In recent years, as energy resources have become more limited, the continuous introduction of energy-efficient equipment and rapid advancements in technology have led to more efficient and widely used power-saving systems. These innovations have significantly improved convenience in both work and daily life.
1. Development Process of AC Motor Speed Control System
1.1 AC Motor Excitation Speed Regulation
In the early stages, a prime mover was used to drive a generator, and the output voltage of the generator was adjusted by controlling its excitation. This allowed for speed control of the driven motor and regulation of its active power output, as well as enabling the motor to start and stop.
1.2 Current Motor Controllable Rectification Speed Regulation
With technological progress, controlled rectifier technology began to be used, where the conduction time of thyristors was regulated to adjust the voltage. This significantly improved the response speed of the speed control system and effectively solved the issue of current interruption at low speeds. Thyristor-based speed control works by changing the conduction angle of the thyristor, which alters the waveform and effective value of the motor’s terminal voltage, thus achieving speed regulation.
1.3 Summary of Key Methods
1.3.1 Frequency Control Method
Frequency control adjusts the frequency of the stator power supply, thereby changing the motor’s synchronous speed. This is primarily achieved through a frequency converter, which can be either an AC-DC-AC inverter or an AC-AC inverter. Most modern systems use the AC-DC-AC type. The basic principle involves adjusting the input frequency to change the motor speed, based on the relationship between speed (n), frequency (f), slip (s), and pole pairs (p).
1.3.2 Pole Number Variation Speed Control
This method changes the number of stator poles in a squirrel-cage motor to adjust its speed. It is suitable for machines that do not require smooth speed adjustment, such as machine tools, elevators, and pumps. The advantages include stability and adjustable speed, making it a promising direction for future AC motor development.
2. Demonstration of AC Motor Speed Control System Programs
2.1 Single-Chip Speed Regulation
With the global shift toward digital control systems, full digitalization of speed control has become essential. This integration enhances the functionality of AC speed control systems and allows them to interact more effectively with information systems. As control strategies grow more complex, components like DSP chips, filters, and adaptive controls are increasingly used in vector and direct torque control. Buses such as STD, industrial PC, fieldbus, and CAN bus also play critical roles in automation applications.
2.2 PWM Speed Regulation
PWM control serves as the core of AC speed control systems, enabling the implementation of any control algorithm. Over time, PWM techniques have evolved from controlling voltage and current waveforms to optimizing magnetic flux and reducing noise. Space vector PWM, in particular, is widely used due to its high voltage utilization, simple control, and low harmonic distortion. While existing systems provide smooth speed regulation, they still fall short of DC systems in dynamic performance, especially under complex load conditions.
Asynchronous motors are inherently nonlinear and multivariable, making direct torque control challenging. However, by using coordinate transformations, the motor model can be simplified to resemble a DC motor, allowing for precise control of torque and flux through vector control.
3. Main Circuit Design of AC Motor Speed Control System
Beyond computers, speed control systems are vital in integrated equipment, machine tools, robots, electric vehicles, and train transmissions. Over time, AC speed control has become the dominant trend, gradually replacing traditional DC systems.
The inverter converts standard AC power into variable frequency power, enabling precise motor speed control. This allows for stable, pure sine wave output, ideal for various applications. The asynchronous motor is connected to a synchronous generator, and its speed is controlled via a frequency converter. Adjusting the excitation current helps regulate the generator's output voltage, ensuring both frequency and voltage are controllable.
PLC acts as the central control unit, sending speed signals to the inverter and managing motor speed. It also coordinates with the excitation unit to maintain synchronization. Real-time communication with a touchscreen provides user-friendly control and data display.
In a variable frequency constant pressure water supply system, the pressure is measured and the frequency is adjusted using a built-in PID regulator. The inverter’s frequency overrun signal helps manage pump operations, preventing water hammer effects by linking the pump’s outlet valve.
For a system with four pumps, one inverter, one PLC, and a pressure transmitter, the system dynamically adjusts the number of operating pumps based on demand. When pressure drops, additional pumps are activated; when demand decreases, some are turned off to save energy. This cycle ensures efficient operation and reduces electricity consumption.
Additionally, the system includes auxiliary circuits to allow manual adjustments in case of failure, ensuring continuous operation and maintaining normal system function.
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