KR19990042262A - Induction motor control method and control inverter - Google Patents

Abstract

The present invention relates to an inverter for controlling an induction motor, and generates an applied voltage of the motor by using a frequency corresponding to the rotational speed of the motor, a minimum voltage, and a slope of a profile, and generates an applied voltage of the motor and a preset maximum of the motor. Generates a modulation rate of the pulse width modulated signal from the voltage, calculates an increase in the rotation angle of the motor corresponding to the frequency, adds the increase in the calculated rotation angle and an increase in a predetermined delayed past rotation angle by Generate a current rotation angle of the motor, calculate a sine value required for spatial vector modulation in response to the current rotation angle, and use the sine value and the modulation rate for the respective pulse widths Generate a duty value indicative of the pulse width of the modulated signal, and generate a pulse width modulated signal of the three phases corresponding respectively to the duty value. This controller is implemented as a low-cost 8-bit microcontroller instead of an expensive 16-bit, reducing overall production costs and replacing peripherals with software to reduce the overall size.

Description

Induction motor control method and control inverter

The present invention relates to a general-purpose inverter for induction motor, and more particularly, to a control method of an induction motor and an inverter for controlling the same.

Inverters for induction motors, which are widely used for industrial use, have diversified their requirements as their needs increase. These requirements include high performance, convenient and rich man-machine interface (MMI), low cost and miniaturization.

Algorithms for controlling such induction motors are classified into a vector control method and a scalar control method. The vector control method is an algorithm that performs both the transient and steady state characteric control of the motor. The vector control method detects the information of the motor using various sensors for precise control of the motor and then provides it to the drive. Closed loop method is used. Because of these characteristics, vector control is used for high performance motor drives such as servo drives. However, in order to form a closed loop, not only various sensors are required, but also the algorithm is complicated, so a high performance processor must be used, and thus, there is a problem in that the price increases and the size thereof becomes relatively large.

The scalar control scheme is an algorithm that performs only static characteristic control of a motor and is also referred to as variable voltage variable frequency (VVVF) control. This method is simpler than the vector control method because it does not require a sensor in an open loop method, that is, a control command is only provided to the motor without knowing the information of the motor. Has the advantage of being cheap. Therefore, the VVVF type inverter is widely used as a general purpose inverter in the industrial field.

The typical inverter described above performs all control algorithms for motor operation, and generates a binary digital value corresponding to a pulse width modulated (PWM) signal or a pulse width modulated signal for the variable speed driving of the motor, a program or A memory element for storing data, the ROM including a sine value used for controlling the induction motor.

Typically, the controller used to control the induction motor typically uses a 16-bit microprocessor. This makes it difficult to program algorithms for controlling induction motors with 8-bit microprocessors, and because they use analog rather than digital methods, they require a lot of peripherals and performance degradation. However, when an 8-bit microprocessor is used in the inverter, a separate external ROM must be used because the capacity of the internal ROM to store the sine value is insufficient, and a digital IC such as a buffer or a latch connecting the peripheral device to the processor is required. There is a problem that it becomes bulky because it is additionally used.

Therefore, the present invention has been made to solve the above-mentioned problem, and an object thereof is to provide an inverter for controlling a small induction motor, which is configured using an inexpensive 8-bit microprocessor instead of a 16-bit microprocessor.

Another object of the present invention is an inverter for controlling a small induction motor configured to replace external memory ROM, RAM and other peripheral circuits for program code and execution, control pulse width modulated signals using an internal memory and a software program in an 8-bit microprocessor. For that purpose.

According to the present invention for achieving the above object, there is provided an inverter of an induction motor having three pairs of semiconductor switch elements each switched by a three-phase space vector pulse width modulated signal having a phase difference, the inverter being the motor. Means for inputting a frequency corresponding to the number of revolutions of the motor, a minimum voltage, a profile of a selected voltage versus frequency for the motor, and a slope value of the selected profile; Means for generating an applied voltage of the motor using the frequency, the minimum voltage and the slope of the profile; Means for generating a modulation rate of the pulse width modulated signal from an applied voltage of the motor and a preset maximum voltage of the motor; Means for calculating an increase in the rotation angle of the motor corresponding to the frequency; An adder for generating a current rotation angle of the motor by adding the calculated increase in rotation angle and an increase in a predetermined delayed past rotation angle; Means for calculating a sine value required for spatial vector modulation in response to the current rotation angle; Space vector modulation means for generating a duty value representing a pulse width of each pulse width modulated signal using the sine value and the modulation rate; Means for generating the three-phase spatial vector pulse width modulated signal corresponding to the duty value, respectively.

