JPH01311889A - Controller for induction motor - Google Patents

Abstract

PURPOSE:To obtain a suitable torque even in a low frequency range and to further prevent a motor from overheating at the time of no load by setting the power factor angle of a load amount to be as a reference in response to the output frequency of an inverter. CONSTITUTION:A delay power factor detector 7 detects a delay power factor angle by using the detecting timing of the zero point of an output current, and a predetermined electric angle of the output voltage of an inverter 4. A discriminator 13 compares the power factor angle of a reference load amount set corresponding to the output frequency of the inverter with the detected delay power factor angle, and discriminates the amplitude of the load amount of a motor. In a V/F controller 5, the ratio of the output voltage corresponding to the output frequency of the inverter 4 is set to be increased or decreased on the basis of the discriminated result of the discriminator 13 responsive to the amplitude of the load amount. Thus, a decrease in the torque in a low frequency range is eliminated, and an induction motor can be operated at a suitable power factor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a control device for an induction motor that varies the rotational speed of the induction motor by V/F control.

[Prior Art] Conventionally, as an induction motor control device that controls the rotation speed of an induction motor, the rotation of the induction motor is controlled by controlling an inverter section using control methods such as vector control, slip frequency control, and V/F III control. There is an induction motor control device that controls the number of motors.

Among these control methods, V/F control keeps the ratio of the output voltage V and output frequency F of the inverter section constant to keep the air gap magnetic flux of the induction motor constant, and improves the efficiency of the induction motor over a wide range of variable speed operation. This is designed to prevent the power factor from decreasing.
Unlike slip frequency control, this method does not require a tacho generator or the like, nor does it require complicated calculations like vector control, so it is highly versatile.

However, in V/F control, in the range where the output frequency is low, the primary resistance r of the equivalent circuit of the induction motor shown in Fig. 11,
There was a problem in that the air gap magnetic flux of the induction motor decreased due to the voltage drop, resulting in a decrease in torque. In V/F control, since the ratio of output voltage is constant, torque cannot be controlled in a low frequency region, which poses a problem, for example, when a large starting torque is required. Therefore, conventionally, the output voltage V1 with respect to the output frequency F of the inverter section is increased in the low frequency region so that the ratio between the induced voltage E, which is related to the amount of air gap magnetic flux, and the output frequency F is approximately constant in accordance with the load torque pattern. A control device for an induction motor is provided in which a V/F function is prepared for correcting the voltage drop across the primary resistance r, and the function is switched and selected according to a pre-known load torque pattern. ing.

However, this control device also requires the load torque pattern to be known in advance, and the switching settings must be made by the user, which poses the problem of poor operability.

Therefore, in order to automatically perform this switching according to the load amount of the induction motor, Japanese Patent Laid-Open No. 61-81'
A control device as shown in Japanese Patent No. 197 has been proposed.

This device generates a sampling pulse at a phase of 90 electrical degrees of the output voltage of the inverter, detects the amount of current corresponding to the amount of sampling pulse generation, and boosts the output voltage of the inverter by a proportion of this current amount. The idea is to do so.

[Problems to be Solved by the Invention] What is disclosed in JP-A-61-81197 is that in a high frequency range, the phase relationship between voltage V and current I is in an ideal state as shown in FIG. Since it is possible to determine when there is no load and when there is a load, it is possible to optimize the control state of the induction motor.

However, in the low frequency range, the primary resistance r1 of the induction motor cannot be ignored, so even when there is no load, the voltage V and the current
As shown in Fig. 12, the phase relationship is φ minus 90°,
As a result, since the no-load magnetizing current is treated like a normal loaded current, there is a problem in that the output voltage of the inverter section is significantly increased even under no load. In addition, in the low frequency range, the no-load current is as large as the overload current during commercial frequency driving, so there are problems such as overheating of the induction motor and high-pitched noise.

In FIG. 12, Ll is the primary inductance, M is the exciting inductance, L is the secondary inductance, and r
2 is the secondary resistance, S is the slip, -s (2) is the primary current, (2) is the primary voltage, and -r2 is the mechanical equivalent output.

