The variants of the scalar control include open-loop control and closed-loop control.
Open-loop The most common approach makes the voltage V proportional to frequency f (so called
V/f control,
V/Hz control,
Constant Volts/Hertz, CVH). Advantage of the V/f variant is in keeping the
magnetic flux inside the
stator constant thus maintaining the motor performance across the range of speeds. A voltage boost at low frequencies is typically employed to compensate for the resistance of the coils. An open-loop V/f control works well in applications with near-constant load torque and gradual changes in rotational speed. The controllers implementing this method are sometimes called
general purpose AC drives.
Closed-loop If sensors are utilized (
closed-loop configuration) for better/faster transitional response, the common approach uses a rotational speed sensor (so called
closed-loop V/Hz control). The speed error is passed through the
proportional-integral controller to create the accumulated
slip difference that is combined with the direct reading of the speed sensor into a frequency control signal. In a torque-control variant (TC, not to be confused with the
direct torque control a.k.a. DTC), the motor
torque is held constant in the steady-state, this requires a
current sensor. Frequency and flux (voltage or current, depending on the type of the drive) control signals are decoupled, with the flux control driven by the flux estimate, and the frequency control driven by the torque estimate and speed sensor data. The increased performance comes at the cost of additional complexity and associated potential stability issues. == References ==