JPH0817598B2 - Electric power converter for variable speed operation of electric motor - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention comprises a rectifier connected to an AC power source, and a voltage source inverter that converts a DC voltage supplied from the rectifier via a DC intermediate circuit including a smoothing capacitor into a variable voltage variable frequency and supplies the AC voltage to an AC motor. The present invention relates to a power converter for variable speed operation of an electric motor.

[Prior art]

In this type of power conversion device, a braking operation can be performed by controlling the voltage source inverter to bring the electric motor into a power generation operation. At this time, the kinetic energy of the electric motor and the load machine is converted into electric energy by the electric motor, and the electric energy is returned to the DC intermediate circuit via the inverter. Therefore, energy processing is required to keep the voltage rise of the smoothing capacitor in the DC intermediate circuit within the allowable range. In order to avoid wasteful power consumption and perform efficient energy processing, it is known to regenerate the electric energy returned to the DC intermediate circuit to the AC power supply during motor braking operation. In this case, a thyristor converter is provided as a rectifier that supplies power to the DC intermediate circuit during motor drive operation, and this thyristor converter is also used for regeneration during motor braking operation, thus omitting the thyristor converter dedicated to regeneration. You can In this case, since the voltage polarity of the smoothing capacitor in the DC intermediate circuit does not change between the driving operation and the regenerative operation, means for switching the connection polarity between the smoothing capacitor and the thyristor converter is provided. FIG. 8 shows a conventional electric power converter for variable speed operation of a motor, which is equipped with such a switching means. According to this, the DC terminals of the thyristor converter 11 connected to the three-phase AC power supplies R, S, T are diodes 13, 15 respectively.
Is connected to the smoothing capacitor 17 via. The voltage source inverter 18 connected to the smoothing capacitor 17 is composed of a three-phase bridge connection of transistors each having a diode connected in parallel, and supplies an AC voltage of a variable voltage variable frequency to the AC motor 19. Further, the thyristor converter 11 and the smoothing capacitor 7 have the opposite polarity with respect to the connection by the diodes 13 and 15 described above, while the transistor 14 has the opposite polarity.
And a resistor 21 in series, on the other hand a transistor
16 and a resistor 22 are connected via a series circuit. In such a configuration, during driving operation, the smoothing capacitor 17 is fed from the AC power supply through the thyristor converter 11 and the diodes 13 and 15 by the forward conversion operation of the thyristor converter 11, and further from the smoothing capacitor 17 through the inverter 18. Power is supplied to the AC motor 19. Also, during braking operation, the AC motor 19
The electric energy is returned to the smoothing capacitor 17 via 18 and the surplus energy accumulated in the smoothing capacitor 17 is converted into resistors 21 and 2 by the reverse conversion operation of the thyristor converter 11.
It is regenerated into an AC power source through 2 and transistors 14 and 16 and thyristor converter 11. In this conventional device, a transistor is used during braking operation.
14, 16 are kept conductive, and the phase control of the thyristor converter regulates the regenerative current flowing through the transistors 14, 16. The forward conversion operation of the thyristor converter 11 during the driving operation is performed with an ignition delay angle of almost zero so that the maximum output voltage of the same value as that of the diode rectifier is generated in order to improve the voltage utilization rate. On the other hand, in the reverse conversion operation of the thyristor converter 11 during braking operation, it is necessary to secure an appropriate commutation allowance angle so as not to cause commutation failure. The magnitude of the reverse voltage that can be generated in the above is smaller than the magnitude of the maximum output voltage during the above-mentioned forward conversion operation. Therefore, during braking operation, the maximum possible reverse voltage of the thyristor converter 11 that opposes this is smaller than the voltage of the smoothing capacitor 17, so that if resistors 21 and 22 are not present, they will pass through transistors 14 and 16. The regenerative current that flows is uncontrollable and diverges, and the transistors 14 and 16 are destroyed by the overcurrent. The resistance value R [α] of each resistor is selected as follows. That is, of the DC reverse voltage values generated by the thyristor converter 11 when the thyristor converter 11 is operated at a predetermined minimum control lead angle, the minimum value that is assumed in consideration of power supply voltage fluctuations is
When E ₁ [V] and the assumed maximum value E ₂ [V] of the voltage of the smoothing capacitor 17, the current value [A] given by (E ₂ −E ₁ ) / 2R is applied to the transistor 14, Select the resistance value R [Ω] so that it stays within the allowable range of 16.