The present invention has been made in view of the above-mentioned problems, and its purpose is to set the power factor angle of the load as a reference according to the output frequency of the inverter section, thereby reducing the power factor angle when there is no load. The load amount of the induction motor can be accurately detected without being affected by the no-load magnetizing current.
In addition, it is possible to digitally detect the lagging power factor angle, which is an index of the load amount of the induction motor, which simplifies the circuit configuration and automatically controls the air-gap magnetic flux of the induction motor without adjustment. To provide a control device for an induction motor, which can obtain an appropriate torque even in the range, and prevents the induction motor from overheating during no-load conditions.

Additionally, it is an object of the present invention to provide a control device for an induction motor that can use a current transformer with a small capacity as a current detection means, thereby reducing manufacturing costs and making the entire device smaller.

[Means for solving the problem] The invention according to claim 1 provides a V/F control for an inverter section.
In a control device for an induction motor consisting of a /F control section, the load amount of the induction motor is determined based on the relationship between the electrical angle at the zero point of the output current of the inverter section detected by the current detection means and the predetermined electrical angle of the output voltage of the inverter section. detecting a lagging power factor angle and comparing the lagging power factor angle with the power factor angle of a reference load amount set corresponding to the output frequency of the inverter section to determine the magnitude of the load amount of the induction motor; Based on the determination result, the V/F control section increases or decreases the ratio of the output voltage corresponding to the output frequency of the inverter section. a V/F control section that performs V/F control on the inverter section; a current detection means that detects the output current of the inverter section; a lagging power factor angle detecting means for detecting a lagging power factor angle due to the load amount of the induction motor from the relationship between the electrical angle at the zero point of the current detected by the current detecting means and a predetermined electrical angle of the output voltage of the inverter section; and determining means for determining the magnitude of the load of the induction motor by comparing the power factor angle of the reference load set in [7] with the delayed power factor angle, and determining the magnitude of the load of the induction motor. The invention set forth in claim 3 is characterized in that the ratio of the output voltage corresponding to the output frequency of the inverter section is increased or decreased by a V/F control section based on the determination result of the determination means according to the magnitude. In the described invention, the determination means is provided with a function of setting the power factor angle of the load serving as a reference at the frequency corresponding to the output frequency of the inverter section. Further, the invention according to claim 4 stores a plurality of functions relating to the ratio between the output frequency and the output voltage in the V/F control section, selects an appropriate function based on the determination result of the determination means, and controls the inverter section. It is equipped with a function generating means for determining the output voltage of the circuit.

In the invention of claim 5, the electrical angle of the output voltage of the inverter section is 0.
A timing circuit that generates a timing signal at ゛ and a zero point detection signal of the detected current detected by the current detection means corresponding to the electrical angle of 0 degrees of the output voltage of the inverter section is used as a set signal to generate a latch output, and the timing circuit generates a latch output. a latch circuit that receives and sets a timing signal, and a lagging power factor angle detecting means for detecting a phase angle as a lagging power factor angle from the latch circuit's latch point to the latch point, and the inverter. The load amount of the induction motor is determined from the relationship between the electrical angle of the sampling signal generated from the sampling circuit and the above-mentioned delayed power factor angle according to the power factor angle of the reference load amount set corresponding to the output frequency of the section. In the invention according to claim 6, the count start timing circuit generates a count start timing signal at an electrical angle of 0 degrees of the output voltage of the inverter section, and a determination means corresponds to the electrical angle of 0 degrees of the output voltage of the inverter section. a count stop timing circuit that generates a count stop timing signal at the zero point of the detected current from the current detection means, which starts time counting when the count start timing signal is generated, and ends the time count when the count stop timing signal is generated. a timer counter, a lagging power factor detection means configured to detect a lagging power factor angle from the phase time difference between the output voltage of the inverter section counted by the timer counter and the output current, and a lagging power factor angle corresponding to the output frequency of the inverter section. and determining means for determining the load amount of the induction motor by comparing the power factor angle of the reference load amount set by the above-mentioned lagging power factor angle.

