Inverter apparatus provided with electric discharge control circuit of DC smoothing capacitor and method of controlling the same

Abstract: In a variable frequency power source unit, a series circuit of a contractless semiconductor switching element (11) and a resistor (10) is provided in parallel to a d.c. smoothing capacitor (3) and a control circuit (12) is provided for receiving a signal for turning on the switching element (11) selectively at any time after the power source is interrupted thereby discharging the electric charge of the d.c. smoothing capacitor (3).

Description: BACKGROUND OF THE INVENTION The present invention relates to an inverter apparatus, and more particularly to an inverter apparatus in which after disconnecting an a.c. power source, residual electric charge of a smoothing capacitor is discharged by turning on a semiconductor switching element at a desired time or at a lapse of an allowable duration time of momentary power stop so that electrical shock by residual voltage may be prevented. In general, an inverter apparatus is provided therein with a d.c. smoothing capacitor having large electrostatic capacity, and a voltage remains in this smoothing capacitor for a long time after the input voltage is cut off unless special measures of discharge are taken. Since a dangerous electrical shock may be encountered in touching the inside of an inverter apparatus at maintenance and the like, electric charge of the d.c. smoothing capacitor is discharged in a conventional inverter apparatus by devices as disclosed in JP-A-59-172984 (1984) (which discloses the features included in the first part of claim 1), JP-A-60-9396(1985), JP-A-61-191299 (1986), JP-A-62-114492 (1987), and JP-A-64-8894 (1989). Fig. 8 shows an example of these prior arts, in which a contact b of an electromagnetic contactor 84 is closed when an a.c. input voltage is cut off, thereby absorb as heat energy the residual voltage of a smoothing capacitor CB through a resistor RB. Any of above-described prior arts, however, is not arranged to discharge the electric charge of the d.c. smoothing capacitor at any desired time, and hence it is impossible to discharge the electric charge of the d.c. smoothing capacitor for the purpose of preventing a danger immediately when some serious situation happens for instance. Further, it is not necessarily recommendable to discharge the electric charge of the d.c. smoothing capacitor every time the power source is turned OFF, because it involves a problem that charge and discharge are repeated frequently in case the frequency of start-stop is high, thus reducing the life of the d.c. smoothing capacitor, a contactor, a rush current preventing resistor and the like. Further, in above-described prior art, since the electric charge of the d.c. smoothing capacitor is discharged independently of the allowable duration time of momentary power stop, it involves a problem that there is a possibility of a large rush current at a time of power restarting if the electric charge of the d.c. smoothing capacitor has been discharged, thereby to destroy elements of a power rectifier. SUMMARY OF THE INVENTION It is an object of the present invention to provide an inverter apparatus in which above-described problems are solved, electrical shock can be prevented and reliability is high. This object is met by the inverter apparatus characterised in claim 1 and the method of claim 5. The allowable duration time of momentary power stop is selected usually at 10 ms to 50 ms, but may be substantially infinite sometimes as disclosed in US. Patent Re. 33,519. On the other hand, many contactors such as electromagnetic contactors operate within 10 ms to 20 ms. Accordingly, it is required to determine the timing of discharge based on the allowable duration time of power stop even when discharging by using a contactor or a semiconductor element. Thus, a command for turning on the element is inputted at a lapse of the allowable duration time of momentary power stop after the a.c. input power source to the inverter apparatus is interrupted. A signal for turning on the semiconductor switching element is outputted from the ON/OFF control circuit in response to the above command. The residual electric charge of the d.c. smoothing capacitor is discharged similarly with the foregoing. Since the residual electric charge is not discharged within the allowable duration time of momentary power stop, a large rush current does not flow even when the power source is restarted within the allowable duration time. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit block diagram of an embodiment of the present invention; Fig. 2 is a detailed diagram of a discharge control circuit and a power source circuit in Fig. 1; Fig. 3 is a circuit block diagram of another embodiment of the present invention; Fig. 4 shows a flow chart of process by software at power stop according to the present invention; Fig. 5 shows an example of circuit connection of a power source; Fig. 6 is a diagram showing still another embodiment; Fig. 7 is a detailed diagram of an inverter; and Fig. 8 is a diagram showing a conventional inverter apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described hereinafter with reference to Fig. 1. Fig. 1 shows an inverter apparatus according to the present invention. Three-phase a.c. electric power is inputted to a rectifier 1 from connecting points R, S and T of the three-phase a.c. input power source 50 and the rectifier 1, and converted into d.c. electric power by means of the rectifier 1. This d.c. electric