[Problems to be Solved by the Invention]

In the above-described conventional device, when the regenerative current flowing through the resistors 21 and 22 is i [A], the resistors 21 and 22 generate a power loss of i ² R [W] and generate heat. Therefore, when actually manufacturing a power converter for variable speed operation of an electric motor, there is a problem in that it is very difficult to process the amount of heat generated by these resistors. That is, this has been an obstacle to downsizing and weight reduction of the device and cost reduction. Therefore, the present invention can be expected to exhibit a control performance equal to or higher than that of the conventional device while having a low number of parts and low cost, and while operating at the maximum output voltage of the thyristor converter during driving operation, While keeping the regenerative current within the risk-free range without the resistors in conventional equipment,
It is an object of the present invention to provide a power conversion device for driving a variable speed electric motor, which enables efficient regeneration of energy to a power supply.

[Means for Solving the Problems]

According to the present invention, the above object is achieved as follows. A motor variable speed consisting of a rectifier connected to an AC power source and a voltage source inverter for converting a DC voltage supplied from the rectifier through a DC intermediate circuit including a smoothing capacitor into a variable voltage variable frequency and supplying the voltage to an AC motor. In the power converter for operation, as the rectifier, a thyristor converter that is operated at a substantially predetermined maximum forward converter output during motor driving operation and is operated at a substantially predetermined maximum reverse converter output voltage during motor braking. And the DC terminal of the thyristor converter and the terminal of the smoothing capacitor are respectively connected via a diode in a polarity relationship corresponding to the forward conversion operation of the thyristor converter at the time of driving the motor, and the motor braking Each semiconductor switch has a polarity corresponding to the reverse conversion operation of the thyristor converter during operation. And an inductance element is inserted in series on the AC side or the DC side of the thyristor converter, and both of the semiconductor switches are blocked or one of them is blocked when the motor is driven. Is operated as a chopper by performing switching control, and one of the semiconductor switches is made conductive during the motor braking operation so that the other is operated as a chopper.

[Work]

According to such a configuration according to the present invention, during the motor braking operation, the high frequency switching operation of the semiconductor switch, which is a transistor, for example, causes the smoothing capacitor voltage and the thyristor conversion that opposes the smoothing capacitor voltage while both semiconductor switches are simultaneously on. The regenerative current flowing through the reactor increases due to the voltage difference from the DC reverse voltage of the device, and the current flowing through the reactor decreases while one semiconductor switch is in the OFF operation. By repeating such operation, regeneration from the DC intermediate circuit to the AC power source is performed safely and efficiently while keeping the regenerative current within the desired range. In that case, the thyristor converter operates in the same manner as the diode rectifier during driving operation to generate the maximum forward conversion output voltage, and during braking operation, generates the maximum reverse conversion output voltage at a predetermined minimum control advance angle. It is possible,
Therefore, a high utilization rate of the thyristor converter can be guaranteed.

