In a closed loop system, the drive monitors the output (for example, pressure or flow rate) and continually adjusts the motor speed to try to maintain the desired value. Applications such as controlling temperature, pressure and flow rate in air handling systems, or pressure in pumping systems are well known and have been described in an earlier article.
But there are plenty of other possibilities. The drive can control a vacuum level in applications such as milking parlours; this has shown significant savings. Other closed loop systems control humidity in greenhouses (adjusting the water misting system) or Carbon Dioxide levels in buildings.
A neat application is shown in Figure 1. The conveyer feeds stones into the crusher; the crusher runs at fixed speed. The variable frequency drive monitors the load on the crusher (by measuring the crusher motor current) and adjusts the conveyer speed to bring supply or less stone. This allows the crusher to run at optimum load all the time, improving throughput. A real improvement in many applications such as sugar crushing, wood powdering etc.
Torque control and systems with encoders (such as cranes and elevators) are special applications of closed loop control. Getting the best out of solar panels (when used with a drive for remote irrigation pumps) is another application where closed loop control makes a big improvement.