JPS60113693A - Controller of ac motor - Google Patents

Abstract

PURPOSE:To reduce the pulsation of a torque by controlling so that the distortion rate of the output voltage of an inverter becomes minimum in a rotating speed range corresponding to the resonance point. CONSTITUTION:When a speed range is N<Nf or NB<N, the DC voltage Ed of the input side of an inverter 4 is controlled to the prescribed value set by a voltage setter RH2, the AC voltage VOUT of the output side of the inverter 4 accords with the set value of a setter 8, and automatically controlled to be proportional to the speed N. When the speed N passes the range of Nf<=N<=NB, the contact of the switch 15 is switched to the reverse opened and closed states to the shown state. Thus, a signal compared with a carrier triangular wave TW is switched to the prescribed reference set by a setter RH1 by a pulse width modulator 11 at the inverter 4 side. When this reference is set to minimize the distortion rate of the output voltage waveform of an AC power source VVVF, the pulsating torque of the motor 6 can be reduced.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a control device for an AC motor, more specifically, a rectifier having a DC voltage control function that converts AC power into DC power, and a rectifier that converts AC power into DC power, and A pulse width modulation controlled inverter that converts DC power into variable frequency AC power and supplies it to an AC motor; The present invention relates to a control device for an AC motor, including a control section that controls the output frequency of the inverter.

[Technical background of the invention]

The above-mentioned rectifier and inverter together constitute a variable voltage/variable frequency direct current helix (hereinafter referred to as VVV'), which is used to drive an AC motor at variable speed. When using the VVVF to drive an AC motor, pulsating torque occurs due to the fact that the output waveform of the VVVF is not a sine wave.

On the other hand, a resonant frequency always exists in a mechanical system as a load connected to an electric motor. (In the case of pumps, fans, etc., the resonance frequency is relatively low, so if the AC motor that drives those mechanical loads is driven with VVVF, the frequency of the pulsating torque mentioned above will increase while accelerating to the operating rotation speed.) A coincidence point may appear between the resonance frequency of the mechanical system and the resonance frequency of the mechanical system, and the resonance of the mechanical system may be promoted.

In general, VVVF output voltage control methods are broadly divided into methods that control the DC voltage value through phase control of the rectifier in the front stage, and methods that control the energizing pulse width using an inverter in the rear stage.
1- I can do something. FIG. 1 shows a control section constructed according to the latter method.

In FIG. 1, a VVVF is constituted by a rectifier 1, a smoothing circuit consisting of an accelerator 2 and a capacitor 3 for smoothing its escape output, and an inverter 4 provided on its output side, and its AC output is It is supplied via an output transformer 5 to an AC motor 6 (for example, an induction motor). The AC motor 6 is assumed to drive a mechanical load (not shown), such as a pump or fan.

Here, the output abrasive pressure of the inverter 40 is detected by the voltage detector 7 on the secondary side "fTx" of the output transformer 5, that is, on the input side of the AC motor 60, to obtain an output voltage signal (2).

The voltage frequency setter 8 generates a 'voltage reference signal (2)' and a frequency reference signal (2) f. Frequency reference signal ■,
is converted via the voltage/frequency converter 9 into a pulse train CI) having a frequency proportional thereto. This pulse train CP is input to the carrier wave generator 10 and converted into a carrier triangular wave TW having a frequency that is an integral multiple of the set frequency. Second
The figure shows a carrier triangular wave T with a frequency six times the set frequency.
W is exemplified. Wake-up pressure reference signal ■1 and output voltage signal ■. A signal corresponding to the deviation of is obtained as the output signal I of the operational amplifier OAI. This signal ■1 and carrier wave generator 10
As a result of the comparison with the carrier triangular wave TW, the pulse width modulation circuit 11 generates a gate signal PW that determines the switching timing of each switching element constituting the inverter 4. This gate signal PW is supplied to the switching element via an amplifier 12.

FIG. 2 exemplifies the relationship between the switching timing of each switching element constituting the impark 4 and the output 'F (pressure).As already mentioned, the carrier triangular wave TW is It has a frequency that is six times the frequency of the side voltage.This triangular wave TW and operational amplifier OA
By switching each switching element based on the signal v from I, the inverter output phase voltage
. -o+VV-o.

