JPH0632561B2 - PWM signal switching device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a PWM signal switching device for a variable voltage / variable frequency inverter using a pulse width modulation (PWM) method.

[Conventional technology]

As a PWM pulse calculation method of this type, a method of generating a PWM pulse by comparing the magnitude of a triangular wave which is a modulation signal with an inverter output voltage reference signal is generally well known.

FIG. 3 is a waveform diagram showing an outline of such a PWM signal generating system. In FIG. 4A, a sine wave S indicates an inverter output voltage reference signal and a triangular wave R indicates a modulation signal. The PWM pulse signal obtained by comparing the magnitudes of the sine wave S and the triangular wave R is, for example, an ON signal in the range where the sine wave is large and an OFF signal (OFF) in the range where the triangular wave is large, as shown in FIG. As a signal, this PWM pulse signal is transmitted to each switching element that constitutes the inverter device.
It is given as an N, OFF signal.

Here, the frequency of the modulation signal (carrier frequency) is _C ,
When the frequency of the reference signal is _I and the ratio (modulation ratio) n of this frequency is n = _C / _I , n indicates the number of triangular waves included in one cycle of the reference signal.

By the way, in the PWM pulse calculation method, as shown in FIG. 3, a periodic expression in which the phases of the triangular wave and the reference signal are matched to make n constant, and a frequency _C of the triangular wave, which is not shown, is made constant and the reference signal _{I is} the number of triangular waves in one cycle
There is an asynchronous method in which the phase of the triangular wave and the phase of the reference signal are not particularly matched, depending on the magnitude of the. The carrier frequency _C is related to the switching frequency of the semiconductor element used in the inverter device, and generally has an upper limit due to the characteristics of the element and the loss of the inverter device.
Especially in the case of asynchronous operation, even if the reference signal _I increases
_{Since C} is constant, the value of n is small. As described above, in the asynchronous method, the phases of the reference signal and the triangular wave do not match, so that as the value of n decreases, lower harmonics occur in the inverter output voltage, and the current ripple increases. Will occur. Therefore, when n is not sufficiently high (n ≦ 10), it is necessary to switch to the synchronous system as shown in FIG.

FIG. 4 shows an example of the relationship between _C and _I. In the figure
_I is asynchronous operation from 0 to _IS , and _I > _IS
Then it is a synchronous operation. That is, when asynchronous, _C
Operates with _C = _CAS constant, and gives a carrier frequency in the relationship of _C = n _{I in} order to keep n constant during synchronous operation. Since the modulation ratio n decreases as the inverter frequency increases in the asynchronous operation region, it is necessary to switch from asynchronous operation to synchronous operation at the point _IS where n ≦ 10. However, when the asynchronous operation is switched to the synchronous operation, the carrier frequency changes significantly.
For example, at the _IS point in Figure 4, _C is from _CAS
Reduce to _CS . Thus, there is a deviation in the carrier frequency, and when switching from asynchronous to synchronous without considering the phase deviation of the reference signal and the modulation signal, discontinuity may occur in the triangular waveform.

FIG. 5 shows an example in which the waveform of a triangular wave becomes discontinuous when switching from asynchronous to synchronous operation. The figure (c) shows the switching operation, and it can be seen that when the triangular wave switches at a point, it becomes discontinuous, and when it switches at a point, it becomes continuous. Similarly, (d) shows a PWM pulse signal, and when comparing the pulse signal when the triangular wave is switched until it is discontinuous and the pulse signal when the waveform is continuously switched,
The width of the ON pulse signal becomes wider only in the area indicated by
The pulse width is narrowing. This increase causes a difference in output voltage, and this voltage difference causes a transient jump.

In this way, the inverter output voltage changes due to the discontinuity of the triangular waveform when switching from asynchronous to synchronous,
As a result, the current jumps and causes an overcurrent.

Therefore, the applicant has proposed the following method in order to solve such a problem (hereinafter, also referred to as a proposed method).

FIG. 6 is a block diagram showing such a proposed system. In the figure, 1 is a voltage / frequency (V / F) converter, 2 is a counter, 3 is a latch circuit, 4 is an adder, and 5-9 are ROMs (read only memories).