power is converted further into an a.c. electric power of variable voltage and variable frequency by means of an inverter 2. An a.c. machine 4 is connected to the output of the inverter 2, and the speed of the a.c. machine 4 is controlled variably. A smoothing capacitor 3 is disposed between the rectifier 1 and the inverter 2 for smoothing the d.c. voltage. When the a.c. electric power is applied at a start of the inverter apparatus, a rush charging current flows in the smoothing capacitor 3. A resistor RS for restricting the rush charging current is provided between the rectifier 1 and the smoothing capacitor 3. During operation, the resistor RS is short-circuited by a normally-open contact 84a of an electromagnetic contactor 84. A series circuit of a semiconductor switching element 11 and a resistor 10, which is connected in parallel with the smoothing capacitor 3 and primarily used as a circuit for regenerative braking of an a.c. motor, is used commonly as a discharge circuit for discharging electric charge of the smoothing capacitor. A reference numeral 9 denotes a discharge control circuit including an ON/OFF control circuit 12 for controlling a drive signal, which in turn controls ON/OFF of a switching element 11 for discharging electric charge of the smoothing capacitor 3. A signal a for discharging electric charge of the smoothing capacitor 3 is inputted at a desired time to the discharge control circuit 9 so as to turn on the semiconductor switching element 11. The semiconductor switching element 11 is of a contactless type. When the signal a is inputted, the semiconductor switching element 11 is turned on, and the electric charge of the capacitor 3 is discharged through the switching element 11 and the resistor 10. On the other hand, a speed command which determines the output frequency of the inverter apparatus is outputted from a frequency setter 6 and inputted to a control circuit 5 including a microcomputer. The control circuit 5 controls the inverter 2 in the pulse width modulation PWM mode (a generally well-known method of controlling a voltage with the pulse width, and drives the semiconductor switching elements of the inverter 2 so as to produce an a.c. power of variable voltage and variable frequency. Further, 7 denotes a control power source which receives an a.c. electric power through terminals R₀ and T₀ and supplies voltage to control circuits of the whole apparatus. This a.c. electric power may be obtained from any two of the terminals R, S and T connected to the three-phase a.c. power source or from a separate power source. Next, the operation of an embodiment according to the present invention will be described in more detail with reference to Figs. 1 and 2. When the a.c. power source connected to the terminals R, S and T is cut OFF, the energy of the smoothing capacitor of a d.c. intermediate circuit is absorbed by a switching regulator circuit 17, as disclosed, for example, in "Practical Electronic Circuits Hand Book" published by CQ Book Co. 1980, pp 274-284, through a path of a positive pole P of the smoothing capacitor 3 → a diode D₁ → a switching regulator circuit 17 → a negative pole N of the smoothing capacitor 3. Since power consumption of the switching regulator circuit is small, however, the voltage of the smoothing capacitor 3 is substantially maintained for a long period of time. When a signal a for turning on or off the semiconductor switching element 11 independently of the OFF state of the a.c. power source is inputted under this state, the semiconductor switching element 11 is driven through a drive circuit 15. A manual signal generated by a push-button or the like may be used for the signal a . Incidentally, a potential V PN of the d.c. intermediate circuit rises by regenerative energy at regenerative braking of the a.c. motor. When the potential reaches a predetermined reference value V PN * higher, an output signal d of a comparator 16 is inverted thereby to turn on the semiconductor switching element 11 through the drive circuit 15. In an embodiment shown in Fig. 1, a contactless semiconductor switching element is used instead of an electromagnetic contactor, in order to discharge the electric charge of the smoothing capacitor 3. Therefore, it is not only possible to provide a compact and highly reliable inverter apparatus, but also to discharge the electric charge at any time, if necessary. That is, a signal interlocked with a push-button or a door of a control board may be used as the signal a . Thus, since the electric charge of the smoothing capacitor 3 will not be discharged unnecessarily, there is also advantage in the life of the smoothing capacitor 3. Fig. 3 shows another embodiment of the present invention. This embodiment differs from the embodiment shown in Fig. 1 in that an a.c. input power detecting circuit 13 and an OR circuit 14 are provided in a control circuit 9. The a.c. input power detecting circuit 13 detects an ON/OFF state of the a.c. input power source and generates a signal b at a lapse of the allowable duration time of power stop after detection of the OFF state of the power source. Then, the OR circuit 14 outputs a signal c in response to any one of the signals a and b applied thereto. The signal a is the same as the signal a explained in the embodiment of Fig. 1 and may be a manual signal produced by a push-button for example. When the a.c. power source is interrupted, the signal b is generated at a lapse of the allowable duration time of power stop. This signal b is inputted to an ON/OFF control circuit 12 through