VW-O is obtained, and as a difference between the former two, a rectangle-shaped interphase voltage VU-V is obtained between the U and V phases. This is a waveform with an off period of .

Assuming that this energization off period is 2θ, the relationship between the voltage ■=VU−V and θ, that is, the voltage control angle, can be expressed by the following equation (1).

K, 1　π　11π　.

V=-7sin T CO5-g (sin -g -
s+nθ)Ed (1) Here, K is a constant, n is the order of the harmonic component, and E4 is the input DC voltage.

Figure 3 shows the n-th harmonic E calculated according to formula (1).
θ” for n (n21, 5, 7, 11)
't211)0. The figure also shows the distortion factor Yn. This distortion factor Y is a value determined according to the following.

As can be seen from Figure 3, each harmonic component E and distortion factor Y
n depends on the voltage control angle θ. When starting an AC motor from VVVF vc, the voltage to frequency ratio is generally 1.
In order to adopt a control method that keeps the voltage constant, the frequency f is increased (the voltage control angle θ is decreased, and the output voltage
Once ax is determined, the value of the voltage control angle θ at the rotational speed on the way to that value will also be uniquely determined. However, in that case, the distortion factor Yn and harmonic components at the voltage control angle θ at a rotational speed corresponding to the resonance frequency of the mechanical system are not necessarily minimized.

[Problems with background technology]

In addition, as a countermeasure to the resonance phenomenon of the mechanical system caused by the coincidence of the frequency of the pulsating torque of the current motor and the resonance frequency of the mechanical system, conventional methods have been to increase the mechanical strength of the mechanical system to withstand the resonance phenomenon. The main circuit configuration is such that the output voltage waveform is improved by increasing the output voltage or by multiplexing inverters to reduce the pulsating torque. However, these measures
In both cases, the cost is high and the external size of the device is also large.

[Purpose of the invention]

The present invention has been made in consideration of the above circumstances, and by reducing the torque pulsation generated by the VVVF at a rotation speed corresponding to the resonance point of the mechanical system, the mechanical strength of the mechanical system and the main circuit configuration of the VVVF can be improved. An object of the present invention is to provide a control device for an AC motor that can reduce design constraints.

[Summary of the invention]

In order to achieve the above object, the present invention has been developed so that the distortion factor of the output voltage of the inverter is at least 1 in the rotational speed region of the AC motor corresponding to the resonance point of the mechanical system consisting of the AC motor and the mechanical load connected thereto. The present invention is characterized in that it is provided with distortion rate control means for controlling the distortion rate.

[Embodiments of the invention]

FIG. 4 shows an embodiment of the present invention.

In FIG. 4, circuit elements that are the same as those in FIG. 1 are designated by the same reference numerals as in FIG. In the device shown in FIG. 4, a rectifier 21 having a voltage control function, that is, consisting of a controlled rectifying element such as a thyristor, is provided in place of the rectifier 1 without a voltage control function. A circuit 13 is provided. Phase control circuit 1
3 is a switch 15 that switches according to the output of the oscillator 9;
The deviation between the voltage reference signal ■□ from the setting device 8 and the actual DC voltage signal ■4 detected by the DC voltage detector j4, or the constant reference voltage and output voltage set by the voltage setting device RH2. It receives the output of the operational amplifier OA, which calculates the deviation from the voltage signal 2 of the detector 7 or p, and operates so that it becomes zero. Modulation circuit 11
A contact of the switch 15 is inserted into one input side of the switch 15, and depending on whether the contact is turned on or off, a constant reference voltage set by the voltage setting device ILI-11 or a signal from the operational amplifier OAI is selectively guided. Based on the frequency reference signal f, the switch 15 sets the contact state as shown in the rotational speed section corresponding to the resonance point of the mechanical system, that is, the region of N<Nf and NB<N in FIG. In the speed section N, ≦N≦NB, the contact state is opposite to that shown.

