69 Frequency Converters 2.7.4 Open Loop and Closed Loop Flux Vector Control Flux vector control is also referred to as fi eld-oriented control. The control methods referred to above control the motor magnetic fl ux via the stator. With fi eld-oriented control, the rotor fl ux is controlled directly. The following motor variables are controlled within this context: • Speed • Torque Once the rated data for the motor has been entered, a magnetic fl ux model can be used to determine the necessary voltage and angle for ensuring optimum motor magnetisation. The measured motor current is converted into a torque- generating current and a magnetising current.
An internal PID controller is responsible for controlling the speed, with the feedback value being estimated on the basis of the measured motor current. 2.7.4.1 Flux Vector (Open loop) Flux control requires accurate information about the condition, temperature, rotor position of the motor. It is a challenge to run open loop while the motor condition is being simulated. Obtaining optimum performance can be diffi cult, especially at low motor speed. Typical shaft output: • Speed setting range 1:50 • Speed accuracy (steady state) ± 0.5% of rated frequency • Acceleration torque 100-150% of rated torque • Speed change response time 50-200 ms • Torque change response time 0.5-5 ms