JPH03117369A - Voltage controller for inverter - Google Patents

Abstract

PURPOSE:To improve voltage control accuracy by providing a dead time compensation circuit for correcting the PWM control signal and a DC voltage compensation circuit for correcting a voltage reference in PWN control according to the fluctuation of DC power supply. CONSTITUTION:In an inverter for controlling an AC motor 3 through a DC power source 1 and an inverter bridge 2, output VR from a ramp function circuit 5 is divided by an output v18 from a voltage detector 18 and the division result v19 is provided to a voltage reference generator 7 thus producing a reference voltage Vu. The reference voltage Vu is then compared with a triangular wave v14 through a comparator 13a thus producing a PWM signal v13a. The signal v13a is provided to a dead time compensation circuit 17a in order to compensate for the fluctuation of pulse width caused by the dead time of an inverter bridge, and the output v17a from the compensation circuit 17a is provided to a drive circuit 15. when the influence of the fluctuation of DC power supply and the distortion caused by the dead time are removed in such a manner, a highly accurate controller a can be obtained.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Field of Industrial Application) The present invention is directed to an inverter that controls the output voltage of a voltage-type PWM inverter used in an AC motor drive power supply or an uninterruptible power supply to a desired value. This invention relates to a voltage control device.

(Prior Art) An example of a conventional voltage control device for a voltage source inverter is shown in FIG.

In FIG. 7, a DC power source 1 is converted into an AC voltage by PWM control via a three-phase inverter bridge 2, and is supplied to an electric motor 3.

The frequency reference voltage set by the frequency setter 4 is outputted via a ramp function circuit 5 to an output VR that changes at a constant rate of change.
Furthermore, the frequency reference f of the inverter is given via the voltage-frequency converter 6, and the voltage reference generator 7
Input 1 and f to set the three-phase AC voltage reference VU"t VV'*
Outputs V-.

On the other hand, the outputs U, V, and W of the inverter bridge 2 are resistors 8a, 8b, 8c, resistors 9a, 9b, respectively.
9c, capacitors 10a, 10b, and 10c are detected through a voltage detection circuit connected in a star shape, and the insulation unit lla.

An isolated phase voltage is obtained via 11b and lie.

The deviation between the voltage reference yL,* and the detected phase voltage VU is amplified by the amplifier 12a, and further compared with the output V14 of the triangular wave generator 14 by the comparator 13a, which is input as a PWM signal to the drive circuit 15, and the voltage reference vU The inverter bridge 2 is controlled by PWM so that * and the detected voltage Vυ become equal.

Similarly, for the V phase, the amplifier 12b compares and amplifies v* and Vv, the comparator 13b outputs a PWM signal, and the W phase also has vw*.
and vchi are compared and amplified by an amplifier L2c, and a PWM signal is outputted by a comparator 13c.

In this method, the output voltage is delayed using capacitors 10a, 10b, and 10c, and the detected voltage is controlled by removing high frequencies with a filter.If you consider the delay circuit using the capacitors as a type of integrating circuit, you can control the magnetic flux. Therefore, it is also called magnetic flux control.The advantage of this method is that it stabilizes the waveform factor deterioration and voltage fluctuation due to the dead time of the inverter bridge, and the voltage fluctuation of the DC 111 source through closed-loop control, and smoothly operates the load motor. Must be able to drive.

(Problems to be Solved by the Invention) Although the above conventional system has the above advantages, it also has the following drawbacks.

The first is that it is expensive because it uses an analog isolation unit for the voltage detection circuit.

Note that the transformer cannot be used when the inverter frequency is low because the V/f ratio increases and saturates.

The second is that as the inverter frequency increases, the sense voltage decreases due to capacitors loa, 10b, 10c, and therefore there is an upper limit on the inverter output frequency.

The third reason is that there is a phase difference between the voltage reference vUx and the phase voltage of the inverter output due to the phase delay caused by the capacitor 10a, and this phase difference changes depending on the inverter frequency. It is necessary to compensate for the voltage drop due to the internal resistance of the motor and the internal resistance of the motor, and the method for doing so becomes complicated. This is because when the inverter output voltage is low, the voltage drop caused by the resistance causes, for example, the magnetic flux of the induction motor to be weakened, resulting in a torque shortage, so it is necessary to compensate for this.