According to another embodiment of the present invention, there is provided a method of controlling an induction motor using a space vector pulse width modulated signal, the method comprising: voltage vs. frequency (V / F) having a linear or quadratic function relationship to the motor. Selecting a profile of; Inputting an operating frequency and a minimum driving voltage of the motor and a slope of the selected V / F profile; Generating an applied voltage (V) of the motor using the operating frequency, the minimum voltage and the slope; The maximum voltage preset for the applied voltage (V) and the motor ( Calculating a modulation rate (m) of the pulse width modulated signal using; Calculating an increase in the motor rotation angle corresponding to the input frequency; Delaying an increase of the calculated rotation angle by a preset period; Generating a current rotation angle of the motor by adding the increase of the rotation angle and the delayed increase; Calculating a sine value required for spatial vector modulation in response to the current rotation angle; And generating a three-phase spatial vector pulse width modulated signal having a different pulse width by using the sine value and the modulation rate.

1 is a block diagram schematically showing the configuration of an inverter for controlling an induction motor according to a preferred embodiment of the present invention;

2 is a waveform diagram showing a three-phase pulse width modulated signal generated by the inverter of the present invention;

3 is a circuit diagram showing in detail the configuration of the power module shown in FIG.

4 is a detailed configuration diagram of the inverter controller shown in FIG. 1;

5 is a waveform diagram of various types of profiles of voltage versus frequency selected in the present invention;

6 is a view for explaining the Piecewise Linear Approximation method used in the present invention,

7 is a timing diagram of a three-phase pulse width modulated signal before synchronization generated in an interrupt period according to the present invention;

8 is a timing diagram of a three-phase pulse width modulated signal synchronized in an interrupt period according to the present invention;

9 is a flowchart illustrating the operation of the inverter of the present invention;

10 is a flowchart for explaining an operation executed in the interrupt routine generated in FIG.

<Description of the code | symbol about the principal part of drawing>

10: power supply unit 20: diode rectifying unit

30 man machine interface unit 60 controller

140: space vector modulator 160: sine value calculator

182, 184: Register 190: delay circuit

192: dead time circuit

The above and other objects and various advantages of the present invention will become more apparent from the preferred embodiments of the present invention described below with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1, there is shown a schematic block diagram of an inverter for controlling an induction motor used in the present invention. The inverter includes a power supply unit 10, a control unit 20 consisting of a ROM 70, a RAM 80, a microcontroller 60 and a control signal unit 40, a man-machine interface unit 30, And an IGBT power module 50.

The power supply unit 10 supplies DC power of + 5V and + 15V required for the controller 60 and other peripheral circuits from an external 110 / 220V AC input.

The MMI unit 30 is a circuit for signal transmission between the user and the drive. The MMI unit 30 includes a button switch, a 7-segment circuit, and the like. The MMI unit 30 commands a command regarding the rotational speed of the motor generated from the user through the button switch 12. 60).

The controller 20 may be implemented using a PIC16C74 chip, the microcontroller 60 constituting the controller 20 performs all control algorithms for driving the motor 90, and drives the variable speed of the motor 90. A binary vector corresponding to the spatial vector pulse width modulated signal or the pulse width modulated signal is generated and provided to the control signal unit 40.

The control signal unit 40 converts a signal generated from the microcontroller 60 into a final pulse width modulation (PWM) signal for driving the motor 90. This pulse width modulated signal includes three pairs of pulse width modulated signals shown in Figs. 2A, 2B and 2C.

As shown, the three pairs of pulse width modulated signals comprise three phase component signals on (U, / U), (V, / V), (W, / W). These three-phase pulse width modulated signals each have a phase difference of 120 ° and are repeated at predetermined periods, that is, at frequencies of the pulse width modulated signals. At this time, "0" and "1" representing a level in each signal represent digital values of the pulse width modulated signal. These three pairs of pulse width modulated signals (U, / U), (V, / V), (W, / W) are provided to the IGBT power module 50.

