JPH0140600B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides an inverter (hereinafter referred to as VVVF inverter) that can generate variable voltage and variable frequency.
The present invention relates to an improvement in an inverter control device in which the efficiency of a three-phase induction motor driven at variable speed is always maximized within a range that does not exceed the rated value of the main circuit elements, and in which smooth speed control can be performed.

Generally, when driving a three-phase induction motor (hereinafter simply referred to as a motor) using a VVVF inverter, the inverter output is controlled so that the ratio of voltage and frequency remains constant, regardless of the size of the load, in order to maintain the motor's set rotational speed. It has become a thing. However, with such a control method, if the voltage/frequency ratio is set so that the efficiency of the motor is maximized at the rated load, the efficiency becomes lower than the maximum efficiency that should be obtained at light loads.

The present invention has been made with attention to the above-mentioned points, and it is possible to make the rotation speed match the set rotation speed and always achieve the maximum efficiency that should be obtained under the operating conditions, regardless of the set rotation speed of the electric motor and the magnitude of the load. The present invention provides a VVVF inverter that signals the inverter output frequency and output voltage as follows. In order to simplify the explanation, an L-type equivalent circuit will be used for the electric motor.

FIG. 1 is an L-shaped equivalent circuit diagram for one phase of the motor. In the figure, V〓 ₁ is the primary voltage, r ₁ is the primary resistance, f is the operating frequency, L ₁ is the primary leakage inductance, r ₂ is the secondary resistance converted to the primary side, S is slip, and L ₂ is converted to the primary side. G ₀ is the excitation conductance, and B ₀ is the excitation susceptance.

Here, when the primary input P ₁ and secondary output P ₂ of the motor are expressed using the constants shown in FIG. 1, they are as shown in equations (1) and (2).

P ₁ = 3 (V ₁ ) ²・{G ₀ + r ₁ + (r ₂ /S) / Z ² }......(
1) P ₂ = 3 (1-S/S)・r ₂・(V ₁ ) ² /Z ² ………(2) However, Z ₂ = {r ₁ + (r ₂ /S)} ² + {2πf (L ₁ +L ₂ )} ² Therefore, the efficiency of the motor η
is expressed by equation (3).

η=P ₂ /P ₁ =1-S/S・r ₂ /G ₀ Z ² +r ₁ +(r ₂ /S) ......(3) Also, the rotation speed of the motor is ω ₀ (rad/sec), When the number of pole pairs is expressed as p, equation (4) is established, and the generated torque γ (N·m) of the electric motor is as shown in equation (5).

S=1−pω ₀ /2πf ………(4) γ=3・(V ₁ ) ² /Z ²・(1−S)・r ₂ /S・1/ω ₀ ………(5) Furthermore Equation (4) can be transformed as follows.

f=pω ₀ /2π(1-S) ...(4') From this, substitute (4') into equation (3) to express efficiency η as a function of rotational speed ω ₀ and slip S, and the rotational speed The relationship between the efficiency η and the slip S with ω ₀ constant is determined as shown in FIG. 2. That is, it can be seen that when the rotational speed ω ₀ is constant, there is one point of slip S ₁ where the efficiency η is maximum. If you want to operate at rotational speed ω ₀ in this way, operate at the operating frequency f ₁ determined by this slip S ₁ and adjust the voltage to set the slip to S ₁ to operate at maximum efficiency. be able to. Therefore, by finding the operating frequency f ₁ at which the efficiency η is maximum at various set rotational speeds ω _M (rad/sec), the third
It will look like the figure.

Furthermore, looking at the voltage and current required for the load, the primary voltage V ₁ is expressed by equations (5) to (5'), and the motor current is expressed by equation (6).

Here, if the motor is operated at the operating frequency f ₁ that maximizes efficiency η for a certain set rotational speed ω _M , and the voltage is adjusted to match the rotational speed of the motor for all loads, then the voltage and current for the load will be The relationship is as shown in FIG. However, in VVVF inverters that use semiconductor devices, power conversion is limited by the rated values of the main circuit elements, so when the load is heavy, the voltage is increased unnecessarily and the motor rotation speed is set to the set rotation speed. It is not possible. The present invention has a technical idea of setting an upper limit of the voltage applied to the motor in view of this point, and increasing the rotational speed by increasing the operating frequency without further increasing the voltage applied to the motor.

FIG. 5 is shown to facilitate understanding of the control principle according to the present invention, and is a graph showing changes in efficiency with respect to load. In other words, the operation is performed at the operating frequency f ₁ that maximizes the efficiency for a certain set rotational speed ω _M , and as the load increases, the rated (V/
f) Efficiency η _A due to the operation method in which the voltage is increased to about 20% more than the ratio, and for higher loads, the speed is controlled by increasing the operating frequency.
And, the efficiency η _B is obtained by the so-called constant (V/f) ratio operation method in which the speed is controlled by changing the operating frequency according to the rated (V/f) voltage regardless of the load size. . Therefore, when the load becomes heavy, efficiency can be further increased by increasing the voltage as high as possible and then controlling the operating frequency and speed. In this way, the speed is controlled by adjusting the voltage applied to the motor, and
If the motor is subjected to a heavy load and the operating frequency is increased without further increasing the voltage, then
Optimal motor operation can be achieved.

FIG. 6 shows the main part configuration of an embodiment according to the present invention, in which 1 is an inverter section that supplies variable voltage/variable frequency output to the motor 2, 3 is a speed detector directly connected to the motor 2, and 4 is a speed detector directly connected to the motor 2. Speed setter, 5 is frequency setter, 6 is voltage regulator, 7 is voltage upper limit setter, 8 is limiter, 9 is incremental frequency signal generator,
10 is an adder. Here, inverter part 1
The motor 2 obtains the output voltage command V ^* and the frequency command F ^* to drive the electric motor 2, but this part is well known and detailed explanation thereof will be omitted.

