JP2522407B2 - Pulse generator for pulse width modulation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention generates a pulse width modulation pulse in a state where a carrier wave is synchronized with a modulated wave when the frequency of the modulated wave is high, and the frequency of the modulated wave is The present invention relates to a pulse width modulation pulse generator that generates a pulse width modulation pulse when a carrier wave is in an asynchronous state and can smoothly switch from synchronous to asynchronous or from asynchronous to synchronous.

[Conventional technology]

Since the pulse width modulated pulse train is obtained by proportionally comparing the magnitude relationship between the sine wave modulated wave and the triangular wave carrier having a frequency higher than this sine wave, the inverter for converting DC to AC By using a pulse width modulated (abbreviated as PWM in the following) pulse, it is possible to efficiently convert a sinusoidal waveform with a low harmonic content rate to alternating current. I am calling it.

By the way, in this PWM inverter, when the frequency of the modulated wave is changed, the frequency of the output AC changes correspondingly, so by using the PWM inverter as the power source for the induction motor, which was difficult to control speed in the past. Since it can be driven at a desired speed, it has become popular.

[Problems to be Solved by the Invention]

In generating the above-mentioned PWM pulse, conventionally, the modulated wave and the carrier wave are asynchronously pulse-width modulated. When driving a large capacity electric motor with a PWM inverter,
Since the semiconductor switch element that constitutes the inverter is a thyristor that cannot increase the switching frequency, the fundamental wave component changes due to the phase difference between the modulated wave and the carrier wave, causing the output voltage of the PWM inverter to beat. There was an inconvenience.

Therefore, an object of the present invention is to avoid the inconvenience of causing beats in the voltage by synchronizing the modulated wave and the carrier wave. However, if the carrier wave is synchronized even when the modulated wave has a low frequency, the frequency of the carrier wave becomes low, and this is also the problem to be solved.

[Means for solving the problem]

In order to achieve the above-mentioned object, a pulse width modulation pulse generator according to the present invention compares a magnitude relationship between a variable frequency modulation wave and a triangular waveform modulation wave to generate a pulse width modulation pulse. In the generating device, pulse generating means for generating a pulse train having a frequency proportional to the frequency command value of the modulated wave, an up-down counter for up-counting this pulse train during forward rotation and a down-counting during reverse rotation, and the carrier wave data. And a memory for inputting the signal from the up / down counter, a signal from this memory, and a modulation rate signal set separately, and by comparing the magnitude relationship between the two, synchronous pulse width modulation A digital comparator for outputting a pulse for use, a timing for emitting a pulse at the first time point when the phase of the carrier wave is 90 degrees and the second time point when the phase of the carrier wave is 270 degrees And Guparusu generating means, the first
First time calculating means for calculating a first time from the time point until the carrier wave and the modulated wave cross each other between the 90-degree phase and the 270-degree phase of the carrier wave;
A second time for calculating a second time until the carrier wave and the modulated wave cross each other between the 270-degree phase and the 90-degree phase of the next cycle.
Time calculation means, and the first time point, the first time, and the second time point
Asynchronous pulse generating means for generating an asynchronous pulse width modulation pulse by inputting a time point and a second time, and switching of the synchronous pulse width modulation pulse and the asynchronous pulse width modulation pulse from synchronous to asynchronous Is a switching command when the phase of the modulated wave is 0 °, 120 ° or 240 °, and the phase of the modulated wave is 0 °, 120 ° or 240 ° from the asynchronous to the synchronous during forward rotation, and the carrier wave Phase of 270 degrees to 360
The switching command is issued at a point in time between 0 degree and the phase of the modulated wave is 0 degree or 120 degree or 240 degree and the phase of the carrier wave is from 0 degree to 90 degree from asynchronous to synchronous at the time of reverse rotation. According to the instruction of the synchronous / asynchronous switching means and the synchronous / asynchronous switching means that issues a switching command at a point in time
A data selector for selecting either the synchronous pulse width modulation pulse or the asynchronous pulse width modulation pulse.

