JPH0341029B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an operating method for improving operating efficiency in variable speed operation of an induction motor using a voltage controlled inverter.

[Prior art]

When an induction motor is operated at a load much lower than its rated value, its efficiency decreases significantly. To improve this, the phase difference φ between the inverter's output voltage and output current (induction motor input voltage and input current) is
Calculate the power factor cosφ from , lower the inverter output voltage when the power factor is low, and improve the power factor.
Improving efficiency.

This method is based on the premise that the changes in power factor and efficiency (increase and decrease curves) with respect to changes in load are similar. In fact, when the load is quite small, the power factor and efficiency tend to increase or decrease in a similar manner, but at medium or higher loads, there are cases where the increase or decrease is the opposite. In this state, efficiency cannot be effectively improved by controlling the power factor.

In addition, in a method that does not rely on power factor calculation, multiple patterns of output voltage V for the output frequency f of the inverter are provided, and an appropriate output voltage pattern (V/f) is manually set according to the load of the induction motor. There are things that you can select and use.

However, that method has two major drawbacks:

The load state of the motor is not quantitatively determined.

Too many voltage patterns are required to enable highly efficient operation at various load conditions of the motor.

[Purpose of the invention]

The present invention aims to eliminate such conventional drawbacks and directly improve the operating efficiency of the inverter by controlling the motor input voltage while monitoring the inverter DC output power. .

[Structure of the invention]

The method of operating an induction motor of the present invention is that in the method of operating an induction motor driven by a voltage-controlled inverter, when the induction motor is accelerating or decelerating, the ratio V/f between a certain set inverter output voltage and inverter output frequency is Adjust the inverter output voltage so that it operates under constant V/f control, and when the induction motor is in steady operation, multiply the inverter output voltage E _DCN and the inverter output current i _DCN . Find the output power P _N , then set the inverter output voltage to the output voltage at the beginning of steady operation.
The difference ΔP ₁ between _{the inverter output voltage P 1 calculated} at this time and the inverter output power P ₀ at the beginning of the steady operation is compared with _a preset constant ΔP _S. If ΔP ₁ <ΔP _S , the inverter output voltage is fixed at the voltage V ₁ ,
If ΔP ₁ ≧ ΔP _S , the inverter output voltage is further decreased by ΔV from V ₁ after a certain period of time, the output power reduction value at that time is compared with the constant ΔP _S , and the inverter output voltage is fixed according to the magnitude. The invention is characterized in that the induction motor is operated at the inverter output voltage when the inverter output voltage becomes the minimum by repeating the steps of increasing or further decreasing the inverter output voltage.

〔Example〕

Hereinafter, the present invention will be explained based on embodiments shown in the drawings. Figure 1 shows the configuration of a PWM (pulse width modulation) inverter control system according to the present invention. In the figure, 1 is an induction motor, 2 is a load, 3 is a PWM transistor inverter circuit for driving the induction motor, and 4 is a main A circuit DC output voltage detector, 5 a main circuit DC output current detector, 6 a microprocessor, 7 a multi-input channel A/D converter, 8 a voltage adjustment calculation section, 9 a PWM waveform calculation section, and 10 the aforementioned This is a base driver of the PWM transistor inverter circuit 3.

In this embodiment, PWM inverter circuit 3
DC voltage E _DC detector 4 is installed in the DC voltage intermediate circuit section of
and a direct current i _DC detector 5, and these two
The input power P of the motor 1 is calculated by multiplying the two detected values.
can be obtained. The magnitude of the output voltage (or V/f value) of the inverter can be changed by the voltage adjustment calculation unit 8.

The voltage adjustment calculation section 8 is controlled by the microprocessor 6 as follows.

During acceleration and deceleration of the electric motor 1, the motor 1 is operated according to a certain set output voltage pattern (constant V/f control), and at this time, high efficiency operation control is not performed.

When the electric motor 1 enters steady state operation, the output voltage of the inverter is reduced by ΔV. If the initial inverter output voltage during steady operation is V ₀ , then V ₁
=V ₀ −ΔV. Electric motor 1 calculated at this time
Using the input power P ₁ of , let P ₀ be the value before reducing the output voltage, then by calculating P ₀ − P ₁ ,
Obtain the reduction value ΔP ₁ of the input power of the electric motor 1. Compare this with the input power reduction value ΔP _S , which is a preset constant, and if ΔP ₁ < ΔP _s , the inverter output voltage becomes V ₁ = V _ppt (V _ppt is the sub-optimal efficiency operation output voltage). Fix it. Furthermore, if ΔP ₁ ≧ ΔP _S , after a certain period of time (discrete control delay) T has elapsed, the inverter output voltage is further decreased by ΔV from V ₁ , and as described above, the input power decrease value ΔP ₂ is Calculate and compare with _ΔPS . From this comparison result,
When ΔP ₂ < ΔP _S , inverter output voltage V ₂ = V _ppt ,
When ΔP ₂ ≧ ΔP _S, inverter output voltage V ₃ = V ₂ −
Control the inverter output voltage like ΔV.

By repeating the above procedure a finite number of times, the inverter output voltage V _ppt can be obtained. This control flowchart is shown in FIG.

Next, FIGS. 3 and 4 show that the inverter output voltage V _ppt is a high efficiency operation (power save operation) point. FIG. 3 shows the general tendency of the relationship between the slip of the induction motor, the motor input power P _io and the motor output power P _put using the inverter output voltage (motor input voltage) as a parameter.

