JPS5886872A - Variable-voltage variable-frequency power source - Google Patents

Abstract

PURPOSE:To prevent an electric motor from being burnt by an overcurrent at low speed without any reduction in torque of the motor at low speed, by varying an overcurrent setting signal in accordance with a speed command signal. CONSTITUTION:A function generator 15 into which a speed command signal from a speed setting device is fed is connected to the input side of an overcurrent setting device 7. The function generator 15 receives the speed command signal and dilivers an overcurrent setting signal regulated in consideration of allowable current-time characteristics in accordance with the number of revolutions of an electric motor 5 and so that a current demanded by an ordinary load will sufficiently be supplied. A comparator 8 and an integrator 9 perform a comparison as to the magnetudes between the overcurrent setting signal set by the overcurrent setting device 7 and a motor current feedback signal, and integrate the difference therebetween.

Description

[Detailed Description of the Invention] 1) Technical Field of the Invention The present invention relates to a variable voltage variable frequency power supply device, and particularly relates to an effective overcurrent detection circuit when the main purpose is to drive an AC motor at variable speed. The present invention relates to a variable voltage variable frequency power supply device.

2)-1 First Prior Art An example of a conventional variable voltage variable frequency power supply device is shown in the block diagram of FIG. In the figure, 1 is a rectifier that rectifies the AC power supply, 2 is a smoothing capacitor that smoothes the DC voltage that is the output of the rectifier 1, 3 is an inverter that converts the DC voltage to AC, and 5
4 is a current detector that detects the output current of 3, 6 is a rectifier that converts the output of current detector 4 to DC, 7 is a motor 8 is a comparator that centers the output of rectifier 6 and the output of overcurrent setter 7, and 9 integrates the output of comparator 8 to determine the overcurrent. An integrator that outputs a signal OL, IO is a speed setter for a speed command signal, 1
1 is a voltage frequency conversion circuit that generates a signal with the same wave number according to the speed command signal, U is a ring counter that converts the output signal of the voltage frequency conversion circuit 11 into a pulse train, and 13 is a speed command signal that converts the output signal of the ring counter U. A pulse width modulation circuit 14 converts the signal into a pulse width modulated signal based on the pulse width modulation signal, and 14 is a drive circuit that drives each element of the inverter 3 based on the pulse width modulation signal and prevents a signal from being applied to the inverter 3 based on the overcurrent signal OLK. It's an amplifier.

In this configuration, AC power input from a power supply means (not shown) is rectified by a rectifier 1 and converted into a DC voltage, which is then smoothed by a smoothing capacitor 2. Inverter 3
A smoothed DC voltage is supplied to the motor 5, and the DC voltage is supplied to the electric motor 5 after being subjected to AC fILK conversion of a variable frequency variable voltage.

On the other hand, the speed command signal from the lotus setting device 10 is connected to a voltage frequency conversion circuit 11, where the speed command signal is converted into a pulse train of the same wave number proportional to the value and inputted to a ring counter 12. The ring counter converts the input pulse into a pulse train with the number of phases corresponding to the number of load phases (six phases in the case of a three-phase motor), and inputs this pulse train to the pulse width modulation circuit 13 together with the speed command signal. The pulse train input to the pulse width modulation circuit 13 is modulated into a pulse train with a pulse width proportional to the speed command signal, and serves as a signal source for each switching element constituting the inverter 3. The output of the pulse width modulation circuit 13 is the drive width i! of each switching element constituting the inverter 3. ) 14 to the inverter 3.

Further, the output of the current detector 4 is rectified by a rectifier 6 and input to a comparator 8. An overcurrent reference, which is the output of the overcurrent setter 7, is connected to one input terminal of the comparator 8, and the obtained difference is integrated by an integrator 9. As a result, when the cumulative value of the difference reaches a certain value, an overcurrent signal is generated.
OL is generated and blocks the output of the drive amplifier 14.

