JPH04105594A - Ac motor controller - Google Patents

Abstract

PURPOSE:To realize stabilized inverter driving of an AC motor even under low speed operation by controlling the power factor based on a current flowing through the DC bus of inverter, i.e., the current flowing into the motor. CONSTITUTION:When a current flows through each phase of a three-phase induction motor 28, a current idc flows through the DC bus 25 of an inverter 27. The current idc is detected through a current detector 29 and a current I corresponding to the current idc is fed to a power factor operating circuit 30. Since the current idc flows into the DC bus 25 when one of three-phase currents of the three-phase induction motor 28 is selected according to the switching state of the inverter main circuit 26, one phase current of the three- phase induction motor 28 can be detected when the current idc is extracted through sampling with the timing of a specific switching state determined by a PWM output voltage waveform.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a control device for an AC motor driven by a voltage source PWM type inverter.

(Prior Art) Conventionally, as a control device for a three-phase induction motor, which is the most common among polyphase AC motors, Japanese Patent Application Laid-Open No. 62-10019
There is something like the one disclosed in Publication No. 2, which is shown in FIG. That is, the voltage source PWM type inverter 1 includes a rectifier circuit 3. It is comprised of a smoothing capacitor 4 for smoothing the rectified voltage and an inverter main circuit 5 to which the smoothed DC voltage is applied. The AC output voltage from the inverter main circuit 5 is then applied to a three-phase induction motor 6. The current tdc flowing through the DC bus of the inverter 1 is detected by the current detector 7, and the detected current I is detected by the low bus filter (L, P).
, F) 8, and the low-pass filter 8 receives the detection current I
The fundamental wave is extracted and output as a frequency correction value Δf.

This frequency correction value Δf is subtracted from f frequency command values in a subtracter 9 and output as a frequency command value f (−f −Δf), which is applied to a pulse width modulation (PWM) control circuit 10. This pulse width modulation control circuit 10 is given V voltage command values obtained by converting f frequency command values by a frequency-voltage (f/V) conversion circuit 11. As a result, the pulse width modulation control circuit 10 provides a base signal to the power transistor of the inverter main circuit 5 based on the frequency command value f and the voltage command value V* to perform pulse width modulation control.

Therefore, in the voltage source PWM type inverter 1,
Since the DC voltage is constant, the average value of the current flowing through the DC bus of the inverter 1 is proportional to the power supplied to the three-phase induction motor 6. In this case, if the rotational speed of the three-phase induction motor 6 is sufficiently high and the rate of change in rotational speed is very small, then the torque fluctuation will be proportional to the power fluctuation, and therefore the DC bus of the inverter 1 will be If the average current of is controlled, the torque of the three-phase induction motor 6 will be controlled.
The generation of vibration can be prevented.

(Problems to be Solved by the Invention) The conventional configuration is effective when the rotation speed of the three-phase induction motor 6 is sufficiently high and the ratio of change in rotation speed is very small, but when the rotation speed is low and the During speed operation, the amount of change in electric power in response to changes in torque decreases, so even if this is divided by the rotational speed or the estimated rotational speed and compensated so that it does not depend on the operating speed, the S/N ratio will decrease. Sufficient accuracy cannot be obtained, and therefore, it becomes difficult to stably drive the inverter during low-speed operation.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a control device for an AC motor that can stably drive the AC motor using an inverter even during low-speed operation.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a multi-phase AC motor driven by a voltage source PWM type inverter, in which a current for detecting a current flowing through a DC bus of the inverter is provided. A detection means is provided, and a power factor calculation means is provided for calculating the power factor of the AC motor from the current values before and after the phase switching point generated in the current waveform detected by the current detection means, and the power factor calculated by the power factor calculation means is provided. The present invention is characterized in that it includes an inverter control means that determines the output voltage or output frequency of the inverter.

(Function) According to the control device for an AC motor of the present invention, the power factor is controlled based on the current flowing through the DC bus of the inverter, that is, the current flowing through the AC motor. Unlike the conventional method, an element that depends on the operating speed is not included, so that sufficient accuracy can be obtained even during low-speed operation, and the AC motor can be stably driven by an inverter.

(Example) Hereinafter, a first example of the present invention will be described with reference to FIGS. 1 and 2.

