JPS5866584A - Current comparison type ac power source - Google Patents

Abstract

PURPOSE:To suppress the variation in the switching frequency of a switching element to the allowable degree by composing a control circuit of a hysteresis width instructing circuit and a hysteresis correcting circuit. CONSTITUTION:A hysteresis width instructing circuit 80 which outputs a hysteresis width correcting command 120 corresponding to the induction electromotive force of a motor and a hysteresis width correcting circuit 82 for correcting the hysteresis width by the command 120 are provided in a control circuit. The circuit 80 has a speed detector for detecting the rotating speed of the motor, and the command 120 responds to the rotating speed of the motor. A comparator 90 compares the command 120 with a reference value 120 from a reference voltage generator 84, the switch 94 of the circuit 82 is controlled in switching by the compared output 124, thereby correcting the switching frequency of the main switching element.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AC power supply device used for speed control of an AC motor, and particularly to a current comparison type AC power supply device.

As this type of device, the following PWM AC power supply device is conventionally well known.

FIG. 1 shows the main circuit of the conventional device. In the same figure, diodes 10, 12, 14゜16.18.2
0 constitutes a full-wave rectifier circuit, and the three-phase alternating current supplied to this circuit is rectified here and smoothed by the smoothing capacitor 22 to become a direct current with less pulsation. The direct current thus obtained is supplied to a PWH inverter in which a main circuit is formed by connecting main transistors 30, 32, 34, 36, 38, and 40 in a pristine manner. In addition, each main transistor 30°32.34
, 36, 38, and 40 respectively have free wheel diodes 42,
44, 46, 48, 50.52 are connected.

Each main transistor 30, 32°34.36 in Figure 1 above
．． 40 is controlled on and off at a predetermined timing, whereby three-phase PWM AC current is supplied to the AC motor 54 to drive the AC motor 54. and,
Each phase current supplied to the AC motor 54 is detected by a current detector 56.
, 58°60.

The first regeneration circuit has the above-mentioned configuration. Next, a control circuit that applies control signals to the bases of the transistors 30, 32, 34, 36, and 38° 40 to control them on and off will be explained based on FIG. .

FIG. 2 shows only the control circuit for one phase, which consists of a trigger circuit. This control circuit includes a current command 100 that commands the instantaneous value (sine wave) of the inverter output current, and a first
Detection signal 102 of current detector (56, 58 or 60)
is entered.

In the figure, a command 100 is transmitted to a comparator 6 through a resistor 62.
4 and the signal 102 is applied via a resistor 66 to the other comparison input. Note that this comparator 64 is composed of an operational amplifier 68 and resistors 70 and 72.

This comparator 64 has a hysteresis characteristic, and its comparison output (current deviation) is output as an on/off control signal 104 via an inverter 74 and as an on/off signal 106 via inverters 74 and 76. This signal 104 is applied to the base of the transistor 320, and the signal 106 is applied to the base of the l-run raster 320, which control the switching of the transistors 30 and 32.

The conventional current comparison type PWM inverter device has the above-mentioned configuration.The operation thereof will be explained below with reference to FIGS. 3 and 4.

The comparator 64 compares the command 100 and the signal 102, and the inverter output current corresponding to the command 100 is determined by the comparator 64.
When the upper limit of the hysteresis width (ΔH) of 4 is exceeded, the output is inverted, and at this time, the transistor 30 is turned off and the inverter output current is controlled in the direction of decreasing.Then, the inverter output current decreases. When signal 102 falls below the lower limit of the hysteresis width (ΔI-()), the output of comparator 64 is inverted again, transistor 30 is turned on, and the inverter output current increases.

As described above, in this type of device, the current deviation of the detection signal 102 with respect to the current command 100 is determined by the hysteresis width (ΔH) of the comparator 64.

In order to improve the characteristics of the device, it is preferable to make the current deviation as small as possible, and normally this hysteresis width (ΔH) is made as narrow as possible.

At this time, main circuit transistor 30, 32° 34.36
, 38.40 switching frequency is the hysteresis width (
ΔH). However, transistor 30
, 32, 34, 36°38.40 has an upper limit, so the hysteresis width (ΔH) cannot be narrowed indefinitely. Therefore, in this type of device, the hysteresis width (ΔH) of the comparator 64 is determined in consideration of the current deviation and the main transistor switching upper limit frequency.

By the way, in this type of device, the main transistor 30.32
, 34, 36. ．． The switching frequency of 38.40 also varies with the rotational speed of the motor 54 according to the characteristic shown at 110 in FIG.

That is, since the inverter output current flows due to the difference between the inverter output voltage and the induced electromotive force of the electric motor 54, when the electric motor 54 rotates at low speed, the induced electromotive force is small and the rate of change of the inverter output current with respect to time becomes large. , therefore the switching frequency of the inverter becomes higher.