The conventional variable voltage variable frequency power supply device configured as described above uses a speed command signal from a speed setter 10 to generate a pulse with a frequency corresponding to the speed command signal by a voltage frequency conversion circuit 11, and converts this pulse into a ring. The counter U distributes the signals into multi-phase signals as respective signal sources for each switching element constituting the inverter 3, and the pulse width modulation circuit B modulates the pulses with a pulse width according to the speed command signal. The drive amplifier 14 performs isolation amplification to drive each element of the inverter 3. As a result, electric power with a frequency and voltage according to the speed command signal is supplied to the electric motor 5, which is a load, and the electric motor 5 is controlled at variable speed.

On the other hand, the current supplied from the inverter 3 to the motor 5 is detected by a current detector 4, converted to DC by a rectifier 6, and then input to a comparator 8, where it is compared with the output of an overcurrent setting device 7. The resulting difference is integrated by an integrator 9,
When the output of the integrator 9 becomes constant and reaches pel, the drive amplifier 14
It prevents the output from reaching the inverter 3 and outputs it to the outside as an overcurrent signal OL. As a result, the electric motor 5 is protected and this can be known from the outside. Furthermore, in this case, the greater the difference between the current of the motor 5 and the overcurrent setting signal, the earlier the overcurrent signal OL appears, so that characteristics similar to those of a thermal overcurrent relay can be obtained.

2)-2 Disadvantage of the first conventional technology However, when the electric motor 5 is driven at variable speed, the cooling effect due to the rotation of the electric motor 5 is different depending on whether the electric motor 5 is rotated at low speed or high speed. The allowable motor current for the motor must be smaller than that at high speed. However, in the conventional variable voltage variable frequency power supply device shown in FIG. 1, the overcurrent setting signal is constant regardless of the speed command signal and is given based on the rated current of the motor 5. The disadvantage was that it could not protect the

3) Second prior art and its drawbacks On the other hand, as another example of the prior art, a variable voltage variable frequency power supply device with a minor loop of current control is known, but this This method employs a method of keeping the current limit value at low rotations low by changing the l peak value of the current reference input of the current control amplifier.
In this case, since the torque at low rotations is also suppressed, there is a risk of insufficient starting torque.

4) Purpose of the Invention Therefore, the purpose of the present invention is to eliminate the drawbacks of the prior art mentioned above,
To provide a variable voltage variable frequency power supply device capable of effectively protecting a load by preventing burnout due to overcurrent at low speeds without reducing the torque of a motor at low speeds.

5) Structure of the Invention In order to achieve the above object, the present invention comprises a variable voltage variable frequency power supply device including an inverter that converts direct current into alternating current with variable frequency and variable voltage and supplies it to a load, and a load current a drive circuit for driving the inverter with a control signal corresponding to the speed command signal; a means for generating a reference signal having a predetermined functional relationship with respect to the speed command signal; calculation means for calculating the difference;
It consists of an integrating means for integrating the difference signal from the calculating means, and a means for stopping the inverter based on the output of the integrating means.

6) Examples of the invention Hereinafter, the present invention will be described in detail with reference to the drawings.

<Configuration> FIG. 2 is a block diagram of a variable voltage variable frequency power supply device according to an embodiment of the present invention. The difference between this embodiment and the conventional example shown in FIG. This is because a function generator which inputs the speed command signal from the speed setting device 10 is connected. This function generator is an overcurrent setting device that inputs the speed command signal, takes into account the allowable current hourly characteristics depending on the rotation speed of the motor 5, and is adjusted to supply enough current required by the normal load. This outputs the number.

FIG. 3 is a circuit configuration diagram showing an example of a detailed circuit of the function generator 15, overcurrent setter 7, comparator 8, and integrator 9 shown in FIG. It is connected to a negative input terminal of an operational amplifier 155, but this negative input terminal is separately connected to a negative power supply via a resistor 152. On the other hand, a parallel circuit consisting of a resistor 153 and a constant voltage diode 154 is connected between the output terminal of the operational amplifier 155 and its negative input terminal. The positive input terminal of operational amplifier 155 is grounded. The output terminal of the operational amplifier 155 is connected to one end of a variable resistor 7' for setting an overcurrent reference, and the other end of the variable resistor 7' is grounded.