First, the overall configuration will be described according to FIG. 21
is a three-phase AC power supply, 22 is a full-wave rectifier circuit that full-wave rectifies the three-phase AC voltage from this three-phase AC power supply 21, and 23 is a DC voltage that smoothes the rectified voltage from this full-wave rectifier circuit 22. a smoothing capacitor applied between bus bars 24 and 25;
Reference numeral 26 denotes an inverter main circuit to which the DC voltage between the DC buses 24 and 25 is applied to an input terminal, and these constitute an inverter 27 of the voltage type PWM system.

In this case, the inverter main circuit 26 is configured by connecting a plurality of switching elements, for example, six power transistors 26a in a three-phase bridge, and connecting a diode 26b in antiparallel to each of these power transistors 26a, so that the output thereof The three-phase AC voltage from the terminals is supplied to a polyphase induction motor, such as the most common three-phase induction motor 28, as a polyphase AC motor. Reference numeral 29 denotes a current detector consisting of a Hall CT as current detection means, which detects the current L flowing through the DC bus 25 of the inverter 27.
It detects dC and outputs a detection current I in accordance with this. Reference numeral 30 denotes a power factor calculation circuit serving as a power factor calculation means, which receives the detected current I from the current detector 29 at its input terminal and performs calculations as will be described later.
The tangent component tanφ of the power factor angle φ is output from the output terminal.

31 is a subtracter, to which the tangent component tanφ from the power factor calculation circuit 30 is given to its negative (-) input terminal;
The tangential component of the four power factor angle command values is input to the positive (+) input terminal.
φ is given, and the difference between them Δtanφ is calculated from the output terminal.
It is designed to output . 32 is proportional integral (PI)
This is a control circuit in which the difference Δtanφ from the subtracter 31 is given to the input terminal, and the voltage correction value Δtanφ is output from the output terminal.
It is designed to output V. 32 is an adder;
This means that the voltage correction value ΔV from the proportional-integral control circuit 32 is given to one positive (+) input terminal, and the voltage correction value ΔV from the frequency-voltage (f/V) conversion circuit 34 is given to the other positive (10) input terminal. A voltage command value V kyō is given to it, and the voltage command value V is output from the output terminal. In this case, the frequency-voltage conversion circuit 34 is configured such that the frequency command value f is given to its input terminal, and seven voltage command values corresponding to one frequency command value are output from the output terminal. It has become. 35 is pulse width modulation (PW
M) A control circuit which is configured such that the voltage command value V from the adder 33 is given to one input terminal, and the frequency command value f is given to the other input terminal,
A pulse width modulation signal corresponding to the voltage command value V and frequency command value f8 is generated from the output terminal, and this is applied to the base of the power transistor 26a of the inverter main circuit 26 as a base signal. And the above subtractor 31. Proportional-integral control circuit 32. Adder 33, frequency -
The voltage conversion circuit 34 and the pulse width modulation control circuit 35 constitute an inverter control means 36.

Next, the operation will be explained with reference to FIG. 2.

At the beginning of the startup of the three-phase induction motor 28, the frequency command value fIK is given to the pulse width modulation control circuit 35, and the frequency command value f* is converted to the voltage command value V flux by the frequency-voltage conversion circuit 34. These seven voltage command values are given to the pulse width modulation control circuit 35 as the voltage command value V via the adder 33. As a result, the pulse width modulation control circuit 35 outputs the voltage command value V (VX) and the frequency command value f.
The power transistor 26 of the inverter main circuit 26 outputs a pulse width modulation signal corresponding to lK as a gate signal.
As a result, the inverter main circuit 26 performs a switching operation and outputs a three-phase AC voltage with an output voltage and an output frequency according to the voltage command value V and frequency command value fl= mentioned above. It is supplied to a three-phase induction motor 28. Therefore, a current flows through each phase of the three-phase induction motor 28, and a current Ldc flows through the DC bus 25 of the inverter 27 as shown in FIG. 2(a). This current Ldc is detected by the current detector 29, and a detection current corresponding to this is detected! It is given to the power factor calculation circuit 30 as follows.

