Loading Process
The compressor loading process is the process of transitioning from an unloaded (no-load) state to constant pressure control, reaching the pressure setpoint, and supplying air to the system. When the control system receives the loading command, it compares the actual system pressure with the set pressure. When the system set pressure is greater than the actual pressure, the inlet guide vanes gradually open from 10% (no-load state), and the motor current increases. When the current exceeds 230A (TL setpoint), the blow-off valve begins to close, and the inlet guide vanes and blow-off valve regulate together to maintain the motor current at 230A until the three-stage pressure rises, the system pressure reaches the setpoint, and enters the constant pressure control state. The speed and success of this loading process depend on the proportional gain and integral time of the control parameters. When the system demand is high, the integral time should be increased to reach the pressure setpoint faster; conversely, when the system demand is low, it should be decreased, otherwise, it will be too sensitive and cause valve oscillation and loading failure.
Control after Successful Loading
Since the system air volume demand is variable, the compressor's task is to maintain constant system pressure through the control system's regulation when the system demand changes, acting as a constant pressure source. This control is achieved by the joint regulation of the inlet guide vanes and the blow-off valve. The control mode is high-load limiting, throttling limiting, plus a proportional-integral algorithm. High-load limiting restricts the maximum opening of the inlet guide vanes when needed to protect the motor, and throttling limiting restricts the minimum opening of the inlet guide vanes to protect the compressor. The introduction of motor current into the pressure control loop is a characteristic of this loop. The first function of introducing motor current is to protect the motor from overload, which is called High Load Limit (HLL); the second function is to determine the threshold at which the blow-off valve begins to open, which is called the Throttle Limit (TL). In fact, the inlet guide vanes and blow-off valve are controlled in a split-range manner.
Control during the Unloading Process
After the unloading command is issued, the controller will smoothly transition the inlet guide vanes from their current opening to the unloading point opening, which is 10%, while the blow-off valve will fully open. This solution implements one-button loading and unloading functionality for the centrifugal air blower, allowing the unit to fully utilize the motor's potential and maximize work output at maximum current. At minimum current, under low load conditions, the system opens the vent valve when approaching the surge limit, reducing energy loss caused by venting. This achieves fully automatic control and one-button startup, realizing truly intelligent control.
