JPH05316735A - Sine wave pwm controlling method for inverter - Google Patents

Abstract

PURPOSE:To optimally operate an inverter to be PWM-controlled with an induction motor as a load to increase an output voltage and to decrease a waveform distortion in an overmodulation state. CONSTITUTION:A time tn in which instantaneous values of a carrier Vc and a sine wave control signal Vs become equal is calculated, whether the time tn is larger than time ((Tc/2)-td) or not is decided, and a larger case is Y, not larger case is N. If an output frequency fi of an inverter is its predetermined value fic or less and a result of decision of the time is N, a time to of I/2 value of a predetermined PWM control pulse width is (tn-td), while if it is Y, the time to is ((Tc/2)-td), and both are controlled to be partly conducted. On the other hand, if the frequency fi is larger than the value fic and a result of the decision of the time is N, the time to is (tn-td) to be controlled to be partly conducted. On the contrary, at the time of Y, the time to is (Tc/2) to be controlled to be entirely conducted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sine wave PWM control method for an inverter that drives an induction motor at a variable speed.

[0002]

2. Description of the Related Art First, sine wave PWM control of an inverter
A case where the control operation is performed as shown in the control operation diagram of FIG. 3 will be described as an example. Now, let V _s be a sine wave control signal whose amplitude E _s is a proportional value of the inverter output voltage and whose frequency is equal to the inverter output frequency.
Also a sawtooth carrier wave and the wave height value 2E _c and its period T _c is, each is a constant and the magnitude of changing repeatedly at between + E _c and -E _c and V _c, for each period of the carrier wave V _c Then, the instantaneous value of the magnitude of the carrier and the control signal V _s is compared, and the elapsed time from the start of the cycle at the time when the instantaneous values of the carrier become equal in the n-th cycle is defined as t _n. Further, when the dead time for preventing simultaneous conduction between the switching elements in the upper and lower arms of the main circuit conversion section bridge structure of the inverter is t _d , the pulse width of the PWM-controlled basic conduction command signal for the respective arm elements is The time t _o that forms a half value of is expressed by the following equation (1).
Further, the time t _n and the modulation ratio λ that constitutes the constituent factor are determined by the following equations (2) and (3), respectively. Note that θ _n in the equation (2) is the average phase angle of the control signal V _s in the n-th cycle of the carrier wave V _c .

T _o = t _n −t _d ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ (1) t _n = (T _c / 4) (λ sin θ _n +1) ‥‥‥‥ (2) λ = E _s / E _c ‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥ (3) Therefore, the basic conduction command signal is n of the carrier wave V _c .
= 1,2, becomes as forming a pulse train consisting of each its width around its 1/2 point for each successive periods as ‥‥ obtained as 2t _o pulses and each respective arm elements of the The conduction command signal for the signal is determined to have a predetermined phase difference with respect to the basic conduction command signal.

Further, in an inverter using an induction motor as its load, in order to carry out constant flux control for the electric motor,
The output of the inverter is generally controlled so that its voltage-to-frequency ratio is constant, so that the modulation ratio λ also increases with a predetermined slope as the inverter output frequency increases. Here, as a conventional sine wave PWM control method of this type, the time t _n defined by the above equation (2) is (T _c / in the overmodulation state in which the modulation ratio λ is greater than 1 as described above. 2) When it becomes larger than -t _d , all the arm elements of the bridge structure are in a full conduction state with a conduction rate of 1, or T _c -2t _d is a conduction period and the conduction rate is 1 It is known to perform partial conduction control of −2t _d / T _c .

FIG. 4 is an example of an operation waveform diagram of various signals during the PWM control as described above, and FIG. 4A shows the relative relationship between the control signal V _s and the carrier wave V _{c in the} overmodulation state. 4B and 4C show basic conduction command signals S _swp and S _swf corresponding to the partial conduction control and the full conduction control, respectively. Further, FIG. 5 is a characteristic diagram of the inverter output voltage versus modulation ratio. As shown in the figure, the inverter output voltage V _i in the region where the modulation ratio λ is large is the characteristic line shown by the characteristic line L _f during the full conduction control. It is larger than that in the partial conduction control shown by L _p .

[0006]

However, according to the conventional sine wave PWM control method as described above, the time t _n is (T _c in one cycle of the sine wave control signal V _s.
/ 2) -t _d or more, when performing full conduction control in which the conduction ratio is 1 in the section, the fluctuation amount of the inverter output voltage V _i per cycle of the carrier wave V _{c in} the section is Since it is larger than that in other sections, the waveform distortion of the sine wave waveform of the output voltage V _i also becomes large, and along with this, the efficiency of the induction motor serving as the load of the inverter is lowered or its vibration and noise are increased. And so on.