The intermediate terminal of the variable resistor 7' is connected to the negative input terminal of the operational amplifier via a resistor.

A resistor 81 is separately connected to this negative input terminal, and the other end of this resistor 81 is grounded. On the other hand, the feedback signal IF of the current of the motor 5 is connected to the positive input terminal of the operational amplifier via the resistor section, but a resistor system is separately connected to this positive input terminal, and this resistor is connected to the positive input terminal. The other end of is grounded. A series circuit of a resistor 91 and a capacitor 92 and a voltage diode circuit are connected in parallel between the negative input terminal and the output terminal of the example operational amplifier.

Note that the output of this operational amplifier becomes the overcurrent detection signal OL.

In other words, the function generator 15 is configured by the resistors 151 , 152 , 154 , the constant voltage diode 153 , and the operational amplifier 155 , and the resistors 81 , 82 , 83 , 84
The comparator 8 is constructed by the resistor 91, the capacitor 92, the constant voltage diode circuit, and the integrator 9 by the example of the operational amplifier.

<Effect> In this configuration, the operation is shown in FIG. 4(a).

This will be explained according to the characteristic diagrams (b) and (c). By the way, Fig. 4 ((r) is a characteristic diagram showing the relationship between the speed command signal VB, which is the input of the function generator 15, and the output signal v1, and Fig. 4 et al.) shows the relationship between the current value and time at the time of overcurrent detection. FIG. 4(e) is a characteristic diagram showing the relationship between the speed command signal VB and the overcurrent detection time TOL.

Since the operational amplifier 1550 input is the speed command signal VB (negative polarity) and a constant negative voltage -V, the resistance 151°152
, the resistance value of 154 is 1Rtst, R,
Rm, the output signal ■1 of the operational amplifier 155 is RI
B4, vRRtsa ” to -””R'TiT-Ii”) ”(1
), as shown in FIG. 4(a), when the speed command signal vR is 101, the output signal V! becomes a constant positive voltage, and as the speed command signal VR increases to the negative side, the output signal V
1. increases, and when the output signal Vx approaches the voltage clamped by the constant voltage diode 153, the output signal v1 does not increase even if the speed command signal VB increases beyond that point, the output signal V! of the operational amplifier 155! That is, since the output of the function generator 15 becomes the input voltage of the overcurrent setting device 70, the output of the overcurrent setting device 7 is proportional to the output signal VIK of the operational amplifier 155.

On the other hand, the comparator 8 and the integrator 9 function to compare the magnitude of the overcurrent setting signal from the overcurrent setting device 7 and the motor current feedback signal IF and integrate the difference between them. The signal is In, and to simplify the explanation, Resistor Co. and U.

Assume that the resistance values of the wings are equal. In this case, while the overcurrent setting signal Iii is larger than the motor current feedback signal Iy, the output of the operational amplifier 94 becomes negative, and the forward drub voltage of the constant voltage diode circuit is approximately -1.
It is clamped to a voltage below v. On the other hand, electric stacking +31
When the feedback signal IF becomes larger than the overcurrent setting signal Ill, the output of the operational amplifier becomes positive, and the capacitor 92 passes through the resistor 91 to reach an equivalent input resistance determined by the resistance value of the comparator 8.

It is charged with a time constant determined by the capacitance of the capacitor 4 and the capacitor 4.