In the invention set forth in claim 7, a current transformer that saturates throughout the variable range of the output frequency of the inverter section is used as the current detection means, and in the invention set forth in claim 8, a current sensor consisting of a Hall element is used as the current detection means. .

[Operation] According to the present invention, the induction motor is controlled by using the detection timing of the zero point of the output current corresponding to the electrical angle of 0° of the output voltage of the inverter section and the predetermined electrical angle of the output voltage of the inverter section. The lagging power factor angle, which is an index of the load amount, is detected, and the power factor angle of the reference load amount, which is set corresponding to the output frequency of the inverter section, is compared with the detected lagging power factor angle to determine the load on the induction motor. The determination means determines the magnitude of the load amount, and the ratio of the output voltage corresponding to the output frequency of the inverter section is set to increase or decrease using the ■/F#J control section based on the determination result of the determination means according to the magnitude of the load amount. Therefore, regardless of the output frequency of the inverter, the output voltage of the inverter is automatically increased or decreased according to the load of the induction motor, eliminating the drop in torque in the low frequency range and maintaining the appropriate power. This makes it possible to operate an induction motor at a high rate. Here, the lagging power factor angle, which is an index of the load amount of the induction motor, can be digitally detected, and the power factor angle of the load amount, which is the reference, can be set corresponding to the output frequency of the inverter section, and this power factor angle can be detected digitally. Since the ratio angle and the detected lagging power factor angle are compared, the output voltage of the inverter section can be controlled appropriately without being affected by the no-load magnetizing current in the no-load state, and the induction motor can be prevented from overheating and high noise. No noise or the like is generated.

[Example code] The present invention will be explained below with reference to Examples.

FIG. 1 shows the overall configuration of Embodiment 1, which includes a converter section 2 that full-wave rectifies a three-phase commercial power supply with a rectifier 1, and a smoothing capacitor 3 that smoothes the rectified output of the converter section 2. , an inverter section 4 whose input power is the DC obtained from the smoothing capacitor 3, and a V/F control section 5 which controls the inverter section 4 by a predetermined ffV/F. A current transformer 6 is used as a current detecting means, and the current detected by the current transformer 6 is used to detect the zero point of the output current corresponding to the electrical angle of 0° of the output voltage V of the inverter section 4. A lagging power factor angle detecting section 7, which is a lagging power factor angle detecting means for detecting a lagging power factor angle from the relationship between the timing and a predetermined electrical angle of the output voltage V of the inverter section 4, and a preset lagging power factor angle detecting section 7, A determination circuit 13 is provided as a determination means for determining the magnitude of the load amount of the induction motor 14 by comparing the power factor angle of the reference load amount set in the V/F control unit 5 based on the output frequency. There is.

The inverter section 4 has three circuits in which two transistors are connected in series, and three circuits are connected in parallel, and the connection point of the transistors in each series circuit is connected to a power terminal of a three-phase induction motor 14.

In this embodiment, the current transformer 6 is constructed of a current transformer that saturates throughout the variable range of the output frequency of the inverter section 4.

The delayed power factor angle detection section 7 includes a low-pass filter 8 that passes only the fundamental wave component of the current detection output output from the current transformer 6, an amplifier circuit 9 that amplifies the output of the low-pass filter 8, and an output of this amplifier circuit 9. a comparison circuit 10 that performs waveform shaping by comparing the output voltage V with a reference voltage; a timing circuit 11 that outputs a timing signal when the output voltage V of the inverter unit 4 matches the electrical angle of 0°; and the output of the comparison circuit 10. and a latch circuit 12 which is set by the timing circuit 11 and reset by the output of the timing circuit 11, and detects the electrical angle from when the latch circuit 12 is reset until it is set as a lagging power factor angle. The reason why the timing signal of the timing circuit 11 is generated at the electrical angle O0 of the output voltage ■ - is because in the latch circuit 12, which is set at the zero point of the output current of the inverter section 4, the zero point of the output current is in phase with respect to the zero point of the output voltage V. It is based on the fact that there is no progression.

