage being detected by the DC voltage detection unit21begins to increase. When the DC voltage on the DC side of the 120-degree conduction mode rectifier11reaches a predetermined value, the operation mode of the 120-degree conduction mode rectifier11is switched from powering operation to regenerative operation (time N inFIG. 4) under the control of the motor drive apparatus1, and the regenerative power energy begins to flow into the AC side of the 120-degree conduction mode rectifier11; as a result, the DC voltage being detected by the DC voltage detection unit21begins to decrease. When a power failure occurs at time O inFIG. 4, the DC voltage being detected by the DC voltage detection unit21suddenly begins to increase, and the frequency being calculated by the frequency calculation unit123begins to fluctuate. The illustrated example shows as an example of the frequency fluctuation the case where the frequency being calculated by the frequency calculation unit123gradually increases. The storage unit24stores the reference value which is the DC voltage detected by the DC voltage detection unit21at the start of the regenerative operation of the 120-degree conduction mode rectifier11(at time N), and the voltage evaluation unit31in the power failure detection unit25determines whether or not the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24. When it is determined at time P inFIG. 4that the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24, the frequency abnormality detection unit32in the power failure detection unit25then determines whether or not the AC voltage frequency calculated by the frequency calculation unit23during the regenerative operation of the 120-degree conduction mode rectifier11is outside the normal frequency range. At time Q inFIG. 4, the AC voltage frequency calculated by the frequency calculation unit23deviates outside the normal frequency range; then, when the frequency abnormality detection unit32has detected that the AC voltage frequency calculated by the frequency calculation unit23has been outside the normal frequency range continuously for the predetermined period of time, the regeneration stopping unit33causes the DC-to-AC conversion operation of the 120-degree conduction mode rectifier11to stop. After that, when the amplitude evaluation unit34has detected that the amplitude of the voltage on the AC side of the 120-degree conduction mode rectifier11has dropped to or below the predetermined value, it is determined that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. More specifically, during the regenerative operation of the 120-degree conduction mode rectifier11, only when it is determined that the DC voltage detected by the DC voltage detection unit21is larger than the reference value stored in the storage unit24and when the frequency abnormality detection unit32has detected abnormality in frequency, does the power failure detection unit25according to the second embodiment of the present invention determine that a power failure has occurred, in the same manner as the first embodiment; as a result, if the normal frequency range is narrowed in order to advance the power failure detection timing, there is no possibility of erroneously determining that a power failure has occurred when actually no power failure has occurred. Further, if the AC voltage frequency calculated by the frequency calculation unit23has exceeded the upper limit of the normal frequency range for any reason at time M inFIG. 4during the powering operation of the 120-degree conduction mode rectifier11, since the 120-degree conduction mode rectifier11is not in regenerative operation, there is no possibility of the power failure detection unit25erroneously detecting a power failure.

In the first and second embodiments of the present invention, the normal frequency range has been defined as a range of frequencies centered about the commercial power frequency (50 Hz or 60 Hz), but by considering the fact that the commercial power frequency fluctuates, the normal frequency range may be defined as a range of frequencies centered about the frequency calculated by the frequency calculation unit23at the start of the regenerative operation of the 120-degree conduction mode rectifier11.

As a modified example of the first and second embodiments of the present invention, the above power failure detection unit may be used in combination with an additional power failure detection unit that calculates the amplitude of the AC voltage on the AC side of the 120-degree conduction mode rectifier11and that determines that a power failure has occurred when the amplitude value is smaller than a predetermined value, and the presence or absence of a power failure may be determined by ORing the two power failure detection units. This will be explained below by taking the first embodiment of the present invention as an example.FIG. 5is a circuit diagram showing a motor drive apparatus according to a modified example of the first embodiment of the present invention. The power failure detection unit25in the first embodiment of the present invention described with reference toFIG. 1is designated by reference numeral25-1inFIG. 5. The additional power failure detection unit25-2determines that a power failure has occurred when the amplitude of the AC voltage on the AC side of the 120-degree conduction mode rectifier11, calculated by the amplitude calculation unit26, is smaller than a predetermined value. When at least one of the power failure detection units25-1and25-2has determined that a power failure has occurred, it is than determined that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier11. A similar modification can be made to the second embodiment of the present invention.

In the first and second embodiments of the present invention, since the power failure detection to be performed is determined based on whether the 120-degree conduction mode rectifier11is in powering operation or in regenerative operation, as described above, the motor drive apparatus1includes an operation mode determining unit27which determines whether the 120-degree conduction mode rectifier11is to be operated in powering mode or in regenerative mode.FIG. 6is a block diagram showing the configuration of an operation mode detection unit in the motor drive apparatus1according to the first and second embodiments of the present invention. For simplicity of explanation,FIG. 6focuses on the configuration of the operation mode determining unit27.FIG. 7is a diagram for explaining how the 120-degree conduction mode rectifier11is switched between powering operation and regenerative operation.

