JP4449283B2 - PWM inverter control method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a control method of a PWM inverter using a power conversion circuit in which each arm is bridge-connected, and in particular, a control method for protecting a semiconductor element constituting a power conversion circuit by an arm current of each arm. About.
[0002]
[Prior art]
Conventionally, in this type of inverter, the detection value of each arm current is synthesized using a drive signal to a semiconductor element constituting the power conversion circuit, and a control method based on this synthesized value is performed. (For example, refer to Patent Document 1.)
However, with the recent high performance and low price of inverters, PWM inverters that convert a DC voltage into an AC voltage with a desired frequency and voltage by PWM control have become mainstream. The problem of using the inverter control method according to the above will be described below with reference to the circuit configuration of the conventional inverter shown in FIG. 6 and its operation waveform diagram shown in FIG.
[0003]
That is, in FIG. 6, 1 is a PWM inverter (showing the main part of its circuit configuration), and 5 is an AC motor as a load fed from the PWM inverter 1.
[0004]
This PWM inverter 1 is composed of a DC power supply 11 such as a rectifying power supply whose DC voltage is Ed, and an anti-parallel circuit of an IGBT and a diode as shown in the figure, and the lower arm is a U-phase arm, a V-phase arm and a W-phase arm. , A shunt resistor r _U , r _V , r _W for detecting the arm current of each of the U-phase arm, V-phase arm, and W-phase arm, and these shunt resistors An AC voltage having an amplitude and a frequency based on a voltage command is output from the DC voltage Ed from the power conversion circuit 12 to each of the IGBTs constituting the power conversion circuit 12 and the arm current detection unit 13 that detects both end voltages. A PWM signal generator 14 for supplying a PWM-controlled on / off drive signal and a carrier signal Fc for performing the PWM control. An overcurrent protection level in which at least one of the U-phase arm, V-phase arm, and W-phase arm current detected by the carrier oscillator 15 and the arm current detector 13 is set in advance. For example, the protection means 16 is configured to prevent overcurrent destruction of semiconductor elements such as IGBTs and diodes that form the power conversion circuit 12 by stopping the operation of the PWM signal generator 14 when the value exceeds the threshold. .
[0005]
In the PWM inverter 1 shown in FIG. 6, in order to prevent element destruction caused by an overcurrent in a semiconductor element such as an IGBT or a diode constituting the power conversion circuit 12, for example, an output current of each phase is detected and an overcurrent is detected. Although there is a control method for performing current protection, in this control method, the current detection circuit for detecting the output current needs to be insulated from the power conversion circuit 12, and this current detection circuit is large and expensive. Therefore, in order to reduce the size and cost, shunt resistors r _U , r _V , r _W that can be formed without being insulated from the power conversion circuit 12 and the arm current detector 13 are used.
[0006]
[Patent Document 1]
JP-A-3-243173 (page 2-3, FIG. 1)
[0007]
[Problems to be solved by the invention]
A case where the PWM inverter 1 shown in FIG. 6 drives the induction motor (hereinafter also referred to as the induction motor 5) as the AC motor 5 at a variable speed will be described below.
[0008]
When the induction motor 5 is driven at a variable speed and the induction motor 5 changes from a driving state to a braking state, the power factor angle of the induction motor 5 viewed from the PWM inverter 1 is generally in a range of about 30 ° to 150 °. Become.
[0009]
FIG. 7 shows the PWM control result PWMu ^* for the voltage command e _U ^* of the U-phase phase voltage in the PWM signal generator 14 and the primary currents i ₃₀ and 150 of the induction motor 5 when the power factor angle is 30 °. The relationship between the primary current i ₁₅₀ at the time of ° and the detected waveforms at the shunt resistor r _U and the arm current detector 13 is shown. As is apparent from this waveform diagram, the waveforms of the current detection values of i ₃₀ and i ₁₅₀ depend on the waveform of the PWMu ^* , and therefore the positive half cycle period of the voltage command e _U ^* of the U-phase phase voltage. In this case, the period in which current can be detected, that is, the period in which current flows, is smaller than the negative half cycle period. As a result, the above-described protection means 16 prevents the semiconductor elements such as IGBTs and diodes constituting the power conversion circuit 12 from being shunted by the shunt resistors r _U , r _V , r _W and the arm current detection unit 13 by the protection means 16 described above. There was a risk that it would be difficult to do.