Under the same conditions as above, the flow rate is 1
When performing partial conduction control formed between -2t _d / T _c, the waveform distortion of the such inverter output voltage is relatively small and the problems associated with the waveform distortion becomes is reduced, the driving characteristics on the induction motor There was a case where the supply voltage became insufficient when a large supply voltage was required. In view of the above, the present invention is
It is an object of the present invention to provide a sine wave PWM control method in which the full conduction control and the partial conduction control are switched and used in accordance with the output frequency of the inverter and the advantages of the both controls are utilized.

[0008]

To achieve the above object, a sine wave PWM control method for an inverter according to the present invention is a sine wave control whose frequency is equal to an inverter output frequency and whose amplitude is proportional to an inverter output voltage. Performs an instantaneous value comparison of the signal and the sawtooth carrier wave with a constant peak value,
In a sinusoidal PWM control method for designating a duty factor in an intermittent operation of an inverter main circuit switching element for each cycle of the carrier wave according to the comparison result, in the amplitude of the control signal with respect to a half value of the peak value of the carrier wave. In the case of overmodulation in which the defined modulation ratio is greater than 1, the conduction ratio is set to a conduction period in which the conduction period is set to 1 and a period in which a predetermined dead time is subtracted from the carrier wave period is used as the conduction period. 1
A smaller partial conduction control is used together with switching, and the switching of the control state is performed at a predetermined value of the inverter output frequency, and none is performed at the frequency higher than the predetermined value. Further, the partial conduction control is performed in a region below the frequency predetermined value, the modulation ratio is switched to the new predetermined value at the same time when the control state is switched, and the new predetermined modulation ratio is set. The value is predetermined as a value corresponding to the inverter output frequency so that the variation of the inverter output voltage due to the switching of the control state is zero.

[0009]

As described above, the output voltage of the sine wave PWM controlled inverter is controlled to be changed according to the amplitude of the sine wave control signal whose instantaneous value is compared with that of the carrier wave in the PWM calculation. It also varies with the modulation ratio given as the amplitude ratio with the carrier. Further, in the inverter using the induction motor as its load, the ratio of the output voltage to the output frequency is controlled to be a constant value in the normal control range, so that the modulation ratio increases with the increase of the inverter output frequency. .. That is, the output voltage of the inverter whose load is the induction motor is controlled in relation to the output frequency and the modulation ratio.

Therefore, when the modulation ratio increases with the increase of the inverter output frequency and the overmodulation state is reached, the control state is switched between the full conduction control and the partial conduction control at the predetermined output frequency of the inverter. And at the same time change the modulation ratio to the new modulation ratio,
Further, by predetermining the new modulation ratio as a value corresponding to the inverter output frequency so that the variation of the inverter output voltage due to the switching of the control state becomes zero, the variation of the inverter output voltage is caused. It becomes possible to switch between the two control states without the need of the above.

In accordance with the above, the present invention provides the inverter output frequency for the full conduction control and the partial conduction control in the sine wave PWM control of the inverter for variable speed control of the induction motor while simultaneously switching the modulation ratio as described above. By performing the corresponding switching, it is possible to use the both conduction controls together while avoiding the fluctuation of the inverter output voltage due to the switching.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to a control flow chart shown in FIG. 1 and an inverter output frequency characteristic diagram of modulation ratio shown in FIG. First, the sine wave PWM control will be described with reference to the control flowchart of FIG. The quantities shown in this flowchart are the same as those shown in the control operation diagram of FIG.

First, in the nth cycle of each successive cycle of the carrier wave V _c (where n = 1, 2, ...), the time t _n is calculated in accordance with the equation (2), and as described above. A predetermined value of the inverter output frequency f _i for switching between the control states of the full conduction control and the partial conduction control is f _ic , which is determined as 80H _Z , for example. Next, it is determined whether or not the time t _n is longer than the time [(T _c / 2) -t _d ], and the time is determined such that the greater state is Y and the lesser state is N. ..

After each of the above preparations, the following four selections are made. That first selection thereof, the inverter output frequency f _i or less, for example, 80H _Z and made the the changeover predetermined value f _ics and the time determination result if the condition of N, the said PWM control The time t _o that forms a half value of the pulse width of the basic conduction command signal is set to t _n -t _d by using the dead time t _d according to the equation (1).

Next, in the second selection, the frequency f _i is equal to or less than the predetermined value f _ic and the result of the time determination is Y.
In this state, the time t _o is (T _c / 2) -t _d . The third choice is the frequency f
_{If i} is larger than the predetermined value f _ic and the result of the time determination is N, the time t _{o is set} to t _n.
-T _d .

Further, in the fourth selection, when the frequency f _i is larger than the predetermined value f _ic and the result of the time determination is Y, the time t _o is T _c / 2. It is something to say. That is, the PWM control is a full conduction control in which the conduction ratio is 1 in the fourth selection, and a portion in which the conduction ratio is less than 1 in the first to third selections. It becomes conduction control.

