JPS6120236B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a control device for an induction motor,
In particular, the present invention relates to a control device capable of operating an induction motor at or near the lowest loss point by changing the voltage/frequency ratio (hereinafter referred to as V/F ratio) of supplied power.

As a control device for an induction motor, the one illustrated in FIG. 1 has been widely used. In the figure, when alternating current power AC is frequency-converted through a frequency converter 1 to drive an induction motor 2, a frequency reference _R set by a frequency setter 3 is used.
is converted to a predetermined frequency by the voltage/frequency converter 4 to control the output frequency of the frequency converter 1, and at the same time, the output _R of the frequency setter 3 is input to the function generator 5 to output the voltage reference V _R and transform The input of the induction motor 2 detected through the device 6 and this voltage reference V
The differential output is amplified by an amplifier 7 and fed back to the frequency converter 1, thereby controlling the induction motor 2 _.

In this case, the V/F ratio is usually selected depending on the load characteristics as shown in FIGS. 2a, b, and c. In the figure, characteristic curve a shows a case where constant torque characteristics are required, and the V/F ratio is increased in the low frequency range to compensate for the voltage drop across the resistor. Characteristic b is applied to general loads, and V/
The F ratio is controlled to a constant value. Characteristic c is applied when the load is proportional to the square of the speed, such as in pumps and fans.The V/F ratio is lowered in the low frequency range, and by lowering the excitation current, the input of the induction motor is reduced. The aim is to reduce current and improve efficiency.

However, with the conventional method described above, it is necessary to adjust the V/F ratio one by one according to the load characteristics, and it is very difficult to set the optimal V/F ratio over the entire range of wave control numbers. If there is a problem, it is difficult to deal with this, and if only the starting torque is large, it is difficult to adjust the V/F ratio.

The present invention has been made to eliminate the above-mentioned disadvantages in conventional devices, and is an induction motor that can automatically control the V/F ratio so that the power factor of the induction motor is always at or near the optimum value. The present invention aims to provide a control device for the following.

By the way, the equivalent circuit of the induction motor is as shown in FIG. 3a. Here, r ₁ and r ₂ represent the resistances on the primary and secondary sides, L ₁ and L ₂ represent the inductances on the primary and secondary sides, and Le represents the excitation inductance. Among these, the primary resistance r ₁ and
If the primary and secondary inductances L ₁ and L ₂ are ignored since their values are small, the equivalent circuit will be as shown in FIG. 3b. Here, the primary current is i ₁ and the exciting current is i ₂
Then, the following equations (1) to (4) hold true.

i ₁ ² = i ₀ ² + i ₂ ² ………(1) T=k ¹・φ・i ₂ ………(2) φ≒k ₂・i ₀ ………(3) T=k ₃・i ₀・i ₂ ………(4) Here, T is the generated torque, φ is the magnetic flux, k ₁ , k ₂ ,
All k ₃ are constants.

In equation (3), the equality sign holds if there is no saturation of the iron core. Therefore, the saturation of the iron core can be ignored in the range where the magnetic flux is weakened and the operation is performed, so that equation (4) holds true.

Figure 4 shows T ₁ = k ₃・i ₀・ when the load torque is T ₁ .
A trajectory A of i ₂ and a trajectory B of i ₁ ² =i ₀ ² +i ₂ ² where the primary current i ₁ becomes the minimum value i ₁ m are shown. From these trajectories, it can be seen that the minimum value of the primary current that balances the load torque T ₁ is when i ₀ =i ₂ , that is, when the power factor angle is 45°. When operating with a constant V/F ratio under light load, the load torque is balanced with the excitation current i ₀ n and primary current i ₁ n, so there is a large difference between the values of i ₁ m and i ₁ n. The most efficient operation can be achieved by controlling the load power factor angle to around 45°.

The present invention has been made with attention to this point. The details of the present invention will be described below with reference to the embodiments shown in FIG. 5 and subsequent figures.