[0010]
As a countermeasure against the above problem, there is a control method for increasing the frequency of the carrier signal Fc output from the carrier oscillator 15, that is, the carrier frequency. In this control method, switching of IGBTs and diodes forming the power conversion circuit 12 is performed. As a result, the switching loss of the IGBT and the diode increases, resulting in a decrease in the rated output power value of the PWM inverter 1, and thus the entire PWM inverter 1 becomes large and increases in price. New problems arise.
[0011]
An object of the present invention is to provide a PWM inverter control method for solving the above-described problems and preventing the semiconductor elements constituting the power conversion circuit from being overcurrent destroyed by the arm current of each arm. There is.
[0012]
[Means for Solving the Problems]
This first invention uses a power conversion circuit formed by bridge-connecting each arm, and in a PWM inverter that converts a DC voltage into an AC voltage having a desired amplitude and frequency by PWM control.
The arm current flowing through each arm is detected, and when the instantaneous value of at least one of the detected arm currents exceeds a preset level, the PWM control is performed for a predetermined setting period . The PWM inverter control method is characterized in that the carrier frequency at the time is increased to a frequency at which the ripple amount of the arm current decreases to a predetermined value.
[0013]
Further, a second invention is the method for controlling a PWM inverter according to the first invention, wherein at least one of the detected arm currents is detected while the PWM inverter is operating at the changed carrier frequency . When the instantaneous value exceeds the level again, the changed carrier frequency is newly continued for the predetermined setting period .
[0014]
According to the present invention, when the arm current exceeds a preset level, the carrier frequency at the time of the PWM control is changed for a predetermined period, so that the excess of the semiconductor elements constituting the power conversion circuit can be achieved. Current protection can be performed more reliably.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a circuit diagram showing a first embodiment of the present invention. Components having the same functions as those of the conventional circuit shown in FIG. 6 are denoted by the same reference numerals.
[0016]
That is, the PWM inverter 2 shown in FIG. 1 (indicating the main part of its circuit configuration) includes a DC power source 11, a power conversion circuit 12, shunt resistors r _U , r _V , r _W , an arm current detection unit 13, and a PWM signal. In addition to the generator 14 and the protection means 16, a level setter 21, a level detector 22, a changeover switch 23, a first carrier oscillator 15 a having the same function as the carrier oscillator 15, and a second carrier oscillator 24 are provided. ing.
[0017]
When the induction motor 5 is driven at a variable speed by the PWM inverter 2, the output current of the PWM inverter 2, that is, the primary current of the induction motor 5 is changed to the load factor and electric constant of the motor and the fundamental wave output voltage of the PWM inverter 2. This is based on the combined value of the fundamental wave current based on the above and the high frequency current based on the electrical constant and the modulation rate in the PWM control by the PWM inverter 2, in other words, the carrier frequency.
[0018]
The above-described high-frequency current is a ripple of the output current, and this ripple Δi is a high-frequency time constant based on the primary winding resistance value R ₁ , secondary winding resistance value R ₂ , and leakage inductance Lσ of the induction motor 5, This value is based on the carrier frequency f _C during PWM control in the PWM inverter 2 and the DC voltage Ed of the DC power supply 11.
[0019]
Hereinafter, a derivation formula for the ripple component Δi will be described with reference to the T-1 equivalent circuit of the induction motor shown in FIG. 2. Here, as a calculation condition for the ripple component Δi, the high-frequency current is a primary resistance value. , Leakage inductance, and secondary resistance, the amplitude of the high-frequency power source shown in FIG. 2 changes stepwise from 0 to the DC voltage Ed, and the period is ½ of the carrier frequency f _C Then, the ripple amount Δi is expressed by the following formula (1).