The determination circuit 13 determines the load amount of the induction motor 14 by determining the positional relationship between the latch output of the latch circuit 12, the output timing of the sampling signal of the sampling circuit 15, and the output of the latch circuit 12. The electrical angle of the sampling signal output from the sampling circuit 15 is set according to the output frequency F of the inverter section 4, and the sampling circuit 15 monitors the output voltage electrical angle of the inverter section 4 and the output frequency F. However, the sampling signal is output at an electrical angle based on the relationship between the output voltage electrical angle and the output frequency F shown in FIG. 2, for example.

The electrical angle of the sampling signal at this time is its output frequency F
It corresponds to the power factor angle that corresponds to the optimal, standard, load amount.

As shown in the specific circuit diagram of FIG. 3, the V/F control section 5 receives a frequency command F' as input, and adjusts acceleration and deceleration times when making the output frequency F of the inverter section 4 follow this frequency command F'. an acceleration/deceleration adjustment circuit 16 that adjusts according to preset contents; and an acceleration/deceleration adjustment circuit 1 that corresponds to the output frequency F.
A function generating circuit 17 receives the frequency signal F'' from 6 as input, and selects and determines the output voltage V of the inverter section 4 according to the output frequency F by selecting an appropriate function of V=f (F) according to preset contents. The voltage signal from the function generation circuit 17 corresponding to the output voltage V of the inverter section 4 and the frequency signal F'' from the acceleration/deceleration adjustment circuit 16 are input, and the period is 1/F with the amplitude V shown in FIG. A three-phase sine wave generation circuit 18 generates a three-phase sine wave signal, and the outputs Vu, Vv, V- from this three-phase sine wave generation port 18 are compared with a reference triangular wave to generate a PWM pulse. well-known PW
It is composed of one M generation circuit and a driver circuit 20 that drives the transistors of the inverter section 4 using the PWM pulses output from the PWM generation circuit 16. In the embodiment, a sine wave signal is used as the modulated signal by the three-phase sine wave generating circuit 18, but there is no particular limitation on signals other than sine waves as long as the same control can be obtained.

Here, the function generating circuit 14 sets a V/F curve group as shown in FIG. 5 as a function example, and sets an appropriate function when the load pattern applied to the induction motor is known in advance. Based on the determination result of the determination circuit 13 described above, the output voltage (2) is increased or decreased in accordance with the load amount of the induction motor 14 with respect to the reference function.

The apparatus of this embodiment operates as follows.

When the inverter section 4 is now operating to supply driving power to the induction motor 14, the timing circuit 11 generates a three-phase sine wave output corresponding to the output voltage ■ shown in FIG. 6(a) of the inverter section 4. The output of the generating circuit 15, for example, the output Vυ, is taken in and a timing signal is generated as shown in FIG. 6(b) corresponding to the electrical angle of 0° of the voltage waveform in FIG. 6(a>).

This timing signal becomes a reset signal for the latch circuit 12.

On the other hand, the output current of the inverter section 4 is delayed according to the load amount as shown by the solid line and broken line shown in FIG. 6(d). In this case, the lagging power factor angle becomes smaller as the load amount increases.

Here, the current transformer 6 that detects the output current of the inverter section 4 is connected to a high resistance as a load so as to be saturated in the variable range of the output frequency F of the inverter section 4, and its detection output is connected to the inverter section 4. The detected output corresponds to the zero point of the output current of the section 4, and when this detected output is passed through the low-pass filter 8 and the amplifier circuit 9, it becomes a waveform as shown in FIG. 6(e).

This amplified output is compared with a preset reference voltage in the comparator circuit 10, and the waveform is shaped as shown in FIG. 6(f).
This becomes a rectangular signal corresponding to the zero point of the output current of the inverter section 4.

This rectangular signal becomes a set signal for the latch circuit 12, and the latch circuit 12 outputs an "H" latch output as shown in FIG. 6(g). The rise of this output is the inverter section 4
This corresponds to the zero point of the output current corresponding to the electrical angle of 0° of the output voltage V.

