JPH0274168A - Pwm inverter apparatus - Google Patents

Abstract

PURPOSE:To obtain a low-noise apparatus by switching over the frequency of a carrier wave to a frequency higher than the audible range, when a duty ratio is small. CONSTITUTION:DC power 2 is converted into AC by an inverter composed of transistors Q1-Q4 and supplied to a load 3. Switching of said transistors Q1-Q4 is performed by the output of a modulation circuit 4. In the modulation circuit 4, a signal wave being a sine wave forming a reference is compared with the carrier wave of a triangular wave to determine the timing for turning ON said transistors Q1-Q4. In this case, when the absolute value of said signal wave being less than a specified value is detected and the frequency of said carrier wave is made higher than the audible range, a low-noise power source can be obtained even by the use of a low-priced switching element such as a bipolar transistor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a PWM inverter device for driving an AC load such as an induction motor or a synchronous motor.

(Prior technology) P as a device for variable speed operation of induction motors and synchronous motors
There is a WM inverter device. This PWM inverter device modulates a carrier wave of a predetermined frequency, such as a triangular wave, with a signal wave corresponding to the frequency of an AC voltage to be applied to an AC load such as the induction motor, and sends the modulated output signal to a bridge-connected switching circuit. It converts a DC voltage signal into an AC voltage signal of a predetermined frequency by inputting it into a circuit and switching in both positive and negative directions in the switching circuit, and is widely used in various 4111 devices such as AN controllers using induction motors and electric vehicles. It is used.

In such an inverter device, several KW to several 100
When driving a KW class induction motor or synchronous motor, a bipolar transistor is usually used as a switching element in a switching circuit. In addition, in the inverter device at this mountain pass, the carrier wave frequency fs (angular frequency =
The higher the value of ωS), the more accurate the detection accuracy can be.

(Problem to be solved by the invention) However, since the switching speed of a bipolar transistor is about several μs to 20 μs, the carrier wave frequency f
S1. The lt number can only be increased to about KH2. However, if the frequency fs of the carrier wave is increased to about several Kl-IZ, the switching operation will be performed within the human audible range, so that a harsh noise will be generated during this switching operation.

This noise can be reduced by M
It can be reduced by using an O8 type FET (field effect transistor) or the like and moving the frequency fs of the carrier wave to a frequency band higher than the audible range. However, M.O.
The output capacity of switching elements such as S-type FETs is limited to several kilowatts.

For this reason, there is a problem in that it is not possible to drive an AC load of several 100 kiloliters, which is the largest in human size.

The present invention was developed in view of these circumstances, and its object is to provide a PWM inverter device that can drive a large AC load with low noise.

[Filming configuration 1 (Means for solving the problem) The present invention includes a comparison means for detecting that the amplitude of a signal wave is smaller than a predetermined value, and a detection output of this comparison means to determine the frequency of a carrier wave! riWi, and switching means for switching to a higher frequency than the riWi frequency.

(Function) As explained later, when the duty ratio of the PWM modulation output signal is small, does it play a big role in the noise production? I am fulfilling J. When the duty ratio is small, it is when the amplitude of the signal wave is small. When the amplitude of the signal wave is small, the frequency of the carrier wave is increased by 1.77 fJ above the audible range. This makes it possible to reduce noise in the audible range.

(Embodiment) Fig. 1 is a circuit diagram showing an embodiment of the carrier frequency switching circuit 5 inserted before the modulation circuit which is the main part of the present invention, and Fig. 2 shows the physical structure of the PWM inverter. It is a diagram.

First, to explain the overall layer structure in FIG. 2, the switching circuit 1 is composed of bipolar transistors Q1 to Q4, Ql and Q2, Q3 and Q4 are connected in series, and these series bodies are connected to the DC power supply 2. are connected in parallel. An AC load 3 is connected to a connection point U between Ql and 02 and a connection point V between 03 and 04, and a modulated output signal S1. S4 switches the transistors Q1 and Q4, and in the half cycle from π to 2π, the modulated output signal 32.83 switches the transistors Q2 and Q3, thereby switching the output voltage Ed /2 of the DC power supply 2 and supplying the AC load 3.
It is configured to apply an alternating current voltage that changes between Ed and -Ed.