[0020]
[Expression 1]
Therefore, among the output current of the PWM inverter 2, the fundamental wave current from the fundamental wave power source shown in FIG. 2 varies depending on the load factor of the induction motor 5, and the high frequency current superimposed on this fundamental wave current, that is, the ripple Δi is PWM. This value is based on the carrier frequency f _C during PWM control in the inverter 2. At this time, as is clear from the above equation (1), when the carrier frequency f _C is increased, the value of the ripple Δi becomes smaller. Further, the fundamental wave current and the ripple are also applied to the arm currents of the U-phase arm, V-phase arm, and W-phase arm of the power conversion circuit 12 detected by the shunt resistors r _U , r _V , r _W and the arm current detector 13. An amplitude appears as a superposition value with the minute.
[0021]
FIG. 3 is a waveform diagram for explaining the operation of the PWM inverter 2 shown in FIG. 1, and this waveform is a partially enlarged view of the waveform shown in FIG.
[0022]
That is, in FIG. 3, a triangular wave-like thin solid line carrier signal Fc ₁ shows an output waveform of the first carrier oscillator 15a, and a triangular wave-like thick solid line carrier signal Fc ₂ shows an output waveform of the second carrier oscillator 24. It is to have a relation of f _C1 <f _C2 between the carrier frequency f _C1 and the carrier signal Fc ₂ of the carrier frequency f _C2 of the signal Fc _1, for example, setting the f _C1 of about 2 KHz, the f _C2 to about 4KHz Furthermore, the PWM control result PWMu ^{* in} each of the carrier signals Fc ₁ and Fc _{2 with} respect to the voltage command e _U ^* of the U-phase phase voltage in the PWM signal generator 14 and the power factor angle of the induction motor 5 are 30 °. the relationship between the primary current i ₃₀ of the electric motor, and each of the detected waveform of the carrier signal Fc _1, Fc ₂ in the shunt resistor r _U and the arm current detecting unit 13 when It is. As is clear from this figure, the positive half cycle of the voltage command (e _U ^* ) of the U-phase phase voltage is detected in the detection waveform at the shunt resistor r _U and the arm current detector 13 at the time of the carrier signal Fc _2. In the illustrated interval t in the period, the current can be detected when the current is generated in the form of a pulse. The number of occurrences of the pulsed current is from 3 when the carrier signal Fc ₁ is generated. As a result, the number of detectable periods can be increased.
[0023]
Therefore, in this PWM inverter 2, at least of the U-phase arm, V-phase arm, and W-phase arm that form the power conversion circuit 12 detected by the shunt resistors r _U , r _V , r _W and the arm current detection unit 13. When the instantaneous value of any one-phase arm current exceeds the level set by the level setter 21, for example, about 150% of the rated current value of the semiconductor element forming the arm, the level detector 22 During the period of about one to two cycles of the voltage command e _U ^* , a changeover signal is output to the changeover switch 23, and the changeover switch 23 opens the path on the first carrier oscillator 15a side by this changeover signal. By closing the path on the oscillator 24 side, the carrier frequency in the PWM control is made higher.
[0024]
As a result, the arm current detectable period of each of the U-phase arm, the V-phase arm, and the W-phase arm is further increased. At this time, the pull amount Δi based on the above-described equation (1) is further reduced. Thus, overcurrent protection of the semiconductor elements constituting the power conversion circuit 12 can be performed more reliably. It should be noted that the period of operation of the carrier signal Fc ₂ of the second carrier oscillator 24 is set to about 1 to 2 cycles of the fundamental wave voltage as described above, so that the IGBT, diode, etc. that form the power conversion circuit 12 during this period The increase in switching loss of the semiconductor element is very small.
[0025]
FIG. 4 is a circuit diagram showing a second embodiment of the present invention. Components having the same functions as those in the first embodiment shown in FIG. 1 are denoted by the same reference numerals.
[0026]
That is, the PWM inverter 3 shown in FIG. 4 (showing the main part of its circuit configuration) includes a third carrier oscillator 31 instead of the second carrier oscillator 24.
