Abstract: This paper proposes an optimized VF control strategy based on the traditional VF control algorithm, incorporating functions such as pre-excitation, stator current suppression, and resonance suppression. Pre-excitation increases the torque output at motor startup; current suppression limits the current amplitude during startup, preventing overcurrent tripping; and resonance suppression eliminates motor oscillations in certain frequency bands. Simulations and experiments demonstrate the correctness and effectiveness of the algorithm.
1 Introduction
Inverter drive control is generally divided into VF open-loop control, vector control, and direct torque control. VF control is an open-loop control method with a simple control algorithm, low implementation cost, no dependence on motor parameters, and high system robustness, but its speed control accuracy is not high and its dynamic response is slow. Vector control can decouple flux and torque through rotating coordinate transformation, giving AC motors excellent control characteristics similar to DC motors, and high speed control accuracy, but this control method is greatly affected by motor parameters. Direct torque control keeps the stator flux amplitude constant and directly controls the electromagnetic torque by controlling the motor load angle, resulting in a faster dynamic response.
Compared to vector control and direct torque control, VF control lags behind in speed control accuracy and dynamic response speed. However, due to its simple implementation, low cost, and high robustness, it remains widely used in AC speed regulation. Furthermore, from a system versatility perspective, VF control is the most basic and widely applicable control method in AC variable frequency drives. Therefore, improving and optimizing the control strategy based on traditional VF control methods to reduce or compensate for its shortcomings in control accuracy and response speed has become an important task. This paper proposes a control strategy that optimizes VF control performance through functions such as pre-excitation, stator current suppression, and resonance suppression. The control strategy is analyzed and verified through simulation and experiments.
2VF control principle
According to the principles of electrical machinery, the expression for the phase electromotive force of an asynchronous motor is:
In the formula, f1 is the stator power supply frequency, N1 is the number of turns per phase of the stator winding, KN1 is the winding coefficient, and φm is the main magnetic flux.
It can be seen that when the value of E1/f1 remains constant, the main magnetic flux φm remains constant. However, the electromotive force E1 cannot be directly controlled. Therefore, we keep the main magnetic flux constant by keeping the ratio of stator voltage U1 to frequency f1 constant. At higher frequencies, the stator voltage is higher, and the voltage drop across the stator resistance can be ignored; however, at lower frequencies, the effect of the stator resistance voltage drop cannot be ignored, and the low-frequency performance of VF control needs to be improved through stator voltage compensation.
The overall block diagram of the control strategy adopted in this paper is shown in Figure 1. Based on traditional VF control, pre-excitation, stator current suppression, and resonance suppression functions are added. The pre-excitation function can establish magnetic flux before the motor starts, ensuring a certain torque output at startup and accelerating the motor's starting speed. The stator current suppression function can suppress the current magnitude during motor acceleration to stay within the allowable range, preventing overcurrent protection tripping. The resonance suppression function can effectively suppress the occurrence of motor oscillations in certain frequency bands.
3. Pre-excitation function
In variable frequency speed control systems for asynchronous motors, the motor often starts slowly due to insufficient starting torque, resulting in excessive starting current. This can cause overcurrent alarms and tripping of the system, and may also damage the power electronic components of the frequency converter or reduce the service life of the device.
An effective method to improve the starting torque under VF control is DC pre-excitation, which involves applying a DC voltage to the stator side before the motor starts, thereby injecting DC current into the motor and establishing a magnetic flux with a fixed direction and amplitude. Since the direction is fixed, the rotor winding will not cut the magnetic lines of force, so it will not rotate. After the pre-excitation time ends, the motor enters the normal VF starting state.
The theoretical analysis of DC pre-excitation is as follows: The single-phase equivalent circuit of the asynchronous motor is shown in Figure 2. In the figure, R<sub>s</sub> is the stator voltage, R<sub>s</sub> is the stator resistance, R<sub>r'/s</sub> is the rotor resistance referred to the stator side, is the stator leakage reactance, is the rotor leakage reactance referred to the stator side, R<sub>m</sub> is the magnetizing resistance, L<sub>m</sub> is the magnetizing reactance, is the stator current, and is the rotor current.