the OR circuit 14, thereby turning on the semiconductor switching element 11. On the other hand, the signal a is inputted at any time in the same manner as that described with respect to the embodiment shown in Fig. 1, thereby turning on the semiconductor switching element 11. Since the semiconductor switching element 11 is not turned on in momentary power stop within the allowable duration time thereof, the residual voltage in the smoothing capacitor 3 is not consumed. Thus, a large rush charging current does not flow into the smoothing capacitor 3 at restarting of power source, so that the operation of the a.c. machine 4 is continued smoothly. On the other hand, when the momentary power stop continues longer than the allowable duration time, the semiconductor switching element 11 is turned on and the residual voltage of the smoothing capacitor 3 is discharged. Incidentally, it is possible to achieve the object of the present invention only by the signal b without using the signal a , but it is useful in view of safety to use the signal a together. The control blocks 5 and 9 may be constructed by hardware, or by software using a microcomputer. Fig. 4 shows a flow chart of a process by software at occurrence of power stop. After occurrence of a power stop, the time is measured in a step S1 so as to monitor whether the allowable duration time τ1, which is 10 ms to 50 ms in many cases as stated previously and may be 30 ms for instance, has elapsed or not, and the signal b is generated at a lapse of the time τ1 thereby turning on the semiconductor switching element 11 in a step S2. Until the allowable duration time τ1 elapses, whether the power stop continues or not is judged in a step S3. If the power stop terminates, the operation of the system is continued (step S4). Here, it is required to secure a control power source after the a.c. input power source is turned off. Namely, it is impossible to turn on the semiconductor switching element 11 unless the control power source is maintained without failure. This matter will be explained hereinafter. In the power source circuit 7 shown in Fig. 2, a higher one of the voltage of the smoothing capacitor 3 and the voltage of the capacitor C₁ is supplied through a diode D₁ or D₂ as an input d.c. voltage V SR to the switching regulator circuit 17. When the semiconductor switching element 11 is turned on after the a.c. input power source is turned off, the residual voltage of the smoothing capacitor 3 becomes zero. It is required for the control power source to supply an electric power to the whole apparatus under this situation in order to operate the control circuits. First, when the a.c. control power source (52) connected to R₀ and T₀ is a separate power source independent of the a.c. input power source connected to R, S and T of the inverter apparatus, an electric power is supplied continuously to the switching regulator 17 from the separate power source even if the voltage of the smoothing capacitor 3 disappears, thereby to maintain the voltage of the switching regulator 17 at its normal value. In the case of momentary power stop (for several seconds), there is no problem with respect to power supply to the control circuit if a separate power source is connected to R₀ and T₀ in a manner as above-mentioned. Next, when R₀ and T₀ are connected to any two of R, S and T of the a.c. input power source as shown in Fig. 5, no electric power is inputted from R₀ and T₀ when the a.c. input power source is cut off. Therefore, means is necessary to maintain the voltage of the switching regulator circuit 17. In this case, the capacity of the capacitor C₁ is made large sufficient to maintain the voltage of the switching regulator circuit 27 even if the energy of the smoothing capacitor 3 is consumed after the a.c. input power source is cut off. Namely, the capacity of the capacitor C₁ is determined so that the time interval during which the voltage V SR of the capacitor C₁ is substantially held at the normal value is not less than a sum of a time constant Td or the discharge time of the smooth capacitor 3 determined by the capacitance of the smooth capacitor 3 and the resistance of the resistor 10 and the allowable duration time of power stop. By this arrangement, there is no problem because an electric power is supplied to the switching regulator 17 from the capacitor C1. As another method of achieving the same effects, the power terminals Ro and To may be connected to a primary side of a breaker MCB through a different breaker. Furthermore, Fig. 6 and Fig. 7 show another embodiment. The same symbols as those in Figs. 1 to 3 indicate the same components. Fig. 6 shows an embodiment in which semiconductor switching elements UP, VP, WP, UN, VN and WN of the inverter 2 are utilized in place of the semiconductor switching element 11 shown in Fig. 1, and windings 21 of the a.c. machine are utilized in place of the resistor 10. Fig. 7 is a detailed diagram of the inverter 2. In this embodiment, the residual voltage of the smoothing capacitor 3 is discharged or consumed as heat energy through the windings 21 of the a.c. machine by controlling ON and OFF of selected two of the switching elements (UN, VN, WN), (UP, VP, WP) of the inverter 2, for example, ON and OFF of UP and VN, thereby maintaining the discharge current at an appropriate value. By this embodiment, the same effects as those of the first-mentioned embodiment are obtained. The discharge current in this case flows in a path of the positive pole of the smoothing capacitor 3 → the semiconductor switching element UP → the windings 21 of the a.c. machine → the semiconductor switching element VN → the negative pole of the smoothing capacitor 3. Here, it is not necessarily required to use only one of the semiconductor switching elements in each of the upper and lower arms. The element UP in the upper arm and the elements VN and WN in the lower arm or the elements VP and WP in the upper arm and the element UN in the lower arm may be used for controlling the discharge current. In short, it is sufficient to select suitable elements of the upper and lower arms to that the discharge current flows through the a.c. machine. Further, it is possible to discharge the smoothing capacitor through the a.c. machine by controlling all the elements (UP to WN) of the inverter in a manner similar to that in switching thereof for usual rotation of the a.c. machine. However, in the above embodiment, only selected elements are ON-OFF controlled in order to prevent such a danger that the a.c. machine at rest rotates inadvertently. As to the power source circuit 7 in this embodiment, it is sufficient either to connect a different power source independent of the input power source to the power terminals Ro and Rs or use a large capacitance of the capacitor C1 in the same way as that in the first-mentioned embodiment. In a control mode of the embodiment shown in Fig. 6, the switching of the semiconductor elements of the inverter 2 may be controlled according to a signal of a current detector 19 or 20 so as to maintain the discharge current within a predetermined range. According to this embodiment, there is such an advantageous effect that since the semiconductor switching elements of the power inverter and the windings of the motor are utilized, the apparatus can be made small in size and the number of required components is reduced, thus improving reliability. Incidentally, the regenerative braking circuit or the switching elements of the inverter and the windings of the a.c. machine are utilized for the discharge circuit in the above-described embodiments. However, it is a matter of course that an independent discharge circuit may be provided for exclusive use therefore. According to the present invention, there is an advantageous effect that appropriate processing may be performed under a given situation because the residual voltage in the d.c. intermediate circuit is discharged at a desired time after the a.c. input power source of the inverter apparatus is cut off. Moreover, there is an effect that an excessive current will never flow even when momentary power stop occurs, thus making it possible to provide a highly reliable inverter apparatus.

Claims: An inverter apparatus comprising a rectifier (1) adapted for connection to the output (R, S, T) of an a.c. power source (50), a d.c. intermediate circuit including a smoothing capacitor (3) connected to the output of the rectifier (1), an inverter (2) having its input connected to the d.c. intermediate circuit and outputting a.c. electric power, a series circuit of a semiconductor switching element (11) and a resistor (10) connected in parallel with the smoothing capacitor (3), and a discharge control circuit (9) for turning on the semiconductor switching element (11), characterised in that the discharge control circuit (9) includes means for turning on the semiconductor switching element (11) in response to a first signal (a) interlocked with a door of a control board, or a second signal (b) produced with a delay after interruption of the a.c. power source (50), the delay being determined on the basis of the allowable duration of power interruption of the a.c. power source (50). The apparatus of claim 1, wherein the discharge control circuit (9) comprises a power detecting circuit (13) connected to the a.c. input power source (50) for producing said second signal at said predetermined time delay upon detection of the a.c. input power source (50) being interrupted, an OR circuit (14) receiving said first and second signals (a, b) and outputting either one of them as an ON command, and a drive circuit (12; 5) responsive to said ON command to produce a signal for turning on the semiconductor switching element (11). The apparatus of claim 1, wherein said series circuit serves as a discharge circuit for discharging regenerative energy. The apparatus of claim 1, wherein semiconductor switching elements (UP, VP, WP, UN, VN, WN) of the power inverter (2) are used as the semiconductor switching element of said series circuit, and windings (21) of an a.c. machine connected to the output of the inverter (2) are used as the resistor of said series circuit. A method of controlling the discharging of a smoothing capacitor (3), which is included in a d.c. intermediate circuit provided in a variably frequency a.c. power source, comprising the steps of monitoring (S1) whether a time interval (τ) has lapsed upon interruption of a input voltage from an a.c. power source (50), the time interval (τ) being determined on the basis of the allowable duration of power interruption of the a.c. power source (50), and producing (S2) a signal (b) for actuating switching means (11) for discharging the smoothing capacitor (3) through a resistor (10) when said predetermined time interval (τ) has lapsed.

Inventors: BABA SHIGEYUKI, FUJII HIROSHI, IBORI SATOSHI, ISHIBASHI AKIRA, KAMBARA TAKATSUGU, YAMASHITA HITOSHI, BABA, SHIGEYUKI, FUJII, HIROSHI, IBORI, SATOSHI, ISHIBASHI, AKIRA, KAMBARA, TAKATSUGU, YAMASHITA, HITOSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H02P7
IPC codes: H02H7, H02P3, H02P27, H02H3