JP2893885B2 - Control method of voltage type PWM inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method of driving an induction motor (hereinafter simply abbreviated as an induction machine) using a voltage-type PWM (pulse width modulation) inverter. The present invention relates to a control method for preventing excitation, outputting an appropriate voltage according to load torque from an inverter, and maximizing induction motor efficiency.

[Conventional technology]

When the induction machine is driven by V / F (output voltage / output frequency) constant control using a voltage-type PWM (pulse width modulation) inverter, the voltage associated with the primary resistance (r ₁ ) of the induction machine, especially at low speeds around several Hz. In order to compensate for the gap magnetic flux shortage due to the drop, for example, as shown in FIG. 4, a method of increasing the output voltage at a low speed such as P1 for a constant V / F control pattern P is generally used.

This method can generate a necessary torque when the load is heavy, but on the contrary, when the load is light, the voltage applied to the exciting inductance of the induction machine increases, resulting in overexcitation. In other words, a phenomenon occurs in which the primary current increases as the load decreases, and the loss of the induction machine increases.

For this reason, attention is paid to the fact that the relationship between the power factor of the induction machine and the load torque and the relationship between the power factor of the induction machine and the slip frequency are monotonic functions as shown in FIGS. 5 and 6, respectively. A control method for generating a necessary torque in accordance with a load torque (such a method is sometimes referred to as torque boost control) has been proposed. FIG. 7 is a block diagram showing a conventional example based on this viewpoint.
It was published in a separate volume, Vol. 108, No. 9, 1988, pp. 829-836.

 The outline will be described below.

In this method, first, a decrease ΔΦ (ω _R , ω _S ) from a magnetic flux Φ _R to the induction machine at a low speed is reduced by a magnetic flux decrease ΔΦω _R depending on an inverter angular frequency and a magnetic flux decrease ΔΦω _S depending on a slip frequency. Think separately. And ΔΦ
ω _S is obtained using a correction gain k _v and a power factor cos φ which is a monotonic function with respect to a slip frequency or a load torque, and ΔΦω _R
This was added to [Phi _R determined from the induction machine constants, further V /
A voltage command is obtained from ω _R · (Φ _R + ΔΦ) from the F constant control.

That is, the zero-crossing point is detected by the zero-phase detector 11 from the primary current of the induction machine. Here, a first-order lag filter is used to reduce a detection error due to current ripple. Also, the zero phase of the modulated wave from the PWM control circuit 12 is detected in consideration of the characteristics of the first-order lag filter, and the phase difference φ is detected by the detector 13 using a counter. Calculates cos φ by power factor calculation. On the other hand, arithmetic unit 1
The power factor of the applied induction machine at no load is calculated in 5 and the difference between the two is multiplied by the gain in the amplifier 16 (this gain is determined via the function generator 17 from the characteristic of the induction machine depending on the inverter angular frequency. given) to obtain a correction amount of ΔΦω _S.
The Derutafaiomega _S in addition to the flux command [Phi _R, further correction value of the flux command only plus decrease Derutafaiomega _R of the magnetic flux is influenced by the inverter angular frequency _{^{Φ * (= Φ R + ΔΦω}} R + ΔΦ
ω _S ) is multiplied by the inverter angular frequency ω _R by the voltage command calculator 18 to obtain a voltage command V ^* . Note that ΔΦω _R is obtained via the function generator 19. The voltage command V ^* thus obtained and ω obtained via the V / F converter 20
Torque boost control is performed using _R. In addition,
7 is a voltage type PWM inverter, and 8 is an induction machine.

[Problems to be solved by the invention]

In the above-described method, the power factor of the induction machine must be obtained. However, as is clear from FIG. 5, since the amount of change in the power factor with respect to the load torque becomes smaller as the speed becomes lower, the resolution becomes higher. Is required. On the other hand, the phase difference φ measures the phase difference between the modulated wave of the PWM control circuit 12 and the primary current using a clock frequency proportional to the inverter angular frequency. Must use a fairly high clock frequency for the V / F converter 20.

In addition, since optimal control cannot be performed unless the characteristics of the induction machine are known in advance, it is effective when an unspecified induction machine is connected, such as a general-purpose inverter, or when a constant cannot be measured, such as an existing induction machine. However, there is a problem that there is a case where it cannot be expected.

Therefore, an object of the present invention is to eliminate the low-speed overexcitation by enabling the induction machine primary current minimum control without securing the induction motor characteristics and detecting the power factor. To maximize induction motor efficiency.

