JPH0332310B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a speed control device for an induction machine capable of secondary excitation control, and particularly to a control device suitable for variable speed control of pumps, fans, and the like.

[Background of the invention]

In the past, induction motors have generally been used as drive motors for pumps and fans, and Servius devices have been used for variable speed control. Figure 1 shows the circuit configuration of the device. This figure shows that the secondary voltage of a wound induction motor (hereinafter referred to as IM) 1 is converted to DC by a forward converter (diode rectifier) 2, and the secondary power converted to DC is further converted to DC using an inverse converter 4. This is regenerated into the AC power supply system. In addition, 6 in the figure is a speed detector directly connected to IM1,
7 is a speed command circuit. 8 is a speed regulator that amplifies the deviation between the signal of the speed detector 6 and the speed command signal and outputs a current command signal; 9 is the current detector 1;
1 and the current command signal from the speed regulator 8. Reference numeral 10 denotes an automatic pulse phase shifter (APPS), which is a circuit that generates a gate signal for adjusting the firing phase of the thyristor of the inverter 4 in accordance with the output signal of the regulator 9.

In this configuration, the slip of IM1 is proportional to the secondary voltage, so the DC circuit voltage V _d is proportional to this.
By adjusting the phase control of the inverter 4, the motor speed is controlled.

However, in this case, near the synchronous speed, that is, when the slip s is close to zero, the secondary voltage of IM1 is almost zero, and the DC circuit voltage V _d is also close to zero. At this time, the control angle of the inverse converter 4 is around 90 degrees, and the power factor is very poor. Furthermore, at the time of startup, a large voltage corresponding to the slip s=1 is generated on the secondary side of the IM 1, so the DC voltage of the inverter 4 also becomes large. Therefore, in order to make the variable speed range from stop to synchronous speed (0 to 100%), the inverter must be able to withstand voltage equivalent to slip s = 1 and flow current equivalent to slip s = 0. Therefore, there is a drawback that the inverter 4 and the transformer 5 have very large capacities. Therefore, it is normal to select a variable speed range of 65 to 100%.

In addition, an automatic pulse phase shifter (APPS) is used to control the phase of the inverter 4, but since the APPS controls the phase of each of the six thyristor arms with high precision, it has a complex configuration and is extremely complicated. It's expensive.

As mentioned above, in this conventional control device,
Requires a large capacity inverter and transformer,
Moreover, the power factor of the inverter is low, requiring a capacitor for power factor improvement, and the control circuit for the inverter becomes complicated.

[Purpose of the invention]

The present invention has been made in consideration of this, and its purpose is to eliminate the drawbacks of the prior art described above, that is, to make it possible to reduce the capacity for improving the power factor of an inverter, and to simplify the configuration of its control circuit. The present invention provides a control device of this kind.

[Summary of the invention]

That is, the present invention includes a self-arc-extinguishing element connected directly to a forward converter connected to the secondary side of an induction machine or via a DC reactor, and a diode connected in series in parallel with this arc-extinguishing element. A capacitor and an inverse converter connected through the capacitor are provided, and the arc-extinguishing element is used to compare the detection signal of the DC output current of the forward converter or the AC input current with the current command signal from the speed regulator. The current is controlled on and off according to the command value, and the inverter is controlled to fire at a constant firing phase with respect to the AC power supply voltage to which it is connected. It was designed to achieve the purpose of

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. In the figure, numerals 1, 2, and 6 to 8 are the same as those in FIG. 1, so their explanation will be omitted. 12 is a DC reactor for suppressing fluctuations in the output current of the diode rectifier 2, and 13 is a self-extinguishing element (GTO:
14 is a reverse current blocking diode, 15 is a capacitor, 16 is a thyristor inverter for regenerating the secondary power of the induction machine 1 into an AC power supply, and 17 is a fluctuation in the input current of the inverter. 18 is a transformer for an inverter, 19 is a current detector for detecting the output current of the rectifier 2, and 20 is a comparison between the current command signal from the speed regulator 8 and the current detection signal. death,
This is a comparator with hysteresis characteristics that outputs an on/off control signal for the GTO element 13. 8,2
Command means 30 for self-extinguishing element 13 at 0,21
is formed. 21 is a gate amplifier for supplying a gate signal to the GTO element 13, and 22 is a control circuit that controls firing of the inverter 16 at a constant firing phase.