Now, when the power required by the load is P _L ,
When the inverter output voltage (motor input voltage) changes from _V0 to _V3 , the input power change curve of the induction motor shows a V-shape as _Pio0 to _Pio3 to _Pio0 '.
Therefore, there is an inverter output voltage V _ioppt that provides the minimum input power. However, when the speed control system is open-loop, it is impossible to operate to the left of P _ioppt (for example, P _io3 ′ to _{P IN0} ′).
Also, at an inverter output voltage lower than this, the induction motor 1 will undergo pullout and the input power will increase.

Furthermore, as a practical matter, in order to be stable against pull-out due to load disturbances, it is necessary to set the inverter output voltage to V _ppt , which is slightly higher than V _ioppt .

Figure 4 shows the ratio of decrease ΔP _i (i = 1, 2...) in motor input power to decrease in inverter output voltage R = ΔP _i /ΔV (proportional to ΔP _i when ΔV = constant)
It shows.

A realistic inverter output voltage V _ppt for high efficiency operation is determined under the condition ΔP _i <ΔP _S.

When attempting to start acceleration from steady-state operation at a reduced inverter output voltage (V ₀ > V _ppt ), the inverter output voltage is adjusted in advance by a signal that determines whether the command frequency is accelerating or steady. can be returned to a voltage pattern (V/f) that can produce acceleration torque (by referring to the ROM table).

[Application variation example] In the calculation of the input power of the induction motor, (a) Detect the input currents i _u and i _v of two phases among the three-phase inputs (U, V, W) of the induction motor, and calculate the following two After performing phase conversion, i _DC is determined by referring to the ROM table, etc.

iα _{= i u} iβ=( ⁱ _u + ²ⁱ _v ) _/ √3 Find the input power by multiplying by E _DC .

(b) In PWM inverters, where the output voltage waveform is generated by distributing the pattern that generates the output waveform from the memory of the microcomputer, the phase of each phase voltage is associated from the address that refers to the pattern. be able to.

It is possible to detect the zero-cross point (with polarity) of the current of any one phase and determine the phase difference φ with the corresponding phase voltage from the address of the output voltage pattern at this time. On the other hand, it is possible to detect the phase current using a peak hold circuit or the like and obtain the effective value of the current, for example, i _urns , from its peak value. The effective voltage value V _urns is determined from the output voltage pattern or the voltage command value. Therefore, the input power P can be obtained by multiplying P=3V _urns ·i _urns ·COSφ.

Here, COSφ may refer to a ROM table.

Modified examples of input power calculation have been shown above. Other methods of adjusting the inverter output voltage are the same as those described in the specific embodiments.

〔Effect of the invention〕

As described above, the present invention has the following effects.

(1) The input power of the induction motor can be operated near the minimum point commensurate with the load, enabling highly efficient operation.

(2) In the method of controlling the output voltage using the motor input power calculated from the voltage and current of the DC voltage intermediate circuit of the inverter, the input is controlled to a minimum, including the power switching loss of the inverter. can do.

(3) Operating with a lower inverter output voltage will reduce noise and vibration.

(4) By applying the control of the present invention only to the steady state of operation, it is possible to avoid deteriorating performance during acceleration/deceleration operation.

(5) In acceleration/deceleration operation in high-efficiency operation mode, which is generally operated at a low V/f value, sudden acceleration/deceleration will cause pullout, but when acceleration/deceleration has large fluctuations in load torque, torque fluctuations will occur. Operating at stronger, larger V/f values improves transient stability.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of the inverter control system according to the present invention, Fig. 2 is a flowchart of control calculations, Fig. 3 is a diagram showing the relationship between the slip of the induction motor and the input/output power of the motor, and Fig. 4 The figure is a diagram showing the rate of decrease in motor input power with respect to decrease in inverter output voltage. 1: induction motor, 2: load, 3: PWM transistor inverter circuit, 4: output voltage detector,
5: Output current detector, 6: Microprocessor,
7: Multi-input channel A/D converter, 8: Voltage adjustment calculation section, 9: PWM waveform calculation section, 10: Base driver.

Claims (1)

[Claims] 1. In a method of operating an induction motor driven by a voltage-controlled inverter, the ratio V/f between a certain set inverter output voltage and inverter output frequency remains constant while the induction motor is accelerating or decelerating. The inverter output voltage is adjusted to operate under constant V/f control, and when the induction motor is in steady operation, the output power is calculated by multiplying the inverter output voltage E _DCN and the inverter output current i _DCN . Find P _N , then set the inverter output voltage to the output power at the beginning of steady operation.
The difference ΔP ₁ between _{the inverter output voltage P 1 calculated} at this time and the inverter output power P ₀ at the beginning of the steady operation is compared with _a preset constant ΔP _S. If ΔP ₁ <ΔP _S , the inverter output voltage is fixed at the voltage V ₁ ,
If ΔP ₁ ≧ ΔP _S , the inverter output voltage is further decreased by ΔV from V ₁ after a certain period of time, the output power reduction value at that time is compared with the constant ΔP _S , and the inverter output voltage is fixed according to the magnitude. 1. A method for operating an induction motor, characterized in that the induction motor is operated at the inverter output voltage at which the inverter output voltage becomes the minimum by repeating increasing or further decreasing the inverter output voltage.