The modulation circuit 4 modulates the carrier wave es of a predetermined frequency fs with a signal eO corresponding to the frequency of the AC heavy pressure to be applied to the AC load 3, thereby generating the modulated output signals 81 to 81.
It is configured to output S4, and its details are as shown in the part surrounded by the broken line in FIG.

In FIG. 1, 50 is a function wave generator that generates a triangular carrier wave es as shown in FIG.
2 and the capacitor C connected to fS
=0.3/RC carrier wave es is output. In this embodiment, as R, R8 is fixedly connected, and RA is selectively connected to this RB via a switch 51.

Next, 52 is an absolute value circuit for determining the absolute value 1e01 of the signal wave eO as shown in FIG. 3(a), and as shown in FIG.
, [)2, is a conjuncture of a known high input impedance type absolute value circuit using resistors R1 to R4.

Next, 53 is a comparator that detects whether the absolute value 1001 of the signal wave is smaller than a predetermined value eth;
If l>eth, (I L sends 1 level output, 1
If e01≦Oth, “send 1 torque output, 1e
When 01≦eth, a switch 51 as a frequency switching means for the carrier wave es is turned on.

Next, 40 in the modulation circuit 4 is a signal wave eO and a carrier wave e.
It is a comparator that modulates es by eO by comparing s with s, and here it corresponds to the transistor Q1 of the switching circuit 1 ( ) is shown as a representative.

In the above configuration, assuming that the switch 51 is always open, the operation of the inverter Hffi is the same as the conventional one. That is, by modulating the carrier wave es generated from the function generator 50 with the signal wave eO, modulated output signals 81 to S4 are generated for each of the transistors 01 to Q4. is switched. As a result, an AC F and voltage that changes as shown in FIG. 3(b) is applied to the AC load 3.

Now, if we pay attention to the waveform in FIG. 3(b), we can see that the signal wave eO
It can be seen that the duty ratio is small in the part where the amplitude is small.

First, the on time of transistors 01 to Q4 is t on,
If the off-time is tor, the minimum switching time tnin that can be realized by the transistors Q1 to Q4 is shown in Fig. 5 (
As shown in a), Iln = tOL+-t
of −(1),
.

Therefore, the pulse width WD of the modulated output signals 81 to $4 is WD
The maximum duty ratio is when =lin. Now, if the duty ratio is 1:h, the period Ts of the carrier wave es is as shown in Fig. 5 <b>, Ts = h tn + n = h
(ton+tof)-(2), and its reciprocal becomes the maximum frequency fslaX of the carrier wave es.

fstgax=1/Ts=1. / (t+ (
ton+tof))... (3) For example, ton= 3 μs, tof= 7
μs, h = 20, then Ts = 200 μs
, “s = 5KH2.

However, here, jlA has a relatively small duty ratio.
For example, when h=4, WD=50 μs and tn
in = greater than 10 μs. This means that when the duty ratio is small, the carrier frequency fs is 4 times 25KH2 (=1/(4x (3+10)μ) as shown in Figure 5(C).
This means that it can be made as high as S)).

The duty ratio 11 is small when the amplitude of the signal wave eo is small, as can be seen from the waveform in FIG. 3(b).

Therefore, in this embodiment, focusing on the above relationship, the comparator 53 compares the absolute value 1e01 of the signal wave eO with a predetermined value eth, and if 1e01≦eth, the switch 51 is turned on, and the resistor RA is connected in parallel to the resistor RB. insert.

Then, the frequency fs of the carrier wave es is changed from fs = 0.3/Re C to fs = 0. 3/ (RA //RB) (
However, // is a parallel resistance symbol), which is higher than when Ie (if>eth).

For example, C=6μ'', RA=5/3Ω, R5-5/2Ω
Then, if 1eol≦eth -fs=20KH2eol>e
In the case of th, -fs=5KH2.