On the other hand, the sampling circuit 15 generates a sampling signal as shown in FIG. 6(c) at an output voltage electrical angle corresponding to the output frequency F of the inverter section 4 based on the relationship shown in FIG. The determination circuit 13 determines the power factor angle of the load amount based on the electrical angle at which the sampling signal is generated, and determines the relationship between this power factor angle and the rising position of the output of the latch circuit 10 to determine the load amount as the reference. If the electrical angle at the rising position of the output of the latch circuit 10 is smaller than the power factor angle, it is determined that the load amount of the induction motor 14 is larger than the reference load amount; If the electrical angle is larger than the power factor angle of the reference load amount, it is determined that the load amount of the induction motor 14 is smaller than the reference load amount. For example, if the load on the induction motor 14 is greater than the reference load and a sampling signal is input between the set time and reset time of the latch circuit 10, a signal as shown in FIG. 6(h) is generated.

This signal is input to the function generation circuit 17 of the V/F control section 5, and the function generation circuit 17 selects a function that increases the ratio of the output voltage V to the output frequency F of the inverter section 4 according to this input signal. A signal is output that increases the output voltages V, J, Vv, and V of the three-phase sine generating circuit 18 based on the selected function.

As a result, the inverter section 4 generates an output voltage ■ whose ratio to the output frequency F is increased, and the induction motor 14 obtains a torque corresponding to the load amount and has an appropriate power factor corresponding to the reference load amount. It will be driven by

Based on the relationship between the reference power factor angle of the load, which is set corresponding to the current output frequency F of the inverter section 4 as described above, and the lagging power factor angle, which is an index of the load amount of the induction motor 14. The output voltage V of the inverter unit 4 is increased or decreased by the determination signal of the determination circuit 13, and the primary resistance r1 in the equivalent circuit of the induction motor 14 shown in FIG. When the voltage drop increases, the output voltage V can be automatically increased to eliminate the influence of this voltage drop, thereby preventing a torque drop.

In the first embodiment, a current transformer 6 that saturates in the entire variable range of the output frequency of the inverter section 4 is used as the current detection means, and the induction motor 14 is detected by using the detected output waveform corresponding to the zero point of the detected current. In order to detect the zero point of the output current of the inverter section 4 corresponding to the lagging power factor angle which is an index of the load amount,
Zero point detection can be performed reliably even in the low frequency range.

Of course, a normal current transformer may be used as the current transformer 6, and zero point detection means may be provided for detecting the zero point of the output current of the inverter section 4 by determining the level of the detected output from the current transformer 6.

Furthermore, as a current detection means other than the current transformer 6, a current sensor 6' using a Hall element or a shunt resistor as shown in FIG. 7 may be used. In this case, the output voltage Vi of the current sensor and the shunt resistor A zero point detection means is provided for detecting the zero point of the output current of the inverter unit 4 by determining the level of the voltage across both ends.

Now, in the first embodiment, the latch circuit 1 uses a timing signal corresponding to the electrical angle of 0° of the output voltage V of the inverter section 4.
A method was used to detect the phase angle from the reset of step 2 until the latch circuit 12 is set at the timing of zero point detection of the output current as a delayed power factor angle, but as shown in FIG. etc., to find the phase time difference between the electrical angle of 0° of the output voltage (■) of the inverter section 4 and the zero point of the output current, and detect the lagging power factor angle, which is an index of the load amount of the induction motor 14. Also good.

Embodiment 2 shown in FIG. 8 will be described below.

First, the V/F control section 5 has the same configuration as the first embodiment and performs the same operation, so a description thereof will be omitted. Further, in this circuit, the current transformer 6, current sensor 6', etc. as described above can be used as the current detecting means, but to simplify the explanation, the saturated current transformer 6 is used as in the first embodiment. In addition, the zero point of the output current of the inverter section 4 is detected using a low-pass filter 8, an amplifier circuit, and a comparator circuit 10 as a current zero point detection circuit.

