JP2625870B2 - PWM inverter - Google Patents

Description

The present invention relates to a PW driven by setting an arm short-circuit prevention period.
The present invention relates to a technique for preventing an influence on an output voltage during an arm short-circuit prevention period in an M inverter.

[Conventional technology]

Pulse width modulation (PWM) is one of the output control methods of power converters used as drive sources for electromagnetic machines such as AC motors
There is a control method.

This PWM control system generally compares a voltage command signal (modulated wave) for commanding the output voltage of the inverter with a triangular wave signal (carrier) to determine the switching time of each switching element constituting the inverter. The inverter control circuit is provided with a PWM signal generating circuit as shown in FIG. In the figure, 1 is a single-phase inverter
INV of the DC power supply (voltage E), the main circuit of the second inverter INV, T _r1, T _r2 are switching-element (transistor), 3
Induction motor as a load is, PWM signal generating circuit for generating a PWM signal 4, switching-element T _r1 of 5 main circuit 2, T _r2
Is a drive circuit (driver) that supplies drive signals to the
The M signal generation circuit 4 basically generates a PWM signal P by comparing the variable amplitude triangular wave oscillator 6 for generating the triangular wave signal _VCP with the voltage command signal V ^* and the triangular wave signal _VCP as shown in FIG. A comparator 7 is provided. vo main circuit 2 the output voltage (average value), T is the period of the triangular wave signal, t ₁ denotes the pulse width of the PWM signal.

The output voltage vo is From the relationship, In expressed, the output voltage vo is proportional to the magnitude of the voltage E of the DC power supply, is inversely proportional to the amplitude of the triangular wave signal V _CP, the DC voltage E is varied, the output voltage vo is varied.
For this reason, conventionally, as shown in FIG. 5, a triangular wave oscillator 6 takes in a DC voltage E through a
By making V _CP , fluctuation of the output voltage vo due to fluctuation of the DC voltage E is prevented. K is the scaling factor between the actual value of the voltage and the signal on the control circuit.

[Problems to be solved by the invention]

However, the main circuit 2 has one or a plurality of serial circuits between the output point and the positive electrode of the DC power supply 1 (positive arm) and between the output point and the negative electrode of the DC power supply 1 (negative arm). since a circuit which included a switching-element, to prevent arm short circuit of switching-element of both arms both ON, the arm short circuit preventing period (Detsudotaimu) provided t _d.

Figure 6 is an illustration of one example of a control circuit of the inverter to set the Detsudotaimu t _d, it is shown waveforms of the respective portions in FIG. 7. In FIG. 6, 10a and 10b are on-delay / off instant timer circuits, 11a and 11b are waveform shaping circuits, 12a and 12b are drivers, i _o is a current flowing through the load 3, v _GA , and v _GB are base drive signals. It is.

In this configuration, since the on-delay / off instant timer circuits 10a and 10b are provided, as shown in FIG. 7, the rising of the base driving signal v _{Gb corresponds to} the falling of the pulse of the base driving signal v _Ga and Since the rise of the base drive signal v _Ga is always delayed by the time t _{d with} respect to the _fall of the pulse of the base drive signal v _Gb , the transistors T _R1 and T _R2
Can be turned on at the same time.

However, during this dead time t _d , both transistors T _R1 and T _R2 are turned off and the output voltage _vo is not managed, so that the output voltage _vo deviates from the voltage command value V ^* by ( Error) Δv _o and when the load current i _o is positive, v _o <V
^* , When negative, v _o > V ^* . That is, It is represented by ,Also, Therefore, the above equation (2) is And Δv _o is Thus, the inverter INV is driven by setting the Detsudotaimu t _d, Yotsute the impact of this Detsudotaimu t _d,
The output value v _o causes an error Δv _o with respect to the command value V ^* . This Delta] v _o is not related to the magnitude of V ^*, such as during low speed and DC braking, when the value of V ^* is small, the Delta] v _o is the relatively large percentage of the V ^*.

For this reason, if the inverter load is an AC motor having a poor cooling state, such as an induction motor for driving a vacuum pump, and performs DC braking for a relatively long time, overheating of the load or overheating of the load may occur. There was a problem that control performance became unstable.