[Structure of the invention]

(Means and effects for solving the problem) The present invention has been made in consideration of the above problems, and it detects the voltage between the inverter output and the fixed potential with a level detector, and compares it with the PWM signal. A dead time compensation circuit is provided to correct errors, and the voltage reference value is corrected by detecting the DC power supply voltage.The dead time compensation circuit is used to correct waveform distortion due to dead time, ensuring faithfulness to the voltage reference value. By obtaining a suitable output waveform and controlling the voltage reference inversely proportional to the DC voltage with respect to DC voltage fluctuations of the inverter bridge, it becomes possible to control the inverter output voltage with high precision.

(Example) An example of the present invention is shown in FIG. Components that are the same as those in the conventional FIG. 7 are given the same numbers and their explanations are omitted.

In FIG. 1, the voltage of the DC power supply 1 is detected by a voltage detector 18.
, and the output VR of the ramp function generator 5 is divided by the divider 19.
is divided by the output Vts of the voltage detector 18, and the output V□ is inputted to the voltage reference generator 7.

AC voltage base 1' determined by the output f of the voltage-frequency converter 6
P! vU*, vv*, vw* and triangular wave Q raw fm14 output Z'14 are compared by comparators 13a, 13b, and 13c, respectively, and the PWM signal V-at Z'□zbt
? □,. Bind and output.

The PWM signal # □aat V tab + 1rtic is the pulse width and dead time of the switching output between the three-phase outputs U, V, W of the inverter bridge 2 and the negative electrode of the DC power supply 1 detected by the level detectors 16a, 16b, and 16c, respectively. The compensation circuits 17a, 17b, and 17c compare the above V1zap f xsbe? l'
Output 917al with tac corrected? /ltb*? tt
e is input to the drive circuit 15.

The PWM signal V□ in FIG. 1 is a voltage reference vu* and a triangular wave V0. as shown in FIG. Which intersection point is given?

This PWM signal '+71. When an inverter bridge consisting of transistors 21, 24 and diodes 22, 23 as shown in FIG. Matches *.

In reality, transistors 21 and 24 are not ideal switches and have a delay in operation.
A dead time To (generally about 15 to 60 t1s) is provided between the base signal 21B of the transistor 21 and the base signal 24B of the transistor 24 so that the transistors 21 and 24 do not turn on at the same time.
The switching is shown in FIG.

Therefore, the output voltage Vυ-0 changes significantly depending on whether the current flowing through the inductance of the load motor 3 is in the direction of the solid arrow or in the direction of the dashed arrow, as shown in (a) and (b) in Figure 3. .

In this case, depending on the phase of the voltage reference vu* and the current I, the output voltage Vυ-0 varies greatly as shown in Figure 4 (a), (b), and (c), resulting in a waveform that is completely different from the voltage reference. Therefore, it becomes impossible to control using the voltage reference V- inside the control circuit.

For this reason, dead time compensation is performed by comparing Vx3a in Figure 3 with the output vU-0 and correcting the pulse widths so that they match.By applying such compensation, the voltage reference vU* inside the control circuit and the output The voltage can be controlled so that the waveforms match.

Next, one of the causes of voltage fluctuation is the voltage fluctuation of the DC power supply 1. If the DC voltage is Vd and the PWM voltage reference value is V East, then the inverter output voltage V is V αvd·V East. Therefore, if the voltage is controlled to be VxccvRhd, it will not be affected by the fluctuation of the DC voltage. The division circuit 19 in FIG. 1 performs this operation.

As explained above, in the embodiment shown in Fig. 1, there is no need to use an analog isolation amplifier to detect the inverter output voltage, and a photocoupler can be used to detect the level, and dead time compensation by level detection allows the voltage reference to be used. It becomes possible to generate extremely faithful waveforms regardless of the inverter frequency.

Furthermore, regarding DC power supply fluctuations, by correcting the voltage reference value using a voltage detector with a relatively slow response, stable voltage control of the inverter with less high frequency components becomes possible.

Another embodiment of the invention is shown in FIG. FIG. 5 shows only the parts that are different from FIG. 1, and the rest is the same as FIG. 1.

In FIG. 5, current detectors 20a, 20b, 2
The motor currents of the U-phase, ■-phase, and W-phase are detected by 0c, respectively, and added to voltage standards Vu"+Vv*, Vv" via adders 23a, 23b, and 23c, respectively.
Are their outputs PWM modulated triangular waves? '24 and comparator 13
A, 13b, and 13c compare and output a PWM signal.

The triangular wave V24 is the output of the triangular wave generator 14?
The output obtained by multiplying /14 and the output V of the DC power supply voltage detector 18 by the multiplier 24 is used.