In the apparatus shown in FIG. 4 configured as described above, when the speed range is N (Nf or NB-N), the speed range shown in FIG.
The DC type 1 voltage E on the input side of the inverter 40 is controlled to a constant value set by the voltage setting devices R and I (2), and the AC voltage vou'r on the output side of the inverter 4 is set by the setting device 8. The voltage control angle θ is automatically controlled in accordance with the value in proportion to the speed N. As shown in the figure, the voltage control angle θ is a decreasing function.

When the speed N passes through the region Nf≦N≦NB, the contacts of the switch 15 switch to the open/close state opposite to that shown in the figure, and accordingly, the pulse width modulation circuit 11 on the inverter 4 side
The signal that is compared with the carrier triangular wave TW is the voltage control signal■■
to a certain standard set by the voltage setting device RHI. If this standard is set to minimize the distortion rate of the output voltage waveform of VVVF, the pulsating torque of the AC motor 6 can be reduced. On the other hand, rectifier 2
On the 1 side, the voltage reference is switched from the constant reference by the setting device RH2 to the voltage reference V by the voltage frequency setting device 8, and the voltage feedback signal also changes from the DC voltage 4 to the output voltage 2.
and the ratio of output voltage to output frequency is V7. Control is performed to keep the value constant. This is expressed in FIG. 5 by making the characteristic line of the output pressure vou'r a straight line with the speed N as the horizontal axis.゛: The pressure control angle θ is a constant value in this section, and is fixed so that the distortion rate of the output voltage is minimized.

In the embodiment described above, as shown in FIG.
Although the case has been described in which pulse width modulation control is performed under an output voltage waveform that has two off periods within the energized period, the present invention is not limited thereto (although various modifications are possible). Needless to say.

Note that if harmonics of a specific order become dominant to torque pulsation, such as when the resonant rotational speed of the mechanical system is a number of times the harmonic of a specific order of the output voltage, it is not the distortion factor (, The output voltage waveform may be controlled by fixing the voltage control angle θ so that the harmonic component of the specific order is reduced.

〔Effect of the invention〕

As described above, according to the present invention, in the VVVF that controls the rotational speed of an AC motor by controlling the output voltage and output frequency, the inverter outputs in the speed region corresponding to the resonance point of the load mechanical system of the AC motor. By controlling the voltage distortion factor to a minimum, it is possible to reduce the influence of pulsating torque on the mechanical system, thereby reducing the mechanical strength of the mechanical system and design constraints of the VVVFO main circuit configuration. can do.

[Brief explanation of drawings]

Figure 1 is a block diagram of a conventional control device, Figure 2 is a time chart for explaining the output voltage waveform of an inverter, and Figure 3 is a diagram showing the relationship between voltage control angle, harmonic components, and distortion rate. , FIG. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a diagram showing the characteristics of control performed by the device shown in FIG. 4. 4... Inverter 6... AC Electric motor, 8... Voltage frequency setting device, 9... Voltage/frequency converter, 10.
...Carrier wave generator, 11...Pulse width modulation circuit, 13
...Phase control circuit, 15...Switcher, OAI, OA
2...Operation amplifier, ItHl, RH2...Voltage setting device. Applicant's agent Kiyoshi Inomata

Claims (1)

[Claims] 1. A rectifier that converts AC power into DC power and has a DC voltage control function, and a pulse width modulation that converts the DC power from the rectifier into variable frequency AC power and supplies it to an AC motor. In a control device for an AC motor, comprising a control type inverter, and a control unit that controls the output DC voltage of the MiJ rectifier and the output frequency of the inverter according to the magnitude and frequency of the AC voltage to be output from the inverter. , further comprising a distortion rate control means for controlling the distortion rate of the output voltage of the inverter to be minimized in a rotational speed region of the AC motor corresponding to a resonance point of a mechanical system consisting of the AC motor and a mechanical load connected thereto. A control device for an AC motor, characterized by: 2. The alternating current according to claim 1, wherein the distortion rate control means is set so that a harmonic component of a specific order of the output voltage of the inverter is minimized in the rotational speed region. Electric motor control device.