The IGBT power module 50 has three pairs of IGBT semiconductor switches 52, 62; 54, 64; 56, 66 that provide three phase power to the motor 90 as shown in detail in FIG. 3. Each pair of semiconductor switches 52, 62; 54, 64; 56, 66 is a switching element, such as a thyristor, each in the form of a back-to-back on each of the motors 90. It is connected. The first semiconductor switches 52, 54, 56 and the second semiconductor switches 62, 64, 66 in each pair are connected to the first pulse width modulation signals U, V, and W provided from the control signal circuit 40. It is alternately turned on and off by the second pulse width modulated signal / U, / V, / W to provide 300V power applied through the lines A and B to the motor 90.

The level "0" provided from the control signal unit 40 means that the semiconductor switch inside the IGBT power module 50 is turned off, and the level "1" means the semiconductor switch is turned on. As will be appreciated, each pair of first and second semiconductor switches requires a dead time required to perform an interaction where the other side is turned off when one side is turned on. This dead time is normally assigned to 3-6 ms.

In this operation, when the user transmits the rotational speed command of the motor to the microcontroller 60 through the MMI unit 30, the microcontroller 60 is a ROM 70 and RAM 80, which are memory elements therein. A control signal for driving a motor is generated by executing a program stored in the controller. This control signal is finally converted into a signal suitable for driving the motor through the control signal circuit 40 to control the IGBT power module 50.

4, a detailed configuration is shown, including a microcontroller 60 and a hardware configuration configured in accordance with the present invention.

Microcontroller 60 is implemented as an 8-bit microprocessor, PIC16C74, which has first and second timers 180 and 186, PWM1 register 182 and PWM2 register 184 in hardware. (See FIG. 4). In addition, the microcontroller 60 includes a modulation rate generator 120, a rotation angle increase generator 130, a space vector modulator 140, an adder 150, a retarder 155, and a sine value calculator 160. And a retarder 170, these configurations may be performed in software in accordance with the present invention.

The MMI unit 10 includes a button 12, a seven segment display 14, and an EEPROM 16. The button 12 is used to select the rotational speed of the motor, and the rotational command generated by the button 12 is converted into a code previously promised by a program in the MMI program storage 110. This converted code generates the frequency f of the motor. This frequency command f Is provided to the V / F profile generator 120 and the K value generator 130.

EEPROM (16) is used to store the most recently used parameters in order to avoid the need to set a new drive information for the motor 90, in case of power failure or after restarting the inverter for a long time, speed and capacity This is not a problem, so a small serial EEPROM is used.

The seven segment display 16 displays various information of the motor such as frequency to the user.

The modulation rate generator 120 calculates a modulation rate of the pulse width modulated signal according to a profile suitable for the characteristics of the induction motor.

The VVVF method used in the present invention is a control method that maintains a constant ratio of frequency and voltage. As shown in FIGS. 5A and 5B, the frequency F and the input voltage V of the motor 90 are linearly proportional to each other. It has a linear function relationship. However, the V / F profile (i.e., slope) between the frequency F and the input voltage V, as shown in Figs. 5C and 5D, is a quadratic function depending on the characteristics and needs of the induction motor. May have a relationship.

In addition, there is a specific minimum frequency (about 3 Hz) and maximum frequency (about 60 Hz) regions in which a constant voltage is maintained without being proportional to frequency and input voltage according to the characteristics of the induction motor. Accordingly, there exists a minimum voltage Vmin that must be supplied to the motor and a maximum voltage Vmax that can be supplied to the motor corresponding to this minimum and maximum frequency. However, the minimum and maximum voltages have different values depending on the characteristics of the motor. Therefore, in order to operate the motor correctly, the user must select the V / F profile for the motor.

According to the present invention, the maximum voltage Vmax is set at a fixed value of about 173V, and the minimum voltage Vmin and the shape of the profile selected and the slope value k of the V / F profile are determined by the button 12. Optionally set to input. Accordingly, the voltage V applied to the motor can be calculated whenever the motor is driven using the input variables, for example, the minimum voltage Vmin, the frequency, and the slope k.

The V / F profile calculation performed by the modulation rate generator 120 may be performed according to Equation 1 and Equation 2 below according to a linear relationship between the frequency f and the applied voltage V of the motor. It is calculated by each.

In Equations 1 and 2, k is a slope, f is a frequency, and Vmin represents a minimum voltage. When the applied voltage V to the motor 90 is determined by the above equation, the frequency modulation rate "m" is calculated from the following equation.

In the above equation, Vmax represents the maximum voltage, and the frequency modulation index m calculated by this equation is provided to the space vector modulator 140.