[0027]
The carrier frequency f _C3 of the carrier signal Fc ₃ output from the third carrier oscillator 31 is set to a motor constant (primary resistance R ₁ , primary resistance R ₁ , frequency at which the value of the ripple Δi based on the above equation (1) is reduced to a predetermined value. Secondary resistance R ₂ , leakage inductance Lσ, etc.) in advance, and setting the obtained carrier frequency (for example, about 4 KHz to 5 KHz), while reducing the ripple Δi to a predetermined value, U Since the arm current detectable period of each of the phase arm, the V-phase arm, and the W-phase arm is increased, it is possible to more reliably perform the overcurrent protection of the semiconductor element constituting the power conversion circuit 12 via the protection means 16. it can. It should be noted that the period during which the third carrier oscillator 31 operates with the carrier signal Fc ₃ is set to about one to two cycles of the fundamental voltage as in the PWM inverter 2, thereby forming the power conversion circuit 12 during this period. The increase in switching loss of a semiconductor device such as a diode is small.
[0028]
FIG. 5 is a circuit diagram showing a third embodiment of the present invention. Components having the same functions as those of the second embodiment shown in FIG. 4 are denoted by the same reference numerals.
[0029]
That is, the PWM inverter 4 (showing the main part of its circuit configuration) shown in FIG. 5 includes a level detection unit 22a instead of the level detection unit 22, and further includes a switching control circuit 41.
[0030]
In this level detection unit 22a, the instantaneous value of at least one of the U-phase arm, V-phase arm, and W-phase arm forming the power conversion circuit 12 is set at the level set by the level setting unit 21, for example, When the value of about 150% of the rated current value of the semiconductor element forming the arm is exceeded, a trigger signal is output from the level detector 22a, and the switching control circuit 41 that receives this trigger signal receives the voltage command e _U. ^The changeover signal is output to the changeover switch 23 for a period of about 1 to 2 cycles of ^* . Here, when the switching control circuit 41 receives a new trigger signal while outputting a switching signal based on the previous trigger signal, the switching signal is continued for a period of about one to two cycles from the new trigger signal. I am doing so.
[0031]
As a result, the PWM inverter 4 can more reliably perform overcurrent protection of the semiconductor elements constituting the power conversion circuit 12 via the protection means 16.
[0032]
【The invention's effect】
According to the present invention, when the arm current of any one of the arms forming the power conversion circuit of the PWM inverter exceeds a preset level, the carrier frequency for PWM control in the PWM inverter is set to a predetermined value. By changing to a carrier frequency higher than the period, overcurrent protection of the semiconductor elements constituting the power conversion circuit can be more reliably performed.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of a PWM inverter showing a first embodiment of the present invention. FIG. 2 is an equivalent circuit diagram of an induction motor for explaining the operation of FIG. 1. FIG. FIG. 4 is a circuit configuration diagram of a PWM inverter showing a second embodiment of the present invention. FIG. 5 is a circuit configuration diagram of a PWM inverter showing a third embodiment of the present invention. Circuit diagram of PWM inverter [FIG. 7] Waveform diagram for explaining the operation of FIG.
1 to 4 ... PWM inverter, 5 ... AC electric motor, 11 ... DC power supply, 12 ... power conversion _{circuit, r U, r V, r} W ... shunt resistor, 13 ... arm current detecting unit, 14 ... PWM signal generator, 15 DESCRIPTION OF SYMBOLS ... Carrier oscillator, 15a ... 1st carrier oscillator, 16 ... Protection means, 21 ... Level setter, 22, 22a ... Level detection part, 23 ... Changeover switch, 24 ... 2nd carrier oscillator, 31 ... 3rd carrier oscillator, 41 ... switching control circuit.

Claims (2)

In a PWM inverter that converts a DC voltage into an AC voltage having a desired amplitude and frequency by PWM control, using a power conversion circuit in which each arm is bridge-connected,
The arm current flowing through each arm is detected, and when the instantaneous value of at least one of the detected arm currents exceeds a preset level, the PWM control is performed for a predetermined setting period . The PWM inverter control method is characterized in that the carrier frequency at the time is increased to a frequency at which the ripple amount of the arm current decreases to a predetermined value. In the control method of the PWM inverter according to claim 1,
When the PWM inverter is operating at the increased carrier frequency, when the instantaneous value of at least one of the detected arm currents exceeds the level again, the increased carrier frequency is newly set. The PWM inverter control method is characterized by continuing the predetermined setting period .

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