【Example】

FIG. 1 shows an embodiment of a power converter for variable speed operation of a motor according to the present invention. According to this embodiment, as a rectifier connected to the three-phase AC power supplies R, S, T, a thyristor U connected in a three-phase bridge is used.
A thyristor converter 1 made up of Z is provided. For example, a voltage source inverter 8 which supplies a three-phase AC voltage of a variable voltage and a variable frequency to an AC motor 9 which is a three-phase squirrel-cage induction motor is formed by connecting transistors, each of which has a diode connected in parallel, in a three-phase bridge connection. PWM control is performed by a known vector control device (not shown). A smoothing capacitor 7 is provided in the DC intermediate circuit between the thyristor converter 1 and the voltage source inverter 8. In this case, the smoothing capacitor 7 and the voltage source inverter 8 are directly connected, whereas the thyristor converter 1 and the smoothing capacitor 7 are, on the other hand, the forward conversion operation of the thyristor converter 1 during driving operation. The corresponding terminals are connected via the diodes 3 and 5 in the polarity relationship corresponding to
On the other hand, the semiconductor switches have polarities corresponding to the reverse conversion operation of the thyristor converter 1 during braking operation.
The corresponding terminals are connected via 4,6. Transistors can be used as these semiconductor switches. Furthermore, reactors 2R, 2S, 2T are inserted in series in each phase on the AC side of the thyristor converter 1. The thyristor converter 1 is controlled to be fired at a control delay angle of substantially zero during driving operation to generate a maximum forward conversion output voltage similar to that of a diode rectifier, and is controlled to be fired at a predetermined minimum control advance angle during braking operation. Generates maximum reverse conversion output voltage. Further, for the transistors 4 and 6, any one of the following is selected according to the voltage specification of the voltage source inverter 8 during the driving operation. That is, when the input voltage of the voltage source inverter 8 is set to the same level as the maximum forward conversion output voltage of the thyristor converter 1, the transistors 4 and 6 are kept in the OFF state, and the input voltage of the voltage source inverter 8 is changed to the thyristor converter 1. When it is desired to make it higher than the maximum forward conversion output voltage of, either one of the transistors 4 and 6 is switched at a high frequency. Further, the transistors 4 and 6 are conduction-controlled during the braking operation, but in that case, at least one of them is switched at a high frequency. First, the operation in the driving operation of the electric motor will be described. The operation at the time of driving operation when keeping the transistors 4 and 6 in the off state is the same as in the case of the conventional device shown in FIG. 3 described above, and therefore the description thereof is omitted. A description will be given of a case where the switching operation is performed and the driving operation is performed while keeping the other transistor, for example, 6 in the off state. During the driving operation, the thyristor converter 1 is ignition-controlled with a control delay angle of almost zero to generate the maximum forward conversion output voltage. Now, it is assumed that the thyristors U and Z are conducting in the thyristor converter 1. When the transistor 4 is turned on, the power source R phase → reactor 2R → thyristor U → diode 3 →
Transistor 4 → Thyristor Z → Reactor 2T → Power supply T
Line voltage of AC power supply e _R −e _T (> 0)
Is applied to the reactors 2R and 2T, and the current in the above path rises. During this period, the discharging of the smoothing capacitor 7 through the conducting transistor 4 is blocked by the diode 5, so that the smoothing capacitor 7 is discharged only to the voltage source inverter 8 side. When the current reaches the predetermined upper limit value and the transistor 4 is turned off, even if the line voltage is lower than the voltage U ₇ of the smoothing capacitor 7, a current is flown into the smoothing capacitor 7 by the stored energy of the reactors 2R, 2T. . That is, the power source R
Phase → reactor 2R → thyristor U → diode 3 → smoothing capacitor 7 → diode 5 → thyristor Z → reactor 2T → power supply T The current flowing through the path decreases while charging the smoothing capacitor 7. When the current reaches a predetermined lower limit, the transistor 4 is turned on again, which causes the current to rise again. By repeating the following operation, the smoothing capacitor voltage can be maintained at a desired level higher than the maximum forward conversion voltage of the thyristor converter 1 while limiting the current. Therefore, during driving operation, the transistor 4 operates as a boost chopper. The voltage source inverter 8 converts the smoothing capacitor voltage into an AC voltage having a variable voltage and a variable frequency and supplies the AC voltage to the electric motor 9. The braking operation of the electric motor is performed by lowering the output frequency of the voltage source inverter 8 below the electric motor rotation frequency and making the slip frequency negative. As a result, the electric motor performs a power generation operation to convert rotational energy into electric energy, and the electric energy is returned to the smoothing capacitor 7 via the voltage source inverter 8. As a result, the voltage of the smoothing capacitor 7 rises,