During the powering operation of the 120-degree conduction mode rectifier11with all the switches OFF, when regenerative power occurs from the motor2, and the regenerative power begins to be fed from the inverter12, the charge is stored on the smoothing capacitor C, and the DC voltage on the DC side of the 120-degree conduction mode rectifier11increases (time R inFIG. 7). Then, when the DC voltage detected by the DC voltage detection unit21has exceeded a predetermined value, or when the potential difference between the DC voltage detected by the DC voltage detection unit21and the amplitude of the phase-to-phase voltage on the AC side of the 120-degree conduction mode rectifier11, calculated by a voltage amplitude calculation unit62, has exceeded a predetermined value, a regenerative operation starting determining unit64in the operation mode determining unit27determines that the 120-degree conduction mode rectifier11is to be switched from the powering operation to the regenerative operation (time S inFIG. 7). In response to this determination, a switching pattern calculation unit66outputs a semiconductor switch ON/OFF signal so as to cause the 120-degree conduction mode rectifier11to perform the DC-to-AC conversion operation (i.e., regenerative operation) and, based on this signal, the 120-degree conduction mode rectifier11performs the DC-to-AC conversion operation (that is, regenerative operation). At this time, as long as there is no power failure on the AC side of the 120-degree conduction mode rectifier11, the regenerative energy stored on the smoothing capacitor C is fed through the 120-degree conduction mode rectifier11back to the commercial three-phase AC power supply3; as a result, the DC voltage being detected by the DC voltage detection unit21drops (time T inFIG. 7) and thereafter is maintained at a substantially constant value. When the supply of the regenerative power from the inverter12ends, the power on the AC side of the 120-degree conduction mode rectifier11, calculated by a power calculation unit63, is equal to a value at least not smaller than zero (that is, a nonnegative value). When the power on the AC side of the 120-degree conduction mode rectifier11has exceeded a predetermined threshold value, a regenerative operation stopping determining unit65determines that the regenerative operation is to be stopped, and notifies the switching pattern calculation unit66accordingly. Based on a command supplied from the regenerative operation starting determining unit64or the regenerative operation stopping determining unit65, the switching pattern calculation block66creates the semiconductor ON/OFF signal to be supplied to the 120-degree conduction mode rectifier11.

The frequency calculation unit23, the power failure detection units25,25-1, and25-2, the amplitude calculation unit26, the operation mode determining unit27, the voltage evaluation unit31, the frequency abnormality detection unit32, the regeneration stopping unit33, the amplitude evaluation unit34, etc., are implemented by an operational processor such as a DSP or an FPGA, and its operation is defined by a software program.

In a motor drive apparatus for driving motors used in such machines as machine tools, forging presses, injection molding machines, and various kinds of robots, including a 120-degree conduction mode rectifier for converting input AC power to DC power and inverters for converting the DC power output from the DC converter to AC power for driving the respective motors, the configuration of the present invention can be applied for detecting the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier.

More specifically, in the motor drive apparatus according to the present invention, the AC power supplied from the AC power supply is converted by the 120-degree conduction mode rectifier to DC power which is further converted by the inverters to the AC power for driving the respective motors, and the presence or absence of a power failure on the AC side of the 120-degree conduction mode rectifier is determined by using the DC voltage detected on the DC side of the 120-degree conduction mode rectifier, the reference value, i.e., the DC voltage detected on the DC side of the 120-degree conduction mode rectifier at the start of the regenerative operation of the 120-degree conduction mode rectifier, and the frequency of the AC voltage detected on the AC side of the 120-degree conduction mode rectifier; with this configuration, the occurrence of a power failure at the AC power supply side can be detected to protect the apparatus even during the regenerative operation of the 120-degree conduction mode rectifier, and any possibility of erroneously determining that a power failure has occurred when no power failure has actually occurred can be eliminated.

According to a first mode of the present invention, during the regenerative operation of the 120-degree conduction mode rectifier, only when the DC voltage detected on the DC side of the 120-degree conduction mode rectifier is larger than the reference voltage and when the frequency of the AC voltage detected on the AC side of the 120-degree conduction mode rectifier is outside the normal frequency range, is it determined that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier; as a result, not only can the occurrence of a power failure be detected more accurately, but any possibility of erroneously determining that a power failure has occurred when no power failure has actually occurred can be eliminated.

According to a second mode of the present invention, during the regenerative operation of the 120-degree conduction mode rectifier, only when the DC voltage detected on the DC side of the 120-degree conduction mode rectifier is larger than the reference voltage and when the frequency of the AC voltage detected on the AC side of the 120-degree conduction mode rectifier is outside the normal frequency range, is a command issued to stop the DC-to-AC power conversion operation of the 120-degree conduction mode rectifier and, when the amplitude of the AC voltage detected at this time on the AC side of the 120-degree conduction mode rectifier is equal to or smaller than a predetermined value, it is determined that a power failure has occurred on the AC side of the 120-degree conduction mode rectifier; as a result, not only can the occurrence of a power failure be detected more accurately, but any possibility of erroneously determining that a power failure has occurred when no power failure has actually occurred can be eliminated.