The speed of voltage rise due to this charging is proportional to the voltage difference between the motor electric fly pack signal IF and the overcurrent setting signal IR. When the output of the operational amplifier 94 becomes a constant positive voltage, the overcurrent detection signal OL is determined to be a signal 1 or 1, and the overcurrent detection signal OL is detected. In this case, the relationship between the time until the overcurrent detection signal OL is detected and the current of the motor 5 is inversely proportional as shown in FIG. This also applies to the conventional overcurrent detection circuit 9. The key point of the present invention is to change the overcurrent setting signal by the speed product command signal, and the overcurrent setting signal In is changed from the output signal V in FIG. 4 (Jl) with respect to the speed command signal VB.
Since the relationship is the same as that of t, when the speed command signal Vm is a low value, the input differential voltage of the operational amplifier increases for the same motor current, so the rising speed of the output voltage of the operational amplifier becomes faster and overcurrent occurs. The time it takes for the detection signal oLy to reach its peak becomes faster. That is, in the case of the same motor current, the relationship between the detection time TOL of the overcurrent detection signal OL and the speed command is as follows. However, this is true only in the section where the speed command signal VR and the overcurrent setting signal IR are proportional. Also, K.
Xl and Kg are constants.

<Effect> Therefore, the overcurrent detection time TOL with respect to the speed of the motor 5
The relationship is as shown in FIG. 4(C), and the lower the rotational speed of the electric motor 5, the shorter the overcurrent detection time TOL becomes, thus making it possible to prevent overloading of the electric motor at low rotational speeds.

7) Modification of the invention In the configuration shown in FIG. 3, the characteristic of the function generator 15 is 1.
Although the case of a polygonal line characteristic having individual breakpoints has been exemplified, it is of course possible to have an arbitrary functional characteristic depending on the motor characteristics. Further, in the embodiment shown in FIG. 2, a case has been illustrated in which the current of the motor 5 is detected on the AC side, but it is also possible to perform this on the DC side and detect the peak value.

8) Effects of the Invention As described above, according to the present invention, by changing the overcurrent setting signal of the electronic overcurrent protection circuit in accordance with the speed command signal, the tolerance corresponding to the rotational speed of the motor can be adjusted during low-speed operation of the motor. It is possible to perform overcurrent detection within the allowable range of current vs. time characteristics, and as a result, the overcurrent detection time is accelerated and shortened in accordance with the decrease in allowable current due to the decrease in cooling effect at low rotation speeds of the motor. By not changing the time current limit value, it is possible to obtain a new variable voltage variable frequency power supply device that can prevent motor burnout due to overcurrent at low rotation speeds without reducing starting torque at low rotation speeds. be.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional variable voltage variable frequency power supply device, FIG. 2 is a block diagram of a variable voltage variable frequency power supply device according to an embodiment of the present invention, and FIG. 3 is a partial configuration of FIG. 2. Circuit configuration diagram showing details, Figure 4 (a), (b)
, (c) is a characteristic diagram illustrating the operation of the configurations shown in FIGS. 2 and 3. DESCRIPTION OF SYMBOLS 1... Rectifier, 3... Inverter, 5... Electric motor, 4... Current detector, 10... Speed setting device, 14...
... Drive amplifier, 7... Overcurrent setter, 8... Comparator, 9... Integrator, 15... Function generator. Applicant's agent Kiyoshi Inomata 1F, 1 Figure '#h20 City 3 Figure City 4

Claims (1)

[Claims] 1. An inverter that converts direct current into alternating current with a variable frequency and variable voltage and supplies the converted alternating current to a load, and means for detecting load current;
A drive circuit that drives an inverter with a control signal corresponding to a speed command signal, a means for generating a reference signal having a predetermined functional relationship with respect to the speed command signal, and a means for generating a difference between the reference signal and a load current. 1. A variable voltage variable frequency power supply device comprising a calculation means, an integration means for integrating a difference signal from the calculation means, and a means for stopping an inverter based on the output of the integration means. 2. The reference signal is adjusted to a functional relationship in which the speed command signal tends to have a small value when commanding low speed and a large value when commanding high speed. Variable voltage variable frequency power supply. 3. The reference signal is adjusted so that when the speed command signal is below a certain value, it is approximately proportional to the speed command signal, and when the speed command signal is greater than a certain value, it is a substantially constant signal. A variable voltage variable frequency power supply device according to claim 2.