When this exceeds a predetermined set value, a command for switching the thyristor converter 1 to the control during braking operation is issued by a voltage monitoring device (not shown). This command can also be generated in advance based on the braking operation start command of the electric motor. With such a command, the thyristor converter 1 is switched to the operation at a predetermined minimum control advance angle β _min, for example, and a maximum reverse conversion output voltage having a polarity opposite to that during the driving operation is generated on the DC side. This maximum inverse conversion output voltage is smaller than the smoothing capacitor voltage. One of the transistors 4, 6, for example 6, is kept in a continuous ON state, and the other, for example 4, is operated for switching at a high frequency. Now, in the thyristor converter 1, it is assumed that the thyristors Z and U are electrically connected. Since the smoothing capacitor voltage is larger than the DC voltage of the thyristor converter 1 in the direction opposite to this, the smoothing capacitor 7 → transistor 4 → thyristor Z → reactor 2T → power supply T phase → power supply R during the ON period of the transistor 4. A regenerative current flows into the power supply through the path of phase → reactor 2R → thyristor U → transistor 6 → smoothing capacitor 7, and the smoothing capacitor 7 is discharged by this current. Up to this point, the regenerative current continues to rise, so the transistor 4 is turned off when the regenerative current rises to a predetermined upper limit value. As a result, the regenerative current due to the stored energy of the reactors 2T and 2R is reactor 2T → power supply T phase → power supply R phase → reactor 2R → thyristor U → transistor 6
→ Diode 5 → Thyristor Z → Reactor 2T is recirculated through the path and decreases. When the regenerative current drops to a predetermined lower limit value, the transistor 4 is turned on again. By repeating such an on / off operation, the smoothing capacitor voltage is maintained at a desired level while the regenerative current is maintained within a desired range and regeneration is performed to the power supply. The on / off operation of the transistor 4 in this case is a so-called step-down chopper operation. The step-up chopper operation during driving operation and the step-down chopper operation during braking operation by high-frequency switching of the transistor 4 are performed by configuring a smoothing capacitor voltage control loop including, for example, a current limiting loop by instantaneous current value control as a minor loop. Can be made. The above step-up chopper operation and step-down chopper operation are
In principle, it is possible to turn on / off the transistors 4 and 6 at the same time. In addition, the roles may be alternately changed in a predetermined cycle in order to equalize the generated losses of both the transistors 4 and 6. Reactors 2R, 2S, 2T have the function of limiting the current change rate during chopper operation of transistors, but these are
The AC reactor may be a known AC reactor originally required for enabling the commutation operation of the thyristor converter 1, and thus does not necessarily need to be specially added. Further, the reactor having such a current changing speed limiting function may be inserted in series also on the DC side of the thyristor converter 1. FIG. 2 shows another embodiment of the present invention in which a reactor 2'having a current changing speed limiting function is also inserted in series also on the direct current side of the thyristor converter 1. When the step-up chopper operation during the driving operation is not performed, if another diode 10 bridging the reactor 2 ′ and the diode 5 (or only the reactor 2 ′) is added as illustrated,
Since little current flows in the reactor 2'when the motor is driven, the efficiency is slightly improved. FIG. 3 shows an embodiment of a power conversion device according to the present invention including a control device. The main circuit components 1 to 9 are exactly the same as those shown in FIG. Reference numeral 31 is a rotation speed detector such as a pulse encoder coupled to the rotation shaft of the electric motor 9, and 32 is a rotation speed target value N ^*.
The rotation speed detection signal output from the rotation speed detector 31 is input to the control circuit 33 together with the rotation speed target value signal output from the rotation speed setting device 32. The control circuit 33 controls ON / OFF of each transistor in the inverter 8 so as to control the magnitude and frequency of the output voltage of the inverter 8 so that the rotation speed of the electric motor 9 matches the target value N ^* . As this control circuit, various ones known in the variable speed technical field of AC motors can be used. DC voltage detection circuit 34 connected to both ends of the capacitor 7