In FIG. 5, power from a DC power supply 10 is PWM-controlled by an inverter circuit 11 and supplied to an induction motor 12 as alternating current with variable voltage and variable frequency. The frequency reference _R output from the frequency setter 13 is frequency-converted by the V/F converter 14, further converted to a lower frequency by the counter circuit 15, and then input to the distributor 16. This input signal has a phase difference of 60° in the distributor 16.
Generates a signal that is a frequency control signal with a width of 180° and selects the electric valves S ₁ to S ₆ of the inverter circuit 11,
The signal is input to a drive circuit 17 that drives an inverter circuit.

On the other hand, the frequency reference _R is compared with the output of the triangular wave oscillator 20 by the function generator 18 in the comparator 19, and is input to the drive circuit 17 to control the output voltage of the inverter circuit 11 by the PWM control circuit 21, and is then input to the drive circuit 17 to control the frequency control described above. It is combined with the signal to control the electric valves S ₁ to _{S 6} of the inverter circuit 11. Thereby, variable voltage, variable frequency power is supplied to the induction motor 12.

In the present invention, the current transformer 22 further detects the load current Iu, the polarity detector 23 detects the current phase, and inputs the current phase to the phase difference detector 24. on the other hand,
The output V _c of the logic circuit 25 that detects a voltage phase angle of 45° from the counter circuit 15 and the phase signal s ₁ ' of the electric valve s ₁ corresponding to the U-phase voltage are transferred to the phase difference detector 2.
4 and outputs an output VPH whose polarity changes at the load power factor angle of 45°. Amplifier 2
6 and input to an output control circuit 28 connected to the function generator 18 through a switch 27 to limit the voltage reference V _R . The load current I _u is rectified by a rectifier 29 and then passed through a level detector 30 to be controlled by a switch 27 .

In the circuit of the present invention configured as described above, when the load decreases and the power factor angle of the load current becomes larger than 45 degrees, the inverter output voltage is lowered by lowering the voltage reference _VR , and the induction motor 1 of the load
The excitation current of step 2 is lowered and the power factor angle of the load current is controlled to operate at an optimal efficiency of around 45°. In addition, when the load current rate is low but the excitation voltage is increased to maximize the output torque, such as during startup, after the load current I _u is rectified by the rectifier 29,
The level detector 30 detects the level, and when the level exceeds a certain value, the switch 27 is opened to stop the power factor angle control of the load current, and the voltage given from the function generator 18 is used to control the inverter. Controls the output voltage so that the output torque of the load does not become too large. Incidentally, in the above, the same effect can be obtained even if the gain of power factor control is reduced by connecting a resistor in parallel to the switch 27.

Next, the details of the operation of the load current phase difference detection section will be explained in the sixth section.
Explanation will be given with reference to FIG. 7 and FIG. In addition, in these figures, PWM control is fully fired, that is, the electric valve is 180
゜It is shown in the energized state, and the phases of S ₁ ', which is energized by the electric valve S ₁ , and the U-phase voltage Vu match. Output of logic circuit 25 taken out from counter circuit 15
Vc changes at an electrical angle of 45° as shown in Figure 6b. From the signal of the output V _I after determining the polarity of the U _- phase current Iu, if the current phase is larger than _{45 °} , as shown in g in the _same figure _, V _r )
If the current phase is smaller than 45°, Y(S ₁・V _c・V _I +
S ₁ ′・V _c・V _I ) is output. These X and Y signals are amplified by an amplifier having a filter effect as shown in Figure 7a, changing the polarity of X and Y, and the output voltage VPH changes as shown in the figure with respect to the power factor angle. b
Changes as shown in .