The present invention has been made to solve this problem.
It is an object of the present invention to provide a PWM inverter capable of maintaining an output voltage at a voltage command value without being affected by the dead time even when an input voltage during DC braking varies.

[Means for solving the problem]

In order to achieve the above object, the present invention has a configuration in which the magnitude of the voltage command signal to be compared with the carrier of the PWM signal is increased or decreased by an amount corresponding to the arm short-circuit prevention period, in accordance with the polarity of the inverter output current. Things.

[Action]

In the conventional PWM control, the voltage pulse width has an error corresponding to the arm short-circuit prevention period according to the polarity of the load current.In the present invention, the DC braking voltage command value is set to correspond to the arm short-circuit prevention period. Since the correction is made by the amount corresponding to the polarity of the load current, the deviation (error) between the voltage command value and the output voltage average value is eliminated.

〔Example〕

 Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

In FIG. 1, reference numeral 13 denotes a coefficient unit (coefficient K). Reference numeral 14 denotes a voltage command value correction circuit, and a function unit (function t _d / T) 15,
It has an adder 16. 17 is a carrier amplitude correction circuit, which has a coefficient unit (coefficient K / 2) 18 and a multiplier 19,
The main difference from the conventional configuration is that the voltage command value correction circuit 14 and the carrier wave amplitude correction circuit 17 are added to the signal generation circuit 4. The same components as those in FIG. 6 are denoted by the same reference numerals.

Next, the signal conversion operation in the PWM signal generation circuit 4 will be described with reference to FIG. FIG. 3 is an arithmetic representation of the signal system in the PWM signal generation circuit 4. The operation of this embodiment will be described below with reference to FIG.

Assuming that the actual output voltage target value is v _o ^* , Thus, in this embodiment, the carrier into a DC voltage E for determining the amplitude of the magnitude of V _CP, Detsudotaimu / carrier period ratio (t _d / T) corresponding voltage value obtained by multiplying the gain command value V
^{Since *} is added to ^* , the influence of the dead time t _{d on} the output voltage _vo is eliminated.

At the time of the DC braking of the AC motor, the polarity of the load current is positive with respect to the output voltage polarity, so that the output voltage v _o is equal to the voltage command value V ^* (when K = 1) ( −), The output voltage vo depends on the DC voltage of the main circuit. Therefore, depending on the DC voltage E, there is a large excess or deficiency in the braking current, and the braking performance is degraded or the AC motor is overheated. However, in this example,
Since v _o = V ^* can be maintained, the overheating and the like can be prevented.

In the above embodiment, the case of a single-phase inverter has been described, but the present invention can be applied to a polyphase inverter to obtain the same effect.

[Effects of the Invention] As described above, the present invention is configured to correct the voltage command value by an amount corresponding to the arm short-circuit prevention period. Since the influence of the period can be eliminated, control performance and control stability can be improved as compared with the related art.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram expressing a signal system for explaining the operation of the above embodiment, and FIG. 3 is a basic circuit of a PWM signal generating circuit. FIG. 4 is a waveform diagram for explaining a PWM signal, FIG. 5 is a diagram showing a basic configuration of an inverter having a PWM signal generation circuit,
FIG. 6 is a diagram showing an example of an inverter in which an arm short-circuit period is set, and FIG. 7 is a diagram showing waveforms at various parts of the inverter in FIG. 4 ... PWM signal generation circuit, 14 ... Voltage command value correction circuit, 1
7 Carrier amplitude correction circuit.

Claims (2)

(57) [Claims] 1. A PWM inverter for driving an AC motor at a variable speed, wherein the PWM control is performed by setting an arm short-circuit prevention period, and the amplitude of a carrier wave to be compared with a voltage command signal in a circuit for generating a PWM signal is controlled by the inverter. In the inverter that changes according to the magnitude of the input DC voltage, when DC braking is performed, the magnitude of the voltage command signal is increased or decreased by the amount corresponding to the arm short-circuit prevention period in accordance with the polarity of the inverter output current. PWM inverter characterized by being performed. 2. A circuit for generating a PWM signal, comprising a voltage command value correction circuit, wherein the voltage command correction circuit converts a voltage command value into an arm short-circuit prevention period of an inverter input DC voltage value.
2. The PWM inverter according to claim 1, wherein correction is performed by an adder circuit by an amount corresponding to a carrier wave cycle ratio.