[Means for solving the problem]

When the induction motor is driven at a variable speed by a voltage-type PWM (pulse width modulation) inverter, detection means for detecting at least the effective value of the primary current of the induction motor is provided, and the output voltage of the inverter is increased or decreased by a fixed amount. Monitor the increase or decrease of the primary current at the time of occurrence, and obtain and control the output voltage at which the primary current is minimized.

[Action]

FIG. 3 shows the characteristics of the primary current and the torque with respect to the rotation speed when the output voltage of the inverter is changed (frequency is 10
Hz). Here, how the primary current at a certain load torque changes with respect to the output voltage is indicated by a broken line. At the inflection point of this dashed line, the primary current is the smallest, so the primary copper loss is the smallest, and since it is not overexcitation, the iron loss is small, and the slip is close to 0 and the secondary copper loss is not large. As a result, the induction machine efficiency is the maximum point for the load. Therefore, by finding this inflection point for each load, it is possible to perform torque boost control with maximum induction machine efficiency, which can eliminate overexcitation at light load and can secure necessary torque.

 Here, consider the operation at point A in FIG.

Now, if the output voltage is increased by a certain amount ΔV, the primary current decreases. Therefore, the output voltage is increased while the primary current continues to decrease. If you keep increasing the output voltage,
Eventually, the primary current starts increasing beyond the inflection point, and stops increasing at the output voltage immediately before the increase starts.

On the other hand, when there is an operating point at point B, conversely, if the output voltage is decreased by ΔV, the primary current decreases, and the decrease is stopped at the output voltage immediately before the primary current starts to increase. Whether the operating point of the induction machine is point A or point B can be determined based on whether the primary current increases or decreases when the output voltage increases or decreases by ΔV.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the present invention.
In the figure, 1 is a frequency setting device, 2 is an integrator, 3 is PW
M pulse generator, 4 is a function generator, 5 is an automatic torque boost calculator, 6 is a current detector, 7 is a voltage-type PWM inverter, and 8 is an induction machine (IM).

That is, the set frequency given from the frequency setter 1
f ^** becomes a frequency command f ^* via the integrator 2 and is input to the PWM pulse generator 3. Frequency set value f ^** is function generator 4
Also given, where to respond to V / F pattern setting voltage lambda ^** for the frequency instruction f ^** is calculated and output. The automatic torque boost calculator 5 uses the set voltage λ
^** and the magnitude I _{1 of the} primary current detected by the current detector 6
The optimum voltage command λ ^* is searched based on The PWM pulse generator 3 receives the frequency command f ^* and the voltage command λ ^* as inputs, calculates a PWM pulse width of a predetermined carrier frequency, and supplies this to the switch element of the voltage-source PWM inverter 7 as a PWM pulse signal. It should be noted that a known appropriate method can be used for the PWM control.

FIG. 2 shows a flowchart in the case of performing software processing using an arithmetic processing device as an automatic torque boost calculator. Here, it is determined by the voltage command λ ^* and the frequency command f ^* calculated in the previous ((n−1) th) routine.
It is assumed that the result of the induction machine driving by the PWM pulse output is reflected on the current (n-th) detection current I ₁ (n). This process is repeated for each calculation cycle.

First, a change amount Δλ of the voltage command at the current command frequency f ^* is calculated by the following equation (see).

f _base : Base frequency λ _max ^* : Maximum value of voltage command a: Ratio of change of voltage command Next, as can be seen from the torque-primary current characteristics in FIG. 3, there is an inflection point of current at no load. Therefore, when the search for the minimum primary current is performed, the primary current becomes almost zero.
Therefore, a no-load limiter is provided. That is, when the magnitude I _{1 of the} primary current is I ₁ (n) <I ₀ −ΔI ₁ , the addition of Δλ is continued, and when the magnitude of I ₀ −ΔI ₁ ≦ I ₁ (n) ≦ I ₀ is λ. ^{If *} is not changed and I ₀ <I ₁ (n), the routine proceeds to the minimum primary current search routine (see). Here, I ₀ represents the no-load current, [Delta] I ₁ represents the hysteresis width of the current.

In the search routine, the previous current value I ₁ (n-1) and the current value
If the difference from I ₁ (n) is within current hysteresis width ΔI ₂ , voltage command λ ^* is not changed, and if larger than current hysteresis width ΔI ₂ , the following operation is performed (see).