During the asynchronous operation, the modulation frequency command _C via the V / F converter 1 is converted into the phase signal θ _C by the counter 2. The output is input to the adder 4 and is added to the phase θ _I ′ of the reference signal to obtain θ. At this time, the value of θ _I ′ shows a fixed arbitrary phase because the latch 3 is closed. θ is a ROM 1 in which a modulation signal (n triangular waves in one cycle of a reference signal) is written, and R is a timing in which switching from asynchronous to synchronous is written
It is used as a common address with OM2. θ _I is the phase signal of the reference signal, and ROM 5 in which the reference signal (sine wave, trapezoidal wave, rectangular wave) is written, and ROM 3 in which the same contents as ROM 2 are written (the phase of the triangular wave during synchronous operation is (Shown) is a common address. In addition, ROM4 the output of ROM2 _{T A} and ROM3 output T of the
_A command T for switching to the synchronous operation is output with _I as an address.

FIG. 7 shows an example in which there are four switchable phases.

That is, with respect to the waveform of one cycle of the triangular wave R as shown in FIG. 8A, the phases that can be switched from asynchronous to synchronous are A, B, C and D, and the phases are as shown in FIG. Assuming that the 3-bit digital quantities O ₂ , O ₁ , and O _{0 are used} for discrimination, when the modulation ratio is n, n points A, B, C, and D exist in one cycle of the reference signal. Phase A of each point
_I , B _I , C _I , and D _I are represented by the following equations.

Here, I represents an integer of 1 to n. ROM2, R
Predetermined data (O ₂ , O ₁ , O _{0 in} FIG. 7) is stored in OM3 by using the digital quantity corresponding to the phase of the above equation as an address.
Is written. ROM4 is this ROM2, ROM
The switching command T is output based on the contents of No. 3, but the contents are tabulated as shown in the following table.

As is clear from the table, the ROM4 address is RO
These are outputs T _A and T _I of M2 and ROM3. And this ROM4 has ROM2 and ROM3 both A, B, C,
Only at the point D, a switching command (“1” at the time of synchronization) indicating that the phase is switchable is output.

Next, a method of switching from asynchronous to synchronous operation will be described.

In FIG. 6, in the asynchronous operation, the triangular phase signal θ
Is θ = θ _C + θ _I ′. That is, at this time, the latch 3 is closed and θ _I ′ is a constant value, and θ _I ′ ≠
is θ _I. Since θ is also the address of ROM2, the output of ROM2 is the phase A, B,
3 bits corresponding to C and D (O ₂ , O ₁ in FIG. 7,
O ₀ ). FIGS. 8A and 8B show the relationship. That is, this figure (a) shows the triangular wave asynchronous time, also (b) shows the output T _A of ROM 2.
θ _I is the phase of the reference signal and also serves as the address of the ROM 3.

On the other hand, in the synchronous operation, the counter 2 shown in FIG. 6 is cleared to zero and θ _C = 0. At this time, since the latch 3 is open, θ _I ′ = θ _I and θ = θ _I. The output of the ROM 3 whose address is the phase θ _I of the reference signal at the time of non-synchronization becomes the data as shown in FIG. 7 corresponding to the points A, B, C and D of the triangular wave at the time of synchronous operation. ). FIG. 8C shows a triangular wave at the time of synchronous operation, and FIG. 8D shows the output T ₁ of the ROM 3 similarly.
The address of the ROM 4 which outputs the switching command consists of T _A and T _I , and when the output T of each phase of the triangular wave at the time of synchronization and the triangular wave at the time of synchronization (points A, B, C, D) respectively coincides, Issue a "1" signal. Therefore, if the asynchronous mode is switched to the synchronous mode at the timing when the "1" signal is output, the triangular waveform is continuous. The relationship is shown in (e) and (e) of FIG. The same figure (e) is ROM4
Of the asynchronous triangular wave (a) and the synchronous triangular wave (c) point RO.
It is formed by being input to M4. The switching signal T indicates that the phases of the triangular waves match, and if the switching is performed at this timing, the triangular waves are switched from the asynchronous triangular wave point to the synchronous triangular wave point. In the same figure (f), before and after the point, a triangular wave is shown asynchronously and a trace is synchronously, indicating that the waveform is continuous at the time of switching.

