JPS62152392A - Pwm inverter current control unit - Google Patents

Abstract

PURPOSE:To compensate distortion in and around a current zero zone caused by dead time by forming a comparison level voltage into rectangular waves synchronized with the triangular wave carrier frequency. CONSTITUTION:The output ec is outputted from a triangular wave carrier frequency oscillator 23, an operational amplifier 24 and a capacitor 25. A frequency demultiplier 26 divides by one half the voltage comparator output frequency of the triangular wave carrier frequency oscillator. A variable resistance 27 varies the output of the frequency demultiplier 26 to meet the length of the dead time of the power transistor base signal and outputs the comparison level voltage 2. The current can therefore be controlled in the large PWM modulation width even in and around the current zero zone. Consequently, except in the region where the PWM modulation width is small and apt to be affected by the on-delay due to the dead time, the current can be so controlled that the distortion of waveform in and around the zero zone of the motor current can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a current control device for a PWM (Pulse Width Modulation) inverter for driving an electric motor.

[Conventional technology]

Inverter devices that drive AC (alternating current) servo motors and DC (direct current) servo motors generally amplify the deviation between an AC or DC current command and a feedback current using a current amplifier, and convert this amplified current deviation into a triangular wave carrier signal. performs pulse width modulation and converts it to the on/off period of the carrier signal.

This on/off signal becomes, for example, a drive signal for power 1 to transistors in the PWM inverter shown in FIG. 3 to which the present invention is applied.

Power transistors have an accumulation time and a turn-off time, and even if the drive signal is turned off, current continues to flow for a certain period of time.

Therefore, in the configuration of power 1 to transistor as described above, a short circuit between the upper and lower arms may occur. As a countermeasure against this problem, an on-delay is generally provided for each power transistor drive signal to delay the rise time.

[Problem that the invention seeks to solve]

This on-delay causes waveform distortion near zero of the pulse width modulation controlled motor current.

As a result, drizzle, rubs, and current dead zones occur, making it impossible to control the motor with high precision.

In order to prevent current distortion due to this dead time, there is a method of feeding back the output voltage of the inverter and adding a voltage loop in addition to the current loop. This is because the output voltage of the inverter is fed back, so it is necessary to insulate the feedback voltage, and the circuit configuration for insulation is complicated.

The above-described compensation method for improving current distortion has disadvantages in that the circuit configuration is complicated and adjustment is required depending on variations in the elements of the discrete transistors and temperature.

On the other hand, there is a measure to increase the current loop gain near zero current. For example, by inserting a diode in series with the current amplifier feedback resistor, and utilizing the fact that the diode is non-conducting below the forward drop of the diode (approximately 0.6 V for silicon diodes), during this time an operational amplifier (operational amplifier 2!t) is used. The feedback resistance becomes equivalently high resistance, and the gain of the current amplifier becomes high.

There is also a method of nonlinearizing the edge of the PWM triangular wave carrier frequency to equivalently increase the current amplifier gain near zero current.

In the former case, since the amplifier gain can be controlled when the diode is not conducting, the stability of the current control system deteriorates. In addition, the latter has a problem in that when the slope of the nonlinear portion is reduced, the comparator oscillates, and there is a region in which it is impossible to obtain a high gain.

That is, in the above-mentioned method of compensating for current waveform distortion, (1) the circuit configuration is complicated;

■ The optimum value changes depending on element variations and temperature.

■ An unstable region of the system occurs.

There were problems such as:

SUMMARY OF THE INVENTION An object of the present invention is to provide a PWM inverter current control device that overcomes the drawbacks of the conventional example and has a simple circuit configuration and a stable control system.

[Means for solving problems]

The present invention includes a current amplifier that amplifies the deviation between the current command and the feedback current, a chirelia signal generating means and a comparator that pulse width modulates the deviation signal, and an on-delay circuit provided to prevent short circuit between the upper and lower arms of the transistor. , a PWM inverter current control device that has a transistor and a drive circuit that power amplifies a pulse width modulated signal, and controls a motor current by pulse width modulating an alternating current or direct current. The method is to apply a rectangular wave that changes as a bias voltage.

[For production]

Compensates for current waveform distortion around zero current due to on-delay time.

〔Example〕

FIG. 3 is a PWM current control inverter to which the present invention is applied, and is a block diagram of a circuit configuration showing an example of DC motor control.

DC motors usually have a speed detector (not shown).

The speed is controlled by the speed amplifier and the current control inverter shown in FIG.