In this way, it is understood that in the conventional device, the switching frequency of the inverter fluctuates sharply depending on the rotation speed of the electric motor, and as shown in FIG. 4, when the electric motor 54 is rotating at the rated speed and when it is rotating at a low speed It is recognized that the switching frequency is significantly different between the two.

As mentioned above, in the conventional device, the hysteresis width (ΔH)
If the switching frequency of the main transistor is taken into account and set within a predetermined current deviation, when the motor is driven and controlled over a wide rotational speed range, 5 switching frequency is the upper limit of the switching frequency allowed for the main transistor. may exceed.

As described above, in conventional devices, when the electric motor is driven within a wide rotational speed range, the main switching element of the inverter may be switched at a switching frequency that exceeds its permissible switching frequency, and in this case, the main switching element may be destroyed. There was a problem that

The present invention has been made in view of the above-mentioned conventional problems,
The object is to provide a current comparison AC power supply device that can suppress fluctuations in the switching frequency of the main switching element to an allowable level even when the speed of the driven motor varies greatly.

In order to achieve the above object, the present invention provides an inverter that drives and controls an AC motor using an output current obtained by switching a main switching element, a current detector that detects the output current of the inverter, and a current detector that detects the output current of the inverter. A control circuit that includes a comparator that performs matching with a detection signal and controls switching of the switching element based on a current deviation obtained from the output of the comparator, wherein the control circuit includes: The circuit is
a hysteresis width command circuit that outputs a hysteresis width correction command corresponding to the induced electromotive force of the AC motor; and a hysteresis correction circuit that corrects the hysteresis width of the comparator in accordance with the command so that the switching frequency of the main switching element decreases. A circuit is characterized in that it includes a circuit.

Preferred embodiments of the present invention will be described below based on the drawings.

A preferred embodiment of the present invention is shown in FIG. 5, in which the same members as in FIG. 2 are given the same reference numerals and their explanation will be omitted.

As shown in FIG. 5, the present invention includes a hysteresis width command circuit 80 that outputs a hysteresis width correction command 120 corresponding to the induced electromotive force of the electric motor 54 to the control circuit, and a hysteresis width command circuit 80 that corrects the hysteresis width using the command 120. A width correction circuit 82 is newly provided. In this embodiment, the hysteresis command circuit 80 includes a speed detector that detects the rotational speed of the electric motor 54, and the command 120 corresponds to the rotational speed of the electric motor. Note that the induced electromotive force of the electric motor 54 has a predetermined relationship with the rotation speed of the electric motor 540, and therefore the detection signal value of the speed detector corresponds to the value of this induced electromotive force.

One of the hysteresis width correction circuits 82 is provided with a reference voltage generation circuit 84, and a reference value 122 outputted from the circuit 84 is set to a value corresponding to a predetermined speed of the electric motor 54.

The command is supplied via a resistor 86, and the reference value 120 is supplied via a resistor 88 to both comparison inputs of a comparator 9o, where a comparison is made between the two.

The correction circuit 82 is further provided with a resistor 92 connected to the resistor 70 and a switch 94, which are connected in series. This switch 94 is the comparison output 1 of the comparison circuit 90.
A preferred embodiment of the present invention in which the switching is controlled by 24 has the above-mentioned configuration, and its operation will be explained below.

In this embodiment as well, the command 100 and the signal 102 are supplied to a comparator 68 having hysteresis, and a current deviation is obtained at its output, thereby controlling the switching of the inverter main circuit. In this respect, there is no difference from the conventional device.

The predetermined value 122 is used in this embodiment to switch the hysteresis width, but if the value of the command 120 is larger than the set value 122, the comparative output 124 becomes the level of "1", and the swing f94 becomes IC. Driven off by. Therefore, in this case, the device of this embodiment performs the same operation as the conventional device.

In the opposite case, the value of command 120 is the reference value 1.
22, the comparison output 124 is L'''
As a result, the switch 94 is turned on. Therefore, in this case, the hysteresis width of the comparator 64 was determined by the resistors 70 and 74 until the switch 44 was turned on; resistance of 70.92
The voltage division ratio between the resistor 72 and the resistor 72 increases, and becomes an even larger value.

In this way, in this embodiment, when the rotational speed of the electric motor 54 is below a predetermined speed, the hysteresis width is switched so that the main transistor 30°32.34,36,3
The switching frequency of 8.40 is transistor 30,3
The switching frequency is suppressed so as not to exceed the upper limit switching frequency allowed at 2, 34, 36° 38.40°.

As explained above, in this embodiment, the characteristic 160 shown in FIG.
As shown in , based on the rotational speed reference value 122,
When the rotational speed of the main transistors 30, 32, 34, 36, 38, .
The switching frequency of the inverter main circuit never exceeds the switching frequency of 40.