[0017] Next versus inverter output frequency f _i characteristic diagram of the modulation ratio λ shown in FIG. 2 shows a following control patterns of the modulation ratio λ corresponding to the variation of the output frequency f _i in the inverter operation, wherein Frequency predetermined value f _ic (eg 80H
_{In Z} ), the control state is switched between the full conduction control and the partial conduction control, and the modulation ratio λ of λ _{1 is set so} that the variation of the inverter output voltage due to the control state switching becomes zero.
It shows that discontinuous switching between λ ₂ and λ ₂ is performed.

The value of the modulation ratio λ with respect to the inverter output frequency f _i as shown in FIG. 2 is determined in advance as a λ (modulation ratio) table, and the sine wave P of the inverter is used.
It is prepared as software in the WM control circuit. Further, in the creation of the λ table, the change of the frequency f _i is not continuous because it is restricted by the resolution. Therefore, the characteristic shown in FIG. 2 actually exhibits a stepwise change with the frequency resolution as the minimum change unit. Here, the resolution is, for example,
25H _Z as is selected.

[0019]

According to the present invention, an instantaneous value comparison of the magnitude of a sine wave control signal whose frequency is equal to the inverter output frequency and whose amplitude is proportional to the inverter output voltage and a sawtooth carrier having a constant peak value is performed. A sine-wave PWM that specifies the conduction ratio in the intermittent operation of the inverter main circuit switching element for each cycle of the carrier wave according to the comparison result.
In the control method, at the time of overmodulation in which the modulation ratio, which is the amplitude ratio between the carrier wave and the control signal, is greater than 1, in a region where the inverter output frequency is greater than the frequency setting value with a predetermined setting value as a boundary, Switching between both control states is performed such that full conduction control with the conduction ratio of 1 is performed, and partial conduction control with the conduction ratio less than 1 by a predetermined value is performed in the region below the frequency setting value. By doing so, it becomes possible to considerably increase the output torque peak value of the inverter load induction motor in the frequency region in the overmodulation state, for example, about 10% of its rated value, and also switch between the two control states. At the same time, the variation of the inverter output voltage due to the switching of the control state is made zero by simultaneously switching the modulation ratio to the predetermined new value. It is possible to smooth a driving.

[Brief description of drawings]

FIG. 1 is a control flowchart showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram of modulation ratio vs. inverter output frequency characteristic showing an embodiment of the present invention.

FIG. 3 is a control operation diagram of sine wave PWM control of an inverter.

FIG. 4 is a control signal operation waveform diagram during sine-wave PWM control of the inverter.

FIG. 5: Inverter output voltage vs. modulation ratio characteristic diagram

[Explanation of symbols]

f _i Inverter output frequency f _ic Setting value of inverter output frequency for conduction control state switching T _c Carrier wave period t _d Dead time for preventing simultaneous conduction between switching elements t _n PWM pulse width of original conduction command signal 1
/ 2 value t _o 1 / of the pulse width of the PWM-controlled conduction command signal
Binary V _i Inverter output voltage λ Modulation ratio θ _n Average phase angle of sine wave control signal in nth cycle of carrier wave

Claims (4)

[Claims] 1. A sine-wave control signal whose frequency is equal to the output frequency of an inverter and whose amplitude is proportional to the output voltage of the inverter and an instantaneous value comparison of the magnitude of a sawtooth carrier having a constant peak value, 1 of the carrier wave according to the comparison result
The sine wave P of the inverter that specifies the conduction ratio in the intermittent operation of the main circuit switching element of the inverter for each cycle
In the WM control method, in the case of overmodulation in which the modulation ratio, which is the ratio of the amplitude of the control signal to the half value of the peak value of the carrier wave, is greater than 1, the full conduction control in which the conduction ratio is 1 is performed. A sine wave PWM control method for an inverter, characterized in that a period obtained by subtracting a predetermined dead time from the cycle of the carrier wave is set as a conduction period and partial conduction control for making the conduction ratio smaller than 1 is switched and used in combination. 2. The sine wave PWM of the inverter according to claim 1.
In the control method, the switching of the control states of the full conduction control and the partial conduction control is not performed at a predetermined value of the inverter output frequency, and the full conduction control is performed in a region larger than the predetermined value of the output frequency. A sine wave PWM control method for an inverter, characterized in that the partial conduction control is performed in a region below the predetermined value. 3. The sine wave PWM of the inverter according to claim 2.
In the control method, when switching the control state as described above,
At the same time, the modulation ratio is switched to the new predetermined value, and a sine wave PWM control method for an inverter. 4. A sine wave PWM of the inverter according to claim 3.
In the control method, the new predetermined value of the modulation ratio is determined in advance as a value corresponding to the inverter output frequency so that the variation of the inverter output voltage due to the switching of the control state becomes zero. Wave PWM
Control method.