What Is Current Load?

Load current refers to the value of the stator current actually detected when the motor is dragging the load. This value varies with the size of the load.

Load current refers to the current when the electrical equipment normally completes its work task. For example, an electric vehicle starts in neutral and the motor turns. At this time, no-load current flows through the motor. When the gear is running, the current is the load current. In general, the no-load current should be significantly less than Load current, otherwise the efficiency of this electrical equipment is low.

The rated current carrying capacity of the cable can be determined in accordance with the IEC60287 standard formulated by the International Electrotechnical Commission, and a large number of research results of relevant personnel at home and abroad can be used as a supplement and improvement to this standard. Cable due to ground fault (

Research Background

The three-phase voltage-type PWM rectifier can realize grid-side current sinusoidalization, unit power factor operation, and two-way energy transmission, which truly realizes "green energy conversion". In the application of PWM rectifiers, the load changes are generally drastic. Although traditional double closed-loop PI control can achieve ideal steady-state performance, due to its inherent shortcomings in the control system structure and the hysteresis of PI adjustment, the DC voltage during dynamic processes will Large deviations are not conducive to high-quality system operation.

Adding load current feedforward control on the basis of double closed-loop control is the main method to improve the dynamic performance of PWM rectifiers against load disturbances. Combining the mathematical model of the PWM rectifier in the two-phase synchronous rotating coordinate system, the structure diagram of the dual closed-loop PI control system of the PWM rectifier is given. From the control perspective, the shortcomings of the dual closed-loop PI control in overcoming the load disturbance are analyzed, and the load is specifically designed. Perturbation feedforward compensator. The analysis shows that the dynamic feedforward compensator that can theoretically achieve full compensation for load disturbances is difficult to implement in practical systems. To simplify the problem, a static feedforward compensator expression is obtained, and the compensation mechanism of the load current feedforward control strategy is analyzed And the factors that affect the effect of static feedforward compensation. Finally, the correctness of the theoretical analysis is verified by experiments, and a method to further improve the dynamic performance of PWM rectifiers against load disturbances is proposed.

PWM Rectifier Load Current Feedforward Control

To improve the ability of the PWM rectifier to overcome load current disturbances,

Figure 2 Voltage loop structure diagram with load current feedforward control

Based on the control system structure, load current feedforward control is added. Let the current loop closed-loop transfer function be W _ci (s) and the feed-forward compensator transfer function be G _f (s). Because the output of the feed-forward compensator is used as part of the given value of the current loop, the load current feed-forward control is added. The voltage loop structure is shown in Figure 2.

In practical systems, due to the nonlinear and time-varying characteristics of parameters such as L and R in the main circuit of the PWM rectifier, it is difficult to accurately determine the expression of W _ci (s), and the denominator order of W _ci (s) is greater than the numerator order. In order to simplify the problem, considering the fast response of the current loop of the PWM rectifier, and because the current loop adopts PI regulation, the current response has no static difference. If the current loop adjustment delay is ignored, the current loop closed-loop transfer function W _ci (s) = 1, then there is

The voltage loop structure after adding the I _L static feedforward control is shown in Figure 3. With reference to Figure 3, it can be seen that

Figure 3 Voltage loop structure diagram with load current static feedforward control

The current response speed of the current loop of the current transformer is much faster than that of the voltage outer loop. After the I _L feedforward control is added, when the I _L changes, the given value of active current i _d * changes rapidly through the feedforward effect. Fast adjustment, id and I _dc change rapidly to compensate for the change in I _L , greatly reducing the impact of I _L changes on U _dc . The feed-forward compensator determined according to the above formula is extremely easy to implement, but since only static compensation for load disturbance is realized, U _dc still has obvious dynamic deviation when the load disturbance is severe. When C is constant, U The magnitude of _dc dynamic deviation mainly depends on the magnitude of the current loop adjustment delay. The introduction of disturbance feedforward compensation does not affect the stability of the original closed-loop system, and the existence of closed-loop control reduces the requirements for the accuracy of load current detection and the accuracy of the feedforward compensator.

Analysis conclusion

Combined with the mathematical model of three-phase voltage-type PWM rectifier in two-phase synchronous rotation d, q coordinate system, the structure diagram of the dual closed-loop PI control system of PWM rectifier was established, and the load disturbance feedforward compensator was designed. The implementation difficulties of the dynamic feedforward compensator are analyzed, and a simplified static feedforward compensator is obtained. The experimental results show that the static feedforward compensation of load current can significantly improve the anti-load disturbance dynamic performance. When the load changes drastically or the anti-load disturbance dynamic performance of the PWM rectifier is highly demanded, the following methods can be used to further improve the anti-load disturbance dynamic performance of the PWM rectifier based on static feedforward compensation: Improve the speed of the current loop adjustment This method can be implemented by appropriately adjusting the parameters of the current loop PI regulator or appropriately reducing the AC side inductance of the PWM rectifier; Estimating the magnitude of the current loop delay and appropriately adding a load that can suppress high frequency noise in the feedforward compensator The differential term of the current is to approximately cancel the delay of the current loop regulation. ^[3]