The formulas for stator flux linkage, rotor flux linkage, and air gap flux linkage of the motor are as follows:
When the excitation current is stable, the rotor current is 0, and equation (2) can be written as:
It can be seen that the stator flux linkage, rotor flux linkage, and air gap flux linkage are in the same direction. According to the vector control theory of asynchronous motors, the torque formula in the synchronous rotating coordinate system is:
It is known that the output torque is maximized when the stator current and stator flux linkage are 90° apart. Therefore, by changing the output voltage vector by 90° at the end of pre-excitation, i.e., outputting a stator current vector that is 90° apart from the flux linkage vector, the output torque at the moment of startup can be increased, thus improving the startup performance.
The specific implementation method of DC pre-excitation in this paper is shown in Figure 3. First, the pre-excitation time and pre-excitation current command are given according to the motor excitation time constant. A given voltage amplitude is obtained using a closed-loop current magnitude output, and an initial voltage vector angle is given. When the pre-excitation time is reached, the output voltage vector angle increases by 90°, making the angle between the current vector and the flux linkage vector at that instant 90°, resulting in maximum output torque. Afterward, the motor gradually returns to the VF curve for startup and operation. The values of the pre-excitation time and the excitation current are specified.
4. Stator current suppression
The function of stator current suppression (Imax control, the same below) is to ensure that the stator current value of the motor does not exceed the set value during the start-up process.
As shown in Figure 2, the increase in motor stator current is mainly due to two reasons: first, the slip *s* increases, resulting in a very small equivalent impedance on the right side of the circuit, thus increasing the stator current; second, the stator voltage increases, leading to a larger stator current even with a constant equivalent impedance. To address the first reason, the slip can be reduced by decreasing the synchronous frequency output of the inverter, thereby reducing the stator current. To address the second reason, the stator current can be reduced by decreasing the amplitude of the output stator voltage.
Based on the above analysis, the control block diagram of Imax control is shown in Figure 4. The entire control function consists of a frequency regulator and a voltage regulator. The output of the former acts on the output frequency, and the output of the latter acts on the output voltage. Both controllers are implemented by PI regulators. To limit the current, the regulator output must be set to an upper limit of 0. This is so that when the actual stator current value does not exceed the current setting value Imax_set, the PI regulator will not function, preventing the current from being increased. When the frequency regulator is active, the output frequency decreases, which can continue to decrease until it reaches the minimum allowable frequency. At the same time, the speed-given ramp output remains constant. If the current still does not decrease to the required range, the voltage regulator is needed to further reduce the current. When the current is limited below Imax_set, the output frequency continues to start along the originally set ramp.
5. Resonance Suppression
When asynchronous motors operate under no-load or light-load conditions in VF open-loop control mode, they often exhibit operational instability within a certain frequency range, characterized by torque fluctuations, significant current variations, and frequency shifts—a phenomenon known as current oscillation. Many papers have analyzed the causes of this phenomenon, which is a highly complex issue related to numerous factors such as motor parameters, DC filter capacitors, carrier frequency, and system resonant frequency. This phenomenon typically leads to overcurrent alarm tripping, reducing system reliability and stability; therefore, effective suppression of this phenomenon is essential.
When a motor oscillates, the stator output frequency remains constant, but the slip fluctuates, meaning the active current component fluctuates. Therefore, by extracting the oscillating active current component and feeding it back to the output frequency, slip fluctuations can be suppressed, thereby achieving the goal of suppressing current oscillations.
This paper proposes an oscillation suppression method based on the active component of stator current, and its specific control block diagram is shown in Figure 5.
The three-phase stator current is collected and transformed using Clark and Park transforms to obtain the active component iq and reactive component id of the stator current. The transformation angle is obtained by subtracting the stator voltage vector angle, which is approximately equal to the rotor flux linkage angle. The active current component iq is filtered (by module M2 in the diagram) and then subtracted from iq to obtain the oscillating component contained in the active current. The oscillating component is then applied to a given frequency fset after passing through a proportional coefficient Kp, resulting in the final output frequency fout. Module M2 in the diagram is used to control the frequency range in which the resonance suppression function operates. By setting the two parameters f1 and f2, the starting and ending frequencies of the resonance suppression function are determined.
6. Simulation Study
This paper presents a PSIM simulation of the aforementioned control strategy. The motor parameters used in the simulation are shown in Table 1. Based on the motor parameters, baseline values for each variable were set for both the simulation and experimental purposes. The voltage baseline value was the peak phase voltage of 310.23V, the current baseline value was the peak phase current of 32.23A, and the frequency baseline value was 50Hz. All other baseline values were calculated from these baseline values. Furthermore, the calculated rated load of the motor was 71.94 Nm.
6.1 Simulation of pre-excitation function
The simulation results of the pre-excitation function are shown in Figure 6. The motor load is the rated load, and the variables are all per-unit values.