As described above, since the triangular wave is continuous at the time of switching, the current ripple is reduced, and the transition from asynchronous to synchronous is smoothly performed.

Here, attention is paid to the delay time of the timing at the time of switching.

FIG. 9 shows an example of the timing delay when switching from asynchronous operation to synchronous operation. FIG. 9C shows a switchable phase signal in which the asynchronous triangular wave (a) and the synchronous triangular wave (b) have the same phase. FIG. 9D shows a switching command from the asynchronous operation to the synchronous operation. here,
If a command to switch from asynchronous operation to synchronous operation is issued immediately after timing A as shown in (c), switching is performed at the timing A '. The maximum delay in the timing from when the switching command is issued to when the switching operation is performed is approximately equal to the time difference ΔT from A to A ′. This difference ΔT time from A to A 'is given by the following equation (4) from the deviation delta _C carrier frequency _CAS and carrier frequency _CS the synchronous operation of the asynchronous operation shown in Figure 4.

If the switching time ΔT is long, the switching operation is stagnant, and the inverter voltage cannot be controlled during this period. Therefore, it is desirable that ΔT be as short as possible. Then, in order to shorten ΔT, _CAS should be made as large as possible and _CS should be made as small as possible from the above equation (4).

[Problems to be solved by the invention]

However, the asynchronous carrier frequency _CAS has an upper limit on the switching frequency due to the characteristics of the switching element used in the inverter device, the loss of the inverter device, etc.
As mentioned above, it cannot be increased so much. next
_{As for CS} , the synchronous carrier frequency _CS at the time of switching from asynchronous to synchronous is set to _CAS.
If it is lowered too much, the voltage control performance in the synchronous system deteriorates. That is, when the carrier frequency is lowered as _CS → _C1 → _{C2 in} FIG. 4, the number of PWM pulses (ON / OFF of the switching elements forming the inverter device is changed).
Signal) is reduced, which makes voltage control difficult. Therefore, in the conventional method, the deviation Δ _C between _CAS and _CS
Cannot be increased, and therefore, the cycle of the switching phase signal becomes long, the switching operation becomes stagnant, and the voltage cannot be controlled during this period. Also,
Delta _{C =} 0 becomes in the case of a _CAS = _CS, there can not switching operation for the case where switching換位phase signal is not output occurs, thus a problem that does not take into synchronous operation.

Therefore, according to the present invention, when the modulation signal is switched from the asynchronous operation to the synchronous operation, the waveform thereof is continuously changed to smoothly perform the transition from the asynchronous to the synchronous.
PWM that can reduce the dead time during this switching
It is an object to provide a signal switching device.

[Means for solving problems]

In addition to the proposed method as shown in FIG. 6 in which the waveform of a triangular wave which is a modulation signal is continuous and switching from asynchronous to synchronous can be smoothly performed, frequency increasing means for further increasing the frequency of the modulation signal is provided.

[Action]

According to the present invention, at the time of switching from asynchronous operation in which the frequency of the modulation signal is kept constant to synchronous operation in which the phases of the reference signal and the modulation signal are kept constant, a switching command is output and the asynchronous carrier frequency is increased at the same time. By increasing the deviation between the asynchronous carrier frequency and the synchronous carrier frequency to reduce the switching dead time, the asynchronous operation can be smoothly switched to the synchronous operation.

[Embodiment of the Invention] FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a timing chart for explaining the operation thereof.

As is apparent from FIG. 1, this embodiment is different from that shown in FIG. 6 in that the integrator 10 and the zero hold circuit (ZH
The feature is that circuits 11 and 13 and an adder 12 are added. Therefore, only the difference will be described below.