[Action]

According to the present invention, in a region where the frequency of the modulated wave is high, the frequency of the carrier wave is set to an integral multiple of 3 of the frequency of the modulated wave, and the PWM pulse is generated in a state where both are synchronized,
The beat of the voltage is suppressed, the switching loss is reduced, and the harmonics are reduced. In the low modulated wave frequency range, the carrier frequency is kept constant to generate asynchronous PWM pulses to improve controllability.
Therefore, it is necessary to switch between the synchronous PWM pulse and the asynchronous PWM pulse. However, since the modulation ratio when the synchronous PWM pulse is generated is an integer multiple of 3 as described above, the phase of the modulated wave is zero degrees or 120 degrees. At 240 degrees, the carrier phase is zero degrees and the slopes are the same.Therefore, at this point, the synchronous PWM pulse and the ratio synchronous PWM pulse are switched to force the carrier synchronization. It is intended to realize smooth switching by avoiding the unreasonableness caused by the above.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the present invention.

In FIG. 1, the frequency signal of the modulated wave sent as n1 bit data is replaced in the pulse generation circuit 11 by a series of pulse trains.

FIG. 2 is a circuit diagram showing the configuration of the pulse generating circuit shown in the embodiment circuit of FIG. 1, which includes a clock 15, a plurality of adders 16A, 16B ... 16C and a plurality of flip-flops 17A, 17B. …… It consists of 17C. With this configuration, the frequency of D ₁ ~
The pulse train of D _n1 is output from this pulse generating circuit, but in the circuit shown in FIG. 2, the frequency of the pulse train continuously changes in response to changes in the data of D _{1 to} D _n1 .

When the pulse output from the pulse generation circuit 11 having the configuration shown in FIG. 2 is used as a trigger, the up / down counter 12 counts up when the rotation direction command is forward rotation, for example. By doing n2
It forms the address data of the memory 13 of bits. Since the memory 13 stores the triangular wave data of the triangular wave carrier, when the output of the memory 13 and the modulation rate signal are input to the digital comparator 14, the both are compared to obtain the synchronous PWM pulse. Is created and sent to the data selector 12. When this PWM pulse is used in a three-phase inverter, the memory 13 naturally outputs three-phase data whose phases are shifted from each other by 120 degrees.

Even in a region where the modulation wave frequency is low, if the carrier wave frequency is made proportional to the modulation wave frequency, the carrier wave frequency value becomes small, which causes inconvenience such as deterioration of controllability. Therefore, even if the frequency of the modulated wave becomes equal to or lower than a predetermined value, the above-mentioned inconvenience is avoided by maintaining the constant value of the carrier frequency, but at this time, the carrier becomes asynchronous with the modulated wave.

FIG. 3 is an operation waveform diagram showing the operation of the asynchronous pulse generating circuit shown in the embodiment circuit of FIG. 1, and FIG. 3 (a) is the waveform of the modulated wave and the carrier wave, and FIG. 3 (b). Is asynchronous
Changes in the PWM pulse, and Fig. 3 (c) shows the situation of the timing pulse.

The timing pulse generation circuit 21 in FIG. 1 inputs timing pulses (see FIG. 3C) for detecting the time points when the phase of the carrier wave is 90 degrees and 270 degrees to the asynchronous pulse generation circuit 24.

The triangular wave carrier and the modulated wave cross once for the phase of the carrier between 90 degrees and 270 degrees, and once for the phase between 270 degrees and 90 degrees of the next cycle. (See FIG. 3 (a)). Therefore, with reference to the timing pulses at 90 degrees and 270 degrees described above, the first time T ₁ until the carrier wave and the modulated wave intersect is calculated by the first time calculation circuit 22, and the second time T ₂ is calculated. The second time arithmetic circuit 23 performs arithmetic operation, and each data is given to the asynchronous pulse generating circuit 24.
Since the asynchronous PWM pulse shown in FIG. 7B is created, it is sent to the data selector 26.

The synchronous / asynchronous switching circuit 25 commands the data selector 26 to switch at an appropriate timing in response to the issuance of the synchronous / asynchronous switching command (that is, when the frequency of the modulated wave passes the predetermined value described above). If so, this switching is performed when the following conditions are satisfied.

a) The switching from the synchronous PWM pulse to the asynchronous PWM pulse is performed at the time when the voltage phase of the modulated wave is zero degrees, 120 degrees, or 240 degrees regardless of the forward rotation and the reverse rotation.

b) Switching from asynchronous PWM pulse to synchronous PWM pulse
During forward rotation, the voltage phase of the modulated wave is 0 to 120 degrees or 240
And the carrier phase is between 270 and 360 degrees.

c) Switching from asynchronous PWM pulse to synchronous PWM pulse
The reverse rotation is performed when the voltage phase of the modulated wave is zero degrees, 120 degrees, or 240 degrees, and the carrier wave phase is between zero degrees and 90 degrees.