Then, the output VU of the three-phase sine wave generation circuit 18 shown in FIG.
When the count start timing signal output from the inverter section 6 is input, the timer counter 23 starts counting the reference clock CLK, and stops counting in response to the zero point detection signal corresponding to the output current of the inverter section 6 shown in FIG. 9(b). When the count stop timing signal outputted from the timing generation circuit 22 is input, the timer counter 23 stops the above-mentioned counting. The count value of the timer counter 23 becomes the phase time difference tp from the electrical angle 0° of the output voltage V of the inverter section 4 to the zero point of the output current, as shown in FIG. 9(c). From this tp, the delayed power factor angle φ, is φ, ;2π・tp−
Required as F. Here, the determination circuit 13' sets the output voltage electrical angle corresponding to the output frequency F of the inverter section 4 shown in FIG. 2, which is prepared based on the frequency signal from the acceleration/deceleration adjustment circuit 16 of the V/F control section 5 The magnitude of the load on the induction motor 14 is determined by comparing the reference power factor angle φref of the load amount and the delayed power factor angle φ1 obtained from the count value of the timer counter 23, and the determination result is used as a function. The output voltage V of the inverter section 4 is outputted to the generation circuit 17, and the output voltage V of the inverter section 4 is outputted to the generation circuit 17, and the output voltage V of the inverter section 4 is
It increases or decreases. Thus, the circuits of current transformers 6.8 to 10 and the circuits of 21 to 23 constitute the delayed power factor angle detection section 7.

Note that the output voltage (2) of the inverter section 4 may be increased or decreased by taking the difference between the power factor angle φrsf corresponding to the reference load amount and the detected lagging power factor angle φ1.

[Effects of the Invention] According to the present invention, the delayed power factor angle is calculated using the electrical angle of the zero point of the output current corresponding to the electrical angle of 0° of the output voltage of the inverter section and a predetermined electrical angle of the output voltage of the inverter section. detect,
The power factor angle of the reference load amount set corresponding to the output frequency of the inverter section is compared with the detected delayed power factor angle to determine the magnitude of the load amount of the induction motor, and based on the determination result, the inverter The ratio of the output voltage corresponding to the output frequency of the
Since the increase/decrease is set in the /F control section, the output voltage of the inverter section is automatically corrected appropriately according to the load amount of the induction motor, regardless of the output frequency of the inverter section, eliminating a drop in torque and maintaining the appropriate level. The induction motor can be operated using the power factor, and since the lagging power factor angle, which is an index of the induction motor load, is detected as described above, it can be detected digitally, and the resulting circuit This has the effect of simplifying the configuration, and in addition, the power factor angle of the reference load is set in accordance with the output frequency of the inverter section, and this power factor angle is compared with the detected lagging power factor angle. The output voltage of the inverter section can be properly controlled without being affected by the no-load magnetizing current in the no-load state, and there is an effect that overheating of the induction motor and generation of high-pitched noises etc. do not occur. In addition, by using a current transformer that saturates throughout the variable range of the output frequency of the inverter as a current detection means, a current transformer with a small capacity can be used, reducing manufacturing costs and making the entire device smaller. This has the effect of

[Brief explanation of the drawing]

FIG. 1 is a basic circuit configuration diagram of the first embodiment of the present invention, and FIG.
The figure is an explanatory diagram of the relationship between the output voltage electrical angle and the output frequency to explain the I setting of the power factor angle of the load, which is the reference for the same as above, Figure 3 is a detailed circuit diagram of the same as the above, and Figure 4 is the same as the one used in the above. An output waveform diagram of the three-phase sine wave generation circuit, FIG. 5 is an explanatory diagram of the relationship between the output voltage and output frequency of the inverter section, and FIGS. 6 (a) to (
h) is a time chart for explaining the operation of the same as above, FIG. 7 is a circuit diagram of the current detection means used in the above, FIG. 8 is a detailed circuit diagram of the second embodiment of the present invention, and FIG. 9 is an explanation of the operation of the same as above. FIG. 10 is an explanatory diagram of a conventional example, FIG. 11 is an equivalent circuit diagram of an induction motor, and FIG. 12 is an explanatory diagram of a conventional example. 4 is an inverter section, 5 is a V/F control section, 6 is a current transformer, 7
1 is a delayed power factor angle detection section, 8 is a low-pass filter, 9 is an amplifier circuit, 10 is a comparison circuit, 11 is a timing circuit, 12 is a latch circuit, 13 is a determination circuit, 14 is an induction motor, 15
17 is a sampling circuit, 17 is a function generation circuit, 13 is a judgment circuit, 21 is a count start timing generation circuit, 22
2 is a count stop timing generation circuit, and 23 is a timer counter. Agent Patent Attorney 1 Seki 1) Chief 7 Figure 2 Ichigen - Evil EF Figure 4 Figure 5 - Out 7J Karakomaki F Figure 6 Figure 9 Figure 10 Figure 11 Figure 12

