Current detection and control signal generation circuit

The three-phase uncontrolled rectifier in Figure 1 is replaced by a controllable converter, and three high-frequency choke reactors are connected in series at the three-phase power input end to suppress possible bidirectional (grid-servo system) electromagnetic interference, and to play the role of an equivalent DC reactor when the converter works in the inverter state, as shown in Figure 2.

When the motor is operating in the electric state, the high-power switching devices S1 to S6 of the controllable converter are all in the off state, and the six freewheeling diodes form a three-phase uncontrolled bridge rectifier, and the working condition is the same as Figure 1.

When the motor is working in the power generation state, the inverter works in the rectification state, and the controllable converter works in the inversion state, so that the motor works in the regenerative braking state. At this time, the filter capacitor stores energy, the DC bus voltage increases, and after exceeding the grid line voltage value, the diodes D1~D6 reverse block; when the DC bus voltage continues to increase and exceeds the set upper limit allowable value UdH, the converter starts to work and inverts the energy on the DC bus and feeds it back to the grid. At this time, the high-frequency choke reactor will balance the difference between the DC bus voltage and the grid line voltage to ensure the inverter state.

Figure 2 Feedback converter main circuit

Figure 1 General servo system main circuit structure

Figure 4 Current detection and control signal generation circuit

Normal operation. When the DC bus voltage drops back to the lower limit setting value UdL, the converter is turned off. The issues that need to be considered for energy feedback are:

1) The feedback current must meet the feedback power requirements and cannot be greater than the maximum current allowed by the inverter;

2) The inverter can only be started for energy feedback when the DC bus voltage is higher than the set value;

3) In order to increase the feedback power, try to perform feedback when the grid voltage is high, because if the feedback current is constant, the higher the grid voltage, the greater the feedback power.

Therefore, the system must have a voltage control circuit, a synchronous control circuit and a current limiting circuit. The current and voltage control is completed by two hysteresis comparators, and the synchronous control is completed by the synchronous detection and control circuit.