On the other hand, the rotation angle increment generation unit 130 calculates the increment Δθ of the rotation angle θ corresponding to the frequency command provided from the button 12.

The rotation angle increment Δθ described above is generated from the following equation.

In the above equation Is a proportional constant determined by the program, which is determined by the program.

Increment of rotation angle θ generated by the rotation angle increase generation unit 130 ( ) Is provided to the adder 150.

The adder 150 generates a current rotation angle that changes every cycle of the PWM signal by adding an increment of the rotation angle and the past rotation angle delayed through the delay unit 155. This process can be represented by the following equation.

In the above equation, θ (k-1) is the past rotation angle passing through the delay unit 155, θ (k) is the current rotation angle, and Δθ is the rotation angle increment.

The current rotation angle, θ (k), generated by the adder 150 is provided to the sine value calculator 160.

The sine value calculator 160 calculates sin (θ) and sin (60 −θ) for spatial vector modulation by using the current rotation angle, θ (k) provided from the adder 150.

The sine values can be calculated by storing the sine table using the internal memory of the processor and the Piecewise Linear Approximation method. Because memory usage requires a large amount of memory, it is necessary to expand the ROM externally and is very poor in accuracy. Therefore, the present invention is configured to calculate the sine value using the Piecewise Linear Approximation method.

The Piecewise Linear Approximation method is a mathematical algorithm that determines a sine value by dividing a curved sine function into several smaller intervals and then forming a linear linear equation by treating the curve as if it is a straight line at small angles. The function is divided into parts, and FIG. 6B shows an enlarged sine function.

According to the present invention, as defined in the following equation, after dividing the sinusoidal interval from 0 ° to 60 ° into 15 sections at 4 ° intervals, the linear equations for each section are obtained.

The following table lists the linear equations corresponding to 15 (fifteen) intervals and the sin (θ) values obtained according to the linear equations, and the equalized intervals are not necessarily the same, It should be noted that the length of the intervals can vary and the number of intervals can be changed to a number of different values.

section θ。 sin (θ) Linear One 0-4 0.0000-0.0698 sin (θ) = 0.01745 (θ-0) + 0 2 4 to 8 0.0698-0.1392 sin (θ) = 0.01735 (θ-4) + 0.0698 3 8 to 12 0.1392-0.2079 sin (θ) = 0.01718 (θ-8) + 0.1392 4 12 to 16 0.2079-0.2756 sin (θ) = 0.01693 (θ-12) + 0.2079 5 16 to 20 0.2756-0.3420 sin (θ) = 0.01660 (θ-16) + 0.2756 6 20 to 24 0.3420-0.4067 sin (θ) = 0.01618 (θ-20) + 0.3420 7 24 to 28 0.4067-0.4695 sin (θ) = 0.01570 (θ-24) + 0.4067 8 28-32 0.4695-0.5299 sin (θ) = 0.01510 (θ-28) + 0.4695 9 32 to 36 0.5299-0.5878 sin (θ) = 0.01448 (θ-32) + 0.5299 10 36-40 0.5878-0.6428 sin (θ) = 0.01448 (θ-36) + 0.5878 11 40 to 44 0.6428-0.6947 sin (θ) = 0.01298 (θ-40) + 0.6428 12 44 to 48 0.6947-0.7431 sin (θ) = 0.01210 (θ-44) + 0.6947 13 48 to 52 0.7431-0.7880 sin (θ) = 0.01122 (θ-48) + 0.7431 14 52 to 56 0.7880-0.8290 sin (θ) = 0.01025 (θ-52) + 0.7880 15 56 to 60 0.8290-0.8660 sin (θ) = 0.00925 (θ-56) + 0.8290

Therefore, when the angle θ is input from the adder 150, the sine value corresponding to the angle is determined. In the linear equation of the above table, the real values are converted to an integer value of 12 bits and then provided to the space vector modulator 140.

The space vector modulator 140 calculates the duty of the PWM1, PWM2, and PWM3 signals (see FIG. 2) on the U, V, and W3 using the sine values provided from the sine value calculator 160. The PWM signal of the microcontroller 60 is registered in the interrupt signal and output. This interruption is repeated every certain period, which corresponds to the frequency of the PWM signal, that is, the switching period of the semiconductor switch. The PWM1 and PWM2 signals on U and V are then provided to adder 172 and adder 174, and PWM3 to subtractor 176, respectively.