The magnitude of the current I ₁ (n) this time and the magnitude of the previous current I ₁ (n−1) are compared (reference). If I ₁ (n) is smaller, it is determined whether this is the result of the addition or subtraction of Δλ at the (n−1) -th time (see). If it is the result of the addition, it is determined that the operating point is located in an area where the slip is large at the inflection point represented by the point A in FIG. 3, and the memory ΔRMD storing the accumulation result of the addition and subtraction of Δλ is determined. Increase by Δλ (see). Conversely, if the result is a subtraction, it is determined that the operating point is located in a small slip region around the inflection point represented by the point B, and the ΔRMD is subtracted by Δλ (see). Then, the set voltage λ output from the function generator 4
^The voltage command λ ^* is obtained by adding ΔRMD to ^** (see).

If the above operation is continued, I ₁ approaches the inflection point and further exceeds the inflection point, so that I ₁ (n)> I ₁ (n−1). At this time, an operation opposite to the (n-1) th addition / subtraction is performed,
Adding or subtracting λ generates a limit cycle (does not converge), so a value smaller than Δλ, for example, 1/2 ·
Converge using Δλ (see,). In this way, the operation is repeated until | I ₁ (n) −I ₁ (n−1) | ≦ ΔI ₂ , and when the minimum value that is the inflection point of the primary current is reached, the search is terminated. Maintain the output voltage of

〔The invention's effect〕

According to the present invention, for example, for a V / F pattern that performs output voltage compensation so as to generate a rated torque at 6 Hz,
It has been confirmed that when the torque boost control as described above is performed from this pattern, the induction motor efficiency of 2.2 KW is improved by about 30% at the maximum. Further, when an inverter is applied to a fan pump or the like, a V / F pattern with a square reduction is often used for the purpose of saving energy. Although this is effective, it is not always a load suitable for square reduction. Even in such a case, the present invention can achieve a greater energy saving effect from the viewpoint of maximizing the intended induction motor efficiency.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a flowchart for explaining the operation of a torque boost calculator, and FIG. 3 is a diagram for explaining speed-torque and primary current characteristics of an induction machine. Graph, FIG. 4 is a graph for explaining a V / F pattern for performing voltage compensation at a low speed, FIG. 5 is a graph showing a relationship between a power factor and load torque, and FIG. 6 is a graph when a slip frequency is changed. FIG. 7 is a block diagram for explaining a conventional example of the torque boost control. 1 ... frequency setting device, 2 ... integrator, 3 ... PWM pulse generator, 4,17,19 ... function generator, 5 ... automatic torque boost calculator, 6 ... current detector, 7 ... … Voltage source PWM inverter, 8… Induction machine (IM), 11… Zero phase detector,
12: PWM control circuit, 13: Phase difference detector, 14: Power factor calculator, 15: Power factor calculator at no load, 16: Amplifier
18: Voltage command calculator, 20: V / F converter.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H02P 5/408-5/412 H02P 7/628-7/632 H02P 21/00

Claims (1)

(57) [Claims] When an induction motor is driven at a variable speed by a voltage-type PWM (pulse width modulation) inverter, detection means for detecting at least an effective value of a primary current of the induction motor is provided. Monitoring the increase or decrease of the primary current when increasing or decreasing by a certain amount, (a) when the effective value of the primary current decreases when increasing the output voltage of the inverter by a first certain amount ,
The output voltage is continuously increased by the first fixed amount, and when the decreased primary current increases again, the output voltage is reduced by the second fixed amount, and the output voltage is held at that value. When the effective value of the primary current increases when the output voltage of the inverter is increased by a first fixed amount,
The output voltage continues to decrease by the first fixed amount, and once the increased primary current decreases and increases again, after increasing the output voltage by the second fixed amount, the output voltage is held at that value. (C) when the effective value of the primary current increases when the output voltage of the inverter is reduced by a first fixed amount,
The output voltage is continuously increased by the first fixed amount. After the increased primary current decreases and increases again, the output voltage is reduced by the second fixed amount, and the output voltage is held at that value. (D) when the effective value of the primary current decreases when the output voltage of the inverter is reduced by a first fixed amount,
The output voltage is continuously decreased by the first fixed amount, and when the decreased primary current increases again, the output voltage is increased by the second fixed amount, and then the output voltage is held at that value. A control method for a voltage-source PWM inverter, characterized in that control is performed based on any one of the voltage values.