The above explanation is based on the case where a voltage source inverter is used, but when a current source inverter or cycloconverter 31 is used, the phase voltage is detected by the transformer 32 and the polarity is determined as shown in Figure 8a. This output V _v and the current polarity output V _I detected by the polarity detector 35 via the current transformer 34 are input to the phase difference detector 36 to determine the phase of the voltage and current. Detect. This detection output VPH and constant value -V ₁ (45° equivalent voltage)
By taking the sum of , it is possible to obtain a voltage whose polarity changes at a phase angle of 45°. (See Figure 8b). In FIG. 8a, since the configuration of the inverter or cycloconverter control section is almost the same as that in FIG. 5, illustration thereof is omitted, and only the phase detection section is shown.

As described above, in the device of the present invention, when the load power factor drops below approximately 45°, this is detected and the load power factor is reduced by lowering the output voltage of the frequency converter.
Since the induction motor current is controlled to be around 45° and balanced with the load torque by minimizing the induction motor current, an energy saving effect can be obtained through operation with the least loss. In addition, when the load power factor is poor, such as during startup, the load current value is detected, and when the value exceeds the set value, the power factor control signal is cut off, allowing the preset V/F ratio to be set. Since it is controlled by returning to its characteristics, maximum torque can be produced. Furthermore, when power factor control is performed by controlling the voltage reference value, it is possible to prevent the voltage value from dropping below a certain value in a very low frequency range where the primary resistance of the induction motor cannot be ignored.

As described above, according to the present invention, when the induction motor is started, the V/F ratio is automatically set to a preset value, and after the induction motor is started, the induction motor current is kept at the minimum by controlling the power factor angle so that it does not become less than 45 degrees. This improves the overall efficiency including the frequency converter. In addition, there is no need to adjust the V/F ratio according to the load as in the past, and when a voltage-type frequency converter is used, the voltage phase can be taken out from the frequency controller, making it possible to realize an economical device. be able to.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the circuit configuration of a conventional induction motor control device, Fig. 2 is a load characteristic diagram, and Fig. 3 is a diagram showing the circuit configuration of a conventional induction motor control device.
Figures a and b are equivalent circuit diagrams of an induction motor, Figure 4 is a diagram explaining the principle of the present invention, Figure 5 is a block diagram showing an embodiment of the present invention, and Figures 6 and 7 are load currents in the present invention. FIG. 8a is a block diagram showing the essential parts of another embodiment of the present invention, and FIG. 8b is an explanatory diagram of the operation. 3,13...Frequency setter, 4,14...V/
F converter, 11, 31...inverter circuit.

Claims (1)

[Scope of Claims] 1. In a control device for driving an induction motor by a variable voltage variable frequency conversion device, a phase difference detector that obtains an output according to a value at which the power factor angle of the induction motor deviates from 45°. By providing the output of this phase difference detector to the variable voltage control system of the converter, when the power factor angle of the induction motor lags behind 45 degrees, the output voltage of the converter is reduced in accordance with this delay. A control device for an induction motor, characterized in that the power factor is controlled so as to maintain the power factor angle at around 45°. 2. The induction according to claim 1, wherein the load current of the induction motor is detected by a level detector, and when the load current of the induction motor is above a certain value, power factor control is automatically stopped or weakened. Electric motor control device. 3 Output Vc of a logic circuit that detects a voltage phase angle of 45° based on the frequency reference device R output from the frequency setter, and the current output from the polarity detector based on the load current Lu detected by the current transformer. The phase VI and the phase signal Si of the electric valve corresponding to the voltage of the U phase are input to a phase difference detector to output an output VPH whose polarity changes at a load force flow angle of 45°, and this output is output limited. 3. A control device for an induction motor according to claim 1, wherein the voltage reference V _R is limited by inputting it into a circuit. 4 Input the phase voltage detected by the transformer into the polarity discriminator to obtain the voltage polarity output _VV , and input the load current detected by the current transformer into the polarity detector. The induction device according to claim 1 or 2, wherein the obtained current polarity output V _I is guided to a phase difference detector, and a voltage equivalent to 45° is subtracted from the output V PH and input to the output limiting circuit. Electric motor control device.