Since the microcontroller 60 composed of the PIC16C74 used in the present invention has only two PWM ports, a three-phase synchronized PWM signal cannot be produced. Thus, the present invention is configured to generate a three-phase synchronized PWM signal using the internal timer of the microprocessor, PIC16C74. This process can be explained as follows.

As shown in Figs. 7A and 7B, the adder 172 and the adder 174 use the delay time of the preset period by the delay unit 170 to solve the synchronization problem of the three-phase U, V, and W PWM signals. Is added to the PWM1 and PWM2 signals, respectively. The above-described predetermined delay period is defined as the time from the occurrence of interrupt T1 until the interrupt decoding and interrupt flag processing and the PWM signal on W using the internal timer. As such, the duty values of the original U and V phases calculated by the space vector modulation scheme for synchronizing the three-phase PWM signal become a final duty value of 10 bits by adding the above-described delay time. When a new interrupt T2 is generated, the duty values of the U and V phases calculated in the previous cycle are latched into respective PWM registers 182 and 184, respectively.

The PWM1 and PWM2 signals on U and V in the PWM registers 182 and 184 are compared with the preset PWM signals in the timer 180. As a result of the comparison, if the value in the timer 180 is larger, the respective PWM1 and PWM2 signal levels are converted from 1 to 0 as shown in Figs. 2A and 2B. This level conversion is performed whenever a comparison result between the PWM1 and PWM2 signals on U and V in the PWM registers 182 and 184 and the preset PWM signal in the timer 180 is generated. The duty or modulated pulse width of the PWM signal being level converted is provided to the delay circuit 190.

On the other hand, as shown in FIG. 7C, the duty value on W in the subtractor 176 is subtracted from the FFFF (hexadecimal) value, converted to 16 bits, and provided to the timer 186. This is done to convert the 10-bit PWM3 signal on W into 16 bits because the timer 186 in the PIC16C74 microcontroller 60 used in the present invention is comprised of 16 bits. When the PWM3 signal converted to 16 bits is input to the timer 186 and the timer 186 reaches the final value (FFFF in 16 bits), a timer overflow in the above-described interrupt iteration periods T1 to T2. Generates a second timer interrupt T3. The signal level by PWM3 is changed from 1 to 0 by the second timer interrupt signal (see Fig. 2C). The signal for PWM3 is provided to the dead time circuit 192 in the control signal unit 40 directly through any port of the microcontroller 60.

The delay circuit 190 in the control signal unit 40 removes the PWM duty outputs on U and V of the respective PWM registers 182 and 184 by the delayed amount by the delay unit 170, thereby removing the W phase of the timer 186. Synchronize with PWM duty output. 8A, 8B and 8C show three phase PWM signal waveforms synchronized via delay circuit 190.

The dead time circuit 192 generates three pairs of PWM1, PWM2, and PWM3 signals having dead time as shown in FIG. 2 by the signals of PWM1 and PWM2 transmitted from the delay circuit 190 and the signals by PWM3. . These six signals are provided to each switch of the IGBT power module 50 to drive the motor.

9, a flowchart of the main routine for explaining the operation for generating the PWM signal of the above-described configuration is shown.

First, in step 210, when the system is operated, a software reset is performed upon power on to reset all data in the microcontroller 60.

In a next step 220, a set up process of the variables is carried out by the user via the button 12 in the MMI unit 10. This parameter setup can be set up directly by the user or indirect setup by communication with the EEPROM 16. Setup by EEPROM 16 can be started after power up without user setup again. Variables set up at this time include operation mode, operation frequency, acceleration / deceleration time, V / F profile and operation reservation time.

When this setup process is complete, at step 230, whenever the first and second timer interrupts are generated, it is switched to an interrupt routine that executes the PWM mode for motor operation described with reference to FIG.

Steps 240 and 250 determine which instructions are recognized during program execution and perform routines to interpret the instructions. At this point, if there is no command, the process proceeds to next steps 260 and 270.

Steps 260 and 270 include an online setup by the EEPROM as a routine to check if a new setup is to be made. If no new setup is required, the process proceeds to the next step 280 to output and display the current motor operating state in the seven segments 17 and then returns to the step 240.

FIG. 10 illustrates an interrupt routine performed according to an interrupt generated during the execution of the main routine shown in FIG. 9.

If an interrupt occurs during the execution of the main routine (step 310), the duty value of each phase calculated in the previous period is transferred to the PWM registers 182 and 184 of the microprocessor 60, corresponding to this duty in the next period. The PWM signal is output (step 320).

