Design of PLC-based variable frequency fan control system
Cai Xueming
(Yantai Environmental Equipment Co., Ltd., Yantai, China)
Abstract: Fans are essential equipment in mining enterprises and are closely related to their production efficiency. With increasing energy scarcity, energy conservation in these enterprises has become particularly important. This design uses a PLC to detect the number of electric saws in operation, and then combines this with a frequency converter to control the fan speed, thereby achieving energy-saving effects.
Keywords: PLC; frequency converter; fan
ToDesignofControlSystemforFrequencyConverterFanBasedonMATLAB
Foreword
my country has relatively limited fossil fuel reserves, ranking third in the world in coal reserves and only tenth in oil and natural gas reserves. Therefore, energy conservation is a fundamental national policy. Statistics show that 60% of the nation's electricity generation is converted into energy through motors. The 11th Five-Year Plan placed great emphasis on energy conservation in motor systems. Energy conservation in motor systems is mainly reflected in two aspects: first, energy-efficient motors, such as permanent magnet synchronous motors; and second, energy-saving motor speed regulation. Variable frequency speed regulation offers advantages such as small size, light weight, high torque, high precision, powerful functions, high reliability, simple operation, and convenient communication.
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert mains frequency power into electrical energy of another frequency. The frequency converters we use today mainly employ an AC-DC-AC method (VVVF frequency conversion or vector control frequency conversion). First, the mains frequency AC power is converted into DC power by a rectifier, and then the DC power is converted into AC power with controllable frequency and voltage to supply the motor. The circuit of a frequency converter generally consists of four parts: rectification, intermediate DC link, inversion, and control. The rectification section is a three-phase bridge uncontrolled rectifier, the inversion section is an IGBT three-phase bridge inverter with a PWM waveform output, and the intermediate DC link is for filtering, DC energy storage, and reactive power buffering.
Frequency converters can save energy in four ways. First, soft starting. The starting current of a typical AC motor is 6-7 times its rated current. After frequency conversion speed regulation, the starting current does not exceed the rated current of the motor. Second, saving design redundancy. General designs are based on extreme operating conditions, thus containing design redundancy, sometimes quite large, resulting in an oversized motor for a small load. Frequency conversion speed regulation can save this redundancy. Third, speed regulation saves energy. According to fluid mechanics principles, shaft power is proportional to the cube of speed; as speed decreases, shaft power decreases. This is the main energy-saving principle of frequency conversion speed regulation. Fourth, high system power factor. Generally above 0.95, saving reactive power and reducing the burden on the transformer.
1. Variable Frequency Drive Speed Regulation Principle and Control Method
1.1 Variable Frequency Drive (VFD) Debugging Principle
The expression for the synchronous speed of an AC motor is as follows:
n = 60f(1 - s)/p(1)
In the formula: n is the speed of the asynchronous motor;
F is the frequency of the asynchronous motor;
S is the slip of the electric motor;
P represents the number of pole pairs of the electric motor.
As shown in equation (1), the rotational speed n is directly proportional to the frequency f. Changing the frequency f changes the motor's rotational speed. When the frequency f varies within the range of 0–50 Hz, the motor's speed adjustment range is very wide. A frequency converter achieves speed regulation by changing the motor's power supply frequency, making it an ideal, highly efficient, and high-performance speed control method.
1.2 Inverter Control Method
The low-voltage general-purpose frequency converter has an output voltage of 380～650V, an output power of 0.75～400kW, and an operating frequency of 0～400Hz. Its main circuit adopts an AC-DC-AC circuit.
1.2.1 Voltage Space Vector (SVPWM) Control Method
Voltage space vector control (VSL) prioritizes the overall generation of three-phase waveforms and aims to approximate the ideal circular rotating magnetic field trajectory of the motor's air gap. It generates three-phase modulated waveforms in a single pass and controls the system by approximating a circle using an inscribed polygon. Feedback estimation of flux linkage amplitude eliminates the influence of stator resistance at low speeds. Further improvements were made through practical application, including the introduction of frequency compensation to eliminate speed control errors and closed-loop control of output voltage and current to enhance dynamic accuracy and stability. However, the control circuitry is complex and lacks torque regulation, thus the system performance is not fundamentally improved.
1.2.2 Direct Torque Control (DTC) Method
This technology largely solves the shortcomings of the aforementioned vector control and has rapidly developed due to its novel control concept, simple and clear system structure, and excellent dynamic and static performance. Currently, this technology has been successfully applied to high-power AC drives for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system, controlling the motor's flux linkage and torque. It does not require equating the AC motor to a DC motor, thus eliminating many complex calculations in vector rotation transformation; it does not require mimicking the control of a DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling.
1.2.3 Vector Control (VC) Method
Vector control variable frequency speed regulation method equates an AC motor to a DC motor, independently controlling the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current, the torque and magnetic field components are obtained. Through coordinate transformation, orthogonal or decoupled control is achieved. The proposal of the vector control method is epoch-making. However, in practical applications, due to the difficulty in accurately observing the rotor flux linkage, the significant influence of motor parameters on system characteristics, and the complexity of the vector rotation transformation used in the equivalent DC motor control process, the actual control effect is difficult to achieve the ideal analytical results.
1.2.4 Sinusoidal Pulse Width Modulation (SPWM) Control Method
Sinusoidal pulse width modulation (PWM) control is characterized by its simple control circuit structure, low cost, and relatively good mechanical stiffness, meeting the smooth speed regulation requirements of general drives. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, reducing the maximum output torque. Furthermore, its mechanical characteristics are ultimately not as stiff as those of a DC motor, resulting in unsatisfactory dynamic torque capability and static speed regulation performance. The system performance is also low, the control curve changes with load variations, torque response is slow, motor torque utilization is low, and performance degrades at low speeds due to stator resistance and inverter dead-zone effects, leading to decreased stability. Therefore, vector control variable frequency speed regulation has been developed.
1.2.5 Matrix-based AC-AC control method
VVVF frequency converters, vector control frequency converters, and direct torque control frequency converters are all types of AC-DC-AC frequency converters. Their disadvantages include low input power factor, high harmonic current, the need for large energy storage capacitors in the DC circuit, and the inability to feed regenerated energy back to the grid. To address these issues, matrix AC-AC frequency converters were developed. Because matrix AC-AC frequency converters eliminate the intermediate DC link, they also eliminate the need for large and expensive electrolytic capacitors. They can achieve a power factor of 1, sinusoidal input current, and four-quadrant operation, resulting in high system power density. Their control mechanism does not indirectly control current or flux linkage, but rather directly uses torque as the controlled variable. The specific method is as follows:
1. Control the stator flux linkage by introducing a stator flux linkage observer to achieve a sensorless speed mode;
2. Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
3. Calculate the actual values corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
4. Implement Band-Band control: Generate PWM signals based on flux linkage and torque Band-Band control to control the inverter switching state.
Matrix AC-AC converters have fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%). They also have high starting torque and high torque accuracy, especially at low speeds (including 0 speed), where they can output 150% to 200% torque.
2. System Hardware Design
2.1 Selection of Frequency Converter
Since the fan used in this design is a three-phase 380V 11KW, the HLP-A series HLPA001143B frequency converter will be selected. The features of this frequency converter are as follows:
With a large-scale motor control IC and IGBT as the core, it has multiple protection functions and high overall reliability.
It has strong adaptability to incoming line voltage fluctuations of up to ±20%, making it particularly suitable for countries and regions with poor power grid quality.
The software is powerful and has multiple built-in control modes, making it widely applicable to control requirements in various industrial settings.
With a built-in PID controller, it can be easily configured into a closed-loop control system.
It has a built-in simple PLC and has multiple functions such as traction, disturbance, multi-speed control, and program operation.
High output torque, up to 150% at 1Hz, with a frequency resolution of up to 0.01Hz.
It has strong overload capacity, 150% (1 minute), 200% (2 ms).
It has a good communication interface and is very easy to form a centralized control system.
2.2 PLC Selection
According to the design requirements, the PLC in the equipment needs to have 7 inputs and 4 outputs; therefore, this design will use the Omron CPM1A series for discussion. Its features are as follows:
1. Rich instruction set; 2. Simple and reliable installation; 3. Compact and small structure;
4. Flexible system configuration; 5. Powerful analog signal processing capabilities; 6. A wide variety of modules;
7. High-performance motion control; 8. Excellent control functions; 9. Unique power-off protection;
10. Practical offline simulation; 11. Standard programming language; 12. Strong communication capabilities;
2.3 Selection of Fuse Type
The type of fuse should be selected based on the protection characteristics of the load and the magnitude of the short-circuit current. For fuses protecting lighting and motors, generally only overload protection is considered, and the melting coefficient of the fusible element should be appropriately small. For high-capacity lighting circuits and motors, in addition to overload protection, the ability to interrupt short-circuit current should also be considered when selecting the fuse. When the short-circuit current is large, fuses with high breaking capacity or even current-limiting functions should be used. Furthermore, the rated voltage of the fuse should be determined based on the voltage of the circuit to which it is connected.
The rated current of a fuse depends on the size and nature of the load. For stable loads without inrush current, such as general lighting circuits and electric heating circuits, the rated current of the fuse can be determined based on the load current. For motor loads with inrush current, to achieve short-circuit protection while ensuring normal motor starting, the rated current of the fuse for a squirrel-cage induction motor is: INP = (1.5~2.5)INM. Since the estimated current is 20A, a coefficient of 2.0 is selected. Therefore, INP = 2.0 * 20A = 40A. Thus, a Delixi RL1 series spiral fuse, model: RL1-40, is selected.
3. Main circuit diagram
In this design, the main function of the PLC is to acquire the chainsaw switch signals, calculate and control the output frequency of the frequency converter. This includes acquiring the chainsaw's operating signal.
When the chainsaw is in operation, a pair of normally open auxiliary contacts of the control contactor control an intermediate relay. The intermediate relay should have at least two pairs of normally open contacts. One pair is connected to an input point of the PLC, and the other pair controls an air valve. The air valve then drives a cylinder, which in turn opens and closes the air vents on the equipment. This achieves both the PLC receiving the signal to activate the chainsaw and the automatic opening and closing of the air vents, making it simple and practical. The overall control diagram of the system is shown in Figure 1.
4. Software System Design
The system designed in this paper uses a signal system to perform data analysis and calculate the air volume required to start different motors and the power consumption of the system. The control flowchart is shown in Figure 2.
5. Conclusion
With the rapid development of my country's economy, microelectronics, computer technology, and automation control technology have all advanced rapidly, and AC variable frequency speed control technology has also entered a new era, with its applications becoming increasingly widespread. The system designed in this paper is applied to a chainsaw workshop, using an 11KW dust extraction fan to clean sawdust. There are five chainsaws in the workshop. The required airflow varies depending on the number of chainsaws operating, meaning the required fan speed also differs. The fan speed is controlled by a PLC-controlled frequency converter, thereby achieving energy-saving effects.