Figure 6(a) shows the current waveform of the motor when it starts directly without the pre-excitation function; Figure 6(b) shows the starting current waveform of the motor after the pre-excitation function is added; Figure 6(c) shows the speed tracking simulation waveform without the pre-excitation function. It can be seen that the actual speed tracking is slow and there is a delay in starting; Figure 6(d) shows the speed tracking simulation waveform with the pre-excitation function. The actual speed tracking is fast and there is basically no delay in starting.
6.2 Simulation of Imax control function
The simulation waveform of the Imax control function is shown in Figure 7. The motor load is the rated load, and the acceleration time is 2s. Without the Imax control function, the current waveform during motor startup is shown in Figure 7(a). During motor startup, the current magnitude exceeds 1.0 in the range of approximately 0.2s to 0.45s, with a maximum value of about 1.7, indicating a relatively large starting current. With the Imax suppression function enabled and the current limit Imax_Set=1.0, the motor startup waveform is shown in Figure 7(b). Throughout the startup process, the stator current magnitude is consistently suppressed to around 1.0 or below, effectively controlling the current magnitude during motor startup and preventing excessive current from causing overcurrent alarms or even tripping.
6.3 Simulation of Resonance Suppression Function
Given a set of motor parameters, it is easy for oscillations to occur in the 20~30Hz frequency band under given system parameters. Based on this, this paper conducts simulation. The motor parameters are shown in Table 2.
Figure 8 shows the simulated waveforms of resonance suppression. Figure 8(a) shows the three-phase current waveforms at 20Hz without resonance suppression, which shows that severe oscillations occur. Figure 8(b) shows the three-phase current waveforms at 20Hz with resonance suppression, which shows that the current becomes symmetrical and sinusoidal across the three phases.
7 Experimental Verification
Based on the simulation, this paper conducted an experiment, and the parameters of the experimental motor are shown in Table 1.
7.1 Pre-excitation experiment
The experimental waveform of the pre-excitation function is shown in Figure 9.
The experimental results are consistent with the simulation results. By pre-establishing a magnetic field for the motor through DC pre-excitation, the output torque of the motor at the moment of startup can be effectively improved, and the motor startup speed can be accelerated.
7.2 Imax Control Experiment
During the experiment, the motor was unloaded and the acceleration time was 2 seconds. Figure 10(a) shows the stator current waveform without the Imax function. It can be seen that the current magnitude is relatively large in the initial 0.5 seconds of motor startup, with a maximum of about 0.75. Figure 10(b) shows the stator current waveform after the current suppression function is enabled. Imax_Set is set to 0.45, and it can be seen that the current is effectively limited to the region of 0.45 and below.
The experimental waveforms show that the Imax suppression function can effectively control the motor acceleration current within the required modulus range, preventing overcurrent tripping.
7.3 Resonance Suppression Experiment
Experiments revealed that the experimental motor oscillated within a frequency range of approximately 10Hz to 33Hz. Therefore, the resonance suppression start frequency was set to 5Hz and the resonance suppression end frequency to 40Hz to cover the oscillation frequency range. Furthermore, by adjusting the filter time constant and proportional control coefficient in the resonance suppression function, the oscillation was effectively suppressed. Figures 11(a) and (b) show the three-phase current waveforms of the motor before and after suppression at a speed of 500rpm, respectively. As can be seen from the figures, after adding the resonance suppression function, the current oscillation was effectively suppressed, and the three-phase currents became symmetrical and sinusoidal.
8 Conclusions
This paper investigates the implementation of functions such as pre-excitation, stator current suppression, and resonance suppression based on the traditional VF control method, in order to optimize the control performance of VF control.
By pre-exciting the motor with DC before starting, the output torque at the moment of motor start-up can be effectively increased, thus speeding up the motor start-up.
By collecting the three-phase stator current and monitoring its magnitude, and using two regulators that act on the output frequency and output voltage respectively, when the current magnitude exceeds the allowable value, the stator current can be limited to the allowable range by reducing the frequency or voltage, thus preventing overcurrent faults.
By performing vector decomposition on the three-phase stator current, its active current component is obtained, and its fluctuation component is extracted and applied to the output frequency, which can effectively suppress the oscillation phenomenon of the motor in a certain frequency band.
This paper presents a simulation study and experimental verification of the above control algorithm. The simulation results and experimental results effectively verify the accuracy and feasibility of the algorithm.
