SPWM (Sinusoidal PWM) is a relatively mature and widely used PWM method. A key conclusion in the sampling control theory mentioned earlier is that narrow pulses of equal impulse but different shapes applied to an inertial circuit produce essentially the same effect. SPWM is based on this conclusion, using a PWM waveform—the pulse width of which varies sinusoidally and is equivalent to a sine wave—to control the switching of devices in the inverter circuit. This ensures that the area of the output pulse voltage is equal to the area of the desired output sine wave within the corresponding interval. By changing the frequency and amplitude of the modulation wave, the frequency and amplitude of the inverter circuit's output voltage can be adjusted.
SPWM variable frequency speed control principle
Working principle of SPWM variable frequency speed control:
Pulse width modulation (PWM) technology utilizes the concept of "modulation" in communication technology, using the desired waveform as the modulating wave and the signal to be modulated as the carrier wave.
SPWM control mode:
The SPWM modulation wave determines the on/off timing of the inverter's switching devices based on the intersection of the triangular wave and the sine wave.
Applications of SWPM
SA8281 SPWM Wave Generator: Principle and Application in Frequency Converters
Pulse width modulation (PWM) technology controls the switching of elements according to a certain pattern to obtain a set of rectangular pulse waveforms with equal amplitude but unequal width, which approximates a sinusoidal voltage waveform. The application of PWM technology in inverters has greatly promoted the development of modern power electronics technology and modern speed control systems. In recent years, due to the continuous emergence of field-controlled self-turn-off devices, the corresponding high-frequency SPWM (sinusoidal pulse width modulation) technology has been widely used in motor speed control. The SA8281, launched by MITEL, is an integrated circuit for three-phase SPWM wave generation and control. It has convenient microprocessor interface, built-in waveform ROM and corresponding control logic. After setting, it can independently generate three-phase PWM waveforms. Microprocessor intervention is only required when the output frequency or amplitude needs to be changed. The microprocessor controls it in a very short time, thus enabling it to perform system-wide detection, protection, and control. Inverters based on SA8281 and 89C52 have advantages such as simple circuitry, complete functions, high performance-price ratio, and high reliability.
microcontroller generation
Many microcontrollers on the market have the function of generating SPWM control waveforms. These generated waveforms can be connected to an external drive circuit to drive the power bridge and achieve the purpose of inversion. It should be said that any microcontroller with a PWM module and a timer module can accomplish this task.
The specific implementation involves first assigning the sine table to an array. Then, the PWM waveform generation module enters an interrupt every PWM cycle, and in the ISR, it changes the value of the PWM comparator according to the sine table, and so on in a loop.
SPWM variable frequency speed control method in AC servo motors
One of the key components in an AC motor variable frequency speed control system is the inverter. Due to the speed control requirements, the inverter must have functions such as continuously adjustable frequency and continuously adjustable output voltage, and maintain a certain proportional relationship with the frequency.
A pulse width modulation (PWM) inverter can achieve both frequency conversion and voltage transformation. As shown in the figure, since the average voltage value is proportional to the duty cycle, changing the duty cycle of the output voltage pulse while adjusting the frequency can simultaneously achieve frequency conversion and voltage transformation. Compared to Figure 1(a), the voltage period shown in Figure 1(b) is increased (frequency decreases), while the duty cycle is decreased, thus the average voltage decreases.
When using PWM to control the switching on and off of the inverter diodes, a set of rectangular pulses with equal amplitude and width can be obtained. Changing the width of the rectangular pulses controls the output voltage, and changing the modulation period controls the output frequency, thus achieving both voltage and frequency conversion. Because the output voltage waveform is a rectangular wave, it contains many high-order harmonic components. For the motor, only the fundamental voltage is useful. To reduce the impact of harmonics and improve the motor's operating performance, a symmetrical three-phase sinusoidal power supply should be used to power the three-phase AC motor.
The output of a sinusoidal pulse width modulation (SPWM) inverter can obtain a set of rectangular pulse waveforms with equal amplitude but unequal width, which can be approximated as a sinusoidal voltage wave. The SPWM waveform is shown in Figure 2. When the sine value is at its maximum, the pulse width is also at its maximum, while the pulse interval is at its minimum. Conversely, when the sine value is small, the pulse width is small, while the pulse interval is large. This series of voltage pulses can significantly reduce the high-order harmonic components in the load current.
SPWM inverters using digital circuits can employ software-based control modes. Their advantages include less hardware requirements, high flexibility, and strong intelligence. Disadvantages include the need to calculate and determine the SPWM pulse width, resulting in some delay and response time. With the development of high-speed, high-precision, multi-functional microprocessors, microcontrollers, and dedicated SPWM chips, microcomputer-controlled digital SPWM technology has become dominant in today's PWM inverters.
The functions of each component in an SPWM variable frequency speed control system are as follows:
Speed setter: Given a signal, it controls the frequency, voltage, and forward/reverse rotation.
Smooth start circuit: The acceleration and deceleration times during startup can be set according to the mechanical load to achieve the purpose of soft start.
Function generator: Keeps the air gap magnetic flux of the motor constant when outputting low-frequency signals, compensating for the effect of stator voltage drop.
Voltage-frequency transformer: Converts voltage into frequency, generates a square wave through a frequency divider and a ring counter, and sends it together with a triangular wave generated by a triangular wave generator into the modulation circuit.
Voltage regulator: Generates a control sine wave with adjustable frequency and amplitude, which is fed into the modulation circuit. In the modulation circuit, SPWM conversion is performed to generate a three-phase pulse width modulation signal. The signal is then output to the base of the power transistor in the base circuit, thus controlling the main SPWM circuit and achieving variable frequency speed control of the permanent magnet AC servo motor .
Current detector: Overload protection.