The GTO element 13 is controlled on and off according to the output signal of the comparator 20. That is, comparator 2
0 compares the current command signal and the current detection signal,
When the latter increases by a predetermined value or more compared to the former, the GTO is turned off. Conversely, if the latter decreases below the predetermined value compared to the former, GTO
turns on. The current relationships at this time are shown in FIGS. 3a and 3b. During the ON period of the GTO, the current increases, and when the DC current increases by the hysteresis width ΔI of the comparator compared to the current command, the GTO turns off and the DC current decreases. Furthermore, when the DC current decreases by ΔI compared to the current command, the GTO turns on and the DC current increases. Like this ±ΔI
The DC current is controlled to follow the current command with a width of . Here, ΔI is designed to an optimal value by considering the GTO on/off switching frequency and current pulsation rate.

In this way, the input AC current of the diode rectifier, that is, the secondary current of the induction machine 1, is controlled in accordance with the current command signal A (FIG. 2) from the speed regulator 8. The current command signal of the induction machine is a signal generated according to the deviation between the speed command 7 and the signal B from the speed detector 6. As described above, since the DC current i _d is controlled by this current command signal, the secondary current i ₂ and torque of the induction machine are controlled to be proportional to the current command. Therefore, the rotational speed is controlled in accordance with the speed command.

Now, regarding the current flowing through the GTO element 13 and the diode 14 at this time, Figure 3 c and d
As shown. That is, while the GTO is on, direct current flows through the GTO 13, and when it is off, it flows through the diode 14.
During this time, continuity of the DC current is maintained by the reactor 12. The current in diode 14 charges capacitor 15, increasing its voltage. As a result, a difference is generated between the voltage and the DC input voltage of the inverter 16, and a direct current flows from the capacitor 15 to the inverter 16. Since the on/off operation of the GTO element is performed at a frequency of several 100 Hz to several KHz, the pulsation is absorbed by the reactor. As a result, the reactor current becomes a substantially constant and smooth current as shown in FIG. 3f. However, the rectification ripple of the inverter 16 is ignored. Since the inverter 16 is controlled by the ignition control means 22 with a constant ignition phase, its DC input voltage is approximately constant. Therefore capacitor 1
5 is also equal in average value to the voltage of the inverter and is kept approximately constant.

Now, looking at the operation of the GTO element 13 described above from another perspective, when the GTO is turned on, the DC current increases, and at this time, the magnetic energy contained in the circuit increases. Next, GTO element 13
When turned off, magnetic energy is released and charges the capacitor 15. That is, GTO 13, reactor 12, diode 14 and capacitor 1
It can be seen that the circuit formed by 5 has the effect of converting the secondary power of the induction machine 1 into a constant voltage with respect to the induction machine 1 whose secondary voltage is changed by slip. Naturally, if circuit losses are ignored, the power transferred to the inverter side will be equal to the secondary power of the induction machine.

Here, secondary power will be considered with reference to FIG. When the load system driven by an electric motor is a pump or fan, the motor torque is proportional to the square of the rotation speed, as shown in this figure. Since the torque T is proportional to the secondary current, the secondary current is also proportional to the square of the rotation speed. Since the secondary power W ₂ is approximately proportional to the product of the secondary voltage and the secondary current, its characteristics are like the W ₂ curve, and the maximum value is shown when the slip s is 1/3. Its value is small, 1/6 of the motor's output.
Furthermore, since the inverter 16 (FIG. 5) is controlled by the constant firing phase described above, the power factor is always maintained at a constant high value (0.7 to 0.8). For the above two reasons, the capacity of the inverter 16 can be reduced to about 20% of the motor output, making it possible to reduce the capacity. At the same time, the capacity of the inverter transformer 18 is also reduced.

Further, since the inverter 16 only needs to have a constant firing phase, a control circuit for varying the firing phase is not required, and the control circuit is greatly simplified. In the figure, the V ₂ curve indicates the secondary voltage, and the W ₀ curve indicates the IM output. From the above, according to this embodiment, it is possible to realize an extremely low-cost device compared to the conventional Cerbius control device.