Has a comparison function to determine whether the capacitor voltage E _C is larger or smaller than a predetermined value E _CM . As a result of the comparison, the logic signal output from the DC voltage detection circuit 34 is given to the control circuit 35 for the thyristor converter 1 and the control circuit 36 for the transistors 4 and 6. The control circuit 35 receives the voltage detected from the three-phase AC power supplies R, S, T as a synchronizing signal that gives a reference point of the ignition phase angle, and according to the logic signal from the DC voltage detection circuit 34, the minimum control delay. Thyristor converter 1 with either angle α _min (≈0) or maximum control delay angle α _max (minimum control advance angle β _min ).
Control. That is, the thyristor converter 1 is operated at the minimum control delay angle α _min when the capacitor voltage E _C is smaller than the predetermined value E _CM , and performs the maximum forward conversion output operation (the same operation as the diode rectifier), Voltage E _C
Is larger than the predetermined value E _CM , the minimum control advance angle β _min
The maximum reverse conversion output operation is performed by operating at. The control circuit 36 includes, in addition to the logic signal from the DC voltage detection circuit 34, an input signal corresponding to the current limit set value I ^* from the setter 37 and a current transformer 38 inserted on the DC side of the converter 1. And the current detection signal from. The control circuit 36 keeps both the transistors 4 and 6 in the blocking state by the logic signal output from the DC voltage detection circuit 34 when the capacitor voltage E _C is smaller than the predetermined value E _CM . When the capacitor voltage E _C becomes larger than a predetermined value E _CM , one transistor (eg 4) is kept on continuously and the other transistor (eg 6) is on / off controlled, and this on / off control is changed. The current I detected by the sink 38 is the limit set value I
It is done so that it is restricted to ^* . Therefore, during motor drive operation, the thyristor converter 1 generates a maximum forward conversion output voltage equivalent to that of a diode rectifier, and the capacitor 7 has a value approximately equal to this voltage.
During the motor braking operation, the capacitor voltage is generated by the power returned from the motor 9 to the capacitor 7 via the inverter 8.
When E _C rises and the capacitor voltage E _C exceeds a predetermined value E _CM , the thyristor converter 1 is switched to the maximum inverse conversion operation, the above-mentioned control of the transistors 4 and 6 is performed, and the AC is controlled under the current limit. Electric power is regenerated to the power supply side, and the capacitor voltage E _C is suppressed near the predetermined value E _CM . 4 and 5 show different embodiments of the control circuit 36 in FIG. In the embodiment of FIG. 4, the current limit set value I ^* set by the setter 37 in FIG. 3 and the actual current value I detected by the current transformer 38 are introduced to the current regulator 41, Current controller 41
Performs a proportional-plus-integral operation on the current control deviation (I ^* -I) to form a control signal for the pulse width modulation circuit 42. The base drive circuit 43 controls ON / OFF of the transistor 6 according to the pulse width modulation signal supplied from the pulse width modulation circuit 42. Further, the logic signal from the DC voltage detection circuit 34 in FIG. 3 is led to the current regulator 41, the base drive circuit 43, and the base drive circuit 44 for the transistor 4. When the capacitor voltage E _C is smaller than the predetermined value E _CM, the logic signal output from the DC voltage detection circuit 34 is given to the current regulator 41 as a zero hold signal and to the base drive circuits 43 and 44 as an output blocking signal. Given. When the capacitor voltage E _C becomes higher than the predetermined DC E _CM , the logic signal output from the DC voltage detection circuit 34 releases the zero hold of the current regulator 41 and releases the output blocking of the base drive circuits 43 and 44. . The output signal of the base drive circuit 43 can also be led to the control circuit 35. This means that when the ignition pulse that the control circuit 35 gives to the individual thyristors in the converter 1 is a so-called "double pulse", the thyristors in the converter 1 which should be conducting at that time when the transistor 4 is turned on are turned on. It is intended to give an additional pulse to ensure the establishment of the current. The control line 40 from the control circuit 36 to the control circuit 35 in FIG. 3 corresponds to this. However, if the firing pulses of the individual thyristors in the converter 1 are substantially continuous pulses over a 120 ° width, then such a control line 39 is unnecessary. In the modification of FIG. 5, the current regulator 41 and the pulse width modulation circuit 42 shown in FIG.
Has been replaced by. In this case, the current I is reciprocated between the upper limit value and the lower limit value centering on the limit set value I ^* as shown in FIG. The input / output characteristic of the DC voltage detection circuit 34 in FIG. 1 is preferably a hysteresis characteristic as shown in FIG. FIG. 7 shows the transition of the logic levels “H” and “L” of the output with respect to the voltage E _C of the capacitor 7.