First, when a synchronous operation command is output at time t _{0 as} shown in FIG. 2 (a), zero hold (Z
H) The signal z replaces the ZH solution signal (see FIG. 2B). As a result, the voltage V is input to the integrator 10, and the integration operation is started. In the adder 12, the integrator 10
Output V _C2 and V _C1 (constant voltage) are added, and the output V _C becomes V _C = V _C1 + V _C2 (see FIG. 2C). This V _C is given as the input voltage of the V / F converter 1, and its output becomes the asynchronous carrier frequency _C.
Therefore, the carrier frequency _C increases as V _C increases, and the operation of the counter 2 becomes faster as shown in FIG. As a result, the timing delay time of the asynchronous and synchronous switching phase signal T is reduced. In FIG. 2, as shown in (f), the timing delay time is shown when the carrier frequency is constant at T ₁ , and the timing delay time is shown when the carrier frequency is increased at T ₂ . That is, the synchronous operation command is output. By detecting the switching phase signal from the time of t = t ₀ , it is possible to shift to the synchronous operation at time t = t _{1 at} which the switching operation from the asynchronous operation to the synchronous operation ends, and the delay time of this switching is T can be shortened as ₂ (FIG. 2 (f), see (g)). When synchronous operation starts, ZH signal z
The ZH circuits 11 and 13 are held at zero again,
V _C = V _C1 . At this time, the counter 2 is cleared by the zero clear signal C as shown in FIG. The latch 3 is unlatched by the latch signal as shown in FIG. 2 (h), and as a result of θ = θ _I , the ROM 5 in which the reference signal is written and the ROM in which the modulation signal is written
1 and 1 move with the same phase signal θ _I , and the phases of the reference signal and the modulation signal match. The integrator 10 is provided with an upper limiter, which determines the upper limit of the carrier frequency during the switching operation.

〔The invention's effect〕

According to the present invention, at the time of switching from the asynchronous operation to the synchronous operation, the asynchronous carrier frequency is increased to increase the frequency deviation from the carrier frequency during the synchronous operation, thereby outputting the switchable phase signal. Since the cycle is shortened and the delay time of the switching operation is reduced to smoothly shift to the synchronous operation, the effect of improving the voltage control response can be obtained. Since the carrier frequency increases only for a short time (a few cycles at the carrier frequency) at which the asynchronous operation is switched to the synchronous operation, there is no fear of causing an inverter loss.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
3 is a timing chart for explaining the operation of the figure, FIG. 3 is a waveform diagram for explaining the outline of a general PWM pulse generation method, and FIG. 4 is a graph showing the relationship between the frequencies of the reference signal and the modulation signal, FIG. 5 is a timing chart for explaining an example in which the modulation signal waveform is discontinuous, FIG. 6 is a configuration diagram showing the proposed system, and FIG. 7 is RO in FIG.
FIG. 8 is a waveform chart for explaining the relationship between M2, ROM3 and the switching phase, FIG. 8 is a timing chart for explaining the operation of FIG. 6, and FIG. 9 is a reference for explaining the timing delay time at the time of switching. It is a figure. Explanation of symbols 1 ... V / F converter, 2 ... counter, 3 ... latch circuit, 4, 12 ... adder, 5-9 ... ROM, 10 ...
Integrator, 11, 13 ... Zero hold circuit.

Claims (1)

[Claims] 1. A pulse width modulation (PWM) signal arithmetic circuit for comparing a reference signal, which is an output voltage command of an inverter, with a modulation signal to calculate ON / OFF signals of respective switching elements constituting an inverter main circuit, In a PWM signal switching device for switching from asynchronous operation in which the frequency of the modulation signal is constant to synchronous operation in which the phases of the reference signal and the modulation signal are synchronized, the modulation signal in asynchronous operation in which the frequency of the modulation signal is constant Detecting means for detecting the phase of the reference signal, the second detecting means for detecting the phase of the modulation signal from the phase signal of the reference signal during the synchronous operation for synchronizing the phases of the reference signal and the modulation signal, and both detection outputs And a third detection means for detecting the coincidence point of each phase of the modulated signal during asynchronous operation and synchronous operation, and frequency increase for increasing the frequency of the modulated signal. Means is provided and a synchronous operation command is given, and based on this, when switching from the asynchronous operation until then to the synchronous operation, the frequency of the modulation signal is temporarily increased by the frequency increasing means. In the PWM signal switching device, the frequency of the modulation signal is switched based on the output from the third detecting means to reduce the delay time at the time of switching.