Reference numeral 1 designates a PWM power circuit, which includes power transistors 3 to 6, a DC motor 2, a capacitor 7, a current detector 17, and the like. Further, the means consisting of members 8 to 16 is a current control circuit, 8 to 12 are resistors, and 1 is a current control circuit.
3 is an operational amplifier, 14 is a voltage comparator, 15 is a triangular wave carrier oscillator, and 16 is a base drive signal circuit.

Conventionally, the comparison level voltage e of the voltage comparator 14 was always zero, but in the present invention, the comparison level voltage e is set to the triangular wave carrier frequency e. This is a means of compensating for distortion near zero current due to dead time by creating a rectangular wave synchronized with the current.

FIG. 1 is a block diagram showing the main circuit configuration of an embodiment of the present invention.

In all drawings, the same reference numerals represent the same or corresponding parts.

15 is a triangular wave carrier frequency oscillator output e. The means for generating is the same as the conventional one, 19 to 22 are resistors, 23 is a voltage comparator (operational amplifier), 24 is an operational amplifier, and 25 forms an integrator.

The present invention consists of 26 frequency dividers, 27 variable resistors and their output e.

The frequency divider 26 may be constructed of a flip 70 circuit using a gate circuit or a bistable circuit using a transistor.

This frequency divider 26 divides the voltage comparator output frequency of the triangular wave carrier frequency oscillator by 1/2.

Further, the variable resistor 27 varies the output of the frequency divider 26 according to the size of the dead time of the power transistor pace signal. Note that this value does not need to be accurate, as will be described later, and only needs to be larger than the dead time.

FIG. 2 is a time chart of voltage waveforms at various parts of the present invention.

(a) shows the triangular wave carrier frequency, and the current deviation output voltage r is used as the comparison voltage of the voltage comparator 14. and compensation comparison voltage e.

(b) is the output voltage of the voltage comparator 23.

(C) shows the output voltage of the frequency divider 26.

(d) is the output of the voltage comparator 14.

The comparison voltage of the voltage comparator 14 is 18°ec in (a), and e
If the voltage of is zero, the base drive signal is (d
) is the solid line of the comparator 14, and the on time of the base signal is τ.

By the way, the on-delay time due to dead time is Δτ
d, and even in the case of Ie-0, the base drive signal is τ
becomes.

Therefore, when the modulated signal, that is, Io in this case, becomes small and the modulation width becomes smaller than the on-delay, only the current can be controlled.

Here, consider the case where a compensation voltage e is added.

(a>, the comparison voltage I of the comparator 14.

The relationship between e and e is equivalently shown by a dotted line (Ie+e)
Compare the voltages of and ec.

The base drive signal is a dotted line shown in (d).

The on time of the base drive signal is τ−2Δ between 11
t, and τ+2Δτ between t2.

Therefore, 1. +12 is the average value τ of the on time of the base drive signal. h = τ, and the change in the on-time of the base drive signal due to the El pressure e is canceled out.

〔Effect of the invention〕

Thus, according to the present invention, by adding a compensation voltage,
Even in a region where the motor current is near zero, control can be achieved where the PWM modulation width is large.

Therefore, since the PWM modulation width is small and it is possible to control the area excluding the area susceptible to on-delay due to dead time, it is possible to improve the waveform distortion near zero of the motor current, which contributes to this field. many.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the circuit configuration of the main parts in an embodiment of the present invention, FIG. 2 is a time chart of voltage waveforms of each part, and FIG. 3 is a motor drive circuit using a PWM inverter to which the present invention is applied. It is a diagram. 1... -PWM current control inverter 2... DC motor 3-6... Power transistor 7.25... Capacitor '8-12. 19-22... Resistor 13... Operational amplifier 14.23 ... Comparator 15 ... Triangular wave carrier oscillator 16 ... Base drive signal circuit 17 ... Current detector 24 ... Integrator 27 ... Variable resistor. Applicant's agent Mr. Sato, Figure 1, Ward 3

Claims (1)

[Claims] 1. A current amplifier that amplifies the deviation between the current command and the feedback current, a triangular wave carrier generating means and a comparator that pulse width modulates the deviation signal, and a device for preventing short circuit between the upper and lower arms of the transistor of the transistor bridge. A pulse width control means for a PWM inverter that has an on-delay circuit provided for the purpose of the invention, a transistor and a drive circuit that power amplifies a pulse width modulated signal, and controls a motor current by pulse width modulating an alternating current or a direct current. A PWM inverter current control device according to the present invention, comprising means for applying a rectangular wave that changes in positive and negative directions in synchronization with a carrier frequency as a bias voltage.