Generally variable frequency AC drive has following control modes: open loop vector control, V/F control, open loop torque control, closed loop vector control, slip frequency control.
Open-loop vector control
Apply to high performance general-purpose application with no PG encoder, one AC drive controls an electric motor only. Such as machine tools, centrifuge machines, wire drawing machine, injection molding machine, etc.
Suitable for the applications do not need high control precision, like fans and pumps. The variable frequency drive can be used for dragging multiple motors.
Open-loop torque control (without PG card)
Suitable for the system require less torque control accuracy, such as wire-wound, drawing machines, etc. In torque control mode, the speed of the motor is determined by the loads on the motor, its acceleration and deceleration speed is no longer controlled by the variable frequency drive acc/dec time.
Tip: in vector control mode, you need to know more about the electric motor parameters, in order to get full advantages of the vector control AC drive.
Slip frequency control
Slip frequency control is direct controlling the torque, which is based on the V/f control mode, in accordance with the actual speed of the induction motor corresponding power frequency, adjusting the variable frequency drive’s output frequency with desired torque, therefore make the induction motor have a corresponding output torque. In this control method, needs to install a speed sensor in the control system, sometimes needs to install additional current feedback to control the frequency and current. So it is a closed-loop control mode, the frequency drive has a good stability, and has a great response characteristics for fast acceleration and deceleration and load changes.