Claims (1)

[Scope of Claims] (1) Determining the lagging power factor angle due to the load amount of the induction motor from the relationship between the electrical angle at the zero point of the output current of the inverter section detected by the current detection means and the predetermined electrical angle of the output voltage of the inverter section. is detected, and the magnitude of the load on the induction motor is determined by comparing the power factor angle of the reference load amount set corresponding to the output frequency of the inverter section with the above-mentioned delayed power factor angle. A control device for an induction motor, characterized in that the V/F control section increases or decreases a ratio of an output voltage corresponding to an output frequency of the inverter section based on the output frequency of the inverter section. (2) Based on the load amount of the induction motor based on the relationship between the electrical angle of the zero point of the current detected by the current detection means corresponding to the electrical angle of 0° of the output voltage of the inverter section and the predetermined electrical angle of the output voltage of the inverter section. A lagging power factor angle detection means detects a lagging power factor angle, and a lagging power factor angle detecting means compares the lagging power factor angle with the power factor angle of a reference load amount set corresponding to the output frequency of the inverter section to load the induction motor. and a determination means for determining the magnitude of the load amount, and a V/F control section increases or decreases the ratio of the output voltage corresponding to the output frequency of the inverter section based on the determination result of the determination means according to the magnitude of the load amount. A control device for an induction motor, characterized in that: (3) The control device for an induction motor according to claim 2, wherein the determination means is provided with a function of setting a reference power factor angle of the load at the output frequency of the inverter section in accordance with the output frequency of the inverter section. (4) The V/F control section stores a plurality of functions related to the ratio of output frequency and output voltage, and selects an appropriate function based on the judgment result of the judgment means to determine the output voltage of the inverter section. Claim 2 characterized by comprising a generating means.
A control device for the induction motor described above. (5) A timing circuit that generates a timing signal at an electrical angle of 0 degrees of the output voltage of the inverter section, and a zero point detection signal of the detected current detected by the current detection means corresponding to the electrical angle of 0 degrees of the output voltage of the inverter section. It consists of a latch circuit that generates a latch output as a set signal and is reset by the timing signal of the timing circuit, and a delay force that detects the phase angle as a delayed power factor angle between the reset time of the latch circuit and the latch time. Based on the relationship between the power factor angle of the sampling signal generated from the sampling circuit according to the power factor angle of the reference load amount set corresponding to the output frequency of the inverter section and the power factor angle of the power factor angle detection means, and the delayed power factor angle. 3. The control device for an induction motor according to claim 2, further comprising determining means for determining a load amount of the induction motor. (6) A count start timing circuit that generates a count start timing signal at an electrical angle of 0 degrees of the output voltage of the inverter section, and a zero point of the detected current from the current detection means corresponding to the electrical angle of 0 degrees of the output voltage of the inverter section. The timer counter consists of a count stop timing circuit that generates a count stop timing signal when the count start timing signal is generated, and a timer counter that starts time counting when the count start timing signal is generated and ends the time count when the count stop timing signal is generated. a lagging power factor detection means configured to detect a lagging power factor angle from the phase time difference between the output voltage of the inverter section and the output current; and a power factor of a reference load amount set corresponding to the output frequency of the inverter section. 3. The control device for an induction motor according to claim 2, further comprising determining means for determining a load amount of the induction motor by comparing the angle and the delayed power factor angle. (7) The control device for an induction motor according to claim 1, wherein a current transformer that saturates throughout the variable range of the output frequency of the inverter section is used as the current detection means. (8) The control device for an induction motor according to claim 2, wherein a current sensor comprising a Hall element is used as the current detection means.