As a result, according to this embodiment, the main transistor will not switch at a frequency higher than its upper limit switching frequency, and therefore the inverter main circuit will not operate inappropriately and cause an unforeseen accident. Another preferred embodiment of the present invention will now be described with reference to FIG. In addition, in this figure, the same members as in each of the above-mentioned figures are given the same reference numerals, and the explanation thereof will be omitted.

According to the present invention, it is also possible to continuously change the hysteresis width, and this embodiment can do this.

In FIG. 6, the hysteresis width command circuit 8° outputs a command 120 with output characteristics as shown in FIG. There is. These commands 120 and 121 are supplied to the other ends of variable resistors 95 and 96, each of which has one end connected to the output side of the operational amplifier 68. The divided voltage outputs of the resistors 95 and 96 are supplied to the anode of the diode 97 and the cathode of the diode 98, and the cathode and anode of each of these diodes are connected to the inverting input side of the operational amplifier 68.

A second preferred embodiment of the present invention has the above configuration, and therefore commands 120, 121 and resistors 95, 96 . By means of the diodes 97 and 98, the output value of the comparator 64 is small when the motor 54 is rotating at a high speed, and conversely, when the rotation speed is low, the hysteresis width can be expanded.

Therefore, the switching frequency of the main transistor can be suppressed as in the previous embodiment. However, in the above-mentioned embodiment, the hysteresis width of the comparator 64 was changed by changing the resistance voltage division, but in this embodiment, the hysteresis width was changed continuously according to the output amplitude value of the comparator 64. ing.

According to this embodiment, since the hysteresis width of the comparator 64 can be continuously changed, fluctuations in the switching frequency of the main transistors 30, 32, 34, 36, 38, 40 of the inverter main circuit can be reduced. and is more suitable for its control.

As explained above, according to the present invention, by changing the hysteresis width of the comparator 64, the switching frequency of the main transistor of the inverter can be set below the permissible switching frequency, so that the speed control of the AC motor is wide. This is particularly suitable when the process is carried out over a range of rotational speeds.

In addition, by continuously switching the hysteresis width in the first embodiment, the same effect as in the second embodiment can be obtained. In the above description, the rotational speed of the induction motor is used as an equivalent to the induced electromotive force of the motor, but the current command or the output current frequency of the inverter may be used instead.

As described above, since the switching frequency fluctuation of the main transistor is caused by the induced electromotive force of the motor, the hysteresis width may be changed by a signal corresponding to this fluctuation. Here, changing the induced electromotive force itself is particularly effective when the magnetic flux of the motor is controlled. This is because when magnetic flux control is performed, the induced electromotive force is not exactly proportional to the output frequency of the inverter or the rotational speed of the motor.

Note that it is also more suitable to use a thyristor, GTO, etc. instead of a transistor as a main switching element in the inverter main circuit in order to increase the current capacity, and in the present invention, these are also included in the main switching element.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the main circuit of the conventional device, Fig. 2 is a circuit diagram showing the control circuit of the conventional device, and Fig. 3 is a diagram showing the main circuit of the conventional device.
4 is an explanatory diagram of hysteresis width switching operation, FIG. 5 is a circuit diagram showing a preferred first embodiment of the present invention, (15) FIG. 6 is a diagram of the present invention. A seventh circuit diagram showing a second preferred embodiment of
The figure is an output characteristic diagram of the hysteresis width command circuit shown in FIG. 6. 30.32,34. ．． 36,38.40... Main transistor, 54... AC motor, 56,58.60...
- Current detector, 64... Comparator, 80... Hysteresis width command circuit, 82... Hysteresis width correction circuit. Agent Patent Attorney Akio Takahashi (16) Figure q

Claims (1)

[Claims] 1. An inverter that drives and controls an AC motor with an output current obtained by switching the main switching element, a current detector that detects the output current of the inverter, and a current command and current for the inverter output current. and a control circuit that includes a comparator that performs matching with a detection signal and performs switching control of the switching element based on a current deviation obtained from the output of the comparator. The circuit includes a hysteresis width command circuit that outputs a hysteresis width correction command corresponding to the induced electromotive force of the AC motor, and a hysteresis width command circuit that corrects the hysteresis width of the comparator in accordance with the command so that the switching frequency of the main switching element decreases. A current comparison type AC power supply device comprising a hysteresis width correction circuit. 2. The current comparison type AC power supply device according to claim 1, wherein the hysteresis width command circuit outputs a hysteresis width correction command having a value corresponding to the rotational speed of the AC motor. 3. The device according to claim 1, wherein the hysteresis width command circuit outputs a hysteresis width correction command having a value corresponding to the inverter output current frequency.