In step 330, the magnitude of the voltage applied to the motor 90 corresponding to the current operating frequency is determined through the V / F profile generator 120 according to the V / F profile set in the setup routine 220 of the main routine. The modulation rate m is calculated using this.

In step 340, the sine value is calculated through the K value generator 130, the adder 150, and the sine value generator 160.

In step 350, the duty of the PWM signal of each phase is calculated by the space vector modulator 140 according to this sine value. This duty value is transferred to the PWM registers 182 and 184 of the microprocessor 60 whenever a new interrupt occurs, and in the next period the PWM signal corresponding to this duty will be output to the motor 90.

As described above, according to the present invention, it is possible to configure an inverter for induction motor as a low-cost 8-bit microcontroller instead of a 16-bit microcontroller. Replacing ROM and RAM with internal memory and software in an 8-bit microcontroller offers the advantage that single-chip control circuits can be configured.

Claims (6)

In an inverter of an induction motor having three pairs of semiconductor switch elements each switched by a pulse width modulated signal having three phases each having a phase difference, Means for inputting a frequency corresponding to the number of revolutions of the motor, a minimum voltage, a profile of voltage versus frequency selected for the motor, and a slope value of the selected profile; Means for generating an applied voltage of the motor using the frequency, the minimum voltage and the slope of the profile; Means for generating a modulation rate of the pulse width modulated signal from an applied voltage of the motor and a preset maximum voltage of the motor; Means for calculating an increase in the rotation angle of the motor corresponding to the frequency; An adder for generating a current rotation angle of the motor by adding the calculated increase in rotation angle and an increase in a predetermined delayed past rotation angle; Means for calculating a sine value required for spatial vector modulation in response to the current rotation angle; Space vector modulation means for generating a duty value representing a pulse width of each pulse width modulated signal using the sine value and the modulation rate; And means for generating the three-phase pulse width modulated signal corresponding to the duty value, respectively. The method of claim 1, wherein the inverter, Means for generating an interrupt of a period corresponding to the frequency of the pulse width modulated signal when the duty value is generated, The pulse width modulated signal generating means, First and second storage means for respectively storing at least two first and second duty values of the calculated three-phase duty values each time the interrupt is generated; First timer means for counting the frequency; Comparing the respective first and second duty values stored in the first and second storage means with a first preset reference value counted by the first timer means, wherein the reference value is greater than the respective duty value. Means for varying the level of the pulse width corresponding to each duty value; Second timer means for starting the counting of the frequency from a third duty value of the other one of the duty values of three phases each time the interrupt is generated; And means for varying a level of a pulse width corresponding to the third duty value when the counted value of the second timer means reaches a second preset value. 2. The inverter of claim 1, wherein the sine value calculating means calculates the sine value using piecewise linear apparoximation. In the induction motor control method using a space vector pulse width modulated signal, Selecting a profile of voltage versus frequency (V / F) having a linear or quadratic function relationship for the motor; Inputting an operating frequency and a minimum driving voltage of the motor and a slope of the selected V / F profile; Generating an applied voltage (V) of the motor using the operating frequency, the minimum voltage and the slope; The maximum voltage preset for the applied voltage (V) and the motor ( Calculating a modulation rate (m) of the pulse width modulated signal using; Calculating an increase in the motor rotation angle corresponding to the input frequency; Delaying an increase of the calculated rotation angle by a preset period; Generating a current rotation angle of the motor by adding the increase of the rotation angle and the delayed increase; Calculating a sine value required for spatial vector modulation in response to the current rotation angle; And generating a three-phase pulse width modulated signal having different pulse widths by using the sine value and the modulation rate. The method of claim 4, wherein the pulse width modulation signal generation step, Generating a duty value representing the pulse width of each pulse width modulated signal using the sine value and the modulation rate; Generating an interrupt of a period corresponding to a period of the pulse width modulated signal when the duty value is generated; Each time the interrupt is generated, at least two first and second duty values of the calculated duty values are compared with respective preset reference values to determine the respective duty when the reference value is greater than the respective first and second duty values. Varying the level of the pulse width corresponding to the value; Varying the level of the pulse width corresponding to the third duty when the interrupt is generated and upon reaching a value counted from the third duty value of the other one of the duty values; Control method. The method of claim 4, wherein the sine value is calculated using Piecewise Linear Apparoximation in the sine value calculation step.