Here, since one of the three-phase currents flowing through the three-phase induction motor 28 is selected by the switching state of the inverter main circuit 26 and flows to the DC bus 25, the current Ldc is the output pulse width modulated voltage. By sampling and extracting at the timing of a specific switching state obtained from the waveform, it is possible to detect the current of one phase flowing through the three-phase induction motor 28. Furthermore, when sampling the current Ldc, if the sampling phase is sequentially switched only during the 60° period of the voltage phase peak, a sampling waveform as shown in FIG. 2(b) will be obtained. In this way, the sampling timing is synchronized with the output pulse width modulation voltage, so if the power factor is 1, the sampling waveform Is
is a waveform obtained by rectifying the three-phase current flowing through the three-phase induction motor 28, and is a continuous waveform.

However, in the case of a normal lagging power factor, the sampling waveform 5 becomes a discontinuous waveform at the time of phase switching, as shown in FIG. 2(b). The current values before and after this phase switching point are closely related to the power factor, and assuming that the three-phase current flowing through the three-phase induction motor 28 is a symmetrical positive wave, it is formulated as follows.

That is, let the current value immediately before the phase change be I^.

In addition, if the current value immediately after is I, φ is the power factor angle (delay is positive), and IM is the peak value of the current flowing through the three-phase induction motor 28, I A −I, ・cos (φ−30 ° )
−・−・−(1) IB−1y −cos (φ
+30°) −−−−−・(2). This (1
) and (2), (IAI B ) / (I A
+ I n ), tanφ−f d”
(IA -18) / (IA + IB)...
...(3) is obtained. That is, the power factor can be determined independently without depending on the current 1M of the three-phase induction motor 28. After obtaining the above-mentioned sampling waveform 15, the power factor calculation circuit 30 calculates
(3) is calculated. However, if the power factor angle φ is 9
When it reaches 0°, (IA + IB) in equation (3) becomes zero and cannot be calculated, and even if the power factor angle φ does not reach 90°, if it becomes large, the error in current detection becomes the power factor. In this embodiment, the setting range of the power factor angle command value φ flux is set to 60 degrees or less (for example, 4
5°) to limit the power factor angle φ from becoming large.

Therefore, the power factor angle φ calculated by the power factor calculation circuit 30
Since the tangent component tanφ of When the difference Δtanφ approaches zero as will be described later, a voltage correction value ΔV is outputted, and this is outputted by the adder 33.
is added to the voltage command value Vkyo to obtain the corrected voltage command value V(
−V ΔV). Here, in the case of the three-phase induction motor 28, when the ratio of voltage to frequency is increased, the excitation current increases, the phase delay of the current with respect to the voltage increases, and the power factor angle φ increases. Therefore, in the proportional-integral control circuit 32, the tangential component ta of the power factor angle φ
When nφ becomes too large than the end of the tangent component tanφ of the power factor angle command value φ bundle, the voltage correction value ΔV is decreased, and conversely,
When the tangent component tanφ becomes too small than the tangent component tanφky, the voltage correction value ΔV is controlled to be large. As a result, in the inverter main circuit 26, the output voltage is controlled by pulse width modulation so that the tangent component tanφ becomes equal to the tangent component tanφ, that is, the power factor angle φ becomes equal to the power factor angle command value φ. be done.

According to this embodiment, the following effects can be obtained. That is, the current Ldc flowing through the DC bus 25 of the inverter 27 is detected by the current detector 29, and the tangential component tanφ of the power factor angle φ is calculated by the power factor calculation circuit 30 based on the detected current I, and the inverter is controlled. The output voltage of the inverter main circuit 26 is controlled by the means 36 so that the tangential component tanφ becomes equal to the tangential component janφ of the power factor angle command value φ. Therefore, unlike conventional methods, the three-phase induction motor 2
Since power factor control is performed based on the current flowing through the three-phase induction motor 8, elements that depend on the operating speed of the three-phase induction motor 28 are no longer included, and as a result, sufficient accuracy can be obtained even during low-speed operation.
Even when the three-phase induction motor 28 is operated at low speed, the inverter 27
This enables stable driving and control. Particularly, in the case of a commonly used induction motor (28), it is well known that the torque efficiency is maximized when the power factor angle φ is around 45°. If it is controlled so that it is equal to the power factor angle command value φ of 60 degrees or less (for example, 45 degrees), high torque efficiency operation becomes possible.