In FIG. 6, a signal of a set rotational speed ω _M is given from a speed setter 4 to a frequency setter 5 and a voltage regulator _6. It has a built-in memory that stores the operating frequency f ₁ at which _the efficiency η is maximum in the adder 10.
It is also output to the voltage upper limit setter 7. On the other hand, the voltage regulator 6 compares the set rotation speed ω _M with the signal of the actual rotation speed ω ₀ sent from the speed detector 3, and increases the voltage command signal V' if the rotation speed ω ₀ is smaller. , when the rotational speed ω ₀ is larger, a smaller voltage command signal V' is given. That is,
The voltage regulator 6 acts to reduce V' when the load is light and to increase V' when the load is heavy.

Further, the voltage upper limit setting device 7 obtains a signal of the operating frequency f ₁ from the frequency setting device 5 output, and at that operating frequency f ₁ , for example, the voltage that is applied to the motor 2 does not cause the current to exceed the upper limit value. The upper limit value is sent to the incremental frequency signal generator 9 and limiter 8 as the voltage upper limit signal V _R . Here, the upper limit value of the voltage is selected such that the inverter section ₁ outputs a voltage that is 20% higher than the rated (V/f) ratio of the motor, which is proportional to the operating frequency f1, for example. Therefore, the limiter 8 compares the voltage command signal V' and the voltage upper limit signal V _R , and if the voltage command signal V' is smaller than the voltage upper limit signal V _R , the limiter 8 outputs the voltage command signal V'.
V ^* , or if the voltage command signal V' is larger, the voltage upper limit signal V _R is sent to the inverter section 1 as the output voltage command V ^* . Further, the incremental frequency signal generator 9 inputs the voltage command signal V' and the voltage upper limit signal V _R , and does not output if the voltage command signal V' is smaller than the voltage command signal V'.
If it is larger than V _R , an incremental frequency signal Δf ₁ proportional to the difference between V' and V _R is output to the adder 10. Furthermore, the adder 10 adds the signal of the operating frequency f ₁ and the incremental frequency signal Δf ₁ to generate a frequency command F ^* .
Therefore, the frequency command F ^* is based on the signal of the operating frequency f ₁ itself when the load is light, and is the sum of the above f ₁ and Δf ₁ when the load is heavy.

To summarize the control operation as described above, when the set rotational speed ω _M of the electric motor 2 is determined, the inverter section 1 is operated at the highest efficiency operating frequency f ₁ that is uniquely determined from the rotational speed ω ₀ . , when the load on the motor 2 is light, the increase or decrease in the rotational speed ω ₀ is corrected by the increase or decrease in the output voltage of the inverter section 1 to match the set rotational speed ω _M ; The output voltage of the motor is fixed at the upper limit voltage, and a decrease in the actual rotational speed ω ₀ due to an increase in load is corrected by an increase in the operating frequency to match the set rotational speed ω _M .
Note that each of the aforementioned signals ω _M , ω ₀ , f ₁ , V′, V _R , V ^* ,
It is obvious that Δf ₁ and F ^* can be used not only for analog signals but also for digital signals.

As described above with reference to one embodiment, according to the present invention, the efficiency of the electric motor is a function of the rotational speed ω ₀ and the operating frequency f as shown in equation (3), and as a result, the set rotational speed ω _M Taking advantage of the fact that the operating frequency f ₁ at which the motor efficiency is maximum is unique to the motor, the inverter output frequency is determined at this maximum efficiency operating frequency using a memory device or other means, and furthermore, when the load is heavy, By suppressing the increased output voltage and increasing the operating frequency above the maximum efficiency operating frequency, the efficiency of the motor is always maximized within the range that the inverter can output, and smooth speed control is achieved. Close
We can provide VVVF inverter.

[Brief explanation of drawings]

Figure 1 is an L-shaped equivalent circuit diagram per phase of the motor, Figure 2 is a graph showing the relationship between efficiency and slip when the rotational speed is constant, and Figure 3 is a graph showing the relationship between efficiency and slippage when the rotational speed is constant. A graph showing the relationship, Figure 4 is a graph showing the voltage and current against the load when operating at the highest efficiency operating frequency,
FIG. 5 is a graph showing the efficiency with respect to load according to the control principle according to the present invention, and FIG. 6 is a block diagram showing an embodiment according to the present invention. 1... Inverter part, 2... Three-phase induction motor (motor), 3... Speed detector, 4... Speed setting device, 5... Frequency setting device, 6... Voltage regulator, 7
... Voltage upper limit setter, 8 ... Limiter, 9 ... Incremental frequency signal generator, 10 ... Adder.

Claims (1)

[Claims] 1. In an inverter that generates a variable voltage/variable frequency output and performs variable speed operation of an induction motor, a frequency setting means for outputting a maximum efficiency operating frequency signal uniquely determined from a set rotational speed of the induction motor;
Voltage adjusting means for providing a voltage command signal based on the difference between the set rotational speed and the actual rotational speed of the induction motor;
an upper limit setter that inputs the highest efficiency operating frequency signal and outputs a voltage upper limit signal; a limiter that inputs the voltage upper limit signal and obtains the voltage command signal to generate an output voltage command of the inverter; A frequency signal generator that inputs a signal and a voltage upper limit signal and outputs an incremental frequency signal, and an adder that obtains the highest efficiency operation frequency signal and the incremental frequency signal and generates an output frequency command of the inverter. Inverter control device.