FIG. 5 shows another embodiment of the invention. This figure differs from FIG. 2 in that a voltage detector 23 for the capacitor 15 and a voltage fluctuation suppressing circuit 24 that generates a signal based on the voltage detector 23 are provided. According to this embodiment, the following effects are obtained as compared to FIG. 2. That is, in the case of FIG. 2, the circuit formed by the capacitor 15, inverter 16, and reactor 17 is a type of oscillating circuit, so the current charging the capacitor 15 via the diode 14 causes an oscillating current in the circuit. Occur. Due to the influence of this oscillating circuit, the voltage of the capacitor 15 and the current of the inverter 16 may become excessive. In the embodiment of FIG. 2, the inverter 16 is controlled with a constant firing phase, so there is no active suppression of this vibration. When such vibration occurs, it is caused by excessive voltage on the capacitor.
Excessive voltage is applied to the GTO. Furthermore, since the current of the inverter becomes excessive, commutation failure may occur. The embodiment shown in FIG. 5 solves this problem. That is, in FIG. 5, when the voltage detector 23 detects the capacitor voltage and the value exceeds the set level, the voltage fluctuation suppressing circuit 24 adds a signal to the addition point 25 to change the current command. control to reduce the direct current.

The voltage fluctuation suppression circuit 24 also includes a capacitor 15
It may also be a circuit that detects voltage fluctuations. Such a circuit can be realized by a differentiating circuit with a predetermined time constant. At this time, when the capacitor voltage increases, the DC current is decreased, and when the capacitor voltage decreases, the DC current is increased. In this way, voltage fluctuations can be suppressed.

Note that the induction machine 1 is not limited to an ordinary wound motor with a brush or a slip ring, but may be any motor capable of secondary excitation. In other words, there is a brushless induction machine in which two induction machines are connected in cascade, stator windings with different numbers of poles are wound around a common stator core, and the rotor bar is wound with the sum of the number of poles of the two stator windings. A brushless induction machine with a special configuration may also be used.

Further, the speed detector for speed control is not limited to a detector attached to the rotating shaft, and the speed can also be detected based on the secondary frequency of the induction machine. In other words, the rotation speed N is N = 120 ₁ / P (1 - s) = 120 / P ( ₁ - ₂ ) However, since P is the number of poles, s is slip, ₁ is the primary frequency, and ₂ is the secondary frequency, N can be calculated and detected from the fundamental frequency of the secondary voltage. According to this method, a speed detector such as a rotational pulse generator is not required, and a cheaper Serbius device can be realized.

Further, in the above-mentioned embodiment, the DC output current of the diode rectifier 2 was detected by the current detector 19, but the same control can be performed by detecting the magnitude of the AC input current of the rectifier. it is obvious.

〔Effect of the invention〕

As explained above, according to the present invention, since the inverter has a circuit configuration that can control the inverter with a constant firing phase, the capacity of the inverter and the transformer can be reduced, and the power factor of the inverter can be improved. Furthermore, the control circuit of the inverter can be simplified.

[Brief explanation of drawings]

Fig. 1 is a wiring diagram showing a conventional speed control device for an induction motor, Fig. 2 is a wiring diagram showing a speed control device for an induction motor according to the present invention, and Fig. 3 is a wiring diagram showing a speed control device for an induction motor according to the present invention. FIG. 4 is a waveform diagram of the secondary current, FIG. 4 is a characteristic curve diagram thereof, and FIG. 5 is a wiring diagram showing another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Induction motor, 2... Forward converter, 6... Speed detector, 12... DC reactor, 13... Self-extinguishing element, 14... Diode, 15... Capacitor, 16
... inverse converter, 17... DC reactor, 19... current detector, 30... command means, 22... ignition control means.

Claims (1)

[Claims] 1. An induction machine in which the secondary side of the induction machine is connected to an AC power supply via a forward converter and an inverse converter, and speed control is performed while regenerating the secondary power of the induction machine to the AC power supply. In the speed control device, a self-extinguishing element is connected between the DC output terminals of the forward converter directly or via a reactor in the coupling circuit of the forward converter and the inverse converter, and A series circuit of a diode and a capacitor is connected between the terminals of the arc-shaped element, and the inverse converter is further connected between the terminals of the capacitor,
The self-arc-extinguishing element is controlled to be turned on and off in order to control the output current of the forward converter according to the deviation between the detected value of the rotational speed of the induction machine and its command value. Induction machine speed control device. 2. The output current of the forward converter is controlled in accordance with the deviation between the detected value of the rotational speed of the induction machine and its command value, as well as the amount of variation in the voltage of the capacitor. A speed control device for an induction machine according to scope 1.