2.1 FOC Based Variable Frequency PMSM Drive

The following figure shows a simple block diagram of a typical FOC-based PMSM variable frequency drive:

The following steps summarize the basic sensorless FOC operation:

1. Measure the current flowing in the motor: There are several ways to measure the three-phase currents flowing in the motor. Some popular options include measurement using hall-effect, transformer-based magnetic sensors, or using strategically placed one, two or three shunt resistors.
2. Estimate the rotor flux angle: The rotor flux angle can be estimated either by back EMF-based algorithms or by high-frequency pulse injection.
3. Transform the measured current to αβ reference frame: The measured phase currents of the motor are transformed to two orthogonal components in the αβ reference frame by the Clarke Transformation.
4. Transform the αβ current to dq current: The αβ are transformed to two orthogonal components in the dq reference frame using the estimated rotor flux angle by the Park Transformation.
5. Compare the measured dq current with the desired current and generate an error signal: The desired q-axis reference current for controlling the torque, and desired d-axis current for controlling the flux are compared with their corresponding measured quantities to generate the respective error signals.
6. Calculate control voltage from the error signal: The error signals are used to calculate correction voltage. Conventionally a closed-loop feedback mechanism using a PI regulator is used for the task.
7. Apply control voltage to motor terminals: The correction voltage in the dq reference frame is transformed back to voltages in the abc reference frame. These voltages are applied to the motor terminals by some power switching techniques. Conventionally in MCU-based systems, PWM modulation techniques, such as Space Vector modulation are used for the task.

The following section describes various blocks of the FOC-based variable frequency drive in greater detail.