Principle of Frequency inverter Current Tracking

The current tracking frequency inverter system contains five parts: the current waveform generation part, the PI regulator, the PWM part, the inverter and the motor, the first three belong to control part, and last two belong to power part, all of these form a current closed loop feedback system. The framework of the system is shown in Figure 1. In the diagram, PWM1 to PWM6 represents 6 PWM signals input to inverter part, U, V and W represent 3 output voltages of the three-phase bridge, M represents an induction machine. In the current waveform generation module, 3 sine waves are generated as the standard three phases by a 32-bit microcontroller, the phase difference between each two phases is 120^o. i_w and i_v are the two output currents of the frequency inverter, which are measured by hall current sensors. The three currents satisfy that i_u + i_v + i_w = 0, so the third phase current can be obtained by the formula: i_u = -i_v -i_w. The proportional-integral regulator (PI regulator) uses the two feedback currents and the three standard sine signals to calculate a new duty cycle for each bridge.

Fig 1. The Framework of Frequency inverter System

Give one of the phases U as an example. If the given standard sinusoidal current signal is i_u, while the actual output current signal is i_u. In each PWM cycle's interrupt, the actual signal is compared with the given signal, and the current deviation △i_u is obtained. When △i_u is passed through an anti-windup PI regulator, the inverter's duty cycle is changed, so as to adjust the output current. The current tracking diagram is shown in Figure 2. As seen from the figure, the higher the sampling frequency is, the closer the actual output current waveform to the standard sine wave, and also the higher the accuracy for the current control.

Fig 2. Current Tracking Diagram

In practical applications, the motor will sometimes start quickly or decelerate rapidly, and then the traditional PI regulator's instantaneous output deviation is too large to saturation, resulting in a system shock, increasing of adjustment time and degradation of the performance of the PI controller. In order to eliminate the adverse impact of such integral saturation, the anti-windup PI regulator is introduced here, as shown in Equation:

Where K_p is the proportional coefficient, K_I is the integral coefficient, e_k is the deviation value of the k-th sampling time input, u₀ is the initial value when the PI controller starts working, u_k is the output value of the k-th sampling time, u_max is the maximum limit amount of control, u_min is the minimum limit amount of control.