In general, it is known that the voltage drop caused by the resistance between the motor and the inverter and the resistance between the motor and the next winding has a large effect, especially in a range where the inverter output voltage is low, and causes a torque shortage.

Conventionally, this voltage drop was compensated in a scalar manner, but this was not sufficient and caused problems such as insufficient torque and overexcitation, but in Figure 5, it is compensated vectorally! Since compensation is performed in the form of υ8+tR, almost complete compensation is achieved.

In this case, the inverter output voltage V is vxcevd・
Since it is expressed as v*・□, y, 4oc vdh y
24 and 'v24, v = y-, and the shadow of DC voltage Vd 2
4. Voltage control of the inverter that is not affected by noise is performed.

Still another embodiment of the invention is shown in FIG.

FIG. 6 mainly shows changes to FIG. 1, and the motor current is detected by current detectors 20a, 20b, 2.
Detected at 0c, voltage control amplifiers 21a, 21b, 2
Current reference V2□ which is the output of 1c, V2□byZ'z
x. and current control amplifiers 22a, 22b, respectively.
22c constitutes a current minor loop that compares and amplifies the current and controls the load current according to these current standards.

Voltage base 1? IVu"+ Vv't Vv" are respectively,
t A IIJ Control amplifier output 9iza+? zab+
8□C and voltage control amplifiers 21a, 21b, 21c
is compared and amplified at the current reference u 21 a r T
z □b + V z t c is output.

Since the inverter output voltage v is faithful to the waveform of Z'22, internal feedback control of V2□ makes it possible to realize inverter voltage control with a minor output current.

Although the above embodiment has been described as controlling each phase by analog control, it is also possible to perform digital control using a microcomputer or control as a rotation vector obtained by converting the three phases into the dr and q axes.

Furthermore, the present invention is applicable not only to inverters for driving electric motors but also to UPS.
It is also possible to apply the present invention to a power supply device such as an uninterruptible power supply (uninterruptible power supply), and it is also possible to use an inexpensive item such as a V/F converter for DC voltage detection.

〔Effect of the invention〕

As explained above, according to the present invention, the inverter output is detected in level with a simple circuit such as a photocoupler,
The voltage reference waveform is faithfully followed by dead time compensation that is compared and corrected with the PWM signal, and the voltage reference or modulated triangular wave is corrected for voltage fluctuations of the DC power supply.

without feedback of inverter output voltage value.
By performing calculation or feedback control within the control circuit, it is possible to realize inverter voltage control with high voltage accuracy without waveform distortion using a simple and economical device.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figures 2 and 3 are circuit diagrams showing one embodiment of the present invention.
FIG. 4 is an explanatory diagram of the operation of the present invention. FIGS. 5 and 6 are circuit diagrams showing other embodiments of the present invention, and FIG. 7 is a circuit diagram showing an example of a conventional voltage control device for an inverter. 1...DC power supply 2...Inverter bridge 3...AC motor 4...Frequency setting device 5...
・Ramp function circuit 6...Voltage-frequency converter 7...
・Voltage reference generator 8a, 8b, 8c, 9a, 9b, 9cm resistor 10a,
10b, LOc・-:] Ndensa 11a, llb,
11c”・Insulation unit 12a, 12b, 12cm
Amplifiers 13a, 13b, 13cm comparator 14... triangular wave generator 15... drive circuit 16a
, 16b, 16cm level detector 17a, 17b,
17c... Dead time compensation circuit 18... Voltage detector 19... Divider 23a, 23b,
23cm adder 24...multiplier 20a,
20b, 20c = current detectors 21a, 21b,
21cm Voltage Controlled Amplifier 22a, 22b, 22cm Current Controlled Amplifier (8733) Agent Patent Attorney Yoshiaki Inomata (One Idiot) Figure 2 Figure 2 Figure 2 Figure 2

Claims (2)

[Claims] (1) In a voltage control device for a voltage source inverter that obtains a desired AC voltage from a DC power source via a PWM-controlled inverter bridge, the level of the output voltage of the inverter and the PWM
a dead time compensation circuit that corrects the PWM control signal to compensate for the dead time of the inverter bridge by comparing it with the M control voltage; and a DC voltage that corrects the voltage reference for PWM control in accordance with voltage fluctuations of the DC power supply. An inverter voltage control device comprising a compensation circuit. (2) Voltage control of an inverter according to claim (1), comprising a load current compensation circuit that detects the output current of the inverter and corrects the PWM control signal to compensate for a voltage drop in the main circuit due to the load current. Device.