【The invention's effect】

As described above, according to the present invention, the two diodes 3,
The combination of 5 and the two semiconductor switches 4 and 6 has not only the function of switching the connection polarity of the thyristor converter 1 and the smoothing capacitor without contact, but also the semiconductor switch 4 or 6
It has the function of adjusting the smoothing capacitor voltage with current limitation by the step-up chopper or step-down chopper operation by high frequency switching. The thyristor converter 1 is used in a switching operation between a predetermined maximum forward conversion output voltage and a predetermined maximum reverse conversion output voltage, and thus has a high utilization rate. Unlike the prior art, it is possible to safely and efficiently regenerate the power supply while keeping the regenerative current within the desired range during the motor braking operation by the high frequency switching operation of the semiconductor switch without using a resistor. Further, since the resistor is not used, it is not necessary to consider the treatment of the amount of heat generated by the resistor, so that a compact device structure design can be achieved, and the device can be made small and lightweight and the cost can be reduced.

[Brief description of drawings]

1 and 2 are circuit diagrams showing different embodiments of the power converter according to the present invention, respectively, and FIG. 3 is a block diagram showing an embodiment of the power converter according to the present invention including a controller portion. 4 and 5 are block diagrams showing different concrete embodiments of the control circuit 36 in FIG. 3, FIG. 6 is a time chart for explaining the operation of the embodiment of FIG. 5, and FIG. DC voltage detection circuit 34 in FIG.
FIG. 8 is a circuit diagram showing an embodiment of a conventional device. 1 ... Thyristor converter, 2R, 2S, 2T, 2 ... Reactor,
3,5 ... Diode, 4,6 ... Semiconductor switch (transistor), 7 ... Smoothing capacitor, 8 ... Voltage type inverter, 9 ... AC motor, 10 ... Diode.

Claims (1)

[Claims] 1. A rectifier connected to an AC power source, and a voltage source inverter that converts a DC voltage supplied from the rectifier via a DC intermediate circuit including a smoothing capacitor into a variable voltage variable frequency and supplies the variable voltage to a AC motor. In the electric power converter for variable speed operation of the motor, the rectifier is operated at a substantially predetermined maximum forward converter output during a motor drive operation, and is substantially operated at a predetermined maximum reverse converter output voltage during motor braking. A thyristor converter that can be operated is provided, and the DC terminal of the thyristor converter and the terminal of the smoothing capacitor are respectively connected via a diode in a polarity relationship corresponding to the forward conversion operation of the thyristor converter during motor drive operation. In addition, the polarity of the thyristor converter corresponds to the inverse conversion operation during motor braking operation. Connected via a semiconductor switch, insert an inductance element in series on the AC side or the DC side of the thyristor converter, and when driving the motor, both of the semiconductor switches are blocked or one of them is blocked. To operate as a chopper by controlling the other to perform switching control, and to operate as a chopper by controlling one of the semiconductor switches to be in a conductive state and performing switching control of the other during motor braking operation. Converter.