By increasing the carrier frequency fs when 1e01≦eth as shown below, the noise is also reduced.

Figures 6 (a) and (b) are waveform diagrams for explaining how noise is reduced, where (a) is the inverter output voltage waveform with period TS, and <b> is the waveform in Figure 6 (a). FIG. 7 is a diagram showing a change in the amplitude coefficient of the fundamental A' wave when τ (a value indicating the phase switching from +[d to -Ed) is changed.

In these figures, τ and duty ratio ・S10
(ωst +φ) Relationship t to h: 2π/τ (O≦τ×π): 2π, '2π
−τ (π×τ≦2π)... (4) There is.

Here, when the voltage waveform in Fig. 6(a) is subjected to hoop 1 transformation, G(t)-Ed((τ/π)-1) +(2Ed/π)Σ[((1-cos nπ/n) ) sin nωs t+
(sin n r /'n) cos
n ωs l: ]...(5) Shout.

'C-, n represents the harmonic order, and the fundamental wave with n=1 has the greatest effect on noise. r)≧2
In the case of smells, the amplitude also decreases, and the frequency itself becomes beyond the audible range.

The fundamental wave Cll (t) is 1 (t) = <2v'2Ed/yr) J'1-cos
r, and its amplitude coefficient (2J2Ed/π) J 1-
The cosr changes depending on τ as shown in FIG. 6<b>. It is clear from the figure that the closer J is to π, the larger the amplitude coefficient becomes, and the louder the noise becomes.

In other words, this shows that when τ is close to π, that is, when the duty ratio is small, the noise becomes large.

Therefore, as mentioned above, when the duty ratio is small, by switching the carrier frequency rs to around 20 KFIZ, which is higher than the audible range, it becomes possible to minimize the noise accompanying the switching operation.

In the above embodiment, the carrier wave es is generated by the function generator 5.
Although the signal is generated by 0, it may be generated by a circuit that is appropriately combined with a p-amplifier, a microcomputer 4-chip, or the like. Moreover, the signal wave eO is one predetermined 1i
Although the comparison is made only with ieth, the frequency f's of two or more 1s and 1s may be changed to 3 or more times.

[51 Effect of Light] As explained above, according to the present invention, when the duty ratio is small, that is, when the amplitude of the signal wave is small, the frequency of the carrier wave is switched to a frequency higher than the audible range. There is an effect that an AC f'J load of a large WFFx of several 100 KW class can be driven with low noise and by using an inexpensive switching element such as a bipolar transistor shade.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of a carrier frequency switching circuit which is a main part of the present invention, Fig. 2 is an overall signal diagram of an inverter device, and Fig. 3 shows signal waves, carrier waves, and inverter output voltage waveforms. Waveform diagram. Figure 4 is a circuit diagram showing an example of an absolute value circuit. Figure 5 is a waveform diagram to explain why the frequency of the carrier wave can be increased when the amplitude of the signal wave is small. Figure 6 is the inverter output voltage. FIG. 2 is an explanatory diagram for explaining the relationship between the duty ratio and noise. 1... Switching circuit, 2... DC power supply, 3...
- AC load, 4... Modulation circuit, 40... Comparator, 50... Function generator, 51... Switch, 52
...Absolute value circuit, 53...Comparator, R^, R
B...Resistor, C...Capacitor, eO...Signal wave, aS...Carrier wave, Q1 to Q4...Bipolar transistor. Figure 1

Claims (2)

[Claims] (1) A bridge-connected switching circuit that applies an AC voltage signal to a load by switching in both positive and negative directions;
A PWM inverter device comprising: a modulation circuit that modulates a carrier wave of a predetermined frequency with a signal wave corresponding to the frequency of an AC voltage to be applied to the load, and inputs the modulated output signal to the switching circuit; A PWM inverter device comprising a comparison means for detecting that the amplitude is smaller than a predetermined value, and a switching means for switching the frequency of the carrier wave to a frequency higher than an audible frequency based on the detection output of the comparison means. (2) The PWM inverter device according to claim 1, wherein the switching elements of the switching circuit are comprised of biborder transistors.