Moreover, when performing such control of the power factor angle φ, it is easy to set the power factor angle command value φ0 so that the power factor angle φ does not become smaller than a predetermined value.
It is also possible to prevent the three-phase induction motor 28 from shifting to a high slip state in which the slip becomes too high above the maximum torque point under heavy loads, and stalling due to overcurrent can be prevented.

FIG. 3 shows a second embodiment of the present invention, and the same parts as those in FIG.

That is, 37 is an arctangent (tan"") calculation circuit, and its input terminal receives the tangent component tan from the power factor calculation circuit 30.
φ is given.

Therefore, the arctangent calculation circuit 37 converts the input tangent component tanφ into a power factor angle φ and outputs it from the output terminal, and this power factor angle φ is input to the negative (-) input terminal of the subtractor 31. Given. The positive (+) input terminal of the subtracter 31 is supplied with power factor angle command values φ instead of the tangent component janφ flux, and therefore the subtracter 31 calculates the difference Δφ (=φ branches − φ) is outputted from the output terminal and given to the proportional-integral control circuit 32. Then, the proportional-integral control circuit 32 adjusts the voltage correction value Δ so that the difference Δφ approaches zero.
Outputs V.

Therefore, this second embodiment also provides the same effects as the previous embodiment.

FIG. 4 shows a third embodiment of the present invention, and the same parts as in FIG.

That is, 38 is a proportional-integral control circuit replacing the proportional-integral control circuit 32, and this is configured so that the difference Δtanφ from the subtracter 31 is given to its input terminal, and it outputs a frequency correction value Δf. It has become. 39 is a subtracter, to which a frequency correction value Δf is given to its negative (-) input terminal, and a frequency command value fX is given to its positive (+) input terminal.
The frequency command value fC-f''-Δf) is output from the output terminal.The pulse width modulation control circuit 35 outputs the frequency command value fC-f"-Δf) to one input terminal. A value f is given, and seven voltage command values from the frequency-voltage conversion circuit 34 are directly given to the other input terminal.

The pulse width modulation control circuit 35 controls the inverter main circuit 26 to output a three-phase AC voltage with an output voltage and an output frequency based on the voltage command value v and the frequency command value f. It is meant to be controlled. Then, the proportional-integral control circuit 38 outputs a frequency correction value Δf such that the difference Δtanφ approaches zero.

Therefore, in this embodiment, the output frequency of the inverter main circuit 26 is controlled so that the power factor angle φ becomes equal to the power factor angle command value φ, and this also provides the same effect as the first embodiment. can get.

The third embodiment is particularly effective when it is necessary to increase the power factor angle φ when the output voltage reaches its maximum value, and therefore it can be used in combination with, for example, the first embodiment. and use the first embodiment when the speed is below the base low speed,
A configuration may also be adopted in which the third embodiment is switched to be used in a constant output operating state exceeding the base low speed.

[Effects of the Invention] As explained above, the AC motor control device of the present invention has the following features:
The power factor is calculated based on the current flowing through the DC bus of the inverter, and the output voltage or output frequency of the inverter is determined from the calculated power factor, so the AC motor can be operated stably even during low speed operation. It can be driven by an inverter, and has excellent effects such as high torque efficiency operation and operation that can prevent transition to a high slip state.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention, FIG. 2 is a current waveform diagram for explaining the operation, and FIGS.
The figures are views corresponding to FIG. 1 showing second and third embodiments of the present invention, and FIG. 5 is a view corresponding to FIG. 1 showing a conventional example. In the drawing, 24 and 25 are DC buses, 26 is an inverter main circuit, 27 is an inverter, 28 is a three-phase induction motor (polyphase AC motor), 29 is a current detector (current detection means), 3
0 is a power factor calculation circuit (power factor calculation means), 32 is a proportional-integral control circuit, 35 is a pulse width modulation control circuit, 36 is an inverter control means, 37 is an arctangent calculation circuit, and 38 is a proportional-integral control circuit.

Claims (1)

[Claims] 1. In a multi-phase AC motor driven by a voltage source PWM inverter, a current detection means for detecting the current flowing in the DC bus of the inverter, and a phase detection means that occurs in the current waveform detected by the current detection means. It is equipped with a power factor calculation means for calculating the power factor of the AC motor from the current values before and after the switching point, and an inverter control means for determining the output voltage or output frequency of the inverter from the power factor calculated by the power factor calculation means. AC motor control device.