As described above, since the modulation ratio is an integral multiple of 3, the carrier wave phase is zero at the time of the modulated wave phase of 0 °, 120 °, and 240 °, so switching between synchronization and ratio synchronization is smooth. Can be carried out.

〔The invention's effect〕

According to the present invention, when a modulated wave frequency is equal to or higher than a predetermined value, a carrier wave frequency is set to an integer multiple of 3 to create a PWM pulse in a state where both are synchronized, and the modulated wave frequency is set to the predetermined value. When it falls below, by setting the carrier frequency to a constant value
A PWM pulse is created. By doing so, harmonics and switching loss can be reduced in the high frequency region, and controllability can be improved in the low frequency region. Further, smooth switching can be performed by switching between synchronous and asynchronous at an appropriate timing when the carrier wave has a zero phase. Therefore, if this PWM pulse generator is applied to an inverter to drive an electric motor, the electric motor can smoothly operate in a wide speed range from low speed to high speed.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a circuit diagram showing the configuration of the pulse generating circuit shown in the embodiment circuit of FIG. 1, and FIG. 3 is an operation waveform diagram showing the operation of the asynchronous pulse generating circuit shown in the embodiment circuit of FIG. is there. 11 …… Pulse generator, 12 …… Up / down counter,
13 …… Memory, 14 …… Digital comparator, 15 ……
Clock, 16A, 16B, 16C ... Adder, 17A, 17B, 17C ... Flip-flop, 21 ... Timing pulse generation circuit, 22
...... First time arithmetic circuit, 23 ...... Second time arithmetic circuit, 24 ...
… Asynchronous pulse generation circuit, 25 …… Synchronous / asynchronous switching circuit, 26 …… Data selector.

Claims (1)

(57) [Claims] 1. In a device for generating a pulse for pulse width modulation by comparing the magnitude relationship between a modulated wave of a variable frequency and a modulated wave of a triangular waveform, a frequency proportional to the frequency command value of the modulated wave. Pulse generating means for generating the pulse train, an up-down counter for up-counting this pulse train during forward rotation and a down-counting during reverse rotation, and a memory for storing the data of the carrier wave and inputting a signal from this up-down counter. , A signal from this memory, and a modulation rate signal that is set separately, are input, a digital comparator that outputs a pulse for synchronization pulse width modulation by comparing the magnitude relationship between the two, and the carrier phase is 90 degrees. Timing pulse generating means for issuing a pulse at the first time point of the second time point of the 270 degree phase, and from the first time point of the 90 degree phase of the carrier wave First time calculating means for calculating a first time until the carrier wave and the modulated wave cross each other up to a phase of 270 degrees, and from the second time point, from a phase of 270 degrees of the carrier wave to a phase of 90 degrees of the next cycle. The second time calculating means for calculating the second time until the carrier wave and the modulated wave cross each other, and the first time point and the first time point and the second time point and the second time point, and Asynchronous pulse generating means for generating an asynchronous pulse width modulation pulse, and switching between the synchronous pulse width modulation pulse and the asynchronous pulse width modulation pulse are switched from synchronous to asynchronous by setting the phase of the modulated wave to zero degrees or 120 degrees.
The change command is issued at the time of 0 degree or 240 degree, and the phase of the modulated wave is 0 degree, 120 degree or 240 degree, and the phase of the carrier wave is from 270 degree to 360 degree from asynchronous to synchronous at the time of forward rotation. The switching command is issued at some point during the period, and the phase of the modulated wave is zero degrees, 120 degrees, or 240 degrees and the phase of the carrier wave is from zero degrees to 90 degrees from asynchronous to synchronous during reverse rotation. Consists of a synchronous / asynchronous switching means for issuing a switching command at a certain time point, and a data selector for selecting either the synchronous pulse width modulation pulse or the asynchronous pulse width modulation pulse according to the command of the synchronous / asynchronous switching means. A pulse generator for pulse width modulation, which is characterized by: