Abstract : Previous simulations of asynchronous motor variable frequency speed control systems mainly focused on the constant frequency and constant voltage state of the frequency converter. However, actual variable frequency speed control systems are often in a dynamic speed regulation process, with output frequency and output voltage changing over time. Using Simulink and PowerSystemBlocksets, a dynamic simulation model of an asynchronous motor variable frequency speed control system was established. By setting the voltage and frequency variation patterns over time, the dynamic speed regulation process can be simulated. This paper introduces the model establishment method and uses the constant voltage-frequency ratio adjustment mode as an example to simulate the constant acceleration condition. Time-frequency analysis of the simulated output voltage shows that the extracted frequency variation pattern perfectly matches the set value; Park vector analysis of the simulated output current shows that the obtained vector trajectory diagram is consistent with the theoretical inference. These findings demonstrate that the model is feasible and effective.
Keywords : asynchronous motor; variable frequency speed control; instantaneous frequency; SPWM
1 Introduction
Variable frequency speed control motor systems are widely used in the transmission field, and their technology is becoming increasingly mature [1]. Due to the strong coupling and nonlinearity of variable frequency speed control asynchronous motor systems, it is difficult to analyze them using analytical methods. As an important means of system analysis and research, simulation technology has been increasingly widely used. Through simulation, the correctness of theoretical analysis and design can be verified, the operation process of the actual system can be simulated, the variation law of system characteristics with parameters can be analyzed, and the state and characteristics of the system can be described.
Reference [2] used the Simulink module in MATLAB software to model and simulate the variable frequency speed control system of asynchronous motor with a torque inner loop and closed-loop vector control of speed and flux linkage. It basically solved the problems of poor speed dynamic characteristics and low motor torque utilization in general speed control methods. References [3-5] established the vector control and direct torque control operation model of the variable frequency system, extracted the waveforms of four key performance indicators, namely speed, stator line voltage, stator current and electromagnetic torque, as a function of time, and analyzed the dynamic response and steady-state tracking accuracy of speed.
The simulation studies in the above literature all focus on the constant frequency and constant voltage operation of the motor. However, actual variable frequency speed control systems are often in a dynamic speed regulation process, with the output frequency and output voltage changing over time. Therefore, constructing a simulation platform for the dynamic speed regulation process of a variable frequency speed control asynchronous motor system and analyzing the changing patterns of electromagnetic parameters during speed regulation is of great guiding significance for the design, application, and fault diagnosis of speed control systems.
This paper will design a general-purpose dynamic simulation platform for variable frequency speed control asynchronous motor systems in the Simulink environment of MATLAB.
2. Principle of Constant Voltage-Frequency Ratio Speed Regulation for Motors
The speed of an asynchronous motor
(1)
Therefore, there are many ways to adjust the speed of a three-phase asynchronous motor, one of which is to change the frequency of the power supply.
Voltage per phase of a three-phase asynchronous motor
(2)
3 Simulation Model of Asynchronous Motor Variable Frequency Speed Control System
In the MATLAB 7.5 environment, a simple AC-DC-AC variable frequency speed control system was built using the Simulink module and the constant voltage-frequency ratio control method, as shown in Figure 1.
Figure 1. Simulation model of the asynchronous motor variable frequency speed control system
3.1 Composition of the Variable Frequency Speed Regulation Simulation Model
The simulation model of the variable frequency speed control system established in this paper consists of a rectifier, an inverter, an SPWM variable frequency control signal generator, a motor, and a detection module. The voltage source is rectified and filtered to obtain a DC constant voltage source Ud. The inverter circuit uses IGBT as a switching device. The SPWM variable frequency signal control generator outputs a control signal with a linear constant acceleration change in frequency to control the conduction and cutoff of the IGBT in the bridge inverter circuit, generating a voltage with the same frequency as the control signal, and completing the constant voltage-frequency ratio speed regulation of the motor. A detection module is provided on the motor side to observe the motor speed, stator current, rotor current, electromagnetic torque and other index parameters. The output voltage information of the inverter circuit includes the distribution of the fundamental and harmonic components and their amplitude changes, which are mainly affected by the modulation ratio M and carrier ratio N in the SPWM module [6].
3.2 SPWM Design
The SPWM module is a key component of the entire constant voltage-frequency ratio model. Based on the area equivalence principle, SPWM uses a sine wave with frequency f increasing linearly with time as the modulation wave and a triangular wave with frequency f1=Nf as the carrier wave. It employs a natural sampling method, sampling the pulse width and gap time at the intersection of the sine wave and the triangular wave to generate the SPWM waveform, which controls the inverter circuit and thus the inverter output voltage and frequency.
The SPWM module is encapsulated, and its internal structure is shown in Figure 2. The input consists of one Clock module and three Constant modules. In constant acceleration simulation, the values of a and b are set as needed. After calculation and conversion by the Fcn function module, a "sine wave" with frequency f = a + bt varying with time and a "triangular wave" with the corresponding frequency f1 can be obtained. k represents the initial phase of the sine wave, and the parameter is set to [0, -1, 1], with each phase differing by 120°. The carrier ratio N is automatically adjusted in segments in the Fcn module according to the range of f, while the modulation ratio M remains constant.
Figure 2 SPWM module
3.2.1 The impact of carrier ratio variation on inverter output
SPWM has an important parameter—the carrier ratio N, which is the ratio of the carrier frequency to the modulation frequency. During constant-speed operation, the carrier ratio N is fixed. Experiments have shown that a larger N results in more pulses per cycle, leading to a reduction in higher-order harmonic components, smaller motor torque ripple, and more ideal torque and current performance. However, if N remains constant, the modulated carrier frequency will be too high when the inverter outputs at high frequencies, making it difficult for the switching devices to withstand the load.
Therefore, segmented synchronous modulation is adopted in this design, that is, the carrier ratio N changes in segments. The entire frequency conversion range is divided into five frequency bands, and the carrier ratio N is kept constant in each frequency band, while different values of N are used in different frequency bands, as shown in Table 1. N is taken as a multiple of 3 to strictly ensure the 120° symmetry between the three-phase output waveforms and effectively eliminate the harmonic components of the inverter output voltage.
Table 1 Segmented Modulation Carrier Ratio
3.2.2 The impact of modulation ratio variation on inverter output
To analyze the effects of the modulation ratio M on the inverter output voltage, as well as on the motor's electromagnetic torque, current, and speed, M = 0.25, 0.3, 0.35, ..., 0.9 was selected in the simulation module shown in Figure 1. Through a series of simulations, it can be found that, under the condition that other factors remain unchanged, the modulation ratio changes as follows:
(1) The amplitudes of the fundamental and harmonic frequencies of the line voltage changed under different modulation ratios M. As the modulation ratio M decreased, the waveform distortion of torque and other components intensified, the harmonic amplitude increased, and the adverse effects on the motor were aggravated. Under the same conditions, the amplitude of harmonics decreased as the modulation ratio M increased. In addition, according to the simulation graph, the vibration amplitude of the stator current waveform and electromagnetic torque waveform output by the asynchronous motor decreased as the modulation ratio M increased, and the corresponding harmonic losses and torque pulsation also decreased. However, in order not to reduce the intermediate DC voltage too much, it is more reasonable to choose a modulation ratio close to 1.
(2) The harmonic distortion of the stator current also decreases as the modulation ratio M increases. Through simulation, it was found that its value is much smaller than the distortion of the inverter output line voltage. This shows that the motor winding has a filtering effect on the voltage, and the magnitude of its filtering effect is related to the parameters of the motor.
3. Simulation Results and Analysis
Figure 3 Simulation curves of current, speed and torque during constant acceleration.
Simulations verify that when the motor is running under constant load, the angular velocity is 120 rad/s when f = 40 Hz and 150 rad/s when f = 50 Hz. As the modulation wave frequency increases linearly from 40 Hz to 50 Hz over time, Figure 2 shows that, ignoring the motor starting state, the motor angular velocity increases linearly from 120 rad/s to 150 rad/s, with the same increasing trend as f over time. The current is a "sine wave" with gradually increasing amplitude, and the motor is running in a linear constant acceleration state.
3.1 Stator Voltage Analysis
In previous voltage signal processing, Fourier and wavelet decomposition methods were commonly used. In 1998, Huang et al. proposed the Empirical Mode Decomposition (EMD) method [7-8]. It decomposes signals based on the time-scale characteristics of the data itself, without the need to pre-define any basis functions. This is fundamentally different from Fourier and wavelet decomposition methods that are based on a priori harmonic basis functions and wavelet basis functions. Due to this characteristic, the EMD method can theoretically be applied to the decomposition of any type of signal, and therefore has a high signal-to-noise ratio in processing non-stationary and nonlinear data, showing significant advantages. Therefore, this paper uses this method for time-frequency analysis of stator voltage.
The stator input line voltage of the motor, as shown in Figure 4, consists of square wave signals with different duty cycles and constant amplitudes. After EMD decomposition, the fundamental signal of the line voltage is extracted, revealing it to be a sinusoidal signal with constant amplitude and linearly increasing frequency. Time-frequency analysis of the line voltage shows that its instantaneous frequency perfectly matches the given value f, thus proving the correctness of the established model.
Figure 4. Instantaneous frequency of stator line voltage fundamental voltage during constant acceleration
3.2 Stator Current Park Vector Analysis
Figure 5. Simulated current Park vector diagram
4. Conclusion
This paper establishes a dynamic simulation model of an asynchronous motor variable frequency speed control system under normal motor conditions. The model is intuitive and realizes the constant acceleration operation of the variable frequency speed control system. Further signal analysis of the output voltage and current verifies that the instantaneous frequency of the voltage after EMD decomposition is the same as the given frequency value, and the current vector diagram after Park transform basically conforms to the theoretical inference, proving the correctness of the established model. This paper only analyzes the linear constant acceleration process. If other motor operating modes, such as constant speed, constant deceleration, nonlinear acceleration, and deceleration, are to be studied and analyzed, only the given frequency of the modulation wave of the SPWM module needs to be modified on this basis, demonstrating strong versatility. Furthermore, based on this model, other fault diagnosis methods, such as wavelets, Fourier transforms, and neural networks, can be combined for further fault diagnosis research.
About the author:
Liu Zhenxing (1965-), male, PhD, professor, doctoral supervisor. His research interests include fault diagnosis technology for rotating equipment and modern signal processing technology and its applications.
Shen Kaixiong (1983-), male, master's student, research direction: motor fault diagnosis.
Li Yuetang (1985-), female, master's degree, research direction: automated instrument testing.
Funding: This work was supported by the National Natural Science Foundation of China (60874109).
Mailing Address: Room 607, Building 10, School of Information Science and Technology, Wuhan University of Science and Technology